Читать книгу Coal-Fired Power Generation Handbook - James Speight G., James G. Speight - Страница 58

As-mined coal (run-of-mine coal) contains a mixture of different size fractions, sometimes together with unwanted impurities such as rock and dirt (Table 3.1). The purpose of coal preparation (often referred to as (coal cleaning, coal beneficiation) is to improve the quality of coal by cleaning to remove inorganic impurities and sizing for handling, process, and combustion requirements.

Thus, another sequence of events is necessary to make the coal a consistent quality and salable. Such events are called coal cleaning. Preparation of coal prior to feeding into the boiler is an important step for achieving good combustion. Large and irregular lumps of coal may cause the following problems: (i) poor combustion conditions and inadequate furnace temperature, (ii) higher excess air resulting in higher stack loss, (iii) increase of unburned carbonaceous material in the ash, and (iv) low thermal efficiency. Thus, effective preparation of coal prior to combustion improves the homogeneity of coal supplied, reduces transport costs, improves the utilization efficiency, produces less ash for disposal at the power plant, and may reduce the emissions of oxides of sulfur.

The relative density, friability, hardness, and strength of different elements within the coal matrix are key parameters for mechanical cleaning processes. The specific gravity of coal ranges from 1.23 to 1.72, depending on rank, moisture, and ash content. Mineral impurities have higher densities and this property is employed by a variety of separation methodologies. A coal preparation plant typically contains different circuits delineated by particle size. The larger particle fraction from 6 to 18 mm will normally contain coarse rock that can be separated by a vibrating jig or dense medium bath. For the smallest particles, those that are <0.5 mm, froth flotation is used. In this process, a conditioned feed pulp is introduced onto the top of the froth bed. Hydrophobic coal particles attach to rising bubbles and stay in the froth while hydrophilic mineral particles pass through it and discharge at the bottom of the floatation cell. Cyclones are used and the lighter coal particles swirl upward to a clean coal discharge while higher density impurities sink to the funnel outlet. Various dewatering screens, thickeners, and filters are used to separate the product and recover the medium.

Table 3.1 Added value to coal through processing (cleaning).

Method	Comment
Mineral matter	Removal of the mineral matter, which is largely noncombustible and may constitute up to 65% w/w of the raw coal, increases the heating value of the coal on a per unit mass basis. Some combustible material may be lost as part of the cleaning process but the removal of unwanted material reduces the mass and volume of coal for a given heating value thereby reducing shipping costs as well as minimizing coal handling and ash management costs for the end user.
Processing	Principally mineral matter removal and drying to remove moisture allows greater control over the quality of the coal, which improves the consistency of the coal for the end user, such as an electricity generator or coke manufacturer. Improved and consistent quality of the coal increases the efficiency and availability of steam boilers and is particularly important for the quality of metallurgical coke.
Physical processing	Physical processing methods can, to some extent, reduce sulfur and trace element contents, particularly on a heating value basis. Typically, coal cleaning is not practiced primarily for this purpose except for the metallurgical coal market.

Coal preparation is the stage in coal production when the run-of-mine coal is processed into a range of clean, graded, and uniform coal products suitable for the commercial market. In some cases, the run-of-mine coal is of such quality that it meets the user specification without the need for beneficiation, in which case the coal would merely be crushed and screened to deliver the specified product. The decision whether or not to process a particular raw coal depends on the coal and its intended market. The subbituminous coal of the Powder River Basin (Wyoming) is almost always shipped to market raw because it has inherently low mineral matter content (low ash-producing propensity) and poor washability. The region has low water availability, which is a critical requirement for conventional coal beneficiation.

By way of definition, the term washability is used to describe the ease with which mineral matter can be separated from the coal, and depends on the degree of incorporation of the mineral matter in the organic matrix of the coal and its specific gravity relative to the coal.

The purpose of coal preparation is to improve the quality of coal by cleaning to remove inorganic impurities and sizing for handling, process, and combustion requirements (Skea and Rubin, 1988; Speight, 2013). The relative density, friability, hardness, and strength of different elements within the coal matrix are key parameters for mechanical cleaning processes. The specific gravity of coal ranges from 1.23 to 1.72, depending on rank, moisture, and mineral matter content. Mineral impurities have higher densities and this property is employed by a variety of separation methodologies.

