PROBLEMS IN HOT DESULFURIZATION OF COAL-GAS WITH LIME

This investigation deals with the use of lime in hot desulfurization of gas produced by coal gasification. Lime has excellent desulfurization power, but the problem is with the subsequent handling of the reaction product which is the sulfide CaS. The regeneration of the spent absorber is difficult. The reactivity of the regenerated lime decreases from cycle to cylce. Modified lime based absorbers containing suitable additives may maintain their reactivity. But the other problem is that the possible regeneration schemes are complicated involving several steps, vacuum treatment, inert gas flushing, or high pressure. Since lime is cheap the conversion of the CaS to CaSO4 is probably a better way than regeneration. The conversion to sulfate can be performed by high temperature oxidation of the sulfide by air, but unfortunately such a process is neither simple. At high temperatures when the sulfatation rate is sufficiently high, the sulfate decomposes also rapidly or reacts with residual sulfide so that SO2 is formed. At low temperatures the SO2 partial pressures are sufficiently low, but also the sulfatation rate is very low However the experiments show that the sulfatation rate at low temperatures can be accelerated by the addition of catalysts. Two possible conversion processes are suggested. One of them is a two-step process. In the first step CaS is oxidized with air at high temperature (e. g. 1150-degrees-C) to form a mixture of CaO and CaSO4 under release of some SO2. The first-step products are then introduced in the second step, together with air into a reactor operating at lower temperatures (e. g. 850-degrees-C). There the gas phase is cleaned from SO2, and CaSO4 is formed as the final solid product. The alternative is a one-step process operating at low temperature and involving reaction acceleration by a catalyst.