Reductive Decomposition And Reaction Mechanism Of FGD Gypsum

FGD gypsum is the main by-product of flue gas desulfurization (FGD), which will not only occupy a large area of land, but also will cause serious pollutions of surface water and soil if we only take a simple treatment. China is one of the countries that use coal as the main energy, so it is particularly important to find reasonable ways to take advantage of FGD gypsum. One of the most widely used ways is thermal chemical treatment technology. FGD gypsum could be decomposed to CaO and SO2which are recyclable. In this paper, the decomposition and reduction mechanism of FGD gypsum were discussed in detail based on large amounts of theory analysis and mechanism research. The main research results of this paper were as follows:XRF, XRD, SEM and laser particle size analysis were used to study the basic characteristics of FGD gypsum. The results indicated that the main component of FGD gypsum is CaSO4-2H2O, and the content of CaO and SO3are40.1%and55.9%, respectively. Besides, the average particle size is39.34μm.The decomposition thermodynamics of producing SO2and CaO from FGD gypsum with anthracite as reducing agent was calculated. The results indicated that the decomposition reaction of FGD gypsum with nothing added could occur at high temperature. C and CO could both lower initial decomposition temperature and theoretical heat consumption.According to the theoretical calculation, the decomposition under different oxidizing, inert atmosphere and with reducing agent added was studied using thermogravimetric analysis. The reaction mechanism was also studied based on TG-FTIR. The results indicated that the smaller the particle sizes, inert atmosphere, adding a reducing agent were conducive to the decomposition of FGD gypsum. The reaction mechanism was different if the molar ratio of C/CaSO4was different. Only solid-solid reaction mechanism did work if the molar ratio of C/CaSO4was relatively low, while both solid-solid and gas-solid reaction mechanism did work if the molar ratio of C/CaSO4was relatively high.In order to get the best condition for the decomposition of FGD gypsum and better understanding of the reaction mechanism, the tubular reaction was used to study the decomposition characteristics of FGD gypsum. The results indicated that the FGD gypsum was almost completely decomposed and the content of CaO of solid production was nearly80%under the condition of1000℃,20min of decomposition time and the molar ratio of C/CaSO4=1.2. The results also indicated that the decomposition of FGD gypsum was apparent solid-solid reaction.The kinetics model of FGD gypsum with anthracite was studied through thermogravimetric analysis for better understanding of reaction process. The results indicated that two stages were involved during the decomposition processes which were3-dimensional diffusion and2-dimensional nucleation.