Coal Trading Blog

Highwall Mining

noreply@blogger.com (Coal Trading) — Fri, 27 Jan 2012 12:08:00 +0000

Highwall mining is the practice of mining coal by tunneling into the exposed highwall face of a surface mine and removing the coal. This is accomplished in a number of ways including auguring, addcar systems, and Archveyor (Arch Technology Corporation) systems. Highwall mining allows the recovery of coal that would otherwise be lost, and offers high productivity measured in tonnage per man-day. (Walker, 1997). While highwall mining has been developed to extend the recovery of coal from surface mines, it does, additionally, provide an opportunity of examining how such methods may be adapted for the underground mining of thin seams.

Highwall auger systems are composed of three main components: the cutting head or heads, flights for moving the coal and the motorized drive. (Fig 3.1) The cutting head cuts into the face at right angles to the exposed face, while being pushed by the flights and turned by the drive unit. (Fig 3.2) As the head cuts coal, the flights carry the coal back out to the base of the highwall. When the flight reaches the extent of its length, the flight is uncoupled from the drive and another flight is added to the string. This is repeated until the drill string reaches the desired cutting length. Then a new hole is drilled further down the face, leaving a small pillar to support the overburden. (Clark, 1982).

The main weakness of this method is difficulty in maintaining straight holes over long distances. New radar technologies are being developed to maintain hole and to increase the auger range to over 600 feet. (Mowrey, Ganoe, and Monaghan, 1995).

Any curvature of the highwall also has serious effect on the attainable recovery. Concave highwalls cause auger holes to fan out, while convex highwalls cause auger holes to intersect. Both of these situations result in lower recoveries, and intersecting holes also pose a risk of collapse. The highest possible recovery comes from having a straight highwall face. (Fig 3.3) (McCarter, 1992).

The addcar system (Figure 3.4) was also designed for highwall mining. The current system is not applicable for thin-seam application, because it can only be used in seams of more than 90 cm or 35.4 inches in thickness. This system recovers up to 60% of reserves, using 12.5 m-long individually powered Addcars. The standard system depth is 365m, but the new upgraded ‘Highwall Hog’ has an extended range to 500m. The Addcar system utilizes a continuous miner and Addcars that utilize chain conveyors to remove coal from the entry. The advantages claimed for this system over its closest competitors are: 150% more penetration; 90% more annual production capacity; 82% more installed horsepower; and it can produce the same tonnage in a reduced length of highwall. (Walker, 1997).

The newest innovation in highwall mining comes in the form of the Archveyor. (Fig 3.5) This mining system receives its cutting power from a highly modified Joy 12CM continuous miner capable of cutting a 3.8m-wide from 1.8 to 4.9m thick. The Archveyor itself follows the miner into the heading, transporting the coal out. The Archveyor has drive units every 7.5m, enhancing both vertical and horizontal flexibility. A chain conveyor is used for coal transportation and when lowered and reversed, to move the system forward. When mining is underway, hydraulic jacks lift the Archveyor clear of the ground allowing the conveyor to transport coal. (Walker, 1997).

Figure 3.1 Highwall Auger Unit (Walker, 1997) reproduced with permission

Figure 3.2 Overburden Removal (McCarter, 1992) reproduced with permission

Figure 3.3 Auger Hole Pattern (McCarter, 1992) reproduced with permission

Figure 3.4 Highwall Addcar System (Walker, 1997) reproduced with permission

Figure 3.5 Archveyor System (Arch Technology Corporation) reproduced with permission

Current Underground Coal Mining Technologies

noreply@blogger.com (Coal Trading) — Fri, 27 Jan 2012 11:55:00 +0000

Introduction

This chapter presents an overview of the current methods of mining that are used in southwest Virginia and how these technologies are selected. These methods are all currently producing coal but, by themselves, do not have the flexibility to extend the life of the Virginia coalfields.

A vital factor in the selection of a mining method is the prevailing geology of the area. The properties of the confining strata and the coal itself, dictate which methods can and cannot be successfully employed in each seam. For example, in a longwall operation, an ideal strata sequence would have coal of a uniform thickness covering a large areal extent and a roof that is strong enough for adequate ground control, but not so competent that it inhibits controlled caving. By contrast, a room and pillar operation benefits from a strong roof, and this method allows greater flexibility for variations in the seam thickness and extent. Consideration must also be given to floor conditions, faulting, water, and depth. Depth is a very important factor, because as depth increases the stress on the support pillars increases, requiring the pillars to be larger. This means that less coal is extracted, causing the extraction ratio to decrease and results in room and pillar operations becoming uneconomical in deeper workings. Longwall operations may be preferred under those conditions.

Economics play a very important role in almost every decision made in the mining industry. Longwall mining systems are very capital intensive, requiring large initial investments and a long mine life to recover that investment. This limits the use of the longwall system to the larger mining companies that also have significant reserves.

On the other hand, room and pillar operations can deal with more variable geology and require less investment, which can be recovered over a shorter mine life. Consequently, this method is amenable to smaller reserve tracts.

Environmental conditions in the mine also tilt the scale in favor of one method or the other. A longwall face has more concentrated releases of gas and dust because the rate of coal extraction from a single face is greater. Room and pillar operations have less immediate emissions of pollutants, but the ventilation is more difficult to control.

Another factor that comes into play is component interdependence of a mining method. In a longwall setup, numerous conveyors, the shearer, and the hydraulic roof supports are all interdependent. If one component goes down, the entire system is off line. With such a high capital investment, downtime causes serious financial losses. In room and pillar mining, the system components are not so interdependent. If an element in the system fails the consequences are less severe.

Elements of the Longwall System

In conditions where roof control is difficult, the coal of significant lateral extent, and of sufficient thickness, longwall mining is preferred. Longwall mining offers the benefits of enhanced safety due to its system of face supports that cover the entire working face. This method also allows higher extraction ratios, conserving valuable coal reserves. Some other advantages of this system are its flexibility in dealing with greater mining depths, multiple seams, and a significant reduction in roof bolting. (Bibb, 1992)

There are also significant disadvantages to longwall mining, such as the high capital cost of the required equipment. It follows that interruptions to production can have a serious economic impact whether they are short term such as starting and stopping the shearer, or longer term as longwall equipment is moved from a depleted section to a new panel. There may be problems with gas well location, seam thickness, and in soft floor and roof.

There is also the possibility of this system being impractical beneath thick strong roof beds, due the size and cost of the required roof support as well as difficulties in controlled caving. Because of the areal extent required by a longwall section, variability in the seam thickness, roof and floor conditions, local faulting and the presence of wells can also limit the potential of this method.

There are two distinct types of longwall mining that are employed, advancing and retreating. While the advancing system is sometimes used in other countries, American companies prefer the retreating system. In the advancing method, the development entries progress slightly ahead of the advancing face and away from the main entries, while in the retreating system the entire section is developed prior to commencement of production from the longwall face at the inby end, and mines back towards the main entries. The focus of this chapter will be on the retreating system of longwall mining.

The development that is carried out for longwall pillars utilizes a continuous miner and the room and pillar method (Fig. 2.1). These entries are called the headgate and tailgate entries, with the ventilation return being the tailgate entry. When one panel is mined out, the headgate entry of the last adjoining panel often becomes the tailgate entry of the next.

The coal is removed from the extraction panel by a machine called a coal shearer that moves back and forth along the working face, riding on the sides of the armored face conveyor, fragmenting coal from the face, and dropping it onto the face conveyor (Fig. 2.2). This conveyor leads down the face and onto a main haulage conveyor in an entry of neutral ventilation. The roof above the working face is supported by automated hydraulic supports that are advanced as the mining progresses. Controlled subsidence is achieved by allowing the top to collapse behind the row of supports. The conveyor is snaked over, after passage of the shearer, by horizontal jacks attached to the self-advancing hydraulic supports.

Moving a longwall set up is a complicated, time consuming process, that entails dismantling, refurbishment and reconstruction of the computer control system, shearer unit, armored-chain conveyor and the support shields,. This process may involve the majority of the underground workforce for a period of one to two weeks at a cost of up to one million dollars. (Suboleski, 1998).

A method of longwall mining that was developed, primarily, in Germany was the coal plough (Fig 2.3). The coal plough replaces the shearer in a typical longwall setup. The coal plough can mine coal as thin as 18 inches, but beneath 30 inches currently available powered supports cannot be employed, greatly reducing the productivity of the system. The main mechanism of the plough is the armored face conveyor that has two main functions, to transport coal away from the face and to guide the plough unit on the face during mining. There are many problems with the plough system, such as the plough cutting into softer floors and deviating from the desired horizon. The plough also has difficulties in seams where the coal hardness is not reasonably constant across the face. The feasibility of this system is questionable in seams less than 24 inches in thickness.

Figure 2.1 Typical Longwall Panel Layout (not to scale)

Figure 2.2 Longwall face section with Conventional Double-Drum Shearer

Figure 2.3 Longwall Plough Arrangement (Clark, Cauldon, and Curth, 1982) reproduced with permission

Elements of the Room and Pillar System

The room and pillar system is the older and traditional method of underground coal mining in the United States. A typical layout is illustrated on Figure 2.5. Coal is removed from the working faces as the rooms are advanced. Cross-cuts, connecting the rooms are also mined leaving pillars of coal for support. The rooms and cross-cuts are typically about 20 feet wide and of a height consistent with that of the seam.

Prior to the development of continuous mining technology, the conventional room and pillar method was composed of undercutting the coal, drilling, blasting, and loading. These steps have largely been supplanted by continuous mining machines that combine the processes of fragmenting the coal from the face and loading it on to haulage equipment for movement to the main coal transportation system. This has greatly reduced the equipment needed to conduct room and pillar mining. The process is now simplified, using only a continuous miner, a means of haulage, and a roofbolter. The latter inserts steel, cable or fiber bolts into the roof, pinning together the immediate overlying strata to provide roof support.

The process is cyclic in the majority of current room and pillar operations. The continuous miner advances on one half of the face width, then the other for a distance such that the operator remains under supported roof. The continuous miner is then trammed to mine in an adjoining room while the roof is bolted. Mining and roofbolting operations alternate in any given room.

Several designs of continuous miners have been manufactured, such as the Joy Ripper Miner, boring miner, milling-head miner, auger miner, Jeffrey 101 MC Helimatic, and the boom-type miner. The most commonly employed type of continuous miner is the milling head miner. (Fig 2.4) This type of miner has a rotating drum that rips the coal from the seam. The fallen coal is then collected by gathering arms and moved toward the back of the miner by an onboard chain conveyor. This conveyor runs up an adjustable boom that is used to load a shuttle car, which carries the coal to the main belt. A typical system uses two shuttle cars traveling sequentially between the continuous miner and the loading point on to a belt conveyor.

In most current room and pillar systems, the coal is transported by shuttle cars from the continuous miner to a feeder breaker which, in turn, loads on to a section conveyor. A shuttle car is effectively a mobile bunker with a built in steel conveyor for the purpose of loading and unloading. (Fig 2.5) Shuttle car traffic is controlled to increase both loading time and mining efficiency. Scheduling the positions of all mobile equipment is very important to the efficient operation of a miner section. The roofbolter must be bolting roof, while the miner and shuttle cars are mining the next cut.

The production section is usually composed of either five or seven entries, employing split ventilation (Fig. 2.6). This means that fresh air is brought in through the central entry or entries and is split and directed across the working face. The return air is taken down the outermost entries and out of the mine. The entries are usually of the order of twenty feet in width. The depth of the overburden determines the size of the support pillars. The cut sequence is determined by pillar size, cut length, and number of entries.

Figure 2.4 Continuous Miner (Tamrock Inc. 1998) reproduced with permission

Figure 2.5 Shuttle Car (Stefanko, 1983) reproduced with permission

Figure 2.6 Room and Pillar Layout (Stefanko,R. 1983)

Development of an Underground Automated Thin-Seam Mining Method

noreply@blogger.com (Coal Trading) — Fri, 27 Jan 2012 11:29:00 +0000

Coal mining has been in progress in Southwest Virginia since before the time of the Civil War. During the period 1880 to 1930, the region changed from one of subsistence farming to economic dependence on the coal mining industry. As in many other mining regions, the following sixty years saw coal production being maintained or increased, using fewer employees, through the introduction of mechanized methods of mining. However, Table 1.1 shows that from a peak of 46.5 million tons in 1990, Virginia’s coal output has declined. This has occurred for a variety of reasons including competition from both domestic and overseas sources, environmental concerns, geological considerations and exhaustion of many of the thicker coal seams. (Holman, McPherson, and Loomis, 1999b).

Table 1.1 Virginia coal production and numbers of production-related employees since 1990+

In 1951 a detailed survey was conducted to determine the state of Virginia’s coal reserves. It was estimated that Virginia had 10.776 billion tons of bituminous coal in seams 14 inches and thicker. (Brown, 1952) Between 1951 and 1996, 1.577 billion tons were mined with estimated losses of 40%. (VCCER, Virginia Coal Directory, 1998) This effectively removed 2.208 billion tons from the resource leaving 8.568 billion tons. The original estimate for measured, indicated, and inferred reserves between seam thicknesses of 14 and 28 inches was 5.041 billion tons2. (U.S.G.S., 1952) Subtracting this from the 8.568 billion tons of remaining resource yields slightly more than3.5 billion tons of coal in seams greater than 28 inches. (Holman, McPherson, and Loomis, 1999b)

Using estimated values from Bureau of Mines studies on Appalachian coal the amount of economically mineable coal can be obtained. From the remaining 3.5 billion tons in seams thicker than 28 inches, 34% will be recoverable but only a little less than 9 percent can be considered as an economic reserve. (Rohrbacher, 1993). The latter represents 321 million economic tons. With an annual production of 36 million tons, this yields some 9 years of mining for the region. As reserves diminish, production will decrease, slightly extending this life.

It is the inevitable consequence of any mining region that the fuel or mineral resources that can be mined economically will eventually be depleted. If the region is to continue as economically viable then other industries and opportunities for employment must be developed well before mining ceases.* This may be encouraged and funded by utilizing some of the taxation revenues generated by the mining, as has occurred in Southwest Virginia through the Coalfields Economic Development Authority (CEDA).

In the case of Virginia’s Coalfields, it is estimated that a resource of over 5 billion tons of high quality coal remains in the ground in seams of between 14 and 28 inches thickness. The life of coal mining in Virginia would be extended greatly if techniques are developed to enable this valuable resource to be extracted in a manner that is both competitive and environmentally benign. An extension in the life of coal mining in Southwest Virginia would also extend the funds and time available to promote the development of other avenues of sustainable economic endeavor. However, it should not be expected that there will, ever again, be large numbers of persons employed in the coal mining industry of Virginia. In order to meet the challenges of the market, coal mining companies have little choice but to utilize increasingly sophisticated and automated equipment. If thin seams are to be extracted then those working places (faces) where the coal is actually won will be entirely manless. (Holman, McPherson, and Loomis, 1999b).

In 1996, the Virginia State Government enacted legislation that provided a tax credit to coal mining companies operating in the State. This was an enticement for those companies to remain in Virginia and to assist them in extracting some of the thinner seams. This has had the result of temporarily inhibiting the continuous decline in coal production. However, it can be no more than a stopgap. The only way that Virginian coal mining can be extended well into the next century, when coal will continue to be needed as a primary fuel source+ is through the development of thin seam mining technology.

Unfortunately, there are significant barriers to the research and development that is necessary for the introduction of new mining technologies. The selection of an underground mining method is influenced by numerous factors, including tradition. A pervasive idea is that what has worked in the past, will continue to work in the future.

This is a mode of operation that maintains the comfort of familiarity. Hard won experience with tried and true methods allows for sound decision making for as long as that method continues to be employed and remains viable. (Holman, McPherson, and Loomis, 1999b)

Many (but not all) American mining companies operate with a conservative outlook, and are reluctant to try something new until they see it working successfully elsewhere. The primary reason given for this unwillingness to innovate is low profit margins. This reluctance has resulted in a traditional lack of research and development by the American mining industries. Another contributing factor to this trend is that market trends often force coal companies to operate on a boom-bust cycle. While business is booming, many companies indicate that they have little need for research and development. Whereas when business is down they say they cannot afford it.

A further influence on this problem is that throughout most of the 20th century mining research and development has been conducted by government agencies in the U.S. and elsewhere. This effectively came to an end in 1996, in the United States, with dismemberment of the US Bureau of Mines, except for matters of safety and health that are now addressed by the National Institute for Safety and Health (NIOSH). The problem we are left with is that it is difficult to introduce new technologies and ideas that have potential to move the coal industry forward into the next century. New pathways that could yield safe, clean, and high production are not being explored, simply because they are new. For an innovative idea to bear fruit, it must first be tried. (Holman, McPherson, and Loomis, 1999b)

Within is presented a brief overview of current technologies utilized for underground coal mining in the United States. This is followed by a review of developments in highwall mining that show promise of having an application in the underground mining of thin seams. Some past attempts at thin seam mining are discussed. The more recent advances in the guidance systems used in autonomous mining machines are introduced. The state-of-the-art products of several manufacturers in addressing the integration of mining and continuous haulage systems are also highlighted.

All of that background is employed in outlining a conceptual mining system for the underground mining of coal seams in the 14 to 28 inch range of thickness. Most of the equipment suggested for this proposed system is currently available. Furthermore, the mining and manufacturing expertise necessary for the cooperative development of such a system is already available in Virginia and its neighboring states.

Underground thin-seam mining in Virginia cannot hope to achieve the productivity (in tons per man-hour) of western surface mines extracting much thicker seams. However, there are distinct incentives to continuing coal mining in Virginia where the coal is not only low in sulfur content, but has a calorific value some 20 percent higher than western coals. About 50 percent of Virginia coal is of metallurgical quality while proximity to the coal terminals in Hampton Roads allows a ready route to the export market. About 37 percent of Virginia coal is shipped overseas.