A coal preparation plant typically contains different circuits delineated by particle size. The larger particle fraction from 6 to 18 mm will normally contain coarse rock that can be separated by a vibrating jig or dense medium bath. For the smallest particles, those that are <0.5 mm, froth flotation is used. In this process, a conditioned feed pulp is introduced onto the top of the froth bed. Hydrophobic coal particles attach to rising bubbles and stay in the froth while hydrophilic mineral particles pass through it and discharge at the bottom of the floatation cell. Cyclones are used and the lighter coal particles swirl upward to a clean coal discharge while higher density impurities sink to the funnel outlet. Various dewatering screens, thickeners, and filters are used to separate the product and recover the medium.

The output from any coal mine usually consists of raw coal (run-of-mine coal, ROM coal) that is wetter and finer, and also contains more impurities than in the past due to (i) mining of lower quality coals, (ii) advanced, continuous and non-selective mining techniques that cause more coal being broke up, and (iii) more impurities being included, and (iv) extensive water utilization for minimizing dust (Lockhart, 1984). While the quality of run-of-mine (ROM) coals is generally decreasing, the necessity for efficient coal beneficiation technology is significantly increasing, resulting in an increased demand for high-quality coals that meet both market and environmental standards.

Thus, the coal delivered to the coal preparation plant consists of coal, rocks, minerals, and any other form of material that is not coal. The coal also varies widely in size, ash content, moisture content, and sulfur content. Thus, coal preparation serves several purposes. One important purpose is to increase the heating value of the coal by mechanical removal of impurities. This is often required in order to find a market for the product. Run-of-mine coal from a modern mine may incorporate as much as 60% reject materials.

Thus, after coal is mined it generally goes through a process known as preparation or coal cleaning to (i) remove the impurities in order to boost the heat content of the coal and thereby improve power plant capacity, which also reduces maintenance costs at the power plant and extends plant life, and (ii) to reduce potential air pollutants, especially sulfur dioxide – the extent to which sulfur dioxide emissions can be reduced varies, depending upon the amount of sulfur in the coal and the form of its occurrence.

Briefly, the grade of a coal establishes its economic value for a specific end use. Grade of coal refers to the amount of mineral matter that is present in the coal and is a measure of coal quality. Sulfur content, ash fusion temperature (measurement of the behavior of ash at high temperatures), and quantity of trace elements in coal are also used to grade coal. Although formal classification systems have not been developed around grade of coal, grade is important to the coal user.

Coal preparation (coal cleaning) is the means by which impurities such as sulfur, ash, and rock are removed from coal to upgrade its value (Speight, 2013). In the process, the undesirable material is removed from the run-of-mine (ROM) coal by employing separation processes which are able to differentiate between the physical and surface properties of the coal and the impurities. The result is a relatively clean uniform coal product.

The energy content of coal is related to its rank (degree of coalification) (Table 3.2) which is influenced by the content of nonfuel components (e.g., minerals and moisture) (Speight, 2013). Thus, a primary objective of coal cleaning is to maximize the recovery of the heat value of the coal, consistent with achieving standard specifications for mineral matter content (as mineral ash), moisture content, and sulfur content.

Furthermore, since transportation costs are usually charged on a ton-per-mile basis (which does not distinguish between coal substance and moisture content), it is preferential to remove as much as possible of the extraneous mineral matter and water prior to shipping thereby reducing transportation costs for an “inferior” grade of coal and providing a higher energy material to the consumer.

Table 3.2 Typical properties of different rank coals (Speight, 2013).

Coal type	Carbon (%)	Hydrogen (%)	Limits of volatile matter (%)	Fixed carbon (%)	Calorific value (Btu/lb)
Lignites	73–78	5.2–5.6	45–50	50–55	<8,300
Subbituminous	78–82.5	5.2–5.6	40–45	55–60	8,300–11,500
High-volatile bituminous	82.5–87	5.0–5.6	30–40	60–70	11,500–14,000
Medium-volatile bituminous	87–92	4.6–5.2	20–30	70–80	>14,000
Low-volatile bituminous	91–92	4.2–4.6	15–20	80–85	>14,000
Anthracite	95–98	2.9–3.8	5–10	91–95	>14,000

In fact, long-range transportation of lignite, more than one-third of which consists of water, can more than triple the initial mine-mouth costs calculated on an energy basis. There may, however, be some trade-off in transportation costs if the low-rank coal is sufficiently low in sulfur which, in turn, means a lower cost in terms of stack gas clean-up (Nowacki, 1980).

