Chemical Kinetics Of NO_x Reduction By Reburning With Coal-Gas

NO_x emitting from coal combustion in power plant is one of the most serious pollutants which are difficult to be dealt with. Fuel reburning technology is effective in reducing NO_x emission. To adequately utilize the existing coal-gas as reburn fuel, the characteristics of this reduction process have been studied.One of the most efficient methods to study NO_x reduction with coal-gas as reburn fuel is to simulate the chemical process using the kinetic mechanism, which can reveal the chemical reaction process and finally achieve the goal of making the chemical reaction to move towards the direction of NO_x reduction. A kinetic model applied on NO_x reduction with coal-gas reburning has been constructed. It consists of 362 elementary reactions and 62 chemical species. In the coal-gas, the main reducing gas is H₂, CO and CH4. The process of NO_x reduction with single H₂, CO and CH4 as reburn fuel has been simulated respectively using the structural mechanism. The structural mechanism was validated through comparison the modeling results with the experimental data. Lots of sensitivity analyses revealed that for the reduction of NO by H₂ at low temperatures in reduction atmosphere, NO is removed by the sequence H₂+NO<=>HNO+H, HNO+NO<=>N₂O+OH. When at high temperatures mainly by the sequence NO+H₂<=>HNO+H, HNO+H<=>NH+OH, NH+NO<=>..., NO+H<=>N+OH, N+NO<=>N₂+O. For the NO reduction by CO at high temperature in reduction atmosphere, the reaction CO2+N<=>NO+CO contribute the NO removal mainly. When use CH4 as reburn fuel, HCCO+NO and CHi+NO are the reactions most important in reducing NO.Experimental research on characteristics of NO_x reduction with coal-gas has been carried out on a gas reactor test-system and the experimental cases are simulated by the structural mechanism. The influences of the factors on NO reduction by coal-gas have been studied. The contrast analysis of the simulation and experimental results show that when the stoichiometric ratio is 0.55 and 0.7, the NO reduced efficiency gradually increases with the increase of the temperature. But when the stoichiometric ratio is 0.9, there is an optimization reaction temperature. Also it shows that they are almost identical when stoichiometric ratio is 0.55. The modeling predictions are in qualitative agreement with the experimental results but tend to underestimate the reduction of NO very much when stoichiometric ratio is 0.7 and 0.9. The NO reduced efficiency gradually decreases with the increase of the stoichiometric ratio under different temperatures. The NO removal can also be improved by increasing the resident time. But it doesn't incerese obviously when the stoichiometric ratio is 0.85, it'll do little benfit to NO reduction if only increasing resident time under such condition. The NO reduced efficiency gradually increases with the increase of the initial NO concentration. Under various reaction temperatures, the increase of reburning fuel ratio can improve the NO reduction efficiency in the reburning process. The higher the pressure in reactor is, the higher NO reduction can get.