Furthermore, the environmental impacts of surface extraction are much greater than those of underground mining, a matter of increasing concern throughout the nation. These are some of the reasons that research into the feasibility of underground coal mining in Virginia is a worthwhile endeavor. (Holman, McPherson, and Loomis, 1999b).

Compositional Studies of Organic Material With Various Maturities

noreply@blogger.com (Coal Trading) — Mon, 05 Dec 2011 13:16:00 +0000

Introduction

Coal is a nonhomogeneous material composed of organic matter, mineral, volatile matter and moisture.
Coalification

- results in rebuilding of the molecular structure of organic substances under the influence of increasing temperature and pressure.

Variation in coal quality is known to be related to geochemical and stratigraphic factors.

Coal rank (ASTM,1977)

Peat ? Lignite ? sub-bitumious coal ? bitumious coal ? semi-anthracite ? anthracite

* Ro% is the best index of evaluativing organic maturity (Hood et al.,1975).
* Ro% increases with the temperature raising. (Stachet al.,1982)

I. Ro% -- 0.65 Diagenesis methane II. 0.654.0 Metamorphism

Research objective

For the purpose of HC potential assessment, statistical methods were applied to characterize the correlation between organic material and thermal maturation.

Sample selection

Analytic procedure

Analysis method (1)

Vitrinite Reflectance (Ro%)
Under Leitz MPV Compact Microscope with photomultiplier tube, use oil-immersion observation.
150 points were measured for each pellet, mean reflectance values were then calculated.

Analytic procedure

Maceral analysis
Each pellet was point counted on a mechanical stage set to 0.5-mm intervals.
300 points counted.
Four groups: Vitrinite, Exinite, Inertinite and Mineral matter.

Analytic procedure

Ultimate analysis
It’s a analysis that provides data on the proportions of C, H, N and S for classification and application purpose.
Variations in chemical composition from Ultimate analysis data reflect variations in the coal type and coal rank.

Analytic procedure

Rock-Eval Pyrolysis

HI (hydrogen index)

HI is a parameter used to characterize the thermal maturity of organic matter.

Result

Ro% vs Vitrinite (1)

Ro% vs Vitrinite (2)

Ro% vs C%

Ro% vs Tmax (1)

Ro% vs Tmax (2)

Ro% vs S2 (1)

Ro% vs S2 (2)

Ro% vs H/C (1)

Ro% vs H/C (2)

Future work

* Complete data analyses.

* Find environmental explanations between material and thermal maturity.

Organic Petrology And Oil Exploration – Organic Matter Type

noreply@blogger.com (Coal Trading) — Mon, 05 Dec 2011 05:10:00 +0000

Toolebuc – Bituminite, Lamalginite, Dinoflagellates, R 0.53%

Birkhead – Mid Jurassic, Abundant Liptinite And Almost No Inertinite. R 0.65%

Nappameri R 0.86%

Nappameri R 0.87%

Organic Petrology And Coking Coals

noreply@blogger.com (Coal Trading) — Sun, 04 Dec 2011 05:27:00 +0000

Organic Matter Type

• Three main groups are recognized

• A. Humic materials preserved without major oxidation – vitrinite

• B. Humic materials preserved after oxidation in the biochemical stage – inertinite

• C. High hydrogen components some containing lipids – liptinite

• Additionally some zooclasts are found

Complexity

• The full list of categories is long

• The basic data require detailed analysis

• Most uses require at most 4 categories

• The most commonly used type data is the vitrinite %

• You can always combine data but you cannot split analyses are only made at the group level

Best Not To Be Like This!!!

Humic Macerals

Liptinite Macerals – Part I

Liptinite Macerals – Part II

And Then There Is Rank

• Which we now measure with vitrinite reflectance

• Coal type (maceral composition) and coal rank can be used to predict most other properties of the coal

Coal Uses Defined By Type And Rank

Setting For Petrographic Analyses

• Follow standards – usually iccp or iso

• However samples should not be analysed blind as with other types of analysis

• Petrology analyses components

• Other techniques are bulk analyses

• Petrographic analyses are different in kind

For Example

• A volatile matter analysis might tell you that the vm yield is 35.6%

• It cannot tell you why

• A petrographic analysis may be able to tell you why it is 35.6% rather than the 42% of other samples in the batch

• Additionally, petrographic data can tell you if there are contaminants

Component Analysis

• Petrology can tell you the coals present in a blend and indicate the properties of each component

• Other types of analysis can only indicate the average of the property across the components present

Use In Blends And More Generally In Commercial Practice

• Petrology is used in blend control as a quality assurance (qa) tool

• More generally it is used to check that the coals delivered are the ones specificed in the contract

• Customers in japan and korea have banks of analysts beavering away checking on the coals being delivered

Blend Analysis – Main Uses

• Check on blend composition

• Trouble shooting errant blends

• Designing and testing blends

• Preparatory work for coke petrography

Best Not To Do This

Coke Petrography

• Vitrinite and inertinite give distinctively different residues in cokes

• Vitrinite forms a component in cokes with a mosaic structure

• Mosaic size varies with coal rank

• Coke texture analysis indicates both the type and rank of the parent coals

The Right Perspective

One View Of A New World

Oil And Gas Exploration – Regional Variation In Maturation Levels

noreply@blogger.com (Coal Trading) — Sun, 04 Dec 2011 04:38:00 +0000

Central Australian Basins

• Permian To Cretaceous Section

• Coal Measures In The

– Permian

– Lower Triassic

– Jurassic

– Cretaceous

• Marine Section In The Lower Cretaceous

Maturation Levels

• Range From Immature On Basin Flanks To Super-Mature In Some Basin Deeps

• Permian Ranges From 0.34% To Approx 7.0%

• Jurassic Ranges From 0.40% To 1.4%

• Cretaceous Marine Section Is Marginally Mature At Best

• Hydrocarbons Found Are Oil And Gas

• Potential For Shale Gas In Some Basin Deeps

Permian Coals

Basin Margin Pedirka At 650m – R 0.37%

Nappamerri Trough At 2800m – R 3.05%

Cooper Eromanga Stratigraphy

Subsidence History For Kirby-1 In The Nappameri Trough

Vitrinite Reflectance At The Base Of The Cretaceous

Vitrinite Reflectance At The Base Of The Jurassic

Reflectance Section For C-D, Note The Contrast Between The Adjacent Patchawarra And Nappameri Troughs

Simpler Pattern For The Poolawanna Trough To The West

Vitrinite Reflectance Profiles For Nine Wells From The Simpson Desert Basin

Note The Break At The Top Transition Beds – Cause Not Known

Vitrinite Reflectance Profile And Maturation History Reconstruction For Cuttapirrie-1

Vitrinite Reflectance Profiles And Maturation History Reconstruction For Poolawanna-1 And Kirby-1

Ash Formation and Behavior in Utility Boilers

noreply@blogger.com (Coal Trading) — Mon, 28 Nov 2011 12:04:00 +0000

Part 1: Coal Composition

Ash formation and behavior in utility boilers is a complex problem that involves knowledge of not only the effects of system geometry and operating conditions, but a detailed understanding of coal and ash as well. This will be the first in a series of articles intended to inform the reader of how ash forms in a combustion system and how it behaves relative to ash deposition, erosion, corrosion and collectability.

Most of the operational problems associated with coal utilization can be traced back to the coal itself. What makes coal so difficult to deal with is that as a raw material coal is both complex and heterogeneous. We could even deal with the complexity of coal if it was not so heterogeneous. If we could rely on coal to be consistent from mine to mine, and train load to train load, we could make operational changes to reduce problems. But because coal is so variable we have to make operational changes "on the fly." To make the problem even worse, the standard ash analyses often don't provide any indications of when ash problems will occur. Often two coals with very similar ash analyses will act very differently in the boiler.

Why does coal behavior change so rapidly and dramatically? The answers lie in the microscopic characteristics of the coals themselves. Because standard ash analyses provide only the average chemistry of the coal, they provide no information on those microscopic characteristics which have such a big effect on coal behavior during combustion. In this first article in the series we will examine the ash-forming constituents in coal and their significance in coal combustion systems.

Coal is a complex combination of organic materials (those that burn) and inorganic materials (those that produce ash). The inorganic materials can be further subdivided into mineral grains and organically-associated inorganics. The mineral grains consist of particles ranging in size from microscopic to sand-sized and even larger-sized materials. Common coal minerals include pyrite, quartz, clay minerals and gypsum. Both the size and chemistry of these mineral particles control where the ash will end up in a combustion system.

High rank bituminous coals from the U.S. typically contain high levels of mineral grains of a variety of types and sizes. Lower-rank sub-bituminous and lignitic coals contain both mineral grains and organically-associated inorganic elements. There has been little progress made in advancing the methods of coal analysis until the past several years. The methods to determine the effects of coal ash on utility boiler performance involved ashing the coal at 750°C, (1382&°F) and analyzing the ash to determine bulk composition. The bulk composition is plugged into various fouling and slagging indices. The bulk ash is also used in the ash fusion testing to measure the melting characteristics of the ash. The primary problems associated with the conventional types of analyses are:

1) that the ashing procedure in no way represents the ash produced in coal combustion systems,

2) that the composition of the resulting ash does not represent what is found in the utility boiler, and

3) that the ash fusion testing is not an adequate indication of the melting behavior of the ash. These general average properties of ash in most cases do not provide sufficient information to predict or identify ash behavior.

This is substantiated by the fact that the examination of fly ash particles show that many different particles are present, each having its own composition and probably its own melting behavior. Therefore, the melting behavior of the individual ash particles may be different from that predicted for the average ash composition. Today advanced methods of analysis such as computer-controlled scanning electron microscopy and microprobe analysis provide composition and size information of mineral grains in coal and ash particles that is essential in understanding and predicting ash behavior in utility boilers.

Part 2: Mechanisms of Ash Formation

In the last newsletter, I discussed the fact that coal does not contain ash but, it contains minerals and other inorganic components that upon combustion or gasification form ash. The conditions that the coal and the inorganic components are exposed to during combustion result in dramatic changes. Due to the high temperatures and reducing environments in the flame during the consumption of the carbon, the inorganic materials are changed or transformed.

The inorganic coal components undergo complex chemical and physical transformations during combustion to produce vapors, liquids, and solids as illustrated in Figure 1. The partitioning of the inorganic components into the vapors, liquids, and solids depends upon the association and chemical characteristics of the inorganic components in the coal, the physical characteristics of the coal particles, and the combustion conditions.

The two primary ash formation mechanisms involved in fly ash formation include (1) melting and reacting of individual mineral grains within a burning coal particle, and (2) vaporization in the flame followed by subsequent condensation of the inorganic components upon gas cooling. A more detailed description of the formation mechanisms is summarized by Benson and others (1993).

As a result of these interactions, the resulting ash has a multi-modal size distribution. The submicron modes are largely a result of the condensation of flame-volatilized inorganic components. The mass mean diameter of the larger particles is approximately 12 to 15 micrometers, depending upon the coal mineral distribution and combustion conditions. The larger size particles have been called the residual ash by some investigators because these ash particles resemble, to a limited degree, the original minerals in the coal.

The physical state, composition, and size of the ash particles directly influences their behavior in a combustion or gasification system. For example, the chemical composition of the ash particles will influence their melting behavior and therefore, their ability to produce slagging and fouling problems. The size of the ash particles will influence their ability to travel through the combustor and impinging surfaces. The flame-volatilized inorganic materials condense upon heat recovery and condense homogeneously to form very small particles or heterogeneously on the surfaces of entrained ash particles or deposits. The small particles may be difficult to collect in air pollution control devices.

Predicting and minimizing ash related problems requires being able to determine the form and size of the inorganic materials in the coal. CCSEM is used to quantitatively determine the size, composition, and abundance of the mineral grains in the coal and chemical fractionation is used to determine the abundance of organically associated cations. Computer software has been developed to predict ash particle size and composition distribution as discussed by Benson and others (1993).

Figure 1. Transformations undergone during combustion by coal particles.

Part 3: Ash Deposit Growth: Ash Transport Mechanisms

In Part 2, I discussed how ash is formed in pulverized coal fired combustion systems. The characteristics of the resulting ash are primarily a function of coal type and are also somewhat dependent upon operating conditions. Ash species, inorganic vapors, liquids, and solids are entrained in the bulk gas flow within the combustion system and have the potential to produce deposits. In order for deposits to form, the ash must first be transported to and retained at the deposition site (likely a heat transfer surface).

The transport of the particles to a heat transfer surface depends upon the properties of the ash species (i.e. physical state and size) and system conditions such as gas velocity, gas flow patterns, and temperature. The ash transport mechanisms are illustrated in Figure 1. Figure 1 illustrates the transport mechanisms for a tube in cross-flow as would be found in the convective pass of a utility boiler. The ash species transport mechanisms in the radiant sections (water walls) are similar to convective pass deposition, however, the gas flow patterns are designed such that the gas/particle impingement of the wall is minimized.

Vapor phase and small particles (1 micron) are primarily transported to surfaces by diffusion mechanisms. These particles are usually enriched in species that were volatilized in the flame. The vapor phase materials may condense in the stagnant boundary layer next to the heat transfer surface. Another small particle transport mechanism is thermophoresis. Thermophoresis is a transport force that is produced as a result of a local temperature gradient from hot to cold.

The thermophoresis mechanism is important for particles less than 10 micron. Thermophoresis is important at high temperatures such as those observed in the radiant section of a utility boiler. Larger particles, usually 5 to 10 micron size range, are transported to heat transfer surfaces due to inertial impaction. The larger particles may impact a surface if they have enough inertia to allow them to leave the gas streamlines such as gas moving around a tube. The smaller particles will have a tendency to follow the gas streamlines because the drag forces on the particles are sufficient to keep them in the flow stream. The primary factor that influences the inertial impaction mechanism is the velocity and direction of the gas flow.

Eddy impaction is also an important transport mechanism to consider because it can cause accumulation of particles on the back sides of tubes. Eddy impaction acts on particles intermediate in size between those that inertially impact and those susceptible to diffusion and thermophoresis. These particles are too small to impact on the front side of the tube, but have sufficient inertia to impact on the backside of tubes as a result of turbulent eddies. The size of these particle are usually 10 micron. So, the particle size of the ash being transported through the system has a great effect on the accumulation of ash particles on heat transfer surfaces. Next time we will discuss why some particles stick and how larger deposits form.

Figure 1. Ash transport mechanisms for a tube in cross flow (not to scale).

Part 4: Ash Deposit Growth: Deposit Growth

The article in the last newsletter discussed how ash particles are transported to a deposition site such as a heat transfer surface. The key factors that influence the transport and sticking of ash particles include the physical state of the gas-entrained ash species (vapor, liquid, or solid) and system conditions such as gas velocity, gas flow patterns, and temperature.

The adhesion and growth of ash deposits on heat-transfer surfaces depends upon the ability of the deposited ash particles to form strong bonds with the heat-transfer surface. The formation of the strong bond depends upon the characteristics of the reacted layers on the steel surface, temperature of the steel surface, melting behavior of the ash particles, and the thermal and chemical compatibility of the deposit and steel surface. The deposit initiating process usually involves deposit growth and erosion/shedding combined with corrosion of the steel surface.

The thermal and chemical compatibility between the steel surface and the deposit influences the deposit's ability to stick and remain after sootblowing and thermal cycling of the utility boiler. If the thermal characteristics (i.e., thermal expansion coefficients) of the heat-transfer surface and the depositing material are similar, the deposit will not shed easily during cleaning cycles. On the other hand, if the thermal expansion coefficients of the ash and the surface are different, the deposits will shed much more easily during cleaning. The strength of the adhesion bond between the oxide and ash layer depends upon the chemical and physical compatibility. Chemical bonding between the ash deposit and the steel surface provides for the formation of the strongest bonds.

Once a strongly bonded initiating layer has accumulated, the temperature of the deposit surface increases resulting in the formation of liquid phase material. The liquid phase can cause the deposit surface to become sticky and act as a collector of impacting ash particles. The sticky surface is described as a captive surface which is a requirement for rapid deposit growth. In addition, the deposited ash particles can react with gas phase components such as Na, S, Cl, and P species resulting in increased melting.

Besides being necessary for deposit growth, the liquid phase material is responsible for deposit strength development. The ability of deposited material to develop strength depends upon the temperature of the system, the chemistry of the deposited ash particles, the size distribution of the ash particles, and the gaseous environment in the vicinity of the deposit. One requirement for the formation of a strong deposit is the formation of a reactive liquid phase that can incorporate solid ash particles into the melt.

The temperature of the system and the chemical and physical characteristics of the liquid phase markedly influence the characteristics of deposits. Deposits range in consistency from a lightly bonded particle to strong well sintered deposits to flowing slag materials as depicted in the figure below. Viscosity is the primary physical property that influences deposit strength. The viscosity is inversely proportional to the development of deposit strength. The viscosity is dependent upon the chemistry and temperature.

Figure 1 illustrates this relationship. Deposit 1 is a weak deposit in which the adhesive strength and deposit strength are not sufficient to hold the deposit in place resulting in shedding under its own weight. The liquid phase viscosity is high in deposit 1. Deposit 2 is a deposit that exhibits rapid growth and significant deposit strength development. The viscosity is low enough to cause significant bonding of particles but no flow of the deposit. Deposit 3 is a deposit that has attained a temperature where the deposit will flow and shed through melting. The viscosity of liquid phase in this case is very low. Next time we will discuss viscosity and deposit strength development.

Figure 1. Relationship between viscosity and strength.

Origin of Trace Elements in Coal

noreply@blogger.com (Coal Trading) — Sun, 06 Nov 2011 11:04:00 +0000

ABSTRACT

The distribution of trace elements in coal is not random. The elements and their concentrations are the result of the all the geological factors that have occurred in the geological past to form the present coal seams. Understanding the character and concentration of trace elements is greatly enhanced by knowing the coal mineral matter assemblages in terms of identity, origins and when they were emplaced.

