Mercury Speciation as a Function of Flue Gas Chlorine Content and Composition in a 1 MW Semi-Industrial Scale Coal-Fired Facility

The complicated chemistry and multiple mechanisms affecting mercury speciation and control make it necessary to investigate these processes at conditions relevant to full-scale boilers. Experiments were performed in a 1MW semi-industrial-scale, coal-fired facility, representative of a full-scale boiler. Southern Research Institute's spike and recovery system and procedures were used to obtain real-time mercury-speciation measurements, with less than 5% uncertainty in the measured values. The focus of this work was on solutions for Powder River Basin (PRB) sub-bituminous coals and high-burnout conditions, where the fly ash contained relatively little unburned carbon. The relationship of mercury oxidation with chlorine concentration was investigated in conjunction with other associated parameters of importance, such as unburned carbon, minerals in ash, and flue-gas composition. The impact of chlorine content was independently evaluated by injecting chlorine through the burner, while firing PRB coal. The concentration of oxidized mercury at the baghouse inlet and outlet increased as chlorine was added to the flue gas. For the high-efficiency (low UBC) conditions investigated in this work, ash composition was found to be more important than chlorine content. The addition of high-iron bituminous ash, either through direct ash injection or through coal blending, was found to significantly increase mercury oxidation before and within the pilot combustor's baghouse, while firing PRB coal. For certain pilot-scale conditions, blending a small amount of high-iron bituminous coal (<10% by mass) with PRB sub-bituminous coal, resulted in oxidized fractions of mercury of >50% before the particulate collection device and >95% at the outlet of a baghouse. Work continues at Southern Research Institute to elucidate the mechanisms observed for Hg-oxidation enhancement by coal-blending and other adjustments to the flue-gas composition. A greater understanding of these mechanisms will allow the design of Hg-mitigation strategies with high-certainty of the desired result.