Simultaneous reduction of NOx and SO2 emissions from coal combustion by calcium magnesium acetate

The potential of calcium magnesium acetate (CMA) as a medium for the simultaneous control of NO, and SO, emissions has been investigated using a pulverized coal combustion rig operating at 80 M A US and a UK coal of significantly different sulphur contents were used as primary fuel and CMA was injected in solution form into the combustion gases by horizontally opposed twin-fluid atomisers at temperatures of 1100-1200 degreesC. SO2 reductions typically greater than 80 and 70% were found for initial SO2 levels of 1000 and 1500 ppm, respectively, at Ca/S ratios greater than 2.5. There did not appear to be significant limitation on sulphation by pore blockage using CMA due to the open structure formed during calcination and there is clear potential for zero SO2 emissions at higher Ca/S ratios. The Ca content of the CMA in the form of CaO, via a droplet drying/particle calcination process, absorbs SO2 by sulphation processes by penetration into the open pore structure of these particles. The effect of primary zone stoichiometry (gimel(1) = 1.05, 1.15 and 1.4) on NOx reduction was investigated for a range of CMA feed rates up to a coal equivalent of 24% of the total thermal input. NOx reductions of 80, 50 and 30% were achieved at a primary zone stoichiometry of gimel(1) = 1.05, 1.15 and 1.4, respectively, for a reburn zone residence time of 0.8 s. At lower equivalent reburn fuel fractions, coal gave greater NOx reductions than CMA but similar levels were achieved above Rff = 18%. The mechanism for NOx reduction involves the organic fraction of CMA which pyrolyses into hydrocarbon fragments (CH), but to a lesser degree than coal, which may then react with NOx in a manner similar to a conventional 'reburn' mechanism where NOx is partly converted to N-2 depending on the availability of oxygen. (C) 2004 Elsevier Ltd. All rights reserved.