The minerals in coals are the minerals normally found in all sedimentary rocks. The most abundant minerals in coals are clays, quartz, various carbonates and sulphates, minor rutile, apatite and sulphides. If the coal basin is associated with volcanic sediments, then the feldspars are common.

The minerals found in coal are related sediment providences, environments of deposition and post depositional events such as folding, faulting and the introduction of minerals by way of waters, both from the surface and from under the coal measures. The latter are hydrothermal waters that can be generated by the folding or deep-seated volcanism.

Understanding trace elements in coals is assisted by reference to geochemical associations, which are determined by the principles of a chemical reactions and elements that have similar chemical behaviour. All this is related back to the Periodic table, but the Goldschmidt classification is a more convenient way of expressing geochemical elemental associations.

INTRODUCTION

The majority of trace elements are associated with the mineral matter (Riley and Dale 2000) as discrete mineral phases, as substituted elements in the major mineral phases or bound up in the organic matter. In the latter case they were a component of the original plant material. Trace elements in discrete mineral phases occur in much the same way as the major minerals, there is just less of them. The substituted elements will follow the geochemical paths of the major elements that the trace elements are substituting. As those elements enter mineral transformations, they will in most cases behave in similar ways to the element that is substituted for.

The trace element content of a coal is therefore intimately related to the major non-organic components (minerals) found in coal (Gluskoter et al 1977, Finkelman 1981, 1982, 1994, Swaine 1990 and Spears and Zheng 1999). The minerals found in coal are related to the sedimentary providences, environments of deposition. and post depositional events such as folding, faulting and the introduction of minerals by way of waters, both from the surface and from under the coal measures. The latter are hydrothermal waters that can be generated by the folding or deep-seated volcanism.

MINERALS IN COAL

The material presented here is taken from Sakarovs et al 2001. The minerals in coals are the minerals normally found in all sedimentary rocks. Coal is an organic rich sedimentary rock. The sediments that the organic materials deposited with can be from a wide variety of sedimentary sources in a wide variety of depositional environments. Table 1 is an abbreviated listing of the minerals found in coals. The list includes iron oxides, phosphate minerals like apatite, rutile and zircon. If the coal has been folded, then the common low-grade metamorphic minerals such as chlorite are also present. The most abundant minerals in coals are clays, quartz, various carbonates and sulphates, minor rutile, apatite and sulphides. If the coal basin is associated with volcanic sediments, then the feldspars are common as the minerals in coal.

Table 1: List of Mineral Species Found in Coal

Certain elements are present in the original living plant tissue. These elements are ultimately trapped in coal in the form of sub-microscopic mineral grains and as organo-metallic complexes. Some may also contribute to the formation of discrete, coarse-grained minerals. These elements are the intrinsic inorganic matter.

Introduced forms of mineral matter can be primary or syngenetic, and are the minerals that accumulated with the peat at the time of it accumulated. After the complex process of coalification forms coal, minerals are precipitated from moving groundwaters. These minerals are secondary or epigenetic, deposited by the percolating waters into fractures, cavities and pores within the coal seam long after initial accumulation if the peat Exhalations from volcanic sources and associated hydrothermal fluids can also form epigenetic mineral assemblages. It is these processes that introduce some of the less desirable trace elements such as arsenic, antimony, mercury, fluorine and chlorine (in part) and a range of heavy metals. Introduced minerals are called extrinsic minerals.

Silicate Minerals: Of all the minerals found in coal none are so widespread and universally ubiquitous as the clay minerals. All other silicate minerals pale in quantitative significance compared with clay minerals which, when concentrated in the form of claystone layers constitute important marker horizons in coal seams. Although they are highly variable in their physical behavior and chemical composition, clay minerals have some common features in that they are all classified as phyllosilicates.

One of the most common clay minerals in coal is kaolinite. It occurs in a variety of forms ranging from infillings in plant cell cavities to finely dispersed inclusions in coal, and as discrete pellets and vermicular aggregates (Kemezys and Taylor 1964)

Montmorillonite and mixed layer clay minerals are common constituents of Australian coals, where they are concentrated mainly in tuff-derived clay bands. They are even more commonly found in inter-seam tuffs. As an example, in the Newcastle Coal Measures in New South Wales there are several meter thick deposits of relatively pure bentonite, the general term used for the montmorillonite clay.

Illite is also a common constituent of Australian coals. Illite is mixed layer clay that contains a significant portion of potassium. It is believed to be a secondary mineral.

Chlorite is rare in coals themselves, but can often be seen as inclusions in shale bands and inter-seam sediments. It seems that most chlorites in these sediments are detrital in origin. High iron content chlorite occurs in some Queensland coals, and in one example is the major iron-bearing mineral in the mineral matter. This one example can be used to illustrate the point that it is minerals that are important, not the oxide list. In this case the slag index, calculated from the oxide list, indicates the coal to be potentially slagging, but in practice it is not. The chlorite is refractory, but if the iron were in the form of siderite or pyrite there would certainly be a slagging risk.

Silica Minerals: Silica occurs in coal predominately as quartz. Quartz can constitute half of the mineral matter in some Australian coals but usually does not exceed 10%. Most of the macroscopically and microscopically visible quartz is detrital in origin, which means that it was transported as mineral grains to the depositional site. For this reason quartz is usually found together with other detrital material; in many cases, together with clay in altered tuffs or epiclastic dirt bands left behind by flood waters in the form of splay deposits.

Authigenic quartz originates from aqueous solutions during the early stages of diagenesis. Some quartz of this type seems to have passed through the stages opal to chalcedony to quartz. Quartz that has derived from the alteration of opal and chalcedony is difficult to detect. It often occurs as minute needles which are under 1 µm in length, and consequently not seen under the microscope. Together with needle quartz true chalcedony with acicular and spherulitic habit occurs.

Fine-grained silica in coal leads to silica fuming. Fine grained silica formed from the silica in the plant structure, are the most common forms of fine quartz. The fine-grained quartz is distributed throughout the pf particles after milling.

Phosphate Minerals: Phosphate minerals in coal consist mainly of apatite and phosphorite. The term phosphorite is a collective name for those phosphates that were precipitated in colloidal form and later underwent partial crystallization. Most of the phosphorites in coal are arranged in small spherulitic nodules of only a few micrometers in diameter, and it is rare to find lenses of phosphorite, which extend more than 20 mm.

Apatite normally occurs as small inclusions in other minerals. Large crystals of apatite are rare but sometimes they occur as secondary minerals on the surface of earlier phosphorites or on fragments of phosphorus-bearing fossils, such as bones, teeth or conodonts, a type of fossil whose origins are obscure.

Sulphide Minerals: Sulphur is a common constituent in coal, where it occurs in varying quantities and forms. Many coal seams contain less than 0.5% sulphur; others carry more than 10%. Likewise, the proportion of sulphide minerals as a percentage of the total mineral content varies from less than 1% in some Australian coals, to more than 30% sulphur in coals from the Illinois Basin in the United States. A distinction is made between organic and inorganic forms of sulphur.

Inorganic forms of sulphur occur as various forms of iron sulphides. Pyrite is the most common inorganic form of sulphur, but overall there have been reports of around 20 metal sulphides in coal.

In general low sulphur coals contain more organic than pyritic sulphur and vice versa but some notable exceptions occur in which coals with greater than 5% organic sulphur contain less than 0.5% pyritic sulphur. The relationship between organic and pyritic sulphur is similar for Australian coals. High sulphur coals are associated with marine roof rocks and the sulphur content of a coal seam is often highest in the upper portion of the seam and near the floor.

Epigenetic pyrite and marcasite are frequently found on cleats and in fissures. Epigenetic pyrite cleat fillings are invariably related to marine sedimentary conditions. Folding and metamorphism can also emplace late cleat filling pyrite.

Other Minerals in Coal: Oxides and hydroxides in coal include primary hematite, primary limonite and secondary goethite. Sulphides other than pyrite are usually secondary and include galena, sphalerite, and chalcopyrite. The less common silicates occur in the coal because of their association with other silicates. Zircon and tourmaline are in this category, as is feldspar, mica, and rarely even some of the more mafic minerals such as biotite. It has to be kept in mind that the passage of the detrital minerals to actually get into the coal includes a long and strong exposure to the oxygenated atmosphere, consequently if a mineral is unstable in oxygenated conditions it will not survive. In more simple terms it will weather and be transformed to minerals stable in an oxygenated environment.

Rutile is a relatively common mineral in coal, and accounts for the bulk of the titanium assayed in coal ash. The rutile is commonly associated with quartz as fine needles within the quartz grains. Rutilated quartz is a common association in plutonic rocks, and if such quartz is the detrital input, so is the rutile contained therein.

Hydrothermal activity is a common source of secondary minerals. The Hunter Valley has examples of a sodium carbonate minerals, dawsonite being an example, that are the result of highly soda ash charged waters that pass through the coal measures. These waters are believed to be part of a low-grade hydrothermal event driven by the emplacement of later intrusions.

Sulphates found in coal are mainly late alteration products of pre-existing sulphides. The most common of this class are a series of hydrated iron sulphates, melanterite being a prime example, which result from the weathering of pyrite and marcasite. This reaction has severe environmental implications in that the weathering of sulphides, which is exothermic, can initiate coal spontaneous ignition. Pyrite and marcasite weathering also produces strongly acidic waters.

Gypsum is a primary mineral that is part of the chemical sedimentary system along ancient sea shorelines. Gypsum can be re-dissolved by groundwaters if those waters are under-saturated with respect to gypsum, and redeposit in other areas, sometimes in the coal.

GEOCHEMICAL ASSOCIATIONS

There are generalisations that can be made about the occurrences of major and trace elements. A fundamental concept in chemistry is the periodic table grouping of the elements which was proposed by Mendeleev in 1869. The periodic table is organized into horizontal sequences of rows called periods, and vertical sequences of groups. Mendeleev recognized the chemical similarities in the elemental groups and the systematic change that took place along the rows. This was the basis for predicting the character of the (at that time) undiscovered elements. Predictions as to geological behaviour are also possible using the Periodic Table, but are generally less satisfactory.

Faced with such dilemmas, VM Goldschmidt, a pioneer investigator of the rules of distribution of the elements, proposed another sort of classification, based directly on observed distributions of elements in earth and stellar materials.

Goldschmidt attempted by observations to answer the question – How would the elements distribute if sometime in the distant past the earth were a molten mass of material? On cooling the elements would separate themselves into the various phases seen in the total range of earth materials. It was known that generally earth materials could be classified, again on observation, into those seen as metal phases, those seen as sulphide phase and those seen as silicate phases.

The fundamental observations were on:

1. The composition of meteorites, assuming a meteorite represents a primitive earth composition.

2. The analysis of metal slags (silicates) and matte (sulphide) phases in metallurgy.

3. The composition of silicate rocks, sulphide ores, and the rare occurrences of native metals in the earth’s crust.

Goldschmidt's observations suggested that the elements could be usefully grouped into those which occur with native iron, which is probably the major element concentrated in the earth’s core, those in sulphides, those with silicates and those with strong affinities to the gaseous state. He coined the terms siderophile, chalcophile, lithophile and atmophile. The siderophile elements are iron associated and are those most likely to occur in the native state. The chalcophile elements are sulphur associated and occur combined with sulphur as sulphides. The lithophile elements are silicate associated and are the elements that that occur in the rock forming minerals. The atmophile elements occur in the gaseous state, and, as the name suggests, are the elements in the atmosphere.

The Goldschmidt classification of elements, as set out in Table 2 closely relates to the periodic table. In general, the siderophilic elements are concentrated in the centre of the periodic table, lithophile elements to the left of centre, chalcophile elements to the right, and atmophile elements on the extreme right of the table indicating the metallic to non-metallic transition. Certain groupings in the Goldschmidt classification reflect directly the periodic group association, an example being the S, Se, and Te triplet.

Table 2: The Goldschmidt Classification of Elements

Grouping of elements can at best express only tendencies, not quantitative relationships. The different groups overlap. Tin is an example of the overlap. Tin’s position in the periodic classification suggests that it has a number of metalloid properties as well as metallic properties. Elements that have two or more groupings often form amphoteric oxides, that is, the element can be either in the cationic or the anionic part of a compound. By way of example lead can occur as lead chloride, carbonate etc, but there are also metal plumbate compounds.

The major use of the Goldschmidt classification is that it gives to the observer a tool to associate the elements. It is therefore expected that the alkali and alkali-earth elements will associate in a slag with silica, but the halogens will fume, they do not associate. That is an extreme example, but subtler is that of Fe Mg and Mn. These elements often follow each other in the geochemical cycles, and where we find one, we can expect the other.

CASE STUDY

Ward et al (1999) has studied the mineral matter and trace elements in coals from the Gunnedah Basin. This study provides an example of how mineral matter and trace elements in coals are correlated and how information on origins of such elements can be gleaned from the data.

The coals of the Gunnedah Basin contain mineral assemblages embracing four different suites:

1. A detrital suite dominated by quartz, feldspar, illite, and interstratified illite/smectite;

2. An authigenic suite dominated by well-ordered kaolinite and/or siderite;

3. A marine suite with abundant pyrite and associated sulphates;

4. A post-depositional suite in which calcite and dolomite are the principal components.

The detrital suite is dominant in the upper part of the succession and the authigenic suite dominates the lower. The pyritic marine suite occurs at intervals that are also marked by anomalously low vitrinite reflectance, which represents major flooding events in the basin’s geological history. Post-depositional carbonates are abundant mostly near the top and bottom of the succession, a feature that may be related to fluid migration paths.

The concentration of a number of trace elements in the coals of the Gunnedah Basin can be related to the proportions of particular minerals and mineralogical suites. Rubidium, for example, is associated with the detrital suite as a component of all potassium-bearing minerals, but within this group does not seem to occur preferentially in any individual mineral species.

The pyrite in the coals is associated with a consistent concentration of arsenic, equivalent to approximately 0.1% of the pure pyrite component. It is also associated with detectable concentrations of thallium. Selenium occurs below the detection limits in most of the samples studied.

Siderite is precipitated within the peat in the absence of any significant sulphur fugacity is not associated with the pyritic elements, indicating that high sulphate values in the swamp waters rather than high Fe is the principal factor in siderite deposition.

The coals of the Gunnedah Basin contain significant concentrations of the elements Zr and Ti that are normally resistant to leaching. Many of the coals are inertinite-rich, and the concentration of these elements may reflect removal of others from the peat with acids generated by plant decay. Ti is associated with authigenic kaolinite, and may represent a product of leaching and re-precipitation along with alumina in the original peat.The relationship of Ti to Zr, Nd and Y also suggests ultimate derivation of the sediment in the Gunnedah Basin coals from an acid volcanic or similar source material.

Figure 1 illustrates this last point well by means of the Zr/TiO2 and Nb/Y ratios plotted for the igneous rock suites (after Winchester and Floyd, 1977). The clustering of the ratios to show an acid igneous provenance for the sediments in the Gunnedah Basin which aligns well with the geology of the basin hinterland.

Figure 1: Zr/TiO2 versus Nb/Y Ratios for Gunnedah Coals

Although they occur in association with P in a number of other coals, in the Gunnedah Basin coal samples Sr and Ba appear to occur mainly as minor components of the post-depositional carbonate minerals, calcite and dolomite. Co and Mn may also be associated with Fe in the earlier-formed siderite accumulations.

Ge and Ga are correlated to each other. Both more are abundant in coals with a low proportion of mineral matter, and this suggests the possibility of an organic association. Cr and V are also correlated to each other, as are Ce, La, Nd and Pr. These elements however, cannot be correlated to any particular minerals in the sample suite.

Significance Of Trace Elements In Coal An Overview

noreply@blogger.com (Coal Trading) — Sun, 06 Nov 2011 10:51:00 +0000

ABSTRACT

The significance of trace elements in coal relates to the potential environmental impact of the waste products from coal combustion. Coal contains a wide range of trace elements, many identified as being of environmental concern.

The focus on trace elements has been highlighted through designations that label a range of trace elements as of concern to the environment. As a result legislative regulations have been introduced to limit their impact. A suite of priority trace elements has been developed based on these designations and includes all trace elements likely to be of concern to power utilities.

Australian bituminous thermal coals have generally low levels of trace elements particularly those of major concern to health and the ecology. A comprehensive database on the levels of priority elements in Australian and international thermal coals has been established which demonstrate the advantage that Australian thermal coals have minimal environmental impact. This provides a competitive edge in international markets. The development of accurate standard methods of analysis guarantees a high level of quality assurance in the specification of the levels of trace elements in product coals.

The environmental impact of trace elements is related, in the first instance, to their modes of occurrence in the coal. Knowledge on the residences is sparse due to the difficulty in analysing discrete mineral particles. Microprobe techniques currently have limited sensitivity for analysing small mineral grains. Selective chemical leaching has been used to study modes of occurrence in a range of coals. The results suggest that many environmental trace elements occur in a number of mineralogical residences.

The environmental impact from waste residues is largely related to modes of occurrence and complex processes in the combustion zone. The generation of volatile species, particularly trace elements of major environmental concern, and their ultimate fate accounts for their presence in air emissions. The enrichment of trace elements in fine particles has important implications for ash disposal.

This overview will address the significance of trace elements in terms of their origin, the focus on environmentally sensitive trace elements through designations and the levels present in product thermal coals. The behaviour of trace elements during combustion and the characteristics of waste residues will also be discussed in terms of environmental impact.

INTRODUCTION

Coal is a very complex heterogeneous material of organic and mineral fractions and the modes of occurrence of trace elements in coal are influenced by the depositional environment and the geological processes prevailing during the formation of the coal beds. Although most trace elements are associated with mineral matter many of them appear to have an organic affinity arising from their association with the original plant material from which the coal was formed.

During coal combustion trace elements are released from their host environment and the more volatile elements such as mercury, selenium and arsenic enter the gas phase. Some elements may remain in the gas phase and be emitted to the atmosphere and others may recombine with the ash particles.

