Emissions of SO3 from a Coal-Fired Fluidized Bed under Normal and Staged Combustion

This paper reports the measurements of SO 3 emissions with and without limestone under unstaged and staged fluidized-bed combustion, carried out on a  m 2 and 2 m high stainless-steel combustor at atmospheric pressure. The secondary air was injected 100 cm above the distributor. SO 3 emissions were monitored for staging levels of 85 : 15, 70 : 30, and 60 : 40, equivalent to a primary air/coal ratio (PACR) of ~0.86, 0.75, and 0.67. Experiments were carried out at 0%–60% excess air level, 1-2 m/s fluidizing velocity, 800–850°C bed temperature, and 20–30 cm bed height. During unstaged combustion runs, SO 3 emissions were monitored for a wide range of Ca/S ratios from 0.5 to 13. However, for the staged combustion runs, the Ca/S ratio was fixed at 3. SO 3 was retained to a lesser extent than SO 2 , suggesting that SO 2 reacts preferentially with CaO and that SO 3 is involved in the sulphation process to a lesser degree. The SO 3 emissions were found to be affected by excess air, whereas the fluidizing velocity and bed temperature had little effect. SO 3 was depressed on the addition of limestone during both the staged and unstaged operations, and the extent of the reduction was higher under staged combustion.