Emission of SO₂ and desulfurization mechanism in 71 MWe PFBC demonstration plant

Desulfurization extent in the 71 MWe PFBC demonstration plant was analyzed in terms of operational parameters and morphology of the desulfurizing bed material. High removal level of SO₂ achieved in PFBC in spite of high CO₂ 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 SO₂ is governed by the decomposition of CaSO₄ flying out from the boiler. The residence time in the fluidized bed did not influence the level of SO₂ emission, indicating the saturation of desulfurization reaction in the bed within rather short time. The sorbent particles in the bed had CaSO₄ layer of 30-40 μm 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 CO₂ partial pressure was sufficiently low for the decomposition of CaCO₃. The skin of material was eroded to expose the active surface for further desulfurization. Such CaO of sufficient amount in the bed adsorbs SO₂ very rapidly to complete the desulfurization in the bed. Hence the concentration of SO₂ in the flue gas is determined by the reaction CaSO₄+CO→CaCO₃+SO₂ in the freeboard. This reaction appears in equilibrium since a fair linear correlation was observed between In (p_SO2/p_CO) and 1/T_g.