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. If practical, the coal should be stored in several small stockpiles instead of one large stockpile to prevent heat buildup and make inspection of the coal easier. Coal piles should not be stockpiled higher than 12 feet and should not contain more than 1,500 tons in a single pile. No point in the interior of the stockpile should be more than 10 feet from an air-cooled surface.
In many countries many stockpiling methods have to take into account climatic conditions, dimensions, and design of the stockpiling area, as well as the type of machinery used for the stockpiling operation. The coal stockpiles formed in open areas can be generally in the form of a cone, prism, or a variety of geometric shapes. Typically, the methods of stockpiling are (i) the windrow method, (ii) the cone shell method, and (iii) the Chevron method.
The general parameters that can affect coal storage are (i) the site, especially the base upon which the coal will be stored, (ii) mitigating the tendency of spontaneous ignition of the coal, and (iii) the moisture content of the coal.
The site must be carefully chosen and prepared – the ground should be cleared of any vegetation and refuse. A hard packed clay or sandy soil is ideal but, if this is 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 the site does not drain naturally, drains should be installed around the storage pile, not underneath it as this may produce upward air currents through the pile, aiding spontaneous combustion. Make sure the site is away from any external heat sources as combustion liability increases with a rise in temperature.
The chief danger in storing coal in stockpiles is the potential for spontaneous combustion – this 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 placed on 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. The coal should be piled so that any part of the pile can be promptly moved if heating occurs. The coal should be spread 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. If practical, the coal should be stacked in several small piles instead of one large one, to prevent heat buildup and allow the coal to be inspected. Coal can be stored covered or in the open. Wetting and drying coal repeatedly may make it more susceptible to combustion. The actions of water may break up the coal, especially after freezing and thawing. Wet coal or mixed wet coal should not be stored with dry coal and coal should not be stored on a damp base. After heavy rains and snows the coal pile should be inspected and monitored carefully.
