Coal-Fired Power Generation Handbook. James G. Speight

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.

3.4 Size Reduction

      Size reduction (sometimes called pretreatment) is, simply, breaking, crushing, and screening of the run-of-mine coal in order to provide a uniform raw coal feed of predetermined top size thereby minimizing the production of material of ultrafine size by excessive crushing or handling.

      Size reduction of coal plays a major role in enabling run-of-mine coal to be utilized to the fullest possible extent for power generation, production of coke, as well as other uses such as the production of synthetic fuels (Bevan, 1981). Run-of-mine coal is the as-received coal from the mine, whether the mining process is stripping, auger mining, continuous mining, short- or longwall mining, or any other method currently practiced (Speight, 2013 and references cited therein).

      Most conventional coal cleaning facilities 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.

      The first operations performed on run-of-mine coal are removal of tramp iron and reduction of size to permit mechanical processing. The run-of-mine coal is first exposed to a high-intensity magnet, usually suspended over the incoming belt conveyor which pulls the iron impurities out of the coal. This magnet sometimes follows the breaker, but always precedes a screen-crusher. The coal then goes to the breaker, which is a large cylindrical shell with interior lifting blades; the shell is perforated with holes (two to eight inches in diameter) to permit passage of small material.

      The breaker rotates on a horizontal axis, receiving material in one end, tumbling it as it passes through the holes in the shell, and permitting the hard, large, unbroken material to pass out the rear of the machine. The small material (four inches) goes to the cleaning plant, and the large rejected material falls into a bin to be hauled away.

      Most commercial circuits utilize dense media vessels of 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.

      Since the mining processes differ in operation and since size reduction actually begins at the coal face (i.e., during mining), the mined coal will exhibit different characteristics. In fact, the mining process has a direct bearing on the size and on the size consistency of the coal. Thus, prior to final utilization of the coal, some degree of size reduction, or size control, is usually required. The number of stages in the size reduction process depends upon the specific utilization of the coal as well as the condition of the coal.

      For example, coal which is destined for power generation may undergo size reduction to produce a product with a top size of 4 inch (1 mm). On the other hand, the size of the coal needed for a coking operation is coarse and the number of stages of size reduction involved in preparing a coal feed for a coking is somewhat less than required to prepare coal as the feedstock for power generation utilization.

      Coal is reduced in size by crushing and pulverizing. Pre-crushed coal can be economical for smaller units, especially those which are stoker fired. In a coal handling system, crushing is limited to a top size of 6 or 4 mm. The devices most commonly used for crushing are the rotary breaker, the roll crusher, and the hammer mill.

      It is necessary to screen the coal before crushing, so that only oversized coal is fed to the crusher. This helps to reduce power consumption in the crusher. Recommended practices in coal crushing are (i) incorporation of a screen to separate fines and small particles to avoid extra fine generation in crushing, and (ii) incorporation of a magnetic separator to separate iron pieces in coal, which may damage the crusher.

The fines in coal produced during the crushing (sizing) operation can present problems in combustion on account of segregation effects. Segregation of fines from larger coal pieces can be reduced to a great extent by conditioning coal with water. Water helps fine particles to stick to the bigger lumps due to surface tension of the moisture, thus stopping fines from falling through grate bars or being carried away by the furnace draft. While tempering the coal, care should be taken to ensure that moisture addition is uniform and preferably done in a moving or falling stream of coal. 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. In cases where the sized coal has an excessive amount of fine coal, blending with predominantly lump coal (depending upon the coal-fired system) may be