The need for coal cleaning can be reduced by choice of suitable mining methods, many mines include the methods by which oversize coal is reduced in size but the cleaning of run-of-mine coal is, more often than not, a separate operation which is performed as a surface operation that is usually close to the mine-mouth. However, the term coal preparation includes, by definition, not only sizing (i.e., crushing and breaking) methods but also all of the handling and treatment methods that are required to prepare the coal for the market.

Thus, by providing a higher concentration of heat in the fuel (lower mineral matter and moisture content), the associated costs of transportation, handling, crushing, pulverizing and residual waste (fly ash) disposal in the electricity generation are reduced because fewer weight units per kilo-watt hour generation will be required.

Coal preparation processes are categorized as either physical cleaning or chemical cleaning. Physical coal cleaning processes, the mechanical separation of coal from its contaminants using differences in density, are by far the major processes in use in modern coal-cleaning plants. Physical coal cleaning techniques take advantage of the differences in specific gravity of the coal and its impurities. Hydrocyclones and gravity concentration devices are examples of such systems. When coal is finely ground, physical processes that take advantage of the surface properties of the coal materials can be used. For example, froth flotation exploits the hydrophilic surface characteristics of mineral impurities and the hydrophobic nature of coal particles to achieve separation.

Chemical coal cleaning processes are currently being developed, but their performance and cost are undetermined at this time. For example, some of the sulfur in coal is actually chemically connected to the carbon backbone of coal instead of existing as separate particles. Several process have been tested to mix the coal with chemicals that break the sulfur away from the coal backbone but most of these processes have proven too expensive and have not been applied to commercial coal cleaning operations (Speight, 2013).

The direct objectives of coal-cleaning practices are reduction (within predetermined limits) of size, moisture, ash, as well as sulfur (Williams, 1981; Couch, 1991). However, coal properties have a direct bearing not only on whether but also on how coal should be cleaned. Indeed, coal rank (rank being a complex property that is descriptive of the nature of the coal and its properties) (Chapters 2, 5, 6) can, and usually does, play an important role in determining the feasibility and the extent of cleaning. Thus, the type of coal beneficiation technology and the extent of beneficiation depend mostly on the type of coal, the means of mining, and the clean coal utilization.

Run-of-mine (ROM) coal has no size definition and consists of pieces ranging from fine dust to lumps as large as 2 feet (0.6 meter) square, or larger depending on the rank of coal (Baafi and Ramani, 1979). It is often wet and may be contaminated with rock and/or clay; as such, the coal is unsuitable for commercial use. At best, the coal seams being worked may be relatively thick, without faults, and uniform, free of associated rock partings, and dry. In such cases the coal may require only some breaking or crushing and screening to produce a pure coal.

Mineral matter content ranges from three to 60% (mineral ash) at different mines. Most of the ash is introduced for the roof or bottom of the mine or from partings (small seams of slate) in the coal seam. This mineral matter (extraneous ash) is heavier than 1.80 specific gravity; the remaining mineral matter is inherent in the coal. The density of coal increases with the amount of mineral matter ash present.

The moisture content of the coal is also of two types. The surface moisture, that which was introduced after the coal was broken loose from the seam, is the easier to remove. This moisture is introduced by exposure to air, wet mining conditions, rainfall (in stockpiles), and water sprays. The remaining moisture (bed moisture, cellular moisture, inherent moisture) can be removed only by coking or combustion. This moisture was included during formation of the coal.

Sulfur in coal occurs as sulfates, organic sulfur, and pyrites (sulfides of iron). The sulfates usually are present in small quantities and are not considered a problem. Organic sulfur is bound molecularly into the coal and is not removable by typical coal preparation processes. Pyrites generally are present in the form of modules or may be more intimately mixed with the coal. Coal preparation plants remove only a portion of the pyritic sulfur; therefore the degree of sulfur reduction depends on the percentage of pyrites in the coal, the degree to which this is intimately mixed with the coal, and the extent of coal preparation.

The size of the pieces of coal ranges upward to that of the size of foreign materials, such as a chunk of rock that has fallen from the mine roof or a metal tie; large pieces of coal from a hard seam are sometimes included. Foreign materials are introduced into the coal during the mining process, the most common being roof bolts, ties, car wheels, timber, shot wires, and cutting bits.