The major issues relating to environmental impact are the release of trace elements to air and water. The partitioning of trace elements among the waste products has a major influence on the potential impact on the environment.

OCCURRENCE OF TRACE ELEMENTS IN COAL

Coal contains a wide range of trace elements due predominantly to the presence of mineral matter arising from the deposition of sedimentary rock during the formation of the coal beds. Trace elements were also present in the plant material that was the precursor to the formation of the coal. Secondary mineralisation caused by the presence of calcareous elements, calcium and magnesium, also accounts for the presence of associated trace elements. Iron-rich sideritic minerals may also be formed by this process.

Many trace elements of environmental concern are present in common minerals due to co-crystallisation (Turekian and Wedepohl, 1961). This is shown in Table 1.

Table 1: Elements which co-crystallise with common minerals

A comparison o the abundances of trace elements in world coals and crustal abundances are given in Table 2.

Table 2: Abundances of trace elements in coal and the Earth’s crust

It is noteworthy that some of the elements of environmental concern (arsenic, cadmium, molybdenum, antimony and selenium) are enriched in coal relative to the Earth’s crust and this may be due to partial association with the original organic plant material.

Lyon et al, 1978, identified those elements that were enriched in coal compared to the Earth’s crust. This is shown in Table 3, which shows the enrichment factors of trace elements in coal with respect to the average composition in the Earth’s crust. It is again noteworthy that all of the elements identified are of environmental concern.

Table 3: Enrichment factors of trace elements

DESIGNATION OF TRACE ELEMENTS OF CONCERN

The significance of trace elements in coal relates to the potential environmental impact of its utilisation in combustion processes. Many elements have been identified as of concern because of their known adverse health and ecological effects. It should be emphasised that the major concerns have been directed towards industrial plant where substantial quantities of trace elements are likely to impact on the environment. These include ore smelting plants and other industrial processes where feedstocks contain substantial quantities of the elements of concern. Coal-fired utilities are included in the definition of industrial plant because of the large quantities of coal processed and the nature of the waste products generated.

US NATIONAL RESEARCH COUNCIL

The environmental awareness of trace elements in coal was first highlighted by the classification of elements of concern by the US National Research Council in 1981. Trace elements were classified by level of concern based on known adverse health effects or because the abundances in coal were greater than the Earth’s crust. The classifications are as follows:

* Major concern: Arsenic, boron, cadmium, lead, mercury, molybdenum and selenium.

Arsenic, cadmium, lead and mercury are highly toxic to most biological systems when they occur in bio-available form at concentrations above critical levels. Selenium is an essential element but is toxic above certain levels. High levels of molybdenum and boron in plants are of concern. Molybdenum affects the lactation of cows and boron is phytotoxic.

Arsenic, selenium, molybdenum and boron become mobile in ash dams because they form soluble oxyanions and there are concerns about the potential to enter the water table. Overflow from ash dams into natural waterways is another concern.

* Moderate concern: Chromium, vanadium, copper, zinc, nickel, fluorine.

These elements are potentially toxic and are present in coal combustion residues at elevated levels. Bio-accumulation is of some concern. Fluorine present has an adverse effect on forage. Significant leaf damage to plant material can occur when foliage is exposed to elevated levels of hydrogen fluoride.

* Minor concern: Barium, strontium, sodium, manganese, cobalt, antimony, lithium, chlorine, bromine, germanium.

These elements are of little environmental concern. They were classified mainly on the basis that they are present in residues at levels greater than crustal abundance.

* Elements of concern but with negligible concentrations: Tn, beryllium, thallium, silver, tellurium.

These elements have known documented relationships to health but the low levels present are considered to have negligible impact.

* Radioactive Elements: Uranium and thorium.

Uranium and thorium are radioactive and the products of their decay are the natural radionuclides present in the environment. Of the naturally occurring radionuclides, radium, polonium and radon are of some concern. Radium and polonum are alpha emitters with long half-lives. Radon is a gas with a short half-life and there has been some concern on the build up of radon in underground coal mines.

US CLEAN AIR AMENDMENTS

More recently amendments to the US Clean Air Act, 1990, identified a range of elements as hazardous air pollutants. Among these were the trace elements: beryllium, chromium, manganese, cobalt, nickel, arsenic, selenium, cadmium, antimony, mercury and lead.

The Act applies to all industrial plant and not specifically to coal-fired power plants. It required operators of power utilities to monitor mercury emissions.

NATIONAL POLLUTANT INVENTORY

In 1999 Environment Australia introduced the Nation Pollutant Inventory (NPI). This requires operators of fossil fuel power plants to publicly report emissions of designated trace elements if the amount of a particular element exceeds certain threshold levels.

The elements designated are: antimony, arsenic, beryllium, boron, cadmium, cobalt, copper, fluorine, lead, manganese, mercury, nickel, selenium and zinc.

Specific details of test procedures and reporting requirements are provided in the Emission Estimation Technique for Fossil Fuel Electric Power Generation from the web at http://www.npi.gov/handbook/index.html

PRIORITY TRACE ELEMENTS FOR COAL SPECIFICATION

The 22 elements set out in Table 4 are considered to be priority elements for coal specification based on the designations, air emission statutory limits and water quality guidelines.

Table 4: Trace elements for coal specifications

Iodine and bromine are included because they are volatile elements. They are of no environmental concern because of their low levels in coal. A Japanese utility expressed concern on elevated levels of iodine in an Australian coal. The problem was the presence of iodide in scrubber water that interfered with the measurement of chemical oxygen demand. This is a critical measurement to determine the impact of disposal on marine life. To date no concern on bromine levels in coal has been expressed. The element is included on defensive grounds.

TRACE ELEMENTS IN AUSTRALIAN THERMAL COALS

The early work of Swaine, 1990, provided a solid foundation for our knowledge on the levels of trace elements in Australian coals. Of major importance was the demonstration that Australian bituminous coals contained significantly lower levels of trace elements than world coals. Much of the early data was on run-of-mine coals that were not necessarily power station feed coals.

With the expansion of Australia’s thermal coal industry to its position as the world’s largest exporter of coal it seemed more appropriate to generate trace element data on product coals used for power generation. This was driven by the increasing importance power utilities were placing on the environmental impact of trace elements from coal-fired power plants arising from stricter government regulations on emissions.

This was addressed by Dale, 1990, with the development of a database on trace elements in Australian and internationally traded coals. The levels of key environmental trace elements in Australian export thermal coals were compared with coals traded by its major competitors in the international coal trade, including South Africa, USA, China, Indonesia, Poland and Colombia. The database was substantially upgraded by Dale, 2003 with additional power station coals.

The database is being upgraded continually in two current ACARP projects. Project C13069 – Maintenance of Database on Trace Elements in Australian and International Coals (LS Dale) in which coals are obtained from a European coal terminal and Asian utility and the data is used to provide upgrades every six months. Project C12060 – Website Database on Trace Elements in Coal (KW Riley) has established a website on trace elements in coal with fact sheets on environmentally sensitive elements (http://www.det.csiro.au/traceelements). The website is upgraded as data are generated in C13069. Table 5 is the current database and contains data for up 54 international and 92 Australian export coals. Typical values for Australian coals are the median values.

Table 5: Trace elements in Australian and International thermal coals

Australian coals contain significantly lower levels of arsenic, boron, mercury and selenium than most coals traded worldwide. These are elements of major environmental concern.

ANALYSIS OF TRACE ELEMENTS IN COAL

Over the past 20 years considerable advances have been made in the application of modern instrumental techniques to the accurate determination of trace elements in coal (Riley, this Symposium). This was driven by the requirement for quality assurance in the specification of coals for trace elements and pressures to assess the environmental impact of coal-fired power stations.

Importantly, the new methods have been adopted by Standards Australia, ASTM and ISO. Standard methods are validated and robust and give confidence to the analytical data.

Analytical methods are now based on very sensitive techniques that are capable of accurately analysing trace elements at the levels normally present in coal products.

Techniques now used include:

* Inductively coupled plasma atomic emission spectrometry

* Inductively coupled plasma mass spectrometry

* X-ray fluorescence spectrometry

CSIRO has also developed referee methods for specific elements:

* Neutron activation analysis (chlorine)

* Proton induced gamma ray emission (fluorine)

Methods based on these new techniques come with a cost and there is still some resistance by coal producers to bear this.

ENVIRONMENTAL IMPACT OF TRACE ELEMENTS

MODE OF OCCURRENCE OF TRACE ELEMENTS

Knowledge of the mode of occurrence of trace elements would aid in the understanding and assessment of the environmental impact of trace elements from coal combustion. Ideally the direct measurement of mineral particles using microprobe techniques would be the best approach. The sensitivity of probe techniques is however insufficient to analyse small mineral grains. Some success has been achieved using X-ray absorption near edge structure (Huggins et al, 1994). This technique is limited to only a few elements and requires access to synchrotron radiation.

Another method used is floats/sink separation whereby trace elements can be assigned to specific mineral density fractions. From the analysis of the minerals and trace elements in each fraction the associations can be inferred. This procedure lacks specificity and is of limited application.

Another strategy is the use of selective chemical leaching. Specific mineral groups are chemically removed from the coal using reagents specially selected to react with mineral types. With such a scheme water soluble and carbonate minerals are removed with dilute hydrochloric acid, sulphides with nitric acid and silicates with hydrofluoric-nitric acids. Analysis of the leachate from each step allows the fraction of trace element associated with the particular mineral group to be determined. Elements remaining after the final leach step are taken to be organically associated. The effectiveness of the leaching of pyrite is shown in Figure 1, an X-ray diffraction scan of a sample of coal containing 1% pyrite before and after leaching.

Figure 1: XRD patterns before and after treatment

The selective chemical leaching approach has been used by Dale et al, 1999, on a selection of Australian bituminous coals. The results showed that, with few exceptions, the trace elements have a number of mineralogical residences. For example, selenium was distributed between the pyrite and organic fractions and mercury was distributed between the monosulphide, pyrite and organic fractions. Zinc was totally associated with monsulphides, antimony with organic fraction and beryllium with silicates.

There are some problems with this approach. Minerals embedded in coal particles may not be chemically attacked. Analytical errors at the low concentrations of elements present, particularly mercury, selenium and cadmium, increase the uncertainty of the recoveries in the leachates. However strong similarities in the associations were found for a range of coals and this gives some credence to the data.

ENVIRONMENTAL IMPACT OF TRACE ELEMENTS IN COAL COMBUSTION

During combustion trace elements present in the feed coal are distributed between the waste products. These waste streams are the bottom ash, fly ash, stack particulates and stack gas. The distribution between the ash products is manifested by an enrichment of a number of trace elements in the fly ash with a corresponding depletion in the bottom ash. Volatile elements including selenium, mercury, boron and fluorine are present in the stack gas together with fine particulates that pass through the ash filtering system. The distribution is illustrated in Table 6, which shows the recovery of trace elements in the waste streams in a power station. Fly ash constituted 90% of the total ash and bottom ash 10%. The poor recovery for selenium is thought to be due to its retention in the stack gas sampling train or deposition within the ducting of the power station.

Table 6: Recovery of Elements in Waste Products

MECHANISM FOR TRACE ELEMENT PARTITIONING

There is evidence to suggest that the mechanism for the redistribution of trace elements in the waste products is dependent on the volatility of the species originally present in the coal. When the coal enters the combustion chamber the mineral matter undergoes very rapid heating to temperatures in excess of 1,500°C. The refractory clays melt to form glassy alumino-silicates, the major component of the ash. More volatile minerals such as pyrite volatilise with the liberation of associated trace elements. The volatiles are carried through the combustion zone and become absorbed onto the surface of ash particles in the cooler zones or the convection path.

Surface absorption is dependent on particle size and hence surface area. Smaller particles therefore have higher enrichments of many trace elements. This is illustrated in Table 7 where the concentration of some environmental elements in sized fractions of the fly ash is shown. Enrichments of up to 5-fold occur in the finer particles.

Table 7: Concentrations of trace elements in fly ash fractions

Highly volatile species are not totally absorbed on to the ash particles and this accounts for their presence in the flue gas.

AIR EMISSIONS

The flue gas contains volatiles and fine particles that are emitted to the atmosphere. Measurement of the volatiles is carried out using USEPA Method 29 and incorporates a series of bubblers containing chemical reagents.

In Table 8, the results of stack gas tests are shown for some of the most environmentally sensitive trace elements.

Table 8: Concentration of trace elements in stack gas

Most of the emission rates were extremely low. For example, the statutory limit for mercury emission in the NH and MRC guidelines for the emission of trace elements from plant is 3mg/m³. The mercury emission was 0.002 mg/m³, about 1000-fold less than the limit.

FLY ASH DISPOSAL

The disposal of fly ash by storage in ash dams is of environmental concern. For wet disposal the mobility of trace elements in the ash dam environment results in the release of trace elements including arsenic, boron, cadmium, chromium copper, mercury, molybdenum, nickel lead, selenium, vanadium and zinc.

Laboratory-based leach tests are used to determine the leaching propensity of trace elements. These tests include the USEPA 1311 using an acetate buffered solution at pH 4.5-5.5 and the ASTM D3987 test in which water at neutral pH is the medium. These tests do not simulate the environment of the ash dam and their use is questionable. Table 9 compares the results on a fly ash using these procedures showing the differences in leaching obtained.

Table 9: Comparison of leach tests

The differences obtained between the leach tests are further illustrated in Table 10, which is a comparison of the two standard leach tests with the leaching obtained with the sluice water used to transport the fly ash to the ash dam. There were some very significant differences in the leaching of arsenic, chromium, copper, molybdenum and vanadium.

Table 10: Concentrations of trace elements in fly ash leachates

A major study by Killingley et al, 2000 using column leaching has provided data on the long term leaching of fly ash (see also Ward and Jankowski, this Symposium).

SUMMARY

There has been increased scrutiny on the levels of trace elements in coal due to the increasing concern about the impact on the environment of coal fired-power plants. This has been driven by legislative regulations on emissions to air and water.

Australian thermal coals have low levels of trace elements of major environmental concern. Consequently air emissions from coal-fired power stations are very low compared to statutory limits. Leaching of trace elements in ash dams is of greater importance particularly the long-term impact of storage. The most important aspect of the impact of leaching is the bio-availability of the species present in the ash dam water.

Better knowledge of the modes of occurrence of trace elements in coal would enable the processes in the combustion chamber and the convection path to be understood. More sensitive microprobe techniques would greatly assist in assigning mineral associations. This would greatly assist in unravelling the mechanisms occurring in the combustion chamber.

Trace elements will continue to be an environmental issue as restrictions on emissions tighten through legislative controls.

ACKNOWLEDGEMENTS

Much of the data presented was obtained at CSIRO Energy Technology through the efforts of staff in the Analytical Science Research Group.

The support of ACARP through the funding of trace element research is gratefully acknowledged.

Coal Liquefaction Technology

noreply@blogger.com (Coal Trading) — Sun, 06 Nov 2011 10:48:00 +0000

I. Introduction

Coal Liquefaction is a rather simple process, which converts coal into liquid in a process solvent at 400-500 C, usually using hydrogen and catalyst. However, its chemistry is very complex reflecting complicated chemical structure of coal. Free radicals produced by thermolysis of covalent bonds of coal. Such kinds of reaction are generally believed to have a primary role in coal liquefaction. In the mean time, the radicals formed during liquefaction undergo various reactions i.e., decomposition to lighter components, stabilization by hydrogen transfer from hydrogen sources, and retrogressive reactions to heavier fractions than themselves. The action of catalyst such as Iron and molybdenum compounds generally does not involve a coal directly, but rather a solvent and primary liquefaction products.

The ultimate goal of liquefying coal is to increase the ratio of hydrogen to carbon and to remove the heteroatoms (S and N) and ash from coal.

II. The Factors which affect to coal liquefied oil Performance

The factors, which can affect to Coal Liquefaction performance, are both Chemical condition and Physical condition. Such as coal properties, coal structure, role of solvent, role of catalyst, Pressure condition, temperature, concentration coal-solvent etc.

II.1 Coal Properties

It is highly desire to correlate physical or chemical properties of coal with liquefaction reactivity. There have been many attempts since Bergius`s work (He reported that coal with over 85% carbon (d.a.f) gave a poor liquefaction yield.) as early as 1920. However, it is difficult to determine what property of coal is influential to liquefaction reactivity, since coal has its inherent inhomogeneous and complex chemical structure. Mostly the reactivity has been estimated by the point yield of some solvent soluble component at a set time at fixed reaction condition, though it is considered insufficient to compare the reactivity of various kinds of coal. A more sophisticated reactivity measure based on a combination of equilibrium and rate of liquefaction was proposed.

Snape, concluded in his review that no one single property adequately predicts liquefaction conversions for all coals, but correlations with combinations of several coal properties such as sulfur content, reactive macerals (Exinite+Vitrinite), Volatile Matter, Vitrinite reflectant, and atomic ratio of H/C have been established for coal from particulars geological regions. For example, distillate yields in relatively severe liquefaction generally increases with increasing H/C ratio and decreasing vitrinite reflectance.

There are so many coal properties that should be considered in coal liquefaction process including Coal rank, Coal Maceral, Elemental analysis, Proximate analysis and Chemical structure of coal.

II.1.1 Coal Rank

Coal rank is consider as one of indication precursor in predicting liquefaction reactivity, since coal rank is defines by several coal properties such as volatile Matter, Fixed carbon, Calorific Value, Vitrinite reflectance, carbon and hydrogen content in coal etc. Even though a single property of coal could not be perfect information in predicting liquefaction yield, however, at least it can be a precursor indication to estimated liquefaction reactivity. As previously mentioned that distillate yield of liquefaction generally increases with increasing H/C ratio and decreasing vitrinite reflectance.

Coal can be classified by several methods such as Vitrinite reflectance, ASTM method (using Fixed Carbon and Calorific Value in certain basis), or using the graph of Coal Band. Table –1, Table –2, and figure –1 are coal classification in various methods.

