| Semi-dry and semi-wet flue gas desulfurization (FGD) belongs to semi-dry FGD. Both CaSO4 and CaSO3 exist in the products of calcium-based FGD. CaSO4, namely the gypsum is a valuable resource. CaSO3 has poorer stability. It can be oxidized to CaSO4 under some conditions. So, it's interesting to increase CaSO3's conversion in the flue and acquire the gypsum directly.How different reaction conditions in the flue affect the CaSO3's conversion in the process of semi-dry and semi-wet FGD is investigated and the effect of five factors—O2's content, reaction temperature, reaction humidity, particle's diameter and CaSO3's content on CaSO3's oxidation and oxidation mechanism on the condition of semi-dry and semi-wet FGD—is analyzed in this research. The result indicates that: The conversion of different-diameter particles can rise as O2's content increases. When the temperature is lower than 98℃, the conversion rises as reaction temperature rises. When the temperature is a little higher than 98℃, the level of conversion is relatively lower. The conversion reaches a maximum at 98℃; And it rises as the relative humidity increases, reaches maximal at 95%, rises as particle's diameters decrease if other factors keep invariable. And the conversion decreases along with CaSO3 content. The result of this experiment shows: in the zone of experimental conditions, O2's content=20%, reaction temperature=98℃, reaction humidity=95%, particle's diameter d＜60 and CaSO3's content=92.38% the conversion curve is most optimal. When reaction time=60s, the conversion is closed to the highest value.According to the data and the time-conversion rate function by Szekely, the dynamics is discussed. In the first phase, when the reaction time is less than 45 seconds, the gas momentum is uppermost because of the high O2 content and CaSO3 content, here the reaction rate is very rapid. Moreover, at this moment, no plenty CaSO4 appears, the reaction pores are not obstructed. The function of chemical reaction, diffusion and mass transfer shows that in this phase, the rates of chemical reaction, O2's diffusion and mass transfer are similar, so we can think that it is all-rate-limiting step in this phase. When the reaction time is more than 45 seconds, the time-conversion function indicates that although the rate of O2's diffusion and mass transfer still keep some high, the chemical reaction rate reduce distinctly. It is speculated that along with reaction's development, CaSO3's content continues to reduce, so does the rate of CaSO3's separation. As a result, the surface chemical rate reduces. So the chemical reaction process becomes rate-limiting step in this phase.Szekely's time-conversion function depends on gas-solid phases reaction. But this reaction touches upon some humidity besides of gas-solid phases. Although there is some differentia about our analysis, the conclusion puts forward a proposaland heads the further investigation.It is calculated that reaction level equals to 2.2. CaSO3 oxidation rate is 0.7 to CaSO3·1/2H2O, 1.5 to O2.The result indicates that it is practicable to increase CaSO3 conversion in the flue and acquire the gypsum directly on the point of technical economics about semi-dry and semi-wet FGD.
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