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Study of Simultaneous Desulfurization and Denitrification with Calcic-Based Absorbent in a Fixed Bed

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Tutor: GaoJiHui
School: Harbin Institute of Technology
Course: Thermal Power Engineering
Keywords: fixed bed,flue gas pollutant,calcium-based absorbent,combined removal,simultaneo
CLC: X701.3
Type: Master's thesis
Year:  2008
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Abstract:
The extensive use of fossil fuel is the main cause of environmental pollution, which has attracted general attention in all the countries of the world. The pollutive substances derived from coal combustion include dust, SOx, NOx and trace heavy metal elements mainly. At present, relatively developed technologies in flue gas cleaning field cope with dust, SOx and NOx respectively. The system with capability to capture several pollutive substances is usually complex and expensive. Thus, the development of technology with capability to capture several pollutive substances within one reactor will be the main trend in future. Considering the physical and chemical properties of SOx and NOx, the simutaneous desulfurization and denitrification technology has been significant as a research direction of combined removal. Presently, the calcium-based absorbents are generally used in FGD systems. Thus, the simultaneous desulfurization and denitrification technology based on calcium-based absorbents has significant value for application.To investigate the simultaneous desulfurization and denitrification reaction, an experiment system consisted of simulated gas system, fixed bed reactor and temperature control system was designed. The characteristics of the fixed bed reactor were analyzed using relative theories. The analysis showed that the wall effect of the reactor was neglectable. The shrinking unreacted core model of gas-solid reaction was appropriate for this experiment. Based on this model, the main step which controlled the reaction rate was analyzed. The formula of reaction rate was deduced and its application conditions were discovered through the experiment with the height of the fixed bed as variable. The simultaneous desulfurization and denitrification reaction was studied through change of variables, such as the composition of the absorbent, the composition of the simulated gas, reaction emperature and relative humidity etc.The results show that the main step which controls the reaction rate of denitrification is the oxidation of NO to NO2. The existence of oxidant in the absorbent can accelerate the oxidation of NO remarkably. The oxidant can not only oxidize NO, but also enhance the reaction between NO and O2 as catalyst. The more oxidative the oxidant is, the higher the rate of NO oxidation is. NO2 can react with NO in certain number of moles and the molar ratio of NO/NO2 is likely to be one. The experiments show that the relation between desulfurization and denitrification is complicated. The relation between the reaction of SO2 and Ca(OH)2 and the reaction of NO and Ca(OH)2 is competitive, while the products of desulfurization can enhance the reaction between NO and O2. The reaction rate of desulfurization increases when the relative humidity of the simulated gas is raised, while the reaction rate of denitrification decreases.
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