Air pollution control often requires partial removal of pyrites with the ash to reduce the sulfur content of the coal (Godfrey, 1994). Ash content often must be controlled to conform to a prescribed quality stipulated in contractual agreements. Because of firing characteristics, it is often as important to retain the ash content at a given level as it is to reduce it. Freight savings are substantial when impurities are removed prior to loading. Finally, the rejected impurities are more easily disposed of at the mine site remote from cities than at the burning site, which is usually in a populated area.

Coal preparation is carried out at a facility that washes coal to remove soil and rock, preparing it for transport to market – a coal preparation facility may also be called a coal handling and preparation plant. During the preparation process, as much waste as possible is removed from the coal to increase the market value of the coal and reduce the transportation costs.

Coal needs to be stored at various stages of the preparation process, and conveyed around the preparation plant facilities. Stored coal (stockpiled coal) provides surge capacity to various parts of the preparation plant. A simple stockpile is formed by machinery dumping coal into a pile, either from dump trucks, pushed into heaps with bulldozers, or from conveyor belt booms. More controlled stockpiles are formed using a stacker (a large machine used to pile the coal into a stockpile) or multiple stackers to form piles along the length of a conveyor, and reclaimers (a large machine used to recover coal from a stockpile) to retrieve the coal when required for product loading. Taller and wider stockpiles reduce the land area required to store a set tonnage of coal. Larger coal stockpiles have a reduced rate of heat lost, leading to a higher risk of spontaneous combustion.

Briefly, the mined coal is loaded into a stockpile, with a reclaim tunnel beneath it. Then, the coal is transported to a raw coal silo, usually 10,000-ton capacity, for feed to the plant at a constant rate. Generally, the first stage is a crushing/screening plant (Figure 3.1), with heavy media processing (for coarse coal sizes – 2-inch x 10 mesh), spirals for the middling sizes (10 mesh x 60 mesh), flotation for the -60 mesh fine coal feed.

Most conventional coal cleaning operations utilize gravity methods for the coarser size fractions and surface treatment methods for the finest particle sizes (Riley and Firth, 1993). The selection of equipment, especially for the finer sizes, depends on the mining method, coal hardness, and size distribution and amounts thereof. Typical of these is a dense media cleaning process (Fourie, 1980) which uses dense media vessels or jigs for the coarsest size, usually +3/8”, dense media cyclones, concentrating tables or jigs for the 3/8” x 28 mesh size, water-only cyclones or spirals and sometimes flotation for the 28 x 100 mesh size and flotation for the -100 mesh coal.

Screening and centrifugal dryers dewater the coarser products while screen-bowl centrifuges and sometimes thermal dryers are utilized to reduce the moisture content of the finest sizes. If the percentage of fines in the coal is high, wetting of coal can decrease the percentage of unburned carbon and the excess air level required to be supplied for combustion (Table 3.3). In cases where the coal lots have excessive fines, it may be advisable to blend the predominantly lumped coal with lots containing excessive fines. Coal blending may thus help to limit the extent of fines in coal being fired to not more than 25%. Blending of different qualities of coal may also help to supply a uniform coal feed to the boiler.

Metallurgical coal cleaning plants utilize thermal dryers – the coal is softer and more friable and thus has a finer size distribution after extraction by the mining machines. Coals for metallurgical use must be thoroughly processed and dried to meet the end user requirements. Additionally, flotation is typically utilized in these circuits due to the quantity of coal and quality of the needed end product (low ash, low sulfur) (Aplan, 1993; Burchfield, 1993).

Figure 3.1 General layout of a coal preparation/coal cleaning plant (Speight, 2013).

Table 3.3 Extent of coal wetting based on fines and surface moisture.

Fines (% w/w)	Surface moisture (% w/w)
10-15	4-5
15-20	5-6
20-25	6-7
25-30	7-8

Powder River Basin coal (Wyoming), although desirable because of other properties that do not leave a large footprint on the environment, is extremely friable and will break down into smaller particles virtually independent of how the coal is transported or handled. The coal represents the extremes of handling problems; dust is an issue when the coal is fine and dry; plugging in bunkers and chutes is an issue when the same fine coal is wet. Once Powder River Basin coal is exposed by mining, the degradation process begins and the majority of the damage can occur in a short time, even as short as a few days. The extent of the degradation that occurs depends in large part on the distance to the plant from the mine, such as the length of time that the coal is exposed to the atmosphere during transportation. Additional factors such as crushed run of mine size and specific handling procedures also impact the degradation process. Additional decomposition occurs during handling and storage in a pile and bunker, bin, or silo. The root cause of the degradation is believed loss of moisture that impacts the coal both mechanically and chemically, through the generation of additional surface reaction area (Hossfeld and Hatt, 2006).

