PROBLEMS IN HOT DESULFURIZATION OF COAL-GAS WITH LIME

This investigation deals with the use of lime in hot desulfurization o f gas produced by coal gasification. Lime has excellent desulfurizatio n power, but the problem is with the subsequent handling of the reacti on product which is the sulfide CaS. The regeneration of the spent abs orber is difficult. The reactivity of the regenerated lime decreases f rom 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 s teps, vacuum treatment, inert gas flushing, or high pressure. Since li me is cheap the conversion of the CaS to CaSO4 is probably a better wa y than regeneration. The conversion to sulfate can be performed by hig h 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 rea cts with residual sulfide so that SO2 is formed. At low temperatures t he SO2 partial pressures are sufficiently low, but also the sulfatatio n rate is very low However the experiments show that the sulfatation r ate at low temperatures can be accelerated by the addition of catalyst s. Two possible conversion processes are suggested. One of them is a t wo-step process. In the first step CaS is oxidized with air at high te mperature (e. g. 1150-degrees-C) to form a mixture of CaO and CaSO4 un der release of some SO2. The first-step products are then introduced i n the second step, together with air into a reactor operating at lower temperatures (e. g. 850-degrees-C). There the gas phase is cleaned fr om SO2, and CaSO4 is formed as the final solid product. The alternativ e is a one-step process operating at low temperature and involving rea ction acceleration by a catalyst.