Coal-Fired Power Generation Handbook. James G. Speight

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СКАЧАТЬ 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 inventories than mine-site power plants. There may also be safety or environmental issues. The amount of coal in stockpiles at the mine-site power plants will be kept to a minimum because of the potential for spontaneous ignition and ensuing combustion.

      Responsible auditing is required to reconcile the actual amount of coal in the stockpiles to the inventory record. By precisely knowing the tonnage of coal present in a stockpile, it is possible to reduce coal inventories that are too large. In addition, coal consumers are more stringent in their demands to both quality and price. However, taking advantage of the cheaper coal available on the market involves purchasing lower quality coal – hence the need for on-site blending operations.

      Coal in storage should be inspected regularly and if the temperature reaches 60°C (140°F), the pile should be very carefully watched. If the temperature continues to rise rapidly, the coal should be moved as promptly as possible and the coal thus moved should be thoroughly cooled before being replaced in storage, or still better, it should be used at once. If the temperature rises slowly the pile should be carefully watched, but it is not necessary to begin moving the coal at as low a temperature as when the rise is rapid, for the temperature may recede and the danger be past.

      Coal should be moved before it actually smokes. Such smoking may begin at temperatures as low as 85°C (180°F) – steaming should not be confused with smoking since steam can be frequently seen coming from a pile and this does not necessarily indicate a danger point. Temperature tests of coal in storage should be made, if possible, and one should not depend on such indications of fire as odor or smoke coming from the coal.

      Inflammable material, such as waste, paper, rags, wood, rosin, oil, and tar in a coal pile often form the starting point for a fire, and every effort should be made to keep such material from the coal as it is being placed in storage. Irregular admission of air into the coal pile around the legs of a trestle, through a porous bottom such as coarse cinders, or through cracks between boards, etc., should be avoided.

      It is important that coal in storage should not be subject to such external sources of heat as steam pipes, because the susceptibility of coal to spontaneous combustion increases rapidly as the temperature rises. The effect of ventilating of coal remains a disputed point, but the weight of evidence in the United States seems to be against the practice. This may possibly be due to the fact that СКАЧАТЬ