Influence of periodically changing oxidizing and reducing environment on sulfur capture under PFBC conditions

摘要

In the literature it has been reported that sulfur capture with limestone (CaCO{sub}3) under atmospheric fluidized bed combustion conditions reaches a maximum at about 850°C. Previously, the maximum has been attributed to the sintering of thesorbent particles which decreases the reactive surface area. Lately, also another explanation has been reported. In this case the sulfur capture decrease at higher temperatures was concluded to be due to fluctuating oxidizing/reducing conditions in theatmospheric combustor.In this paper the influence of alternating oxidizing/reducing conditions on SO{sub}2 capture at atmospheric and elevated pressure (15 bar) is reported. In the pressurized case, the CO{sub}2 partial pressure was kept high enough to prevent CaCO{sub}3 fromcalcining and therefore the CaSO{sub}4 would not form CaO but CaCO{sub}3 under reducing conditions. The experiments were done with a pressurized TGA by periodically changing the gas environment between oxidizing (O{sub}2, SO{sub}2, CO{sub}2 and N{sub}2)and slightly reducing (CO, SO{sub}2, CO{sub}2 and N{sub}2) gas mixtures at different temperatures. The results showed that under normal pressure and slightly reducing conditions CaO formation from CaSO{sub}4 increased with temperature as expected.However, no significant amounts of CaCO{sub}3 were formed from CaSO{sub}4 at elevated pressure. It was also concluded that since the formation of CaO from CaSO{sub}4 was relatively slow it could not explain the sharp sulfur capture maximum at about 850°C. Therefore, it was assumed that the strongly reducing zones, where CaS thermodynamically is the stable compound, may play a more important role than the slightly reducing zones, concerning the sulfur capture in fluidized bed combustors.