Numerical Simulation On Classification Combustion In A Coal-fired Utility Boiler

NO_x emission mostly come from coal-fired electrical power plant increasingly influences the environment, weather and human health. A lot of technologies of controlling NO_x emission have been proposed such as staded combustion. The numerical simulation on air-staged combustion and fuel-staged combustion were producted in this paper.The main work and conclusions are as follows..Base on the pulverized coal burning mode and the pollutant NO_x production deoxidization model, eguations describing the combustion process were set up and the process of staged-air combustion in coal-fired boiler is simulated with finite volume method, the influences of the parameters such as staged-air proportion, primary zone length and excess air coefficient on NO_x emission and boiler heat efficiency were obtained: (1) the NO_x emission reduces with the incresse of staged-air proportion and primary burning length, but when the staged-air proportion is larger than 20%, the incomplete combustion productions increase rapidly; with the increase of excess air coefficient, NO_x emission presents the tendency of increase first and reduction afterwards; The NO_x emission slightly increases along with the degree of fineness of powdered-coal. (2) Compared with conventional combustion, the air-staged combustion can effectively reduces NO_x emission. When the air-staged proportion is 80~90 %, primary zone length is 3~3.5m and excess air coefficient is 1.3-1.5, the comprehensive effect is good.In order to reduce NO_x emission and maintain the boiler thermal efficiency, the fuel-staged combustion technology with natural gas is proposed in the foundation of the air-staged combustion technology. The influences of the parameters such as natural gas proportion, reburning zone length and excess air coefficient in the reburning zone on NO_x emission and boiler heat efficiency were obtained by simulating the natural gas reburning process in coal-fired boiler: (1) The NO_x emission reduces with the increase of natural gas proportion, but when the natural gas proportion is larger than 20%, the scope of NO_x reduction is not obvious; The increase of reburning zone length causes NO_x emission obvious reduction; the NO_x emission increases gradually with the increase of excess air coefficient of reburning zone increasing. When the natural gas proportion is 10~20%, reburning zone length is 2m~4m and excess air coefficient of reburning zone is 0.8~0.9, the comprehensive effect is good. (2) Compared with powdered-coal reburning, the natural gas reburning can get a lower NO_x emission rate than the powdered-coal. (3) Compared with staged-air combustion, the reburning technology can get a better comprehensive effect.