Читать книгу Coal-Fired Power Generation Handbook - James Speight G., James G. Speight - Страница 89

Coal storage in stockpiles is essential in ensuring continuous supply of feedstock for large capacity power units. In fact, it has been estimated that coal stockpiles at electric power plants in the United States in March 2012 (EIA, 2012) were approximately 196 million tons, almost 18% w/w above the level in March 2011 and above the five-year range. Coal stockpile levels typically decline during summer and winter months as power plants burn through stocks to meet seasonal peak electric demand for cooling and heating load, respectively. However, mild weather during the winter of 2011/2012 combined with decreasing natural gas prices decreased the demand for coal-fired electricity. There are disadvantages to storing large quantities of coal because of the characteristics of coal and important problems may emerge because of its long time storing in open areas.

By way of explanation, there are other forms of coal storage – while coal is typically stored in stockpiles, which are piles or storage locations for the bulk materials. More controlled stockpiles are used in many different areas and are formed using stackers to form piles along the length of a conveyor and reclaimers to retrieve coal when required for product loading. On the other hand, a coal bin or a coal bunker is a storage container for coal awaiting use or transportation. This can be either in domestic, commercial, or industrial premises, or on a ship or locomotive tender, or at a coal mine or processing plant. Domestic coal bunkers are associated with the use of coal in open fires or for solid-fuel central heating. Free-standing bunkers were commonly made of wood or concrete and are currently sold in materials including plastic or galvanized metal. Coal bins or bunkers could also be partly or fully underground. Coal bins formed part of industrial plants and were also used for coal storage on steam ships.

A stockpile (stockpiling machine, commonly referred to in this text as a stacker) is a bulk stockpiling machine that is used to stockpile coal either at the mine site before transportation or at the power plant before use. The stockpiling machines come in different shapes according to specific requirements and the function of the machine is to pile or stack the coal on to a stockpile for later reclamation and use – a reclaimer is generally used to reclaim or recover the stockpiled material. The stockpiling unit typically moves on a rail between stockpiles in a yard and usually has three directions of movement, depending on requirements (i) horizontally along the rail, (ii) vertically by raising or lowering the boom, sometime referred to as luffing, and (iii) slewing, which is rotation of the stacker around its central axis and may not always be a requirement.

Because of the tendency of coal to self-heat followed by spontaneous ignition and spontaneous combustion (Sloss, 2015), there are cautions that need to be observed and stockpiling (sometimes referred to as stacking) of coal has to be done consciously and by respecting basic rules and procedures. The storage site must be properly prepared in order to minimize the risk of fire. Several rules of thumb in coal storage can be enumerated: the coal should be stored in several small piles instead of a large one in order to prevent temperature buildup and facilitate the inspection; stockpiles should not be higher than 12 feet and should not contain more than 1,500 tons of coal; storage of coal with high content of moisture should be avoided. The storage volume and the storage time are parameters depending on safety and continuity of coal supply to the power plant. Since the coal stockpile acts as a buffer between the coal extraction unit and power plant the stockpile volume and storage time cannot be properly controlled.

Although stockpiling is generally done in open areas, there are also covered stockpile areas or completely closed coal silos. Storage of coal is an important part of coal handling systems at coal-fired power plants, particularly since the advent of the unit-train concept in transportation. With this in mind, coal storage is generally practiced in order to accomplish one, or a combination, of the following objectives: (i) to be ready for use in the power plant promptly, (ii) to facilitate blending in order to even out chemical and physical inconsistencies that exist in such a heterogeneous material and to produce a combustible feedstock that has the necessary uniform quality, and (iii) to store coal of preferential sizes where the demand is seasonal (Barkley, 1942; Berkowitz and Speight, 1973).

In a very general sense, the self-ignition process occurs naturally due to the low- temperature oxidation reaction. The main factors that favor the self-heating are humidity and presence of oxygen. Continuous supply of oxygen as in the case of a coal stockpile exposed to wind increases the rate of the low-temperature oxidation reaction. Significantly higher temperature values and hot-spot sites can occur in the vicinity of the side slope of the stockpile that is exposed on the windward side.

Oxygen penetration depth in the coal bed is influenced by the coal porosity and hence the hot-spots may occur at different depths in coal stockpiles with different values of the coal porosity. In addition, the humidity of the coal plays an important role in the dynamics of the self-ignition process. Low-grade coal with high humidity content is more prone to self-heating. In such case, the temperature increases rapidly at the beginning of the storage process. As the temperature increases the evaporation will absorb heat and the temperature may approach a steady state (Akgun and Essenhigh, 2001).

Coal stockpiles are prone to spontaneous combustion especially where large quantities are stored for extended periods. Coals that exhibit the greatest tendency to self-heat (that is lignite, subbituminous coal, and brown coal) are rarely stored for any length of time at the power station. Self-heating of coal is a naturally occurring process caused by the oxidation of coal. Natural oxidation is uncontrolled and can lead to emissions and spontaneous combustion (Berkowitz and Schein, 1951; Berkowitz and Speight, 1973; Carras and Young, 1994; Arisoy et al., 2006; Speight, 2013). Unless handled correctly, the results can be catastrophic in damage to power plant equipment. This is a reflection on the relative length of storage time involved at each stage. Spontaneous combustion in stockpiles poses significant safety, environmental, economic, and handling problems.