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

This paper reports the measurements of SO3 emissions with and without limestone under unstaged and staged fluidized-bed combustion, carried out on a ？m2 and 2？m high stainless-steel combustor at atmospheric pressure. The secondary air was injected 100？cm above the distributor. SO3 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, SO3 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. SO3 was retained to a lesser extent than SO2, suggesting that SO2 reacts preferentially with CaO and that SO3 is involved in the sulphation process to a lesser degree. The SO3 emissions were found to be affected by excess air, whereas the fluidizing velocity and bed temperature had little effect. SO3 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. 1. Introduction The presence of SO3 in flue gas corrodes the equipment and ducts of combustion system and therefore needs to be removed [1]. In order to control emissions of SO3, more studies on its formation and dissociation are required under air-fired and oxy-fired combustion conditions. The simulation study of Zheng and Furimsky [2] shows that SO3 emissions would be unaffected during oxy-fuel combustion, being governed only by oxygen concentration. The kinetics of reactions occurring in the combustor were studied by Burdett et al. [3] using a TGA microbalance. They proposed the following mechanisms for the formation of SO3. 1.1. SO2/SO3 Homogeneous Gas Phase Reaction SO2 may be oxidized to SO3 by two reactions: where M is a chaperon third body molecule. The large temperature dependence of reactions (1) and (2) ensures that the rate of production falls rapidly with decreasing gas temperature, and, in fact, 90%–95% of SO3 is formed in the bed and freeboard and the remaining 5%–10% in the region between the freeboard and sampling point. SO3 increases sharply with temperature, but the homogeneous reaction cannot account for all the SO3 produced. 1.2. Heterogeneous Catalysis of SO2 on Bed Particles and Heat Transfer Surfaces In a coal burning combustor, a more effective catalytic material, iron oxide, is present in fly ash. While the SO3 formation in this process