Generally it can be predict that the higher rank of coal the lower yield of liquefaction will be obtained, since the higher rank means the increase of vitrinite reflectance and lower H/C ratio as well as lower volatile matter. Another property of higher rank coal that can reduce liquefaction yield is more condensed aromatic ring in coal molecule.

II.1.2 Coal Maceral

Basically coal are contain three maceral group i.e. Vitrinite, Exinite (Liptinite),and Inertinite. Vitrinite and Exinite are very reactive maceral, whereas Inertinite is un-reactive maceral. Therefore, coal that, contain higher Vitrinite and Exinite is preferable in coal liquefaction. Because the higher reactive maceral contain in coal, the higher oil yield will be obtained in coal liquefaction. Coal macerals is an important property of coal since its property is indicating the origin of coal derivatives that also indicating the structure of coal molecule. Table –1 shows the origin of plant material of each maceral group in coal.. Figure –1 shows the structure of each coal precursor..

TABLE – 1 ASTM COAL CLASSIFICATION

TABLE - 2 Rank Classification by Vitrinite Reflectance

Figure – 1 The American Coal Band : plot of hydrogen versus carbon (after Macrae, 1966)

TABLE - 3 Classification of Coal Maceral

FIGURE -2 Structures of coal precursors (Whitchurst [217])

II.1.3 Elemental analysis.

Elemental or Ultimate analysis are including Carbon, Hydrogen, Nitrogen, Sulfur, and Oxygen. C, H, N, and S are determined through laboratory analysis, whereas, Oxygen is obtained by different of 100 % and the sum percentage of Carbon, Hydrogen, Nitrogen, and Sulfur content.

Carbon and Hydrogen as described above are the important parameters in coal property, since these parameters can indicate the yield of liquefaction by its H/C atomic ratio.

Nitrogen in coal is not significant parameters in coal liquefaction process, as in the process of liquefaction its constituent can be removed in the hydrotreater. However, Nitrogen in product oil is one of the important parameter to be considered since, the presence its constituent in the oil can reduce the stability of oil as it can generate gum.

Sulfur content in coal is one of parameters is desired in coal liquefaction process, since Sulfur could enhance the reactivity of catalyst such as limonite. Therefore the higher Sulfur content is preferable in coal liquefaction.

II.1.4 Proximate analysis.

Proximate analysis are included Moisture, Ash content, Volatile matter, and Fixed carbon. Fixed carbon is determined by different. Moisture content of coal is not significantly affect to liquefaction process, since in the process of coal liquefaction there is slurry dewatering that very effective and efficient in removing moisture from coal.

Ash content is affect to liquefaction process if coal contains severe ash content, as in the process ash content and some of catalyst could be accumulated in the bottom of reactor. Such accumulation and sedimentation decreased the effective reaction volume in the liquefaction reactor, resulting in a significant pressure drop and decrease of residence time of the slurry in the reactor. Those deposition of solid materials in the connecting pipe is also can reduce the flow area of the slurry in down stream units. As a result, long-term operation is inhibited.

The maximum ash content may be 10 % still can tolerable in liquefaction process, however, lower ash content is preferable to prevent the above circumstance.

Volatile matter and fixed carbon are also the parameter of coal property that can to be an indication in predicting liquefaction yield. Fuel ratio is calculated as fixed carbon divided by Volatile matter (FC/VM). The higher fuel ratio the lower coal reactivity, resulting relatively lower liquefaction yield. Volatile matter is mostly derived from aliphatic hydrocarbon. The higher volatile matter content is indicating more aliphatic hydrocarbon present in coal molecule, resulting more reactive in coal liquefaction process.

II.2. Chemical Structure of Coal

The chemical structure of coal is very important factor in coal liquefaction process. In order to predict the chemical reaction that may be occurred in liquefaction process, we have to know the chemical structure of whole materials involved in the liquefaction process including the chemical structure of coal.

The structure model of coal is very complex. The structure of coal molecule is dependent on the origin of coal precursor and coal rank. The higher rank of coal the higher condensed aromatic present in coal structure. Figure 2, is illustrates the comparison of basic structural units for coal of various ranks.

FIGURE - Basic Structural Unit For Coal of Various Rank

It can be seen that the higher rank of coal the higher condensed aromatic ring present in coal molecule. Anthracite has more condensed aromatic ring in its molecule structure than bituminous, and it decrease as the coal rank is lower. The higher rank of coal has more phi-phi bonding (condensed bonding) inter molecule. This type of bonding is very strong and only break in liquefaction process in the reactor. Whereas, the weaker bonding such as hydrogen bonding and charge transfer bonding, break in the preheater. Figure 3 shows the various bond in coal and its relation ship between rank of coal and rate of bonding interaction.

FIGURE - 4 Various Bond In Coal

It can be seen that the higher rank of coal the higher p- p and covalent bonding, and decrease in hydrogen bonding. Therefore, the higher rank of coal more difficult to break such kind of coal molecule into smaller molecule, and consequently the oil yield in liquefaction will be lower.

II..3. Coal Solubility

Coal solubility is considered as closely related to coal structure and indispensable for coal structural study. Understanding of coal structure is also important in considering coal liquefaction. Therefore, it seems to be essential for the consideration of coal liquefaction to grasp the nature of coal solubility based on coal structure.

Coal may be extract and dissolved at temperature of 625 – 725 K by the action of hydrogen donor solvents with or without catalyst or molecular hydrogen present. During the coal dissolution, hydrogen from the donor solvent reacts directly with coal molecule to produce hydro carbon liquid. Other reaction such as depolymerization, hydro cracking, and sulfur removal reaction also occur in the dissolver. Hydrogen-donor compound exist in various boiling ranges of coal liquid, but may be more concentrated in the higher boiling point fractions and the hydrogenated coal -derived solvents.

II..4. Role of Solvent

In order to enhance the efficiency of coal liquefaction, the performance of solvent has been widely studied. The following subjects are the among recent studies on coal solvent :

1) Function of solvent during liquefaction

2) Evaluation of hydrogen donation ability

3) Design of solvent for efficient hydrogen consumption

4) Effect of additives containing oxygen functional group.

The function of solvent is confirmed as follows: swelling and dissolution of coal particles, hydrogen donation to fragments from coal macromolecule, dissolution of de-polymerized products and hydrogen shuttling.

II..5. Role of Catalyst

During the coal liquefaction, coal fragments evolved from the thermal cleavage of bonds in coal molecule can be stabilized to liquefied products by hydrogen atoms. As well as donor solvents that act as hydrogen transferring medium from the gaseous hydrogen to the radicals, catalyst plays important roles in coal liquefaction. The roles of catalyst can be summarized as follow :

1) Re-generation of donor solvents

2) Hydrogenation of the radicals directly with gaseous hydrogen

3) Removal of heteroatoms containing in the liquefied product.

4) Hydrogenation and hydrocracking of primary products to the lighter oil.

Conventionally, catalysts have been used in order to re-hydrogenate the dehydrogenated solvents formed from hydrogen transferring to coal fragments. Since the solvents have no longer hydrogen donation ability, it is necessary to re-hydrogenate the solvent with gaseous hydrogen to maintain stable liquefaction. Iron compound as a cost effective catalyst, e.g. Iron sulfides or iron ore have been widely studied by many investigator. The catalyst can hydrogenate directly coal fragments with absorbed hydrogen atom on the surface of catalyst.

The experiment results of liquefaction of Yallourn coal with ?-FeOOH show that the atomic ratio of H/C in preasphaltene is higher than that with pyrite. These results suggest that the most important role of catalyst is to transfer gaseous hydrogen to coal fragments.

III. THE PROCESS OF BROWN COAL LIQUEFACTION AND ITS MECHANISM

Figure - 5 Basic Scheme of Coal Liquefaction

BROWN COAL LIQUEFACTION PROCESS

Surfactant Pile Test

noreply@blogger.com (Coal Trading) — Fri, 04 Nov 2011 10:00:00 +0000

I. BACKGROUND

There are so many chemical agents that have been used as dust suppressant, either as wetting type agent or coating type agent. The effect of those chemical agent are also believed could prevent the occurrence of oxidizing of coal by oxygen in the air due to the agent cover coal surface or binds fines coal particle to a larger particle resulting smaller surface area and higher density. Notwithstanding, the effect of surfactant agent against certain coal is not necessarily will the same performance to other coals due to the chemical and physical properties as well as condition of coal stockpile may vary.

Therefore, prior to use of a certain surfactant agent, we need to prove that the surfactant is suitable to our coal with its characteristic at the live condition.

There are several surfactant agent that have been used in Berau Coal i.e. PIC 103 (wetting type), SOLINDO 2000(Coating Type), and ALPHENATE (wetting Type). Those surfactants are free from NONYL PHENOL, as each producer has declared it.

Recently, there are some product of surfactant being offer to Berau Coal by LION Corp. namely: COALCOAT L-15 and PROTOTYPE. Those products demonstrate good performance in laboratory test especially its wet ability to coal to binds fine coal particles.

In order to test the performance of the product against coal in live condition on stockpile, recently we conducted spontaneous combustion test involved 6 stockpiles with different surfactant to make a comparison of its performance. The surfactants involved in the test are comprised of :

1. Coal without surfactant

2. Coal sprayed by COALCOAT 15 L (4PPM)

3. Coal sprayed by COALCOAL 15 L (8PPM)

4. Coal sprayed by PROTOTYPE 83 PPM

5. Coal sprayed by PROTOTYPE 250 PPM

6. Coal sprayed by ALPHENATE

All surfactants is tested its performance against Lati Coal –T seam.

II. SPONTANEOUS COMBUSTION TEST

II.1 Objective

The Primary objective of this test is to determine the Chemical agent (surfactant) which most effective for Lati coal mine to Minimize dust dispersion and also to prevent spontaneous combustion.

II.2 Methodology/ Summary of the test

The numbers of quantity of crushed coals are sprayed with variation Surfactants. The sprayed coals then stacked in a location, and allow them within certain period of time. The temperature of each coals are monitored on daily basis during the test. There are two concern measures of the test i.e. Self Heating and Spontaneous Combustion.

II.3 Procedure

II.3.1 Coal Preparation

Coals, which used for the test was derived from Lati Coal T-Seam.

Coal was crushed by ROXON Crusher to get nominal particle size of 0-50 mm.

The Spraying by each surfactant was carried out during coal being crushed.

Coal samples were also taken during each coal stock being throughput to falling stream.(See fig-4) The formula of the surfactant were added onto coals are described below:

1. Coal coat 15-L (4ppm) : 2.1 kg dilute with water to 1100 L

2. Coal coat 15-L (8ppm) : 4.2 kg dilute with water to 1100 L

3. Prototype 83 ppm : PR-A 8.4 kg + PR-B 104 kg dilute with water to 4000 L

4. Prototype 250 ppm : PR-A 25.2 kg + PR-B 312 kg dilute with water to 4000 L

5. Alphenate : 11 L liquid dilute with water to 1100 L.

II.3.2 Stockpile Construction

The coal test was at minutes 50 mm on particle size and had been previously crushed trough ROXON crusher and sprayed by each surfactant.

The stockpiles were constructed at ROM on July 30 – August 1, 2002.

Six stockpiles were built, each of approximately 400-ton coal.

Stockpiles profile we made were three meter high, and had triangular cross section.

The bases of stockpiles were approximately 5 M wide, and 12 M in length. (See fig 8)

The coal stacks were at its natural angle of repose in the completed stockpiles.

II.3.3 Weather Records

During the course of the monitoring, the following weather data were also collected:

1. Daily rainfall during the course of monitoring.

2. The weather description every measurement of the temperature (observed)

3. The wind velocity and direction did not measure by instrument, however, the prevailing wind and wind velocity were observed visually during the course of the test.

4. Daily ambient temperature did not measure due to the thermometer were not available.

II.3.4 Temperature Monitoring

One steel probe was placed into each stockpile except coal without surfactant stockpile, due to the steel probes available only 5 pieces.

Wood sticks also marked on each stockpile with small flag at each stick. (See fig 8 & fig 6).

The stick marks were used to define points measure coal temperature using Gun Thermometer (IR-Thermometer).

The temperature measurement was carried out twice a day i.e. AM (9.00) and PM (16.00) the data of temperature monitoring is presented on table: 2,3,4,5,6,7,and 8.

III. TEST RESULTS

III.1 Weather records

III.1.2 Rain

The total rain days occurred during the course of the test are 7.i.e August 2, August 5, August 9, and August 20 – August 24. Details of rainfall monitoring are resented on table 8.

The total rain day during August is the lowest in year up to August i.e. average 0.94 mm/day. The highest rain days in this year of up to August were recorded occurred on January and March. The averages per day of both months are of 206.5-mm and 105.5-mm respectively. (See table-8).

III.I.3 Wind

The wind velocity observed was relatively high with the prevailing direction from northwest to southeast, and from southeast to northwest. The trapezium surfaces of stockpiles were faced wind direction at both sides. (See fig 9).

III.2 Physical of coal stockpile inspection

The surfactant of coal after sprayed with surfactants was very clean and no dusty observed during they on falling stream when being throughput on to stockpile through belt conveyor chute that relatively high. (See fig. 5).

There was coal particle segregation occurred on all coal stockpile being tested. Where the lump particle size were segregated and laid on the foot of the pile at four sides, and most of the smaller particle were laid on the middle to top of the pile, after one week, the lump coal particle on each stockpile has become brittle and crack, some of them had broken to be smaller particle even some has become fine coal. (See fig 10).

III.3 Temperature Monitoring

The entire data of monitoring are presented on table 1 – 6.

The summary minimum-maximum temperature during monitoring is also presented on table – 7.

According to the data obtained thermometer probe, the highest maximum temperature was achieved by coal coat 15-L (8ppm) coal pile i.e. 79 oC. Whereas according to infra red thermometer, that the highest maximum temperature is L-15 (4 ppm) i.e. 98 C followed by alphenate i.e. 97 oC.

III.4 Self-Heating and Self-Combustion.

III.4.1 Self-Heating.

III.4.1.2 Thermo Probe

Self-Heating had been occurred on all coal piles since day 4th as shown on Graph – 8.

The temperatures of all coal pile were steady increase after day 4th, then finally the temperature reaches the critical temperature (70 oC) after day 5th to day 30th. The coal pile which firstly reaches the critical temperature, was L-15 (8ppm) i.e. on day 18th followed by L-15 (4ppm) i.e. on day 23rd, and the longest one was prototype 250 ppm which, reaches the critical temperature on day 30th. The coal pile of Alphenate and Prototype 83 ppm had been stopped prior to reach the critical temperature due to spontaneous combustion had been occurred on both coal piles. However, if we plot the line to the right Y line, the critical temperature will be reached after prototype 250 ppm. The coal pile without surfactant was not measured by thermo-probe due to the thermocouple was not available.

III.4.1.3 Infra Red Thermometer

To define the attainment of critical temperature by each coal pile-using IR-Thermometer could be seen on daily maximum temperature graph. (See graph –7) The lines of the graphs is more fluctuate than the thermo-probe graph due to there was disturbance of the point of measurement on coal pile during measuring the temperature of each point. Where the points of measurement of thermo-probe temperature were not disturbed, hence, the data which more accurate determination the performance of self-heating are obtained by thermo-probe measurement. According to the graph – 7, generally, the critical temperature were attained on day 20th by all coal piles.

III.4.2 Spontaneous Combustion

Spontaneous combustion of all coal piles was occurred at the foot of piles and beyond the point of temperature measurement. Therefore, the temperatures were spontaneous combustion occurred were not recorded on the table of temperature monitoring data. The date of the occurrence of spontaneous combustion of each coal pile respectively is as described below:

7) Alphenate on day 14th at PM

8) L-15 (4 ppm) on day 19th at AM

9) L-15 (8 ppm) on day 24th at PM

10) Prototype 83 ppm on day 28th at PM

11) Without Surfactant on day 31st at AM

12) Prototype 250 ppm on day 34th at AM

The point of were spontaneous combustion occurred were mostly at the wider surface which faced to wind direction i.e. the trapezium surface faced to north west, except L-15 (4ppm) and L-15 (8 ppm) that coal combustion were occurred on both side trapezium surfaces (NW and SE). See fig 13.

IV. DISCUSSION

* In order to see the performance of each surfactant against coal self-heating in the pile more accurate could be seen from temperature data, which measured using thermocouple probe. The temperature monitoring data from infrared Thermometer is less accurate than thermo-probe due to disturbance against coal pile during measurements using IR Thermometer. It could be seen from the difference between graph profile of thermo-probe and IR-Thermometer. Where, the IR-Thermometer results more fluctuate and wider ranger than thermo-probe results.

* All Surfactant are effective in suppress fine coal dust. It was proved when coal being stacked through sufficient high falling stream, and no significant dust being observed.

* Although coal has been sprayed with surfactant, but coal especially which lump coal particle size remains easy to weathered as it exposed on stockyard, resulting cracking on it and even breaks to smaller particle when lump particle size breaks, and subsequently the unsprayed surfactant coal is exposed, resulting larger surface area on coal pile surface. See fig 10 (Coal undergoes crack and break after one week exposed).

* In the mean time, when coal is cracking, coal pile become more porous and such circumstance allow oxygen or air easy to penetrate into coal pile that causing more coal is oxidized and temperature rise of coal pile is accelerated accordingly.

* The ease of weathering of coal with surfactants as easy as coal without surfactant. Thus, the surfactant is no effect to lump particle size in terms of protecting coal against weathering.

* The initial spontaneous combustion was occurred on the coal pile where laid on more lump coal particle size, which has undergone cracks and breaks. And firstly occurred on the surface which facing wind direction. The factors that influence to spontaneous combustion occurred on coal stockpile are comprised of coal pile porosity and wind velocity. Coal pile porosity is relates to the ease of air to penetrate into coal pile. Whereas, wind is relates to oxygen supply into coal pile. The higher wind velocity impact in to coal pile means more oxygen supplied in to coal pile, resulting more coal is oxidized and generates heat that leads to spontaneous combustion. See fig 11 (stages of coal spontaneous combustion).

