Читать книгу Coal-Fired Power Generation Handbook - James Speight G., James G. Speight - Страница 95

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 ventilation has been imperfect and has done more to promote oxidation than cooling of the stockpile.

The majority of the coal stockpile fires appear to have occurred within ninety days after the coal was placed in storage. Hence particular attention should be given to the pile during the first three months that it is in storage. The greater the area of the pile exposed to the air the more quickly will the danger be passed.

Finally, a storage plan must consider all of the conditions, and not only a part of the ambient conditions at the site. For example, clean, lump coal of a certain kind may be stored with safety in high piles – while the same coal, run-of-mine or unscreened, may not be safely stored at all, or at least only in smaller piles. Lack of attention to details during storage or failure systematically to inspect storage piles and to be ready for any emergency that may occur may result in safety hazards and losses from fires. It must also be obvious that as the amount of coal stored increases, increased care must be taken in the method of storing and in watching the coal after storage.