On the other hand some steam coals, especially the harder ones (low Hardgrove index), and some coals produced from surface mines have smaller quantities of the –100 mesh size. In many plants there is such a small quantity of the -100 mesh size that this material is sent to disposal and is considered uneconomical to recover.

Coal flotation is a physiochemical process which exploits the differences in the wettability of hydrophobic clean coal and that of hydrophilic foreign particles (Arnold and Aplan, 1989; Fecko et al., 2005). It is, therefore, subject to the surface properties of coal pyrite and other types of commercially worthless material present in coal which plays a major role in determining separation of such material from coal (Luttrell and Yoon, 1994; Luttrell et al., 1994).

Oxidation also leads to the formation of various oxygen functional groups and soluble inorganic that can adsorb on the coal surface and modify its wettability and floatability. These groups have remarkable impacts on surface charge, which controls film-thinning process and thus flotation kinetics (Sokolović et al., 2006). Decreased coal recovery and increased concentrate ash content may be explained by oxidation of coal. In fact, a good correlation exists between the zeta potential and floatability and electrochemical tests confirm the negative effect of oxidation on the coal recovery and also the final effect of coal flotation process (Fonseca et al., 1993; Sokolović et al., 2006).

The scheme used in physical coal cleaning processes varies among coal cleaning plants but can generally be divided into four basic phases: (i) initial preparation, (ii) fine coal processing, (iii) coarse coal processing, and (iv) final preparation (Figure 3.1).

For most coal-fired power plants, coal is prepared for use by first crushing the delivered coal into pieces less than 5 cm in size. The crushed coal is then transported from the storage yard to in-plant storage silos by rubberized conveyor belts. In the initial preparation phase of coal cleaning, the raw coal is unloaded, stored, conveyed, crushed, and classified by screening into coarse and fine coal fractions. The size fractions are then conveyed to their respective cleaning processes.

In plants that burn pulverized coal, coal from the storage silos is fed into pulverizing units that grind the crushed coal into the consistency of talcum and mix it with primary combustion air which transports the pulverized coal to the steam generator furnace (Chapters 7, 8). A 500 MW coal-fired power plant will have approximately six such pulverizing units, five of which will supply the steam generator at full load with approximately 225 tons per hour. In plants that do not burn pulverized coal, the crushed coal may be directly fed into cyclone burners, a specific kind of combustor that can efficiently burn larger pieces of coal (Chapters 7, 8). In plants fueled with slurried coal, the slurry is fed directly to the pulverizing units and then mixed with air and fed to the steam generator. The slurry water is separated and removed during pulverizing of the coal.

Fine coal processing and coarse coal processing use similar operations and equipment to separate the contaminants. The primary difference is the severity of operating parameters. The majority of coal cleaning processes use upward currents or pulses of a fluid such as water to fluidize a bed of crushed coal and impurities. The lighter coal particles rise and are removed from the top of the bed. The heavier impurities are removed from the bottom. Coal cleaned in the wet processes then must be dried in the final preparation processes.

Final preparation processes are used to remove moisture from coal, thereby reducing freezing problems and weight and raising the heating value. The first processing step is dewatering, in which a major portion of the water is removed by the use of screens, thickeners, and cyclones (Hee and Laskowski, 1994; Nowak, 1994). The second step is normally thermal drying, achieved by any one of three dryer types: (i) fluidized bed drying, (ii) flash drying, and (iii) multi-louvered drying. In the fluidized bed dryer, the coal is suspended and dried above a perforated plate by rising hot gases. In the flash dryer, coal is fed into a stream of hot gases for instantaneous drying. The dried coal and wet gases are both drawn up a drying column and into a cyclone for separation. In the multi-louvered dryer, hot gases are passed through a falling curtain of coal, which is then raised by flights of a specially designed conveyor.

Although inherent moisture cannot be changed, the surface moisture can be reduced to any level that is economically practicable. Considerations include the possibility of re- exposure to moisture during transportation and subsequent storage and the fact that intense thermal drying increases ideal conditions for re-adsorption of moisture.

