Mercury emission control from coal combustion systems: A modified air preheater solution

Results are presented of pilot-plant testing of a new approach to resolve the mercury control problem primarily from coal combustors. Performed at the Western Research Institute’s facility in Laramie, Wyoming, the recorded data reflect the previous laboratory results that proved that untreated steel or platinum surfaces in a specific downstream temperature range can significantly convert gaseous atomic mercury to its water-soluble gaseous dichloride without a need for catalysis or amalgamation. This was confirmed by utilizing various steel inserts in a pilot-plant scale combustor fueled by a low-chlorine Powder River Basin coal. The results and mechanism help to explain why it is in full-scale combustors that large excesses of chlorine generally are necessary to convert their very low concentrations of mercury. These efforts have further validated the underlying predominance of mercury’s heterogeneous chemistry. Most importantly it has illustrated that the efficient laboratory proven mechanism extrapolates to the low mercury parts per billion by volume (ppbv) concentrations relevant to full-scale coal combustion. Depending on conditions, chlorine can become less controlling, removing the need for halogen addition or coal blending approaches as possible methods for mercury control. Additional testing now is establishing the exact surface area of the inserted thin metallic surfaces and the residence time required to attain specific conversion efficiencies. Optimal surface temperatures are about 230 ± 30 °C (450 ± 50 °F) for the conversion mechanism. As a result, this temperature region generally will occur in the location of air preheaters in many coal combustions. This also explains previous reports of mercury oxidation across these and other lower temperature devices. Suggestions now are made for modifying or retrofitting air preheater designs. This enhancement of what is mercury’s natural chemical mechanism now raises the possibility for a one-time modification of the combustion train to resolve this otherwise rather expensive problem.