* The coal piles formation were made in the worst condition, where the wider surfaces i.e. trapezium surface were faced to prevailing wind direction. In such condition coal piles encountered high impact of heavy wind, and it means the oxidation of coal is more aggressive, consequently, spontaneous combustion was faster occurred on coal pile.

* On the previous spontaneous combustion test report that had been conducted, where the coal piles were formed in windrow shape, alphenate coal pile started burned on day 40th. It far different compare to this test. Where in the last test had only within 14 days, coal pile sprayed with alphenate had been combusted.

* There was the interesting that the coal pile without surfactant was even burned on the second last of the sequence of coal piles spontaneous combustion, i.e. just three days before prototype 250 ppm. It means that coal without surfactant more stand against spontaneous combustion than alphenate, L-15 and prototype 83 ppm. This Phenomenon due to the coal pile was protected from the wind (see fig 9) the location of without surfactant coal pile was laid on between prototype 250 ppm and ROM stockpile. During the course of the test, ROM stockpile at southeast from coal pile was getting high and coal pile was protected from southeast wind accordingly. In the meantime, the impact of northwest wind was minimizing by prototype 250-ppm coal pile that lay on the northwest from coal pile without surfactant.

* The chemical agent sprayed on to coal, cannot afford to protect lump coal particle against weathering, and therefore, it only effective to bind dust, which generated from coal crushing, in order to enhance the performance of chemical agent especially in protecting against self combustion, it should be sprayed at least twice time, i.e. when coal being crushed, and sprayed on to surface of coal after the surface has undergone weathered and crack.

* Actually the oxidation reaction upon coal in bulk, moreover in incompact (free stack) stockpile, could not be stopped event spraying by chemical agent. it shown on the graph-8 (Temperature monitoring thermo-probe) The temperature were steady increase after only day 4th . It could because of two reason, i.e. firstly, the spraying of surfactant did not wetted all coal particle, and secondly, surfactant could not cover or coating coal surface against oxygen resulting coal remain reacted with oxygen, generated heat that leads to self heating and spontaneous combustion.

* We considered, that the affection of surfactant in protecting coal against spontaneous combustion is because of its binding performance against fine coal particle that resulting coal surface area reduced, and therefore the access of oxygen against coal is minimize accordingly.

CONCLUSION

* The best performance of surfactant in terms of inhibiting self-heating is prototype-83 ppm. It shown on graph-8, where the lowest line (blue one) is prototype 83 ppm coal pile, although, is initial temperature was not on the lowest one.

* The best performance of surfactant in terms of inhibiting spontaneous combustion under condition of the pile test is prototype 250 ppm.

* All surfactant are very effective in terms of binding of fine coal particle, hence, it can afford to inhibit dust dispersion problem.

* All surfactant cannot afford to protect lump coal particle against weathering and crack. Therefore, surfactant has less effect in protecting coal in bulk against spontaneous combustion.

* The most effective way to prevent coal stockpile against spontaneous combustion is protecting coal pile against wind and minimize pore of coal pile by means of compacting it. However, we still need dust suppression to prevent dust, and also to minimize coal oxidation because not all condition can be handle in the same manner, e.g. during coal in transporting.

* Coal oxidation by oxygen unable to stop at all in any way as long as the pores in coal pile are exist, the effort we can do is only to minimizing it in order to enhance coal pile’s endurance against spontaneous combustion, at least to make coal is safe during it being transporting and storage in Costumer stockpile prior to utilized.

Coal Spontaneous Combustion

noreply@blogger.com (Coal Trading) — Fri, 04 Nov 2011 09:56:00 +0000

* Coal will reacts with oxygen in atmosphere as it exposed to the air.

* The oxidation reaction rate increase as the coal rank decrease.

* The oxidation of coal generates heat that will accelerates oxidation reaction itself.

* When the heat generated greater than of that heat dissipation either through evaporation or heat conduction, then self heating of coal will occurs, and finally coal will be combusted spontaneously.

The propensity of Coal Spontaneous Combustion

• Coal Rank

• Coal Properties

• Stockpile Environment

• Coal size segregation on stockpile

• The period of time of coal been exposed

Stages of coal spontaneous combustion

The Theory of Spontaneous Combustion

• The main theory which have been advanced to explain the phenomenon of spontaneous combustion include the following :

* The Pyrite Theory

* The “Coal Oxygen”Complex theory

* The humidity theory

* The bacteria theory

PYRITE THEORY

• Iron disulfide is associated with coal in two form, i.e. cubic yellow pyrite and rhombic marcasite.

• Marcasite has been reputed to be more reactive towards oxygen than cubic yellow pyrites. Li and Parr (1926) found that both form oxidized about the same rate.

• It has been suggested that pyrite helps to promote spontaneous combustion only when it is present in a finely divided state. (Bowes, 1954)

• The heat of oxidation of pyrites was determined by Lamplough andHill (1912-13) who found that a mean value of 13.8 J per mL of oxygen consumed.

• Although difference of opinion have existed about the role of pyrites in spontaneous combustion, it is generally accepted now that :

– The heat of oxidation of pyrites contributes to the heat of oxidation of coal.

– The oxidation of pyrites to ferrous sulphate causes disintegration of the coal and thus provides a greater surface area of coal for oxidation.

• FeS2 + 7O2 + 16 H2O ---> 2H2SO4 + 2FeSO4.7H2O

• Since there is a volume expansion on oxidation of pyrites to ferrous sulphate, fragmentation of the associated coal will occur.

THE “COAL OXYGEN OXYGEN” COMPLEX THEORY

• The formation of a coal oxygen complex during low temperature oxidation of coal was postulated by a number of early workers including Wheeler (1918), Davis and Byrne (1925), and later Schmidt (1945)

• Research conducted by Sevenster (1961) has confirmed the earlierfindings of these workers,

• The stages in the oxidation of coal may be summarized as follows:

1) Physical adsorption of oxygen

2) The chemisorptions step, formation of a complex containing an active form of oxygen called “per-oxygen”.

3) Rapid chemical reaction in which CO,CO2, and H2O are produced bydecomposition of “Per-Oxygen”

• Step 1 takes place readily at low temperature and requires a lowenergy of activation (of the order of 1 kcal per mole -Glasstone, 1954)

• Step 2 and 3 are definite chemical reaction, Jones and Townend (1946) state that step 3 occurs between 70 o C and 80 o C.

• Van Krevelen and schuyer (1957) emphasized the importance of microscopic examination in studying the behavior of coal towardsmolecular oxygen. He concluded that the primary reaction betweenoxygen and coal proceeds with a low activation energy (12-16 kJ per mole) and results in the formation of a primary chemisorptions complex.

THE HUMIDITY THEORY

• Scott (1956) has stressed the importance of humidity in relationship to spontaneous combustion. By means of a specially design micro calorimeter, he was able to show that the quantity of heat liberated by atmospheric oxidation of coal was much less than the quantity required to remove water from coal. He concluded that if evaporation of water can be induced at the heat of a heating, then the temperature of the heating would decrease.

THE BACTERIA THEORY

• Since bacterial activity is known to be the cause of spontaneousignition, many workers examined the role of bacteria in the spontaneous combustion of coal.

• Coward reviewed of this subject (1957) include references of sixinvestigations by different workers between 1908 and 1927. In four of these investigations definite evidence was presented to show that bacteria were capable of living on coal, and in some cases such bacteria could cause a slight rise in temperature of the coal.

• However, Graham (1914 -15) who found that sterilized coal oxidized at the same rate of as the unsterilized coal, and concluded thatthe mechanism of oxidation did not include bacterial activity. A similar conclusion was drawn by Winmill (1914-15), and Scott (1944) quoted the committee on spontaneous combustion in mines thus : The self heating of coal is not in any way due to the presence of bacteria.

Experience facts

• Experiences in coal handling operation shows that the spontaneous combustion occurs when meet the following condition:

– Coal has been stored too long time on exposed stockpile either raw coal or crushed coal without compaction (poor stockpile management)

– High wind speed impact to coal stockpile.

– Poor monitoring of stockpile temperature.

– Ignorance of size segregation stockpile, i.e. the stock under falling stream.

Prevention of Coal Spontaneous combustion

• Spontaneous Combustion has to be prevented as it decrease the value of coal, besides its produce some gases that hazard to health.

• Spontaneous combustion is also causes loss of money either due to lower the value of coal or cost should be spent to extinguish it and removed some coal. Besides, it impact to environment

The Preventive

Action

• Storage of coal over the period of time on open stockpile should be avoided by means of implement good stockpile management (first in –first out)

• Determines the critical period of storage by means of conduction a field spontaneous combustion test.

• Stockpile Temperature monitoring.

• Minimize the impact of wind against coal stock

• Limits the height of stockpile

• Compaction of stockpile which will remain on stockpile longer (certain period of time)

• The usage of chemical agent

HANDLING OF COAL COMBUSTED

• If Spontaneous combustion can not be avoided and occurs in coal stockpile, then evacuation of such burned coal has to be done immediately.

• Be careful when handling burned coal, the necessary safety equipment has to be worn.

• The treatment of burned coal can be undertaken as follows:

– Spray the flame on the surface of stockpile with water until no flame appear on the surface

– Remove of coal ash appeared on the surface of burned coal

– Dig out and spread the area suspected have undergone spontaneouscombustion with either excavator or wheel loader. Be careful when digging out of the hot coal cause it could be results a flame explosion.

– Relocate all hot coal on to safe stockpile location, and spread out to dissipates heat. Allow the spread coal until its temperature decrease to the ambient. If necessary spray these coal by water to speed up the cooling down its temperature.

– Re-stack the cooled down coal and compacts if these coal not to be loaded out in certain period of time.

CONCLUSION

* The propensity of spontaneous combustion increase as the rank of coal decrease.

* Coal Spontaneous combustion can be avoided by implement a good stockpile management

* The usage of chemical additive is an alternative method to minimize coal spontaneous combustion, provided, such chemical agent has proved effectively reduce the propensity of spontaneous combustion through an actual experiments

Slagging

noreply@blogger.com (Coal Trading) — Wed, 02 Nov 2011 11:10:00 +0000

1. Influence of ash fusion temperature :

Slagging is a phenomenon that ash melts in combustion furnace (boiler) and stick to radiation walk in liquid form. It is judged by the fact that whether ash fusion temperature is higher or lower than gas temperature around radiation wall. It is generally believed that coal having ash fusion temperature higher than 1300oC would not have slagging.

2. Influence of ash content :

For the coal with high slagging propensity, amount of slaggs building up on radiation wall has linear relationship with amount of ash to be fed into boiler. Special consideration is to be taken when low calorific coal with high ash content is to be burned.

3. Ash Slagging index

For your information Berau Coal is categorized in lignitic type ash coal.

4. Influence of Sulphur content:

If sulphur content is high slagging propensity would increase because Sulphur (S) combines with alkaline ingradients and form sulphate with low fusion temperature. Sulphur content less than 20% would be desirable in terms of preventing slagging problem.

A. Bituminous type ash ( Fe2O3 -- CaO + MgO )

B. Lignitic type ash ( Fe2O3 -- CaO + MgO)

Where:

TH: Hemisphere ash fusion temperature (higher of oxidizing or reducing atm.)

TID: Initial deformation temperature (lower of oxidizing or reducing atm.)

Impact Of Coal Properties On Power Plant

noreply@blogger.com (Coal Trading) — Sun, 30 Oct 2011 09:14:00 +0000

1. General

Coal fired power station are very capital intensive ventures. Their capital cost is of the same order as the present worth of coal consumed during the lifetime of the plant. In countries using imported coal such as Japan life cycle fuel cost (present worth) could easily exceed the total capital cost. The capital cost of the plant is greatly affected by the quality of coal and the environmental restriction. A modern coal fired power station consistence of 4x700 MW unit, burning low sulphur coal, presently cost about US$ 1200 per KW to install including engineering and financial charges.

A similar plant, burning high sulphur coal, requiring flue gas desulphurization plant (FGD) would typically cost about US$ 1400 per KW.

Capital Cost (million US$)

4 x 700 MW coal fired power plant

1. Boiler Plant US $ 1100

2. Coal Handling including Ship Unloading US $ 2000

3. Ash Collection Plant US $ 130

4. Ash Plant US $ 100

Total Cost of P.P US $ 3330

5. Desulphurization Plant US $ 620

6. Denitrification Plant US $ 50

Total cost with FGD/De-NOX US $ 4000

Operating and maintenance cost amount typically to US $ 28 per KW per year for a plant without FGD and approximately twice as much for a plant with FGD. Large capital savings can be made if high quality coal can be secured for the lifetime of the power plant.

2. Coal properties

As naturally occuring material, coal shows a wide range of properties. For the purpose of steam raising almost all coals can be used however different properties will requires the use of different plant designs. In designing or operating equipment it is essential that the mean and probable range of the various coal properties are to be known and allowed for. The coal properties that affect plant are listed on exhibit 7.

a. Calorific Value

Calorific value indicates the inherent quality of the coal determining the total coal volume to be consumed at the power station for a given energy generation. Calorific value depends on the composition of the coal indicated by the ultimate analysis and adversely affected by the moisture content and ash content. Exhibit 10 compares the boiler furnace sizes for different fuels, gas, oil, coal and brown coal (lignite). It is noted that from these figures lower grade fuels require dramatic increase in size and consequently cost of boiler.

b. Ash Content

Ash content has an adverse effect on power station plant and costs. An increase in ash content results in a direct decrease of the calorific value, requiring greater volume to be handled by the coal plant, boiler, electrostatic precipitators (ESP), and ash plant resulting in larger and expensive plant and increases operating and maintenance cost. Ash is responsible for erosion problem in boiler pressure parts and duct, resulting in additional operating and maintenance cost and loss of availability. It has been found that erosion in boiler is proportional velocity to the power of 3.5 (ash % x velocity 3.5). Accordingly, during the design stage of a boiler the gas velocities are selected as a function of the ash content in the design coal range, affecting the boiler cost.

c. Moisture Content

Moisture not only affects directly the calorific value of coal but also the efficiency of the boiler due to the fact that the latent heat of water is lost with the flue gas. For 1% increase in moisture efficiency drops by 0.1%. A greater volume of coal is also required

Grindability of Coal

noreply@blogger.com (Coal Trading) — Sun, 30 Oct 2011 09:02:00 +0000

1. Particle size

Objective of milling coal before feeding to boiler is to make the coal burned out completely. Followings are general yardstick about acceptable level of over 200-mesh fraction for different type of coal.

• Anthracite -- 10% ~ 15%

• Bituminous Coal -- 15% ~ 35%

• Subbituminous Coal -- 35% ~ 45%

• Lignite -- 45% ~ 55%

Grindability of coal is normally evaluated by moisture content and HGI (Hardgrove Grindability Index) based on ASTM.

2. HGI

Followings are general criteria to achieve favorable grindability.

• Coal with fuel ratio ± 1.0 HGI : 35 ~ 45

• Coal with fuel ratio ± 2.0 HGI : 45 ~ 75

• Coal with fuel ratio ± 3.0 HGI : 75 ~ 100

As HGI becomes smaller, grindability would become inferior and larger capacity pulverizer is needed. Coal with HGI higher than 40 would be desirable in general.

3. HGI’s influence on pulverizing capacity

Graph 1. demonstrates relationship between HGI and pulverizing capacity based on following conditions.

• Constant power at pulverizer (kWh)

• Take Chinese coal with HGI 50 as standard

4. HGI’s influence on power requirement at pulverizer

Graph on page 3/5 demonstrate relationship between power requirement and HGI to achieve same pulverizing capacity. Left curve is the result of theoretical calculation but right curve is the actual performance record at EPDC power station.

• Pulverizing capacity 70 ton/H

• 200 P % means 200 mesh pass weight %

The graph shows that more power is required at pulverizer as HGI goes down taking South African Witbank coal with 54 ~ 56 HGI as standard, coal in order to maintain same pulverizing capacity.

It is noted that HGI’s increase by 10 ( say from 50 to 60 for example ) would result in 10% power cost saving at pulverizer.

5. TM’s influence on pulverizing capacity

Most high moisture content coal would act as a cushion between rollers and rotating table in the pulverizer during grinding process and reduce pulverizing capacity.

In order to get rid of surface moisture and dry coal, most power stations these days use hot air supplied from air heater. Specific temperature, say 80oC, is being set up at the outlet of pulverizer to ensure dried coal to boiler is to be fed.

If the temperature goes down below the set up temperature, more volume of hot air using additional fans is needed or alternatively reduce the volume of coal by regulating coal feeder when volume of hot air reaches the maximum capacity, thus reduces generating capacity.

Graph 2. demonstrates relationship between surface moisture content and pulverizing capacity on the basis of constant power requirement and constant size distribution of pulverized coal.

Graph 3. demonstrates relationship between surface moisture content and power requirement at pulverizer based on constant pulverizing capacity. It is noted that 1% additional moisture in pulverized coal would reduce boiler efficiency by 0.1%.

Fouling

noreply@blogger.com (Coal Trading) — Sun, 30 Oct 2011 08:19:00 +0000

Fouling is a phenomenon that ash condense, adheres and building up on super heater or reheater surface where heat transfer takes place in connective manner. Sometime it bridges the tubes and jeopardieses heat conductivity. Depending on nature of ash being built up, it can normally be removed by shootblower.

1. Influence of alkaline in ash :

Chemicals ingradients which have severe impact on fouling are alkalines such as Na2O, K2O and CaO. Particularly for the coal having high Na2O content in ash, special consideration on fouling is to be taken. In general term, young coal such as lignite and low rank subbituminous coal, have relatively high alkaline content in ash.

2. Influence of Sulphur

Sulphur in coal has propensity to combine with alkaline to form sulphate thus promotes fouling.

