Raw or run of mine coal is the lowest cost per ton coal on the market. Coal washing removes rocks and pyrites, FeS2, from coal, and makes a more consistent product, but comes at a cost. Today’s coal buyer looks to provide the lowest cost heat to a boiler, and using our 1880’s vintage ASTM/ISO standard type coal testing you only get average quality of coal. Coal may on average meet a given SPEC, but with raw coal or coals where rocks from the mining process can get mixed in may have some portions of low ash with pockets or portions of high ash. Power plants mostly have problems with coal quality fluxuations or changes, not average quality coal. We do not properly measure coal fluxuations when we get one analyses for a large (<4,000 tons) shipment. Coal with the same average look the same on paper. Coal with high standard deviations of properties like CV, ash, sulfur, and moisture, have all caused derates and shutdowns of coal plants. The mines and buyers do business; the plant can suffer due to swings in coal quality. Many of the slag outages I have reviewed had root causes of high ash due to “out of seam dilution”, OSD, or rock mixed in with coal during mining. These rocks (and pyrites) are not always consistent, but on average the shipment meets the average coal spec. This suggests improving our coal testing to include looking at these fluxuations. I have tried for over 40 years to solve slag at power plants, and this is an issue I have not been able to change. The boiler tells us something is wrong, but the coal testing does not show it.
