Coal-Fired Power Generation Handbook. James G. Speight
the chemistry of inorganic constituents, (iii) chemistry of the organic constituents, and (iv) the combustion properties of the coals as well as (v) the behavior of the coals to be included in the blend. Also, during combustion, it is really necessary to understand the physical conditions and coal properties during heating of the particles, devolatilization, ignition and combustion of the volatile matter and ignition and combustion of the char. It is also equally important to know the phase changes in mineral matter and other inorganics present in coal. The combustion efficiency and carbon loss will have to be also addressed during blending of coals. It is also necessary to investigate the various aspects of slagging, fouling, and the emission characteristics such as the sulfur oxides, the nitrogen oxides, and the particulate matter. However, caution is advised because of the complexity of the combustion process and the number of variables involved (Chapter 7), it may be difficult to extrapolate small-scale (laboratory) results to the full-scale plant. Also, predicting the risk of spontaneous combustion of coal stocks is another important aspect of coal behavior since the inherent dangers of uncontrolled burning can lead to the release of pollutants, while the economic issues associated with the loss of a valuable energy resource is also a concern.
The presence of trace elements in coal combustion has also received increased attention throughout the world during the last few years, with elements such as mercury of particular concern. One way to reduce trace element emissions is cleaning the coal prior to combustion. The use of cleaner coals – such as coals with a low content of mineral matter and low sulfur content – can have the added advantage of substantially reducing operating costs. Again, however, some effects may be detrimental since the effects on corrosion and precipitator performance are uncertain, which makes testing vital.
A limitation to blending coals is the compatibility of the coals themselves and problems are more likely when blending petrographically different coals or coals with different ash chemistry. Non-additive properties make blend evaluation for power generation inherently complex and it is necessary to understand the manner in which the inorganic components of coals in the blend interact and how this affects behavior of the ash.
In summary, blending decisions should be based on the knowledge of the specific behavior of a given pair of coals, rather than an assumption of linear variation of properties with blend traction. Stringent constraints (such as environmental regulations, maximum efficiency at a reduced cost of power generation, improved availability, and reliability) that are placed on coal-fired power stations and the continuing development of new technologies means that the issue of quality improvement of the feedstock will remain a primary factor.
4.3 Effect of Storage
Coal in storage in stockpiles (or in any from when it is exposed to the air) has a tendency to lose heating value and coking quality. In general, high-rank coal (safely stored so as to limit oxidation to a minimum) will lose only about 1% of the heat value per year. On the other hand, improper storage can result in a 3 to 5% loss in the heat value during the first year (Rees et al., 1961).
In addition, the coking characteristics of many coals and coal blends are so seriously affected by aging in storage that they may be totally worthless as a coke oven charge (Landers and Donoven, 1961). However, the data and claims can vary and range from (i) there is no effect on the coking properties after months and years of storage to (ii) there is significant loss of coking properties in as little as one month of storage time. Storage of low-rank coal presents particular problems in that it is usually accompanied by loss of strength, degradation, and some loss of heating value (Jackman, 1957; Mitchell, 1963).
4.3.1 Long-Term Storage
When coal is stockpiled in the open and is to remain in storage for long periods of time, such as through a winter or during extended periods of diminished sales, the area(s) selected for storage should be dry and be constructed to permit good drainage. The area must then be made free of all combustible material having low ignition temperatures, such as wood, rags, dry hay, and the like (Allen and Parry, 1954).
Clay or firmly packed earth, upon which fine coal is rolled, should form the base of the storage pile and the coal should be spread over the entire area in thicknesses of approximately 1 to 2 feet and compacted. The formation of conical piles should be avoided and the top and sides of the pile should be compacted or rolled to form a seal and exclude air. An effective seal of a coal pile is afforded by a continuous layer of fine coal followed by a covering of lump coal to prevent loss of the seal through the action of wind and rain.
Larger sizes of screened coal can be stored with little difficulty. Loose storage that allows natural ventilation to dissipate the small amount of heat produced is usually adequate. In addition, seals of compacted fine coal may be employed. This can minimize the undesirable production of fines if the coal has a tendency to slack. Run-of-mine coal and stoker-sized coal should be stored by the layering method, with sides sloped for drainage. Oil treatment of smaller sizes of coal (at ambient temperature or by use of a thermal method) is at times desirable as it slows the absorption of moisture and oxygen (Berkowitz and Speight, 1973).
When heating and fires develop in storage piles and it is impractical to spread out the coal to cool, smothering by compaction with heavy equipment is generally the best procedure since water flooding may wash out voids in the pile, allowing the fire to spread. Heating in storage piles can be detected, before it becomes serious, by driving small- diameter pipes at intervals vertically into the pile and using thermometers or thermocouples to measure the temperature. The pipes should be driven completely through the pile to avoid a chimney effect.
4.3.2 Short-Term Storage
Similar actions to those described for long-term storage can (should) also be applied to the short-term storage of coal in stockpiles, particularly at unit train loading facilities with reclaiming tunnels. The major hazard associated with coal recovery tunnels is the possible formation of an explosive atmosphere originating from accumulation of methane and coal dust (Stahl and Dalzell, 1965). Methane often will accumulate despite what appears to be adequate ventilating practice; dust accumulations vary with the surface moisture of the coal.
The release of emanation of methane from coal forms a sluggish atmosphere and may inhibit low temperature oxidation, exceptionally in coals with high content of gas but methane is also a potential as a source of energy (Thomas, 1992). Furthermore, as the methane desorption decreases sharply with time, more of the coal surface will be exposed to oxidation.
Closed-end coal recovery tunnels should be equipped with adequate escape passages that, if properly constructed, can also serve as ventilation ducts. Tunnel walls should be washed down frequently to prevent dust accumulation and welding, and electrical repair work should not be conducted in the tunnel during reclaiming operations or if gas or dust is present in the tunnel. Fire-fighting and respiratory protective equipment should be readily available.
4.3.3 Disadvantages
In addtion to the benefits of having a ready source of coal for the plant, coal stockpiling also presents also some disadvantages, some of which are (i) stacked coal can be uneconomical because of the costs of the stockpiling operation and the maintenance of the stockpiles, (ii) as a result of oxidation, the coking propensity and the calorific value of the coal may be decreased, (iii) oxidation of coal causes an increase in ignition temperature, (iv) if the coal is fragile, it will be fragmented and the percentage of the small particle size material is increased, (v) oxidized coal decreases the performance of washing plants, and (vi) as a result of storage of the coals containing high percentage of methane in closed silos which are not ventilated as required, explosive gas compositions can be formed.
However, the most important of these disadvantages are the fires caused by oxidation and self-ignition of the coal. These fires in stockpiles cause the loss of feedstock to the power plant and the gases formed by the combustion of the coal (as well as any waste material formed, such as