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.
4.2 Stockpiling
As with the other fossil fuels – natural gas and crude oil-based fuels – there is the need to store coal in order to accommodate the possibility of a disruption in supply. Like the other fossil fuels, coal is a material which can be stored in large quantities because of some necessities. For safety reasons and for convenience, stockpiling of coal is typically done in open areas but there are also covered stockpile areas or completely closed coal silos. Some reasons for coal storage are (i) decrease of demand for coal in the market, (ii) to be ready for the bottlenecks caused by the interruptions that may occur during production, (iii) to meet the demand by the consumer without interruption, (iv) to produce the coal in more congenial climate conditions – such as during the non-winter seasons of spring, summer, and autumn – to be available for the winter market, (v) to decrease the moisture content of coal through the elevated temperature or elevated winds of the non-winter seasons, (vi) to lessen any defects – disruptions in the operation – that may occur in coal washing plants and in thermal power stations, (vii) to ensure that the feedstock coal for the thermal power stations retains the specified properties. However, some negative developments are observed in various characteristics of coal and important problems may emerge because of its long time storing in open areas. Consequently, stockpiling of coal has to be done consciously and by respecting basing rules.
Initially, produced coal is generally loaded in trucks or wagons by excavators and loaders to be transported to the storage areas. Use of a conveyor belt system is another transportation alternative. In recent years, the increased capacity of trucks, their ability to function in topographic irregularities, and their easy adaptation to the changes in working areas are the reasons for preference of transportation by trucks.
In the enterprises where bucket wheel excavators are used, the transportation of coal to the storage area by means of conveyor belt bridges becomes possible. Same operations are relevant for the transportation of the coal carried by ships from the harbor to the storage area. The coal transported to the storage area is spread by movable or fixed belt systems and according to desired stockpile geometry.
Stockpiling is carried out at coal mines, coal preparation plants, transshipment facilities (including export/import facilities) and end user sites such as coal-fired power plants. Thus, after arrival at the destination (the power plant), it is often necessary to place the coal in stockpiles until it is required – this is not a long period and an approximate rule of thumb is to use within the week of arriving at the power plant site (Narasiah and Satyanarayana, 1984).
There is always economic pressure to minimize the amount of capital tied up in stockpiles with little return on the investment. Thus, there is a need to optimize coal inventories wherever coal is stockpiled. Issues such as (i) optimum stockpile size, (ii) stockpile turnover period, (iii) timely stock management, and (iv) the ability to take advantage of cheaper coal when such coal is available on the market have assumed greater importance. Thus, coal stockpile systems perform two main functions: (i) they serve as a buffer between coal material delivery and processing, and (ii) as a source of coal for blending.
The stockpile site must be carefully chosen and prepared – typically stockpiles are designed as open store areas. The ground should be cleared of any vegetation and refuse. A hard packed clay or sandy soil is ideal; if not available, a concrete pad can be installed to keep dirt out of the coal. The site should be dry, level and well drained. If drainage is not a natural phenomenon, drains should be installed around the storage pile, not underneath it as this may produce upward air currents through the pile, aiding spontaneous combustion. The site should be away from any external heat sources as combustion liability increases with a rise in temperature.
However, if conditions dictate (such as heavy rainfall or melting snow (as happens frequently in North America) or if the stockpiles are near residential areas), there is the option to cover the stockpiles. However, the cost of storing large amounts (up to several hundred thousand tons of coal in an enclosed system can have a negative influence on the economic scale.
4.2.1 Stockpile Construction
The chief danger in storing coal is spontaneous combustion and its risk can be greatly reduced if dust and fine coal are kept out of the pile. Cleaned and sifted coal with uniformly large lumps stores better than mixed sizes. Sized coal should not be stored on top of a layer of fine coal and the coal should be handled carefully to prevent breakage and dust formation – friable dusty coal should be piled in small low piles. In addition, the coal should be piled so that any part of the pile can be promptly moved if heating occurs. It is preferable to spread the coal in horizontal layers and not in conical piles to prevent the finer coal from clustering in the center and the lumps rolling to the bottom.