
Reducing S02 by Buying Cleaner Coal
Coal quality is very variable, which is one reason that we take the measurements of proximate, ultimate analysis along with calorific value. Shown in the graph below is the acceptable line for emissions of 1.2 pounds of sulfur dioxide per million BTU of chemical input. If you purchase a coal below the green line and you have an efficient utility (normal efficiency should be >35%) then you will not have to spend money on cleaning the coal or installing a scrubber. As in the case of our crude oil, the quality of our coal is decreasing because we have consumed much of the cleaner coals. Many of our coal regions are not under the green line and so a sulfur reduction strategy is necessary to ensure compliance with the Clean Air Act. One of the easiest methods of reducing the sulfur dioxide emissions is to buy the cleaner (and thus more expensive) coal from another coal seam.

Unlike most coal properties, the S content (from the ultimate analysis) has very little to do with rank. Anthracite does have a low S content because the high temperatures necessary for anthracite creation would have been sufficient to remove the S as H2S. However, increasing in rank has very little to do with S content, rather you need to go back millions of years and look to see where the sea shore was. Seawater infiltration into the existing coal seam was responsible for much of the enhanced S content of some coals. Fortunately for the Western states, their coal was not exposed and they tend to have the cleaner (lower S) coals. So as Wyoming coal has a lower S content there was a massive switch to this cleaner coal.

Coal use is declining in the U.S. as we use more natural gas for electricity generation and industries also move towards natural gas. A smaller contribution comes from more wind power generation.