3. Fouling Index (see attached below)

For your information Berau Coal is categorized in lignitic type ash coal,

A. Bituminous type ash ( Fe2O3 -----> CaO + MgO )

B. Lignitic type ash ( Fe2O3 -----> CaO + MgO )

Electrostatic Presipitator (ESP)

noreply@blogger.com (Coal Trading) — Sun, 30 Oct 2011 04:32:00 +0000

1. General

Coal combustion generates more than hundred times of dust compared to oil burning thus requires a dust collecting facilities to meet stringent environmental restriction. Coal being consumed by Japanese power companies has relatively low sulphur content, say less than 1.0%, which gives high apparent electric resistibility of dust thus high efficient electrostatic precipitator (ESP) is required. In general high sulphur content of coal increases dust collecting efficiency thus requires relatively simple electrostatic precipitator.

2. Principle of ESP

Basic principle of ESP is that all dust particles in flue gas are electrically charged by corona discharge and attracted by electrodes and pile up on electrode plates. When the dust piled up on electrodes becomes thick, it would be removed by hammer shock and drop to the hoppers. Electrodes would normally being connected by link chain which rotates with relatively low speed (0.5 m/min) and piled up dust is removed by rotating brushes above the hoppers.

3. Electric resistibility of dust and dust collecting efficiency

Dust collecting efficiency is largely affected by electric resistibility of dust and electric resistibility changes according to followings as:

a. flue gas temperature

b. moisture and SO3 contents in flue gas

c. coal properties and ash composition

In general electric resistibility ranging 106~1011O-cm would normally give favorable collecting efficiency.

4. Type of ESP

a. Low temperature ESP

Low temperature ESP is installed after air heater where gas temperature would normally be 120oC~150oC.

Electric resistibility of dust with the temperature range of 120oC~150oC is relatively high which would results in reducing dust collecting efficiency. Even with some improving technologies 30 mg/Nm3 at the outlet would be considered the limits.

b. High temperature ESP

High temperature ESP is installed after economizer where flue gas temperature would normally be 320oC~380oC.

Original aim of this system was to increase dust collecting efficiency in relatively high gas temperature where electric resistibility of dust would be relatively low. However it is observed that this system has turned out to be no significant advantages compared to low temperature ESP. 30 mg/Nm3 at the outlet of ESP would be considered the limit.

c. High efficiency ESP

This system has been developed to cope with very stringent environmental restriction. ESP is installed after air heater and GGH where flue gas temperature would normally be 95oC~105oC.

5. Ash distribution

Flue Gas Desulphurization Plant (FGD)

noreply@blogger.com (Coal Trading) — Sat, 29 Oct 2011 13:46:00 +0000

1. General

As SOX gas, mainly consist of SO2, is health hazardous and causes deforestation due to acid rain, environmental restriction in SOX has been stringent and most of coal fired power stations in Japan these days have come equipped with FGD. The most commonly prevailing process to eliminate SOX in flue gas these days is to use limestone (CaCO3) to absorb SO2 and convert to gypsum (CaSO4 2H2O). Depending on restriction standard, more than 95% of SO2 in flue gas is removed and gypsum can be sold to construction material industries to produce plasterboard.

Exhibit 25 shows relationship between required SO2 removal efficiency (%) and emission standard (mg/m3) for different sulphur content of coal.

Exhibit 26 shows relationship between sulphur content of coal (%) and FGD efficiency (%) to meet 1.00 ppm SO2 emission standard for example.

2. Type of FGD

a. Conventional Type FGD

SO2 absorption tower and oxidation tower are separately installed and chemical reaction are as follows:

Absorption:

SO2 + CaCO3 + ½ H2O CaSO3. ½ H2O + CO2

(Calcium sulphate)

Oxidation:

CaSO3.½ H2O + ½ O2 + 3/2 H2O CaSO4. 2H2O

(Gypsum)

Exhibit 30 shows a general concept of FGD.

b. Advanced type of FGD

Absorption and oxidation take place in a single tower, which can save capital investment. SO2 in flue gas reacts with limestone (CaCO3) in slurry spray and becomes Ca (HSO3)2 in the first place. Then in the tank following chemical reactions take place gypsum is discharged.

3. Location of FGD

FGD is normally installed in the back end of the whole system. Flue gas temperature after FGD would be in the order of 45oC~55oC. In order to enhance dispersion effect of remaining SO2, NOX and dust out of chimney stack flue gas would normally be heated up by GGH (heat exchanger).

Flue Gas Denitrification Plant (De-NOxPlant)

noreply@blogger.com (Coal Trading) — Sat, 29 Oct 2011 13:32:00 +0000

1. General

NOX is health hazardous chemical ingredient, same as SOX, environmental restriction on NOX has been stringent and most of coal fired power plant in Japan there days have come equipped with low NOX burners and de-NOX plant. NOX is classified into two categories; one type of NOX is mostly fuel NOX and the other is originated from Nitrogen in air, which is called thermal NOX. Nearly 95% of NOX is believed to be NO. Nitrogen content of the coal less than 2.0% dry ash free basis is acceptable to most coal fired power stations in Japan these days.

It is generally noted that NOX under the furnace temperature of less than 1400oC is considered fuel NOX and 25% – 30% of NOX under the furnace temperature of above 1600oC is considered thermal NOX.

Graph above shows the relationship between NOX level and Fuel Ratio with different Nitrogen content at constant furnace temperature of 1350oC. It should be appropriate interpretation that NOX level has something to do with Nitrogen content of the coal, but at the same time NOX level has also something to do with fuel ratio as well. Graph above clearly illustrates that higher the fuel ratio higher the NOX level regardless of nitrogen content of the coal.

2. Low NOX burner (Nozzle)

Basic principle of Low NOX burner is to minimize NOX generation during combustion by changing concentration of coal particles in primary air at each burner using the characteristic that either ban concentration or dense concentration would normally give less NOX generation.

3. De-NOX plant

Basic principle of de-NOX plant is that NO is to be converted to N2 and H2O using ammonia gas (NH3) and its chemical reaction (reduction) is as follow :

Catalyst

4 NO + 4 NH3 + O2 ------> 4 N2 + 6 H2O

NO + NO2 + 2 NH3 ------> 2 N2 + 3 H2O

Catalyst

NH3 in liquid form is vaporized then injected into flue gas. Type of catalysis being used is normally porous ceramic mainly made of Aluminum and Titan.

4. Location of De-NOX plant

a. De-NOX plant is installed after economizer where flue gas temperature would normally be 300oC~400oC. Flue gas after de-NOX plant is then cooled down by air heater to approximately 150oC and goes to low temperature ESP.

Integrated Coal Gasification Combined Cycle (IGCC)

noreply@blogger.com (Coal Trading) — Sat, 29 Oct 2011 13:08:00 +0000

1. General

It is a kind of new coal utilization technology for next generation, major difference of this technology compared to PFBC is to use coal gasification plant to generate gas instead of pressurized bed combustion. 40 ton/day coal gasification pilot plant test started at 1981 in Japan. And 2 ton/day gas cleaning pilot plant test started in 1991. to be able to meet same emission standard as for oil fired power plant in terms of NOX, SOX and dust level and that be able to achieve 43.5% thermal efficiency is achieved, it would be efficiency improvement by 14% compared to conventional coal fired power plants of which thermal efficiency would normally be around 38%. Which means 14% of coal volume can be saved, SOX and NOX are reduced by 14% and dust is reduced by 56%.

If ceramic turbine to bear 1500oC gas temperature is developed in future, 47% thermal efficiency will be expected. Desulphurization and dust collection are carried out under the pressurized condition so that gas volume can be reduced and size of the plant be compact, which would be require only one third of space compared to conventional coal fired power plants. However IGCC is still in experimental stage, no commercial plant has been built yet in Japan.

2. Concept of Integrated Coal Gasification Combined Cycle (IGCC)

Concept Of Coal Combustion

noreply@blogger.com (Coal Trading) — Sat, 29 Oct 2011 13:01:00 +0000

Pulverized coal dispersed in primary air is fed to the boiler through the nozzles then heated directly by flame or by radiant heat from high temperature slag adhered to the boiler wall and starts ignition then forms primary combustion zone.

Primary combustion zone is the one where combustion of volatile matter primarily takes place. CH4, He and CO evaporated from volatile matters combine with O2 in primary air and form flame around the coal particles.

Secondary combustion zone is for char combustion where char and unburned gas from primary combustion zone is burned dispersed by secondary air. Time required for char combustion accounts for 80% – 90% of total coal burnout.

Ignition

Pulverized coal fed through nozzles receives heat from flame directly or radiant heat from other heat sources then temperature around surface of coal particles goes up and ignition takes place. Ignition characteristics would be largely affected by coal properties.

Depending on coal properties, type of nozzles, design of nozzles, necessity of auxiliary fuel and maximum load are to be considered. The temperature when ignition takes place is defined as radiant ignition temperature.

As shown in Fig. 2 radiant ignition temperature goes down as volatile matter of the coal increases. Volatile matters of the coal which Japanese power companies would normally consume are in the range of 30% - 50% on dry ash free basis, so range of radiant ignition temperature would be 600oC – 700oC.

Figure 1. demonstrate the relationship between boiler temperature and distance from nozzle where ignition takes place. It is generally noted that as boiler temperature goes down the distance increases and that ignition becomes unstable.

FIGURE. 1

FIGURE. 2.

Coal Combustibility

noreply@blogger.com (Coal Trading) — Sat, 29 Oct 2011 12:36:00 +0000

Proper measure to judge combustibility of coal is to use fuel ratio. Fuel ratio is defined as follows :

Fixed carbon content

Fuel Ratio = -----------------------------

Volatile matter content

If the fuel ratio goes up, combustibility would be inferior to minimize unburned carbon in ash, fuel ratio ranging 2.5 ~ 3.0 would be desirable. In general term lignite and sub-bituminous coal having fuel ratio less than 1.0 would be superior in combustibility but inferior in mills grindability and slagging/fouling.

Low Volatile bituminous (Fuel Ratio appr. 4.0)
Middle Volatile bituminous (Fuel Ratio appr. 2.8)
A High Volatile bituminous (Fuel Ratio appr. 1.5)
B High Volatile bituminous (Fuel Ratio appr. 1.2)
C High Volatile bituminous (Fuel Ratio appr. 1.1)

• ) Higher the coal rank, lower the combustibility

Volatile Content :

To maintain stable combustibility a certain countermeasure is to be taken when the coal with less than 20% volatile coal is to be burned.

Moisture Content :

Coal having high inherent moisture is in general term, would be inferior in combustibility. A countermeasure is to be taken when coal with less than 35 combustibility index is to be burned.

Heat Value (Kcal/kg) – 81 Fixed Carbon (%)

Combustibility index = -----------------------------------------------------

Volatile matter (%) + inherent moisture (%)

Coking Property/Agglomeration

Coking property would generally be measured by CSN (Crusible Swelling Number), higher ranking coking coal with higher CSN would possibly cause clogging at burner nozzle and increase unburned carbon in ash due to agglomeration during combustion, so special boiler design should be investigated when high CSN of 6-7 coal is to be burned.

Birth of a Mud Volcano

noreply@blogger.com (Coal Trading) — Tue, 25 Oct 2011 10:26:00 +0000

ABSTRACT

On 29 May 2006, an eruption of steam, water, and, subsequently, mud occurred in eastern Java in a location where none had been previously documented. This “pioneer” mud eruption (the first to occur at this site) appears to have been triggered by drilling of overpressured porous and permeable limestones at depths of ~2830 m below the surface. We propose that the borehole provided a pressure connection between the aquifers in the limestones and overpressured mud in overlying units. As this was not protected by steel casing, the pressure induced hydraulic fracturing, and fractures propagated to the surface, where pore fluid and some entrained sediment started to erupt.

Flow rates remain high (7000–150,000 m3 per day) after 173 days of continuous eruption (at the time of this writing), indicating that the aquifer volume is probably significant. A continued jet of fluid, driven by this aquifer pressure, has caused erosion and entrainment of the overpressured mud. As a result, we predict a caldera will form around the main vent with gentle sag-like subsidence of the region covered by the mud flow and surrounding areas. The eruption demonstrates that mud volcanoes can be initiated by fracture propagation through significant thicknesses of overburden and shows that the mud and fluid need not have previously coexisted, but can be “mixed” within unlithified sedimentary strata.

INTRODUCTION

Understanding how Earth recycles elements, compounds, minerals, or even sediment is a major scientific quest, which transcends several disciplines, including chemistry, biology, and earth science. In sedimentary geologic systems, the cycle time can be particularly significant. For instance, the burial of sediment (and pore fluid) to depths in excess of 5 km, and their remobilization and transport back to Earth’s surface, can take millions to tens of millions of years (e.g., Kopf et al., 2003).

One prerequisite for this long-term recycling process is the development of elevated pore fluid pressure (overpressure). The excess fluid provides the required energy for the breach of seals and for the transport of a fluid-sediment mix back to the surface, where it is redeposited as sediment (e.g., Stewart and Davies, 2006; Deville et al., 2006). Mud volcano systems are one of the many expressions of this process, and many have been documented globally (Kopf, 2002; Milkov, 2000). Significant eruptive edifices can develop, which are often grossly similar in form to their more intensively studied igneous counterparts (Stewart and Davies, 2006), although substantially smaller.

However, many of the fundamental processes involved in the recycling of buried fluid and sediment through mud volcano systems are poorly understood, and studies are still in their infancy. Elementary questions remain; for instance: (a) Do the fluid and mud come from the same beds, or is the fluid transported from deeper levels into mud source beds where mud is entrained? (b) How is the plumbing system that feeds mud and fluid to the surface initiated and sustained? and (c) What is the three-dimensional architecture of the feeder systems and how do they evolve through time?

On 29 May 2006, a mud eruption was observed in the Porong subdistrict of Sidoarjo in eastern Java (Fig. 1). At the time of this writing, the erupted mud pool (a) has a volume of ~0.012 km3, (b) covers an area of ~3.6 km2 and is up to ~10 m thick, (c) has buried 4 villages and 25 factories, and (d) displaced 11,000 people. There have been 13 fatalities as a result of the rupture of a natural gas pipeline that lay underneath one of the holding dams built to retain the mud. The eruption has unofficially been named “Lusi” (Lumpur “mud” Sidoarjo), and this name is adopted here. It occurred during the drilling of a nearby exploration borehole (Banjar Panji-1); therefore, in this case several factors (e.g., pressure, depth, stratigraphy) that are normally not constrained in natural mud volcano systems are calibrated. Although we propose that Lusi is man-made, it does offer a unique opportunity to address the mechanisms of initiation and maintenance of a mud volcano. The aims of this paper are to consider why the eruption occurred, compare it to other natural examples, and evaluate what we can learn about how mud volcano systems work.

MUD VOLCANO SYSTEMS

Mud volcanoes are common on Earth (Milkov, 2000), but particularly so in compressional tectonic belts (e.g., Azerbaijan:

Figure 1. Map of Java, showing the location of the eruption in the Porong subdistrict and Purwodadi and Sangiran Dome, where other mud volcanoes have been documented.

Planke et al., 2003; Indonesia: Ware and Ichram, 1997), within deltas (e.g., Mississippi: Neurauter and Bryant, 1990), and submarine slopes undergoing gravitationally driven detachment (e.g., Niger delta: Graue, 2000). The volcanoes can be longlived features, composed of a series of mud “cones,” which indicate a pulsed eruptive history (Evans et al., 2007) that can occur over 104–106 yr time spans.

The term “mud volcano system” was coined by Stewart and Davies (2006) to describe the set of structures associated with a constructional edifice (mud volcano) and feeder complex that connects the volcano to its source stratigraphic unit (Fig. 2A).

The system is driven by pressure and a source of fluid, which may or may not coexist with mud source beds (see Deville et al., 2003). Above the fluid source is a feeder conduit (Fig. 2B), the detailed structure of which is largely unknown. It probably consists of a complex system of fractures and mud-filled dykes (Fig. 2C) that feed a fluid-sediment mix to Earth’s surface (e.g., Morley, 2003). The fluid-sediment mix then erupts to form the “mud volcano”—a term we only use to describe the edifice (Fig. 2D).

The plumbing of mud volcano systems is poorly constrained. For instance, the mud and fluid could coexist at the time of initiation, analogous to magma (e.g., Davies and Stewart, 2005), or the fluid could be transported from a deeper source, remobilizing mud at shallower stratigraphic levels (Deville et al., 2003; Kopf et al., 2003; You et al., 2004). Some mud volcano systems are thought to comprise multiple mud chambers at different stratigraphic levels (Deville et al., 2003; Planke et al., 2003) whereas other models propose that mud volcano systems comprise significant masses of mud, in the form of bulbous- shaped diapirs (Brown, 1990; Milkov, 2000).

A “pioneer mud volcano” (e.g., Fig. 2A) is a term used by Davies and Stewart (2005) to describe the first mud volcano that erupts in a location where no mud volcano system previously existed. They envisage that if a substantial mud volcano develops, a positive feedback loop can become established where subsidence of the overburden due to loading, conduit wall-rock erosion, and volume loss at depth causes new fractures and faults to form in the overburden stratigraphy.

These structural apertures provide new pathways for a fluid-mud mix.

GEOLOGIC SETTING

The East Java basin is an inverted extensional basin (Matthews and Bransden, 1995). It comprises a series of east-west– striking half-graben that were active in extension during the Paleogene and reactivated in compression during the early Miocene to Recent. The Oligo-Miocene to Recent basin was filled with shallow marine carbonates and marine muds, some of which are known to be “overpressured” (see Osborne and Swarbrick, 1997). As a result of the compressional inversion, these strata are gently folded with normal and reverse faults cutting the inversion anticline crests (see Matthews and Bransden, 1995). A small section of one of these east-west–trending anticlines was targeted by the Banjar Panji-1 exploration well.

