Coal-Fired Power Generation Handbook. James G. Speight
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.
4.2.1.1 The Windrow Method
Windrow stockpiles can be created using different techniques of stockpiling. One method is to use a bridge and tripper conveyor system, though this alternative is feasible only for stationary applications. One significant disadvantage of stationary conveyor systems is that they are typically fixed in height, which can result in segregation by wind, as discussed earlier. Another method is to use a telescoping conveyor. Telescoping conveyors are typically preferred over stationary systems because they can be relocated when necessary, and many are actually designed to be road-portable.
In the process, the stockpiling machine moves on rails and spills the coal in parallel rows along the length of the silo by changing the boom angle from the ground level. The stockpiling operation is achieved by the back-and-forth movement of the unit along the stockpiling area and beginning to spill the first rows then the second, third rows, and as many rows are as required.
A good blend can be obtained when the coal is taken by a reclaimer from the stockpile formed with this method. The disadvantage of this method is collection of rain water between the coal rows and penetration in the stockpile as a result of long-lasting and continuous rainfall.
4.2.1.2 The Cone Shell Method
In cone shell method, coal is added to the pile in a cone shape until the final pile height is reached. The stockpiling unit begins to spill the first cone, then moves one step forward to spill the second cone until the stockpile height and continues the operation step by step. This method can be applied in areas where long and rigorous winter conditions prevail in order to ensure that stockpiled coal is affected by rain water at minimum level. Stockpiling in a single cone tends to cause size segregation, with coarser material moving out towards the base. In raw cone ply stockpiling, additional cones are added next to the first cone.
A good blend is obtained when the coal is taken from the stockpile by a reclaimer. For an optimum blend, the reclaimer has to work perpendicularly to the long axis of the stockpile. To adjust the calorific value of the blend, high calorific valued coal can be added during the stockpiling operation.
4.2.1.3 The Chevron Method
In the Chevron method, the stockpiling unit moves along the storage area on an axis which divides the area in equal parcels and spills the coal in triangular prism-shaped stockpiles (like the chevrons on a military uniform). The stockpiling operation is first performed along the first prism. The machine spills the second layer on its way back and continues the same operation until the desired final stockpile height is reached.
When this method is used, the rain water flows down on the slopes of the stockpiled coal. In summertime, since the surface area exposed to the hot air is larger, drying effect becomes more significant. In addition, the rock particles not picked out in the production process roll down on the slopes during stockpiling and consequently separate from the coal.
4.2.2 Stockpile Management
Other than the gernal adverse effect of the deterioration of the coal due to aerial oxidation, the most important aspect of stockpile management is avoidance of the fire caused by self-oxidation followed by self-ignition of the coal leading to combustion of the stockpile. Stockpile fires are a serious safety issue and cause an economic imbalance in the power plant operation. In addition, the gases formed during the fire and the wastes as a result have harmful effects on the environment (Okten et al., 1998; Speight, 2013).
Furthermore, the growing economic constraints, the need for smaller stockyards with the ability to blend coals with the accuracy demanded by consumers, and the increasing use of timely delivery has increased the significance of stockpile management within the international coal market. Terminals are required to handle more throughput and more grades of coal, at higher handling rates and with less impact on the environment, and to do it at lower cost. All of these issues require improved stockpile management in order to avoid supply disruptions and the consequences of interruptions in the power supply.
The size of stockyards varies from several thousand tons to more than six million tons at coal export terminals. The level of stockpile management sophistication can, therefore, range from simple coal piles at some sites to highly automated stockyards used by major coal exporting ports or large tonnage consumers. Stockpiles are also employed for long-term storage, typically at coal-fired power plants, to guarantee supply. Thus the management of stockpiles is specific to the site and depends on the purpose of the facility. In addition, the actual cost of coal storage and security of supply can be difficult to determine.
Good stockpile management is an important part of the coal supply chain from mine to customer. In fact, most coal producers and consumers make use of stockpiles at their respective facilities. Typically, more coal is being produced and traded internationally, providing a wider choice of sources to consumers. In addition, excess production can (and does) drive prices down, which has forced a greater focus on stockpile management. Issues such as (i) optimum stockpile size, (ii) stockpile turnover periods, and (iii) timely stock management have assumed greater significance to coal producers and coal users. However, there is a balance between security of supply and the cost of the stored coal. The optimum inventory is site specific because each site is governed by a unique set of factors. For example, power plants that import coal need to carry larger