Research On Magnesium-based Wet Flue Gas Desulfurization Technology Based On The Resources Of The Product

After more than10years of research and development of the magnesium-based wet flue gas desulfurization technology in China, the reliability of the process, the economy of the construction and operation as well as the feasibility of desulfurization byproducts resources have been widely recognized, which is expected to become the dominant desulfurization technology in our country in the future. This paper is based on the concept of circular economy, focused on the oxidation and oxidation inhibition processes of magnesium sulfite in the magnesium-based wet flue gas desulfurization process, systematically studied the magnesium-based wet flue gas desulfurization process in different reactors, and explored the reaction mechanism of magnesium sulfite oxidation and sulfur dioxide absorption process based on the experimental results, which provided the theoretical basis for the industrial application of the magnesium-based wet flue gas desulfurization technology. The main contents are as follows:Using the intermittent airtight homogeneous reactor, the intrinsic oxidation kinetics of sodium sulfite under different water quality has been studied and the corresponding kinetic equations have been established. The results confirmed that the sodium sulfite oxidation process is very sensitive to the water quality. Using the aeration stirred reactor, the macroscopical oxidation kinetics of magnesium sulfite has been studied and the corresponding kinetic equation has been established. The results confirmed that the oxygen diffusion is the controlling step of the magnesium sulfite oxidation. Using the intermittent airtight homogeneous reactor, the intrinsic oxidation kinetics of sodium sulfite under the presence of different inhibitors has been studied and the corresponding kinetic equations have been established. Sodium thiosulfate was determined as the optimal oxidation inhibitor for the desulfurization product. Using the aeration stirred reactor, the impact of various parameters on magnesium sulfite oxidation process under oxidation inhibition condition with sodium thiosulfate has been studied and the corresponding kinetic equation has been established. The results confirmed that sodium thiosulfate has a good inhibition effect on the magnesium sulfate oxidation and the oxidation inhibition efficiency could reach92.8%with a concentration of6.67mmol/L of sodium thiosulfate.Using the bubbling reactor, the impact of various parameters on the magnesium-based wet flue gas desulfurization process with conventional oxidation of the product has been studied. The results confirmed that the bubbling reactor could adapt to the impact of various parameters on the desulfurization efficiency and the flue gas flow rate, the flue gas SO2concentration, the flue gas O2content and the slurry MgSO4concentration could affect the MgSO3content in the product. The slurry pH value which changes during the reaction process can be divided into three parts, the stable stage, the slow decline stage and the rapid decline stage, respectively. The desulfurization efficiency began to fell rapidly when the pH value was less than5.0. Using the bubbling reactor, the impact of various parameters on the magnesium-based wet flue gas desulfurization process with oxidation inhibition of the product has been studied. The results confirmed that the bubbling reactor could adapt to the impact of various parameters on the desulfurization efficiency and the lower flue gas flow rate, the lower flue gas O2content and the higher Na2S2O3concentration could improve the MgSO3content in the product while the flue gas SO2concentration has less impact on it. The slurry pH value changes during the reaction process can also be divided into three parts and the desulfurization efficiency began to fell rapidly when the pH value was less than5.5.Using the spray scrubber, the impact of various parameters on the magnesium-based wet flue gas desulfurization process with conventional oxidation of the product has been studied. The optimum operating conditions were determined as follows:flue gas flow rate as20Nm3/h, flue gas SO2concentration as1000-2000mg/Nm3, liquid-gas ratio as8L/Nm3, and using the1nozzle which located in the higher position. The slurry enrichment experiment indicated that the slurry utilization efficiency and the desulfurization efficiency could reach the highest level when MgSO4concentration was close to the saturation state and the optimal operation pH value range was5.3-5.8. Using the spray scrubber, the impact of various parameters on the magnesium-based wet flue gas desulfurization process with oxidation inhibition of the product has been studied. The optimum operating conditions were determined as follows:flue gas flow rate as20Nm3/h, flue gas SO2concentration as2000mg/Nm3, liquid-gas ratio as8L/Nm3, using the3#nozzle, inhibitor concentration as20mmol/L, and pH value around6.5-7.0. The molar ratio of Na2S2O3to MgSO3should be maintained above1:4in order to maintain the MgSO3content in the product above90%. The magnesium-based wet flue gas desulfurization mass transfer model in spray scrubber has been established based on the four parts of gas-liquid equilibrium of SO2, gas side mass transfer, liquid side mass transfer and external mass transfer of SO2. The results confirmed that the model can be reasonable simulate the magnesium-based wet flue gas desulfurization process with conventional oxidation or oxidation inhibition of the product under normal operating conditions and the SO2absorption process was found to be controlled by gas-film diffusion.The analysis results of the magnesium-based wet flue gas desulfurization products under different slurry conditions showed that the product composition was related to the terminate pH value and was independent to the initial Mg(OH)2concentration. Using the thermogravimetric analysis method, the maximum mass loss temperature of the removal of external water and structural water, the thermal decomposition of magnesium sulfite, the thermal decomposition of magnesium bisulfite, and the thermal decomposition of magnesium sulfate were obtained as100℃,540℃,640℃and990℃, respectively. The difference of the desulfurization products components from air pump filtration and centrifugal separation was not significant, which indicated that the solid-liquid separation method has less impact to the product.