The effects of ammonia on SO₂ emissions in a fluidized bed combustor under pseudo-staged combustion

The reduction of SO₂ by ammonia gas addition during staged combustion of bituminous coal has been studied in a 2 m high fluidized bed combustor of 30 cm static bed height and a freeboard height of 100 cm. The coal was introduced to the combustor at 42 cm above the distributor and the ammonia gas was injected at 52 cm above the distributor by an uncooled stainless steel injector. Experiments were carried out to investigate effects of ammonia gas injection on SO₂ emissions of (i) air staged levels, (ii) excess air levels, (iii) primary air factor, PAF (ratio of primary to stoichiometric air), (iv) NH₃:SO₂ molar ratio, and (v) fluidizing velocity. Experiments were carried out under a new technique of air staging called Pseudo-staged Combustion, maintaining the excess air level and fluidizing velocity between 17 and 70% and between 0.7 and 2.0 m sec^-1 respectively. A maximum reduction of 92% was obtained at 37% excess air, at NH₃:SO₂ molar ratio of 5.5. The effective NH₃:SO₂ molar ratio was found to be between 3.0 and 5.5, which is true for all staging and excess air levels. A greater removal of SO₂ with NH₃ injection during staged combustion is probably due to this new staging technique. The Pseudo-staging reduces temperature through the freeboard and flue for the occurance of as NH₃ + SO₂ reactions. These reactions are reported to be low temperature reactions. The NH₃ carry over was less than 83 ppm for all operating conditions. The present study demonstrates that staged combustion coupled with ammonia injection can reduce SO₂ emissions.