The free sulfur in the coal is subject to removal only by chemical treatment, which is not a coal preparation process, or by combustion. The reason that the pyrites can be partially removed in washing processes is that they are heavy enough to be removed with the ash. The processes can remove only 30 to 60% of the pyrites, however, because some pyrites are not broken free of the coal and are present in a given piece in a quantity too small to increase its weight enough to be rejected.

Foreign metals can be removed relatively easily. Most wood fragments can be removed although a few small pieces of wood cause no particular harm because they are combustible.

Thus, coal preparation is, of necessity, an integral part of the production and use of coals. The effect on costs can be as important as the planning of mine layouts; decisions concerning mining systems should be an essential element in all mining feasibility studies, especially in view of new (and/or renewed) environmental regulations such as the Clean Air Act Amendments in the United States (Elliott, 1992; Tumati and DeVito, 1992; Rosendale et al., 1993; Paul et al., 1994).

In more general terms, the primary aims of preparing coal for the market depend upon the nature of the raw coal (Table 3.4; Figure 3.1) but, essentially, are (i) the reduction in size and control of size within the limits determined by the needs of transportation, handling, and utilization; and (ii) the removal of extraneous mineral matter to a point that is satisfactory for the customer and specifications are met. This latter operation is more often referred to as control of ash content.

In the early days of the industry, coal was sold as it came from the ground but as the century advanced there was always the possibility of dispute related to payment if the coal contained visible impurities, including excess water above the amount specified in the purchasing contact. Thus, in the early-to-middle decades of the last century, some effort was made to physically remove the impurities from the coal as evidenced by the employment of belt boys. It was the sole purpose of these boys, hired straight from school, at age 12 to 15 (sometimes even younger!) to stand at the side of the underground conveyer belts and remove (by hand) the pieces of rock and slate as the coal passed to the mine cars. By this means, much of the large-sized impurities were sorted and left in the waste area.

Table 3.4 General methods of coal preparation and levels of cleaning (Speight, 2013).

Level	Raw coal weight (%)	Raw coal content (%)	Reduction potential	Comments
Ash	Sulfur
1	98–100	99–100	None to minor	None	Crushing and breaking of raw coal to 3-in. size
2	75–85	90–95	Fair to good	None to minor	Coarse coal cleaning of 3 in. × 3/8 in. coal
3	60–80	80–90	Good	Fair	Moderate coal cleaning of 3 in. × 28 mesh coal
4	60–80	80–90	Good to excellent	Fair to good	Fine coal cleaning of 3 in. × 0 mesh coal
5	60–80	85–95	Deep-cleaned coal: excellent; middle-cleaned coal: none so far	Multiple-stream coal preparation of two cleaned coal products: “deep-cleaned” coal and “middle grade” coal

In many mines, the waste areas are called tipples because of the operation which transferred the coal from the mine cars to picking or sorting screens and where visible impurities were removed by hand. Tipples also segregated the run-of-mine coal into size groups and, as the larger sizes could be more carefully hand-cleaned and were burned with greater ease and cleanliness in fireplaces and hand-stoked furnaces, size became associated with quality. The tipples grew to become environmentally unsuitable mountainous heaps of rock which still disfigure many coal-mining areas. Recently, there have been efforts to take back much of the tipple rock into the worked-out underground seams for storage.

The importance of adequate coal pretreatment technologies must be emphasized; many of the operating problems in cleaning plants are attributed to inadequate (inefficient) pretreatment, which results in large quantities of oversize (or undersize) material in the feeds to the various cleaning units which cause loss of cleaning efficiency, blockages, and even plant shutdown.

Conventional coal cleaning plants are quite efficient for Btu recovery, as well as ash and pyritic sulfur reduction. Btu recovery is generally between 85 and 90% and the ash reductions on a lb. of ash/MM Btu basis are usually in the 70 to 80% range for Pittsburgh seam coals, and in the 85 to 90% range for Illinois and central Appalachian coals (Rosendale et al., 1993).

Thus, preparation of coal prior to feeding into the boiler is an important step for achieving good combustion. Large and irregular lumps of coal may cause the following problems: (i) poor combustion conditions and inadequate furnace temperature, (ii) higher excess air resulting in higher stack loss, (iii) increase of unburned coal constituents in the ash, and (iv) low thermal efficiency.