Mud volcanoes have been documented before in East Java. For example, they are found within the crest of the Sangiran Dome (part of one of the east-west–trending Neogene folds: Watanabe and Kadar, 1985) and near Purwodadi, which is 200 km west of Lusi (Fig. 1). Overpressured lower Miocene clays probably equivalent to the Tuban or Tawun Formations (similarage to the Kujung limestone—see Matthews and Bransden, 1995) and the Upper Kalibeng Formation are considered to be the source of the mud (Watanabe and Kadar, 1985).

Figure 2. Components of a mud volcano system revealed by three-dimensional seismic data and outcrop. (A) Schematic illustration of the main components of a mud volcano system. Mud volcano systems can be divided into intrusive and extrusive structural domains. Fluid may either coexist with the mud source or enter from a deeper source (blue arrows) causing remobilization of shallower mud and entrainment of other overburden lithologies.

The mud-fluid mix is transported through fractures and faults to the surface, where stacked cones form due to episodic eruptive and quiescent periods. (B) Seismic coherency cube (see Bahorich and Farmer, 1995) across the Gunashli mud volcano (South Caspian Sea, from Davies and Stewart, 2005), showing feeder conduits, the detailed internal structure of which is unknown. (C) Mud-filled dykes from the Jerudong anticline in Brunei (see Morley, 2003). These types of mud-filled fractures are potentially what allow for the transport of the mud-fluid mix to the surface. (D) Photograph of mud volcano terrain from Azerbaijan comprising several gryphons from which small mud flows emanate.

OBSERVATIONS

Volumes, Rates, and Dimensions

The typical eruption volume, duration, rate, spatial extent, and aspect ratio of selected naturally occurring mud volcanoes can be compared to Lusi (Tables 1 and 2, respectively). These comparisons show that the Lusi eruption has a significant volume, duration, and spatial extent. The average eruption rate is not particularly high. Lusi has an anomalously high aspect ratio (Table 2). It is also worth noting that long-lived mud volcanoes that consist of several cones that develop as a result of multiple eruptive and non-eruptive developmental stages (Evans et al., 2007) are known to have volumes of up to ~22.5 km3—dwarfing the current but still highly active Lusi edifice (Stewart and Davies, 2006).

Key Events and Subsurface Data

Banjar Panji-1 was an exploration well that was targeting gas within Oligo-Miocene age Kujung Formation carbonates within the East Java Basin. The well reached a depth of 2834 m, after which an eruption of steam, water, and a minor amount of gas was observed at 5:00 a.m. on 29 May 2006, 200 m southwest of the well. On the second and third of June 2006, two further eruptions started 800–1000 m to the northeast of the well, but both of these stopped on 5 June 2006 (United Nations Final Technical Report, 2006). It is reported by local villagers that the water-mud mix at the surface had a temperature of 70–100 °C; a continuous plume of steam seen on early to recent photographs of the eruption supports such high temperatures. An earthquake of magnitude 6.3 occurred at 5:54 a.m. local time on 27 May 2006, with an epicenter 280 km west-southwest of the Lusi eruption, near Yogyakarta (U.S. Geological Survey, 2006). The eruption of a dilute mud-water mix has persisted from the site of the initial eruption, and mud now covers an area of ~3.6 km2 (Fig. 3).

Unreleased geologic data (lithological log, biostratigraphic determinations, gamma ray, sonic, density logs) indicate that the well drilled the following (shallowest first): (a) the Pleistocene age Pucangan and Kabuh Formations, (b) then ~1000 m of overpressured muds with some sand interbeds (the Upper Kalibeng Formation [Pleistocene age]), (c) ~1300 m of interbedded sands and muds, and finally (d) the well penetrated a limestone (presumed to be the Kujung Formation), which was also overpressured. There was no casing set between the bottom of the hole (the Kujung Formation) and ~1743 m of the overburden, including part of the 1000 m of overpressured Upper Kalibeng Formation mud and the entire 1300 m of interbedded muds and sands (Fig. 4A). We know that (a) in the Banjar Panji-1, the pore pressures at 2130 m (700 m above the Kujung limestone) are 38 MPa (5500 psi); and (b) that in a well 5 km away called Porong-1, the pressure within the Kujung limestone aquifer was 48 MPa (6970 psi) at 2597 m.

VOLCANO INITIATION

Model

Given the pore pressure of 38 MPa (5500 psi) at 2130 m in the Banjar Panji-1 well, we calculate an overpressure of 16 MPa (2300 psi) at this depth. In the Porong-1 well, we use the pressure of 48 MPa (6970 psi) at 2597 m to calculate an overpressure of 21 MPa within the Kujung limestone. On the assumption that the Kujung limestone is a regional aquifer (which seems likely given the high continuous flow rates at Lusi), we predict the overpressure was ~21 MPa at the base of the Banjar Panji-1 at 2830 m.

We propose that the drilling of the overpressured Kujung limestone caused an influx of pore fluid into the well bore (known as a “kick”). The well bore itself provided the pressure connection from the limestone to any shallower aquifers as well as the overpressured muds of the Upper Kalibeng Formation. The eruption started with steam and water, and this did not come to the surface through the well bore, but instead took place 200–1000 m away (Fig. 4B).

Therefore, the transport route for the steam and mud was not through the wellbore but through the surrounding overburden. High pore-pressure causes natural hydraulic fracturing of the sedimentary overburden (see Engelder, 1993) when pore pressures exceed the fracture strength. These conditions for the creation of hydraulic fractures are most likely to form in the shallowest strata not protected by steel casing. We propose that the fractures probably formed within the Upper Kalibeng Formation and propagated from 1–2 km depth to the surface over a period of hours. The depth is backed by the temperature of the erupted mud-water mix, which is 70–100 °C, indicative of rapid transport from 1.5 to 3 km depth, assuming a geothermal gradient of 25 °C/km and a surface temperature of 28 °C. Such drilling-induced fracture and fluid flow processes, where the well bore provides the Figure 4. Schematic three-dimensional representations of the Lusi mud volcano showing four main developmental stages.

The first three diagrams depict the evolution between May 2006 and Dec. 2006 (A–C), and the fourth diagram (D) shows the predicted next phase of evolution. (A) March to May 2006: Banjar Panji-1 well drills toward Kujung Formation, through overpressured mud (Kalibeng Formation) and interbedded sands and muds. (B) May 2006: Kujung Formation carbonates are penetrated, which leads to a “kick” (influx of fluid into the well bore). The kick causes hydrofracturing of overlying strata (probably initiated within the Kalibeng Formation). Drilling mud and pore-fluid enter the well bore, driven by the excess pressure upward, through porous and permeable strata and the fracture system.

Entrainment of overpressured Kalibeng Formation muds occurred. (C) May to December 2006: entrainment of Kalibeng Formation muds causes a subterranean conduit to form, the walls of which undergo period collapse. (D) Post-2006: caldera forms around the vent, and gentle sag-like subsidence of the region where the flow extends. Smaller mud cones may be erupted as a result of conduit establishment due to foundering of the overburden stratigraphy.

TABLE 1. VOLUME, DURATION, AERIAL COVERAGE, AND RATES OF SELECTED LARGE-SCALE MODERN ERUPTIONS FROM THE SOUTH CASPIAN SEA AND TRINIDAD COMPARED TO LUSI*

TABLE 2. ASPECT RATIOS* FOR MUD VOLCANOES FROM THE SOUTH CASPIAN SEA AND LUSI†

Figure 3. Satellite images of the Lusi eruption taken ~100 days after the eruption started. (A) Entire area of eruption. (B) Close-up of the main vent (marked by clouds of steam [white]), which appeared 200 m southwest of the exploration well. Both images taken September 2006, courtesy National University of Singapore Centre for Remote Imaging, Sensing and Processing (CRISP).

necessary initial pressure communication, has been witnessed elsewhere; for example, in subsurface blowouts that occurred in Brunei in 1974 and 1979 (see Tingay et al., 2005). At Lusi, the influx of pore water into the well bore may have initially come from the Kujung limestones, but once the heavy drilling mud had been displaced into the new fractures, fluid would have also started to flow from porous and permeable formations in the overburden.

The passage of fluid into overpressured (and therefore undercompacted) mud would lead to entrainment of the unlithified sediment (Fig. 4C), which would also contribute its pore water to the mix. Mud is cohesive, and in a similar way to the entrainment of mud in sedimentary settings, the shear stress imposed by the adjacent moving water has to overcome the sediment’s cohesive yield strength (e.g., Dade et al., 1992; Kranenburg and Winterwerp, 1997) for it to be entrained. Such an entrainment process has been proposed for mud volcanoes in the UK, for instance, where water from an underlying aquifer passes through mud-rich overburden, causing the formation of a subterranean cavern system (Bristow et al., 2000).

The same general process has also been proposed by Deville et al. (2003) for mud volcanoes in Trinidad. We envisage that collapse of the Upper Kalibeng strata will contribute to the mixing process. It is also conceivable that the hot water in large caverns will allow convection cells to develop, which will contribute to the mixing process (e.g., Deville et al., 2003). The resultant dilute water-mud mix is moving up fractures to the surface as a fluidized sediment flow with the mud in suspension.

The mix started to erupt at the surface, driven by the pressure of the pore fluids in the Kujung limestones. Erosion of the walls of the fractures is also likely (it occurs in other mud volcanoes), and therefore a major conduit would grow upward and laterally, periodically collapsing inward. This particular mixing mechanism for mud volcanism has probably led to the very dilute composition of the mud-water mix and the high aspect ratio of the edifice.

Pressure Drive

If a continuous 2830 m column of an erupting mud-water mix has a density of 1.3 gcm-3, based on an assumed water: mud ratio of 80:20, the mud column would exert a pressure of 36 MPa (5225 psi) at the bottom of the Banjar Panji-1 exploration hole. This pressure is 12 MPa less than our estimate of the pressure within the Kujung limestone (48 MPa); therefore, it is most likely that the flow that is being witnessed is driven by this pressure difference. Gas exsolution and expansion (Brown, 1990) are not considered important lift mechanisms at present.

NEXT DEVELOPMENTAL STAGES

Maintenance of flow depends upon one of two factors. If there is a continuous pathway to the surface due to the subsurface erosion of the conduit walls, the influx of the pore fluid and eruption will continue until the aquifer pressure equals the pressure due to the vertical column of erupting mud-water mix (i.e., 12 MPa). Alternatively, if mud gains access to the surface through fractures that remain open against the minimum stress, flow will reduce substantially only when the fracture closure pressure is reached; this pressure will depend on the depth at which the fracture(s) occur. Once the pressure drive abates, the compaction of the extruded and intruded mud can cause low levels of mud-water eruption, potentially for years or decades to come, as noted in other mud volcanoes such as Piparo in Trinidad and many mud volcanoes in Azerbaijan between violent (active) eruptive phases.

If our model of entrainment of the mud within the Upper Kalibeng Formation is correct, then unless the pore pressure drops to allow flow to stop, the subterranean caverns will undergo collapse (Fig. 4D). We predict that the region around the vent will form a caldera and that the area of the mud flow will undergo less significant sag-like subsidence. This subsidence pattern is consistent with the behavior of other mud volcanoes (Stewart and Davies, 2006). The subsidence that caused the fracture of a gas pipeline buried by the mud volcano and dam system indicates that collapse may have already started.

DISCUSSION

Induced by Drilling or Earthquake? We propose that Lusi is the direct result of connection of a high-pressure fluid at depth with shallow sediments at a depth at which fractures can be initiated. Once initiated, the fractures would have propagated to the surface, driven by the deep pressure. Drilling activity has allowed this connection, and our preferred model is that the earthquake that occurred two days earlier is coincidental. The primary reasons for not considering an earthquake to be the trigger or contributing factor are (a) no other mud volcano eruptions were reported in Java at the same time; (b) the earthquake preceded the eruption by two days; seismogenic liquefaction usually occurs during earthquake- induced shaking of sediment (e.g., Ambraseys, 1988); (c) there are no reports of a “kick” during the earthquake or immediately afterward; and (d) sand, rather than mud, is more conducive to liquefaction due to earthquake shaking because it is a non-cohesive, granular sediment. An earthquake could have generated new fractures and weakened the uncased section of the well, but it would be highly coincidental for an earthquake-induced fracture to form 200 m away from this well and provide the entire fracture network required for an eruption on the Earth’s surface.

Initiation and Subterranean Mixing

A fundamental question in mud volcano system studies is how they are initiated. The model proposed by Brown (1990), Van Rensbergen et al. (1999), Davies and Stewart (2005), and Stewart and Davies (2006) is that hydrofractures can penetrate several kilometers of the crust and transport a fluid-sediment mix that erupts to form a pioneer volcano. Because in this case we know that the mud-water mix has been transported ~2 km through the overburden, through new or reactivated fractures, the Lusi eruption supports the models proposed by these authors.

The Lusi eruption also strengthens the concept that rather than the source water and the source mud coexisting in the same stratigraphic unit (mudrocks at 2.0 km depth have strength and not the porosity of 70%–80% required for the Lusi sediment composition), the fluid has a deeper source, and mud is entrained from within the overburden (e.g., Bristow et al., 2000; Deville et al., 2003; Kopf et al., 2003; You et al., 2004).

This subterranean mixing model differs from the concept of mud and fluid coexisting (Davies and Stewart, 2005; Stewart and Davies, 2006) and contrasts with models for the subsurface remobilization of sands where coexistence of sand and fluid is the general assumption. The mud is particularly susceptible to entrainment due to overpressure, which does not allow normal compaction (Osborne and Swarbrick, 1997). The aquifer pressure provides the pressure drive.

Common Phenomena?

Subsurface blowouts are not uncommon events (e.g., Tingay et al., 2005) and can involve sediment entrainment, but this scale of sediment mobilization, triggered by drilling activity,has not been documented before. A combination of factors account for this being so rare: (1) the penetration of an overpressured mud that is susceptible to erosion followed by (2) the penetration of an aquifer that releases large volumes of pore water and (3) the man-made pressure linkage provided by 1.7 km of open hole section.

CONCLUSIONS

It is very likely that Lusi was initiated as a result of access by a high-pressure aquifer at depths in the region of 2.5–2.8 km through an open-hole section of the Bajar Panji-1 well to depths at which fractures could be initiated. Lusi indicates that mud volcanoes can be initiated by fracture propagation from multi-kilometer depths, which triggers fluid flow and the rapid establishment of a subterranean mixing system, into which water is transported from deeper successions.

Prediction of the next developmental stages is fraught with difficulty, but the unabated 173 days of very active eruption indicate a large aquifer has been penetrated, and we can be confident that some sort of eruptive activity (perhaps lower-level) will continue for many months and possibly years to come. A region several kilometers wide should undergo sag-like subsidence over the coming months with more dramatic collapse surrounding the main vent. Modeling and direct measurement of the inevitable land subsidence will help to predict what the future impact the Lusi mud volcano has on the local population.

ACKNOWLEDGMENTS

R. Davies is grateful to Anthony Mallon for discussion. Mark Tingay is thanked for the photograph in Figure 2C. We are extremely grateful to the National University of Singapore Centre for Remote Imaging, Sensing and Processing (CRISP) for permission to use the satellite images. We thank Achim Kopf, Peter Van Rensbergen, and an anonymous reviewer for thoughtful and prompt reviews and Gerry Ross for guidance during the preparation of this paper.

Introduction of Auger Mining

noreply@blogger.com (Coal Trading) — Fri, 21 Oct 2011 11:44:00 +0000

Histrory

Introduction of Auger Mining to Liddell Open Cut in 1992 at the first time in NSW.
AAM was set up in 2001
Augering at Wambo Open Cut

Muswellbrook Open Cut
Camberwell Open Cut

AAM Auger Equipment

Auger 1: 2439-60 modified to 2142-60A coal recovery Auger

675 hp, capable of augering to ? 23º, with a 1.52m (5’) head

Auger2: 2348-72A coal recovery auger

850 hp, capable of augering to ? ˜10º, with a 1.35m, 1.52m and 1.83m (4’6”, 5’ and 6’) head

Auger3: 2233-60 coal recovery auger

650 hp, with 1.52m (5’) head

various conveyors with the largest having the capability of loading direct up to Caterpillar 785 size trucks

Caterpillar loaders (sized to suit specific work requirements)

Auger 1, 2 And 3

Auger 1

Auger 2

Auger 3

Auger Mining Operation

Location of Auger Mining (1)

Location of Auger Mining (2)

Stratigraphy

Workforce and Equipment :

Num of workforce 3 people
Auger Machine 1
Cutting Head 1
Auger Flights
Stucker Conveyor 1
Loader 1

Auger Machine

Auger Machine Front

Auger Machine Back

Cutting Head

Auger Flight

Stack out Conveyor

Loader

Augering Operation

1. Moving of Auger Machine

2. Adjustment of the Location, Direction and Dip direction

3. Setting a Stacker Conveyor

4. Start Augering

5. Put additional Flights

6. Recover Flights

Hole Layout

Coal Recovery : Up to 50%

Augering at steep seam

Production

Coal production will vary significantly depending upon:

seam dip;
depth of auger holes;
coal hardness;
length of production shift; and
use of multi-passing.

In General,

Auger 1: Dip 5-23°

600t-800t/shift (10hrs), Penetrating Depth 60m-90m

Auger 2: Dip <10° 1000t-1300t/shift, Penetrating Depth 80m-90m Issue of Augering Operation Roof and Floor Hit Unexpected Fault Roof Fall Recovery rate Detail Geological and Geotechnical Study will be needed Strengths of AAM Augering Low manning requirements (2 - 3 people) and High productivity Lower capital cost and High economics Selective Mining for High grade coal Flexible to fit in with conventional open cut mining Augering up to approx. 23 degree coal seam Small work area for operation Easier to move Recover the remained coal by Open Cut Auger Mining in Indonesia Are there potential areas in Indonesia ? Final Highwall exposed Seam Dip < approx. 23° Seam thickness > 1.5m
Suitable Geological and Geotechnical Conditions
High Grade Coal would be preferable