Many coal power stations have recently been operating at historically low capacity factors or have even undergone extended economic shutdowns. This can result in coal stockpiles that are exposed to the elements for much longer times than anticipated, resulting in a loss of usable coal energy by several mechanisms.
Coal begins to lose energy almost the minute it’s mined, through a variety of means.
Upon exposure to the atmosphere, a slow oxidation process begins, essentially burning away heat content. Once the coal is handled—through loading, shipping, unloading, stockpiling, and reclaiming—it will lose energy from dust and fines at each step of the process. In addition, moisture is typically gained during shipping and storage, which results in a variety of negative impacts at the power station. While the coal lies in storage, the oxidation process will continue, accelerated by exposure to the elements, potentially leading to spontaneous combustion.
Although most coal power station personnel understand that these three mechanisms—loss of energy through oxidation, dust and fines, and added moisture—will affect their coal stockpiles, the magnitude of the degradation is often unknown.
Methods to reduce coal moisture gain are limited. Covered transport is a good first step (and it also reduces coal fines losses), but it can increase transportation costs. Reducing the time from the mine to stockpile is imperative for reducing moisture gain, so avoid having your coal spend long layovers at coal terminals. Covered storage at the power station is ideal, but it often incurs a very high capital cost and thus is uncommon. Opinions are divided over whether coal pile compaction helps prevent moisture gain, with most believing compacted piles are best overall.
Wind exposure should be kept to a minimum, with covered storage being preferred. Covered storage should utilize heat and combustion sensors, as well as fire and dust suppression systems, to allow plant staff to prevent fires and explosions.