| The focus of atmospheric pollution control is desulfurization and denitrification for flue gas from plant boiler. Some technologies of desulfurization and denitrification have been investigated in the past decades. However, technologies of simultaneous desulfurization and denitrification are rarely reported. So the main purpose of this paper is trying to develop a new technology of simultaneous desulfurization and denitrification for flue gas, which is economical and easily operated.Urea, a sort of alkalescency, is easily soluble in water. Urea solution possesses good absorptivity on sulfur oxide and nitrogen oxide. So it can be used as a kind of economic and environmental absorbent for simultaneous desulfurization and denitrification.Fly ash, a sort of solid waste with pozzolanic reactivity and porous structure, is a good absorbent for gas. In addition, calcium oxide, iron oxide and aluminum oxide contained in fly ash make it become oxidative and catalytic. Owing to these characteristics, fly ash can be used in simultaneous desulfurization and denitrification. Considered characteristics of urea and fly ash respectively, a new sort of absorbent, which are made by adding fly ash into urea solution with a ratio, can be used in simultaneous desulfurization and denitrification. With the help of this new absorbent, cost of urea can be lowed and recycling can be achieved.XLT fly ashes are selected as additive to prepare the new absorbent for simultaneous desulfurization and denitrification. Firstly, the instruments of XRF, XRD, SEM and grain size analyzer are used to detect the main components of fly ash and the surface feature of fly ash and mixed particles. At the same time, the technology of BET is used to detect the specific surface area value of urea, fly ash and mixed particles. After being compared of different values, the variation pattern of specific surface areas of mixed particles can be obtained. Based on the test results of main components of fly ash, the specific surface area and surface feature of fly ash and mixed particles, the effect of fly ash, which is added into urea solution to make new absorbent, on simultaneous desulfurization and denitrification can be deternmined. And the feasibility of this kind of new absorbent used in simultaneous desulfurization and denitrification can also be determined.Secondly, reaction mechanisms of simultaneous desulfurization and denitrification by urea/fly ash solution are investigated based on study of elementary reactions. The feasibility of reactions of simultaneous desulfurization and denitrificaiton by urea/fly ash solution can be estimated based on thermodynamic law, which shows that reactions can occur spontaneously when the change of entropy and enthalpy are both minus and decrease at the same time, Gibbs free energy is minus and equilibrium constant is bigger than105. While the partial pressures of sulfur oxide and nitrogen oxide are also considered to detect the feasibility of simultaneous desulfurization and denitrificaiton by urea/fly ash.Finally, simulated calculation is carried out by Aspen Plus according to the investigation on reaction mechanisms of simultaneous desulfurization and denitrificaiton by urea/fly ash solution. And then the influence of parameters on desulfurization and denitrificaiton efficiency can be investigated and the feasibility of simultaneous desulfurization and denitrification by urea/fly ash solution is determined again,In sum, the following results are reached based on above investigations:(1) There are many microspores on the surface of fly ash particles. The main components in fly ashes are CaO, Al2O3and Fe2O3, which are absorbent and catalyzed oxidation for sulfur oxides and nitrogen oxides. When fly ashes are mixed with urea in a ratio of1:1, the specific surface area of compound reaches a maximum value, which is15.32m2/g. This value is bigger than the value of pure urea that is1.85m2/g and the value of fly ash that is5.76m2/g. So a conclusion can be obtained that is urea/fly ash solution is conducive to gas absorption because of its big specific surface area.(2) In the same reaction situation, the change of entropy,enthalpy and Gibbs free energy are minus, and the value of entropy and enthalpy decrease at the same time, In addition, equilibrium constant is bigger than105. The feasibility of simultaneous desulfurization and denitrificaiton by urea/fly ash is determined in perspective of theory according to the variation of above thermodynamic parameters. At equilibrium state, the partial pressure of sulfur oxides in desulfurization by urea/fly ash solution is smaller than the value in desulfurization by urea solution, which means that urea/fly ash solution is beneficial to desulfurization. Compared with the partial of sulfur oxides, the partial of nitrogen oxides in denitrification by urea/fly ash is close to the value in denitrificaiton by urea solution, which means that there is no obvious effect on denitrificaiton by urea/fly ash solution. Therefore, the feasibility of simultaneous desulfurization and denitrification by urea/fly ash is determined in perspective of theory, and an optimum reaction temperature range is obtained, which is from298K-380K.(3) The simulated calculation and sensitivity analysis results based on Aspen Plus show that concentration of urea solution, reaction temperature, mass concentration of sulfur oxides in inlet gas, mass concentration of nitric oxide in inlet gas, liquid-gas ration and mass flow of mixed solution are main parameters which influence greatly on the efficiency of desulfurization and denitrification by urea/fly ash. When flue gas volume is constant, the maximum efficiency of desulfurization and denitrification can be reached at the following conditions:the concentration of urea solution is10%, the reaction temperature is358K, the mass concentration of sulfur oxide in inlet gas is2000mg/m3, the mass concentration of nitric oxide in inlet gas is400mg/m3, the liquid-gas ration is8L/m3and the mass flow of mixed solution is1400kg/m3.At this condition, the efficiency of desulfurization and denitrificaiton are98%and47%respectively.In conclusion, the feasibility of simultaneous desulfurization and denitrification by urea/fly ash solution is determined in theory. All investigated results in this paper can be used as a reference for further development on a new absorbent of simultaneous desulfurization and denitrification of flue gas. |
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