An evaluation of coal preparation technologies for controlling trace element emissions

Recent studies suggest that low-cost coal preparation technologies can play an important role in reducing the emissions of air toxics at electric utilities. To evaluate this approach, a detailed study was conducted at Virginia Tech to quantify the capabilities of a variety of conventional and advanced precombustion cleaning processes for removing hazardous air pollutant precursors (HAPPs). Characterization data obtained from this study indicate that most HAPPs associate with mineral impurities commonly found in run-of-mine coals. Some elements (e.g., mercury) were found to associate with pyritic sulfur, while others (e.g., manganese) were more closely associated with ash-bearing minerals. Furthermore, pilot-scale tests conducted as part of this work demonstrated that conventional cleaning processes could achieve HAPPs rejections of 50–80%. The use of advanced processes further improved these rejections, particularly for HAPPs associated with pyrite. The data also showed that trace element rejections could be improved by pulverizing the feed coal to liberate mineral matter. However, mathematical simulations indicated that these apparent gains are often not realized in industrial operations due to the lower separation efficiencies of the finer coal cleaning circuits.