HAPs-Rx: Precombustion Removal of Hazardous Air Pollutant Precursors

Abstract

CQ Inc. and its project team members--Howard University, PrepTech Inc., Fossil Fuel Sciences, the United States Geological Survey (USGS), and industry advisors--are applying mature coal cleaning and scientific principles to the new purpose of removing potentially hazardous air pollutants from coal. The team uniquely combines mineral processing, chemical engineering, and geochemical expertise. This project meets more than 11 goals of the U.S. Department of Energy (DOE), the National Energy Strategy, and the 1993 Climate Change Action Plan. During this project: (1) Equations were developed to predict the concentration of trace elements in as-mined and cleaned coals. These equations, which address both conventional and advanced cleaning processes, can be used to increase the removal of hazardous air pollutant precursors (HAPs) by existing cleaning plants and to improve the design of new cleaning plants. (2) A promising chemical method of removing mercury and other HAPs was developed. At bench-scale, mercury reductions of over 50 percent were achieved on coal that had already been cleaned by froth flotation. The processing cost of this technology is projected to be less than $3.00 per ton ($3.30 per tonne). (3) Projections were made of the average trace element concentration in cleaning plant solid waste streams from individual states. Average concentrations were found to be highly variable. (4) A significantly improved understanding of how trace elements occur in coal was gained, primarily through work at the USGS during the first systematic development of semiquantitative data for mode of occurrence. In addition, significant improvement was made in the laboratory protocol for mode of occurrence determination. (5) Team members developed a high-quality trace element washability database. For example, the poorest mass balance closure for the uncrushed size and washability data for mercury on all four coals is 8.44 percent and the best is 0.46 percent. This indicates an extremely high level of reproducibility of the data. In addition, a series of ''round-robin'' tests involving various laboratories was performed to assure analytical accuracy. (6) A comparison of the cost of lowering mercury emissions through the use of coal cleaning technologies versus the use of post-combustion control methods such as activated carbon injection indicates that, in many cases, coal cleaning may prove to be the lower-cost option. The most significant disadvantage for using coal cleaning for control of mercury emissions is that a reduction of 90 percent or greater from as-fired coal has not yet been demonstrated, even at laboratory-scale.