| Among the developed technologies for reducing emissions of NOx, selective non-catalytic reduction (SNCR) is characterized by considerably lower capital cost, moderate NOx removal efficiency and easy installation. A serious limitation of this process is that the temperature range over which NOx can be reduced is relatively narrow. This research focuses on developing cheaper and easily available gas additives which could widen the SNCR temperature window.Firstly, the influences of CO, CH4 and H2 on NOx reduction and the transform of NH3 and the additives in SNCR process were investigated experimentally in an electricity-heated tube reactor. All the three additives can shift the temperature window to lower temperature. Comprehensive considering the decreasing extent of the optimal temperatue for NOx removal, the width of the temperature window and NOx removal efficiency, CH4 is the most effective additive of the three, and its disadvantage is to causes CO emission due to its incomplete oxidation, the maximum conversion rate of CH4 to CO could be more than 50% near the optimum temperature for NOx reduction.Based on the study of single component additive, the composite additives composed of CO, CH4 and H2 were investigated experimentally in order to providing a solid background for the application of coal gas and other industrial gas mixtures as additives in SNCR process. The results show that while the mole ratio of the components is comparative, the effects of composite additive composed of CO and CH4 on SNCR temperature window is closed to that of its component CH4. The contribution of CO component is relatively minor. The temperature window with composite additive composed of H2 and CH4 is distinct from that with its each component, so both H2 and CH4 component make important contributions. While the fraction of CO is no more than that of H2 in composite additives composed of them, the performance of composite additives is dominated by H2 component; while the fraction of CO becomes larger, the influence of CO component becomes notable. The performance of composite additive composed of CO, CH4 and H2 depends mainly on CH4 and H2 component. The function of CO component is relatively minor.To analysis the reaction mechanism and predict the influence of the additives, a primary elementary reaction mechanism for promoted SNCR process by CO, CH4 and H2 was developed. By reaction mechanism analysis, the kinetic data of several elementary reactions was revised referring the related literatures, so as to make more exact predictions of the experimental results. By mechanism analysis, the effects of these additives on NOx reduction are achieved principally by promoting the production of OH and other radicals through chain reaction in their own oxidation process. The difference of the additives in changing the temperature window may be attributed to the distinction of their reaction path and reaction rates. While these additives are coexisted, their consumptions and their influences on SNCR are achieved by the same way as they are added individually. The distinct contribution of CO, CH4 or H2 component on the property of composite additive are mainly caused by their different competence with respect to reacting with OH and producing OH radical by chain reaction.A simple approach for mixing previously proposed together with the elementary reaction mechanism were used to calculation the SNCR process promoted by the additives. The results show while H2 additive is used, if the mix of reducing gas(NH3 together with H2) and the simulated flue gas is not fast enough, the maximum NOx removal efficiency will decrease remarkably, which give a reasonable interpretation on the experimental results.Two steps overall reactions for SNCR process previously proposed and the method for considering the effects of additives by adjusting the reaction temperature were adopted. The kinetic data of the overall reactions model for SNCR process promoted by additives was obtained by data regression based on the calculation results of the elementary reaction model. The overall reactions model is validated by experimental results and the predicted results of the elementary reaction model.Numerical simulation of SNCR process controlled by fluid-dynamic and chemical kinetics of reactants in a 600MW utility boiler is performed using CFD code Fluent together with the overall reactions model developed in this work. The numerical model was validated by industry experimental results and the design value. The researches in laboratory scale reactor indicate that the rate of SNCR reaction can be enhanced by the investigated additives greatly, so higher NOx removal efficiency and lower NH3-slip can be obtained under low temperature. The calculation results in 600MW utility boiler show that NH3-slip can be abated significantly by injecting some additives to furnace. The maximum predicted NH3-slip is 59μL/L while no additive is used, and it decreases to less than 14μL/L while CO additive is injected. The predicted NOx removal efficiency is between 27 and 35% under different boiler load. The NOx removal efficiency is not raised largly by injecting CO and its variation caused by CO addition is less than 2%. However the effects of additives on SNCR process in large scale boilers need further researches in future.
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