Desulfurization extent in the 71MWe PFBC demonstration plant was analyzed in terms of operational parameters and morphology of the desulfurizing bed material. High removal level of SO
2 achieved in PFBC in spite of high CO
2 partial pressure was correlated not with the temperature of the bed but with the temperature of the exhaust gas (T
g), suggesting that the emission of SO
2 is governed by the decomposition of CaSO
4 flying out from the boiler. The residence time in the fluidized bed did not influence the level of SO
2 emission, indicating the saturation of desulfurization reaction in the bed within rather short time . The sorbent particles in the bed had CaSO
4 layer of 30-40μm thickness on the surface . Since the calcium minerals produced are porous in the core as well as in the skin, such observation suggests the formation of CaO at the bottom of the furnace where the CO
2 partial pressure was sufficiently low for the decomposition of CaCO
3. The skin of material was eroded to expose the active surface for further desulfurization. Such CaO of sufficient amount in the bed adsorbs SO
2 very rapidly to complete the desulfurization in the bed . Hense the concentration of SO
2 in the flue gas is determined by the reaction CaSO
4+CO→CaCO
3+SO
2 in the freeboard. This reaction appears in equilibrium since a fair linear correlation was observed between ln (P
SO2/P
CO) and 1/T
g.
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