Study On The Effect And Reaction Mechanism Of Additives On The Selective Non-Catalytic Reduction

Coal combustion in power plants is one of the major sources of nitrogen oxides (NO_X) emission in China. Controlling NO_X emission is becoming more and more necessary as a result of increasing concern and more stringent environmental regulations. Selective non-catalytic reduction (SNCR) has some advantages of low capital and operation cost, good retrofit in nature, medium efficiency and easily being coupled with other NO_X emission control technologies, which becomes one of the optional NO_X control technologies adapted to the situation of coal-fired plants in China. Considering the narrow temperature window and some by-products of SNCR, the results of theory experiments and chemical kinetics modeling have been analyzed to present the reaction characteristics, additives with potential to improve NO_X reduction and by-products emission, and their reaction mechanisims, which provide theory foundation and guidance for an insight into important influence parameters and the optimization of the performace of SNCR process.An experimental study has been performed to present the effect of some major parameters on NO_X reduction, including temperature, NSR and O₂ concentration, with the comparison of performance and features of tow classic N-agents (or reducing agents), urea and ammonia. Due to the complex kinetic behavior of NO_XOUT process, its optimal efficiency and selective non-catalytic reduction is poorer than Thermal DeNO_X process, despite of its wider temperature window.The effect of various sodium salts on SNCR has been investigated experimentally with different amouts and different O₂ concentration. The experimental results indicate that small amounts of sodium additives significantly improve the performance of the SNCR process with a increased NO removal efficiency and a widened effective temperature range. A smaller promoting effect is founded at lower O₂ concentration. In general, sodium salts have a greater promotion effect on NO_XOUT process than Thermal DeNO_X process.A detailed chemical kinetic model has been established for the SNCR process. Modeling results exhibit characteristics of NO_X reduction and by-products emission, which make up for the deficiency of the experimental studies. The kinetic model reproduces the experimental trends qualitatively and often the predictions are in quantitative agreement with the observation. Through sensitivity analysis and rate of production analysis, the major reactions and reaction paths of by-products are identified.Reaction subsystems of sodium species and hydrocarbon have been founded. Effects of additives, including Na₂CO₃, CO and hydrocarbon, have been investigatied by chemical kinetic modeling, and the promotion meehanism of additives have been analyzed. Na₂CO₃ enhances NO removal, extends tempertature window to a lower range and suppresses byproduct formation. The addition of CO and hydrocarbon shifts the effective temperature profile to the lower region and enlarge the width of the temperature window without compromising the maximum efficiency of NO_XOUT process.