| Nitrogen oxides(NO_x)is one of the major air pollutants emitted by coal-fired power plant,whose emission standards differ with respect to the types of utility boilers in different regions of China.For operating boilers of some non-priority areas,NO_x emission limit is 200 mg/Nm3 nowadays.The combination of air staging combustion technology and SNCR denitration technology is usually applied to achieve the emission standard considering their advantages of low cost and easy transform process for the existing units.However,present application is simply overlaid of these two technologies as their interactions have not been taken into consideration,thus numerous reductants are usually occupied to meet the emission standard,which doesn't only increase the consumption of reductants,but causes the excess emission of ammonia and extra loss of boiler efficiency.To solve this problem,a 670t/h tangential firing boiler was chosen as studying object in this thesis,of which air staging combustion and SNCR denitrification processes were numerical simulated as a combination based on the actual operating data of these processes to propose optimization strategies.The simulation of air staging combustion process of the boiler was carried numerically,whose results were compared with the measured data to verify the reliability of computational models,thus opyimizing the operating conditions.Simulation results show that NO_x emission was 296.9 mg/Nm3 at the exit of boiler under BMCR load which is consistent with the actual operation situation.The area where the temperature window of SNCR reaction locates is suitable for the installation of SNCR system.Carbon in fly ash at the furnace exit was 3.24%and obvious temperature deviation appears in both sides of the horizontal pass.The results of numerical optimization towards combustion conditions show that burnout rate of coal in the boiler can be improved and temperature deviation in the horizontal pass can be reduced without increasing NO_x emission and narrowing down spatial domain of the temperature window for SNCR reaction significantly when excessive air coefficient of 1.18,separated over fire air(SOFA)angent angle of 15° and SOFA wind rate of 25%were chosen.The simulation of SNCR process was conducted based on the combustion results of BMCR load condition to research the state distribution of fume and its effect on the gas-liquid mixing process and denitrification reaction,thus discussing the optimization methods for injection characteristics of reductant based on fume parameters.It is found that the most suitable average diameter of spray drops varies at different heights to guarantee sufficient reaction time of SNCR process in the temperature window.Furthermore,a whirl flow remains in the SNCR area,along with which reductants are enriched to part of furnace,thus causing an uneven distribution of NH3/NO in the furnace area.Accordingly,optimization strategies of atomization parameters are proposed:(a)Optimizing the average diameter of spray drops at different heights diversely and ensuring a larger average diameter of spray drops in the high temperature zone;(b)Adjusting the distribution of spray flux in different regions according to the whirlwind characters under certain fixed molar ratio of NH3/NO.The simulation results indicate that the optimization plan can promote NO_x removal efficiency to 42.1%from 36.8%and reduce NH3 slip by 49%compared to those of fiducial injection condition in the boiler under a NH3/NO molar ratio of 1.2.The ultimate NO_x emissions was 171.9 mg/Nm3 after SNCR optimization process,which has been able to meet the relevant emission standards.In addition,the optimization method gains preferred denitrification results under higher NH3/NO molar ratios,while the effects of optimization descend simultaneously. |
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