Control of coal combustion SO{sub 2} and NO{sub x} emissions by in-boiler injection of CMA. Final project report, July 1, 1992--December 31, 1994

Abstract

A study was conducted to determine the efficacy of carboxylic calcium and magnesium salts (e.g., calcium magnesium acetate or CMA, CaMg{sub 2}(CH{sub 2}COOH){sub 6}) for the simultaneous removal of SO{sub 2} and NO{sub x} in oxygen-lean atmospheres. Experiments were performed in a high-temperature furnace that simulated the post-flame environment of a coal-fired boiler by providing similar temperatures and partial pressures of SO{sub 2}, NO{sub x} CO{sub 2} and O{sub 2}. When injected into a hot environment, the salts calcined and formed highly porous {open_quotes}popcorn{close_quotes}-like cenospheres. Residual MgO and/or CaCO{sub 3} and CaO reacted heterogeneously with SO{sub 2} to form MgSO{sub 4} and/or CaCO{sub 4}. The organic components - which can be manufactured from wastes such as sewage sludge - gasified and reduced NO{sub x }to N{sub 2} efficiently if the atmosphere was moderately fuel-rich. Dry-injected CMA particles at a Ca/S ratio of 2, residence time of 1 second and bulk equivalence ratio of 1.3 removed over 90% of SO{sub 2} and NO{sub x} at gas temperatures {>=} 950{degrees}C. When the furnace isothermal zone was {<=} 950{degrees}C, Ca was essentially inert in the furnace quenching zone, while Mg continued to sorb SO{sub 2} as the gas temperature cooled at a rate of -130{degrees}C/sec. Hence, the removal of SO{sub 2} by CMA could continue for nearly the entire residence time of emissions in the exhaust stream of a power plant. Additional research is needed to improve the efficiency and reduce the cost of the relatively expensive carboxylic acid salts as dual SO{sub 2}-NO{sub x} reduction agents. For example, wet injection of the salts could be combined with less expensive hydrocarbons such as lignite or even polymers such as poly(ethylene) that could be extracted from the municipal waste stream.