Reduction of SO₂ emissions by ammonia gas during staged combustion

To reduce SO₂ emissions, ammonia gas was injected into a coal-fired fluidized-bed combustor under staged commbustion condition. The combustor was 2 m high with a 30 cm static bed height and a freeboard height of 170 cm. The ammonia gas was injected at 52 cm and 65 cm above the distributor, which had a temperature of ca. 700°C, by an uncooled stainless-steel tube injector. The experiments investigated the effects of ammonia gas injection on sulphur dioxide emissions at staged conditions, varying the: (i) excess air level, (ii) secondary air ratio, (iii) fluidizing velocity, (iv) ammonia injection position, and (v) the ammonia:sulphur dioxide molar ratio. A maximum reduction of 66% in SO₂ emissions was found at 40% excess air, 65:35 staging, 1.5 m/s fluidizing velocity, 65 cm injection height, and an NH₃:SO₂ molar ratio of 1.2. The onset of SO₂ reduction occurred at an NH₃:SO₂ ratio of 0.5. The fluidizing velocity, excess air, level of staging, and ammonia injection height all have a significant influence on SO₂ reduction. It is difficult to determine how the SO₂ reduction varies with the operating conditions when ammonia is injected in the high temperature zone of the combustor. As sulphur oxides-ammonia reactions are low temperature reactions, the findings confirm the occurrence of reactions above the freeboard or near the exit to the cyclone.