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_x and SO_x emissions has been investigated using a pulverized coal combustion rig operating at 80 kW. 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 °C. 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 (λ₁=1.05, 1.15 and 1.4) on NO_x reduction was investigated for a range of CMA feed rates up to a coal equivalent of 24% of the total thermal input. NO_x reductions of 80, 50 and 30% were achieved at a primary zone stoichiometry of λ₁=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 NO_x reductions than CMA but similar levels were achieved above Rff=18%. The mechanism for NO_x reduction involves the organic fraction of CMA which pyrolyses into hydrocarbon fragments (CH_i), but to a lesser degree than coal, which may then react with NO_x in a manner similar to a conventional ‘reburn’ mechanism where NO_x is partly converted to N₂ depending on the availability of oxygen.