| Based on the principle of re-burning combustion,taking a 660MW ultra-supercritical coal-fired boiler as the research object,using the Fluent simulation software to build a biomass(gas)re-burning combustion model,the effects of the re-burning system transformation of the boiler on the combustion status and pollutant emission reduction in the boiler influence have been studied.The main research contents are as follows:(1)The characteristics of combustion in the furnace,the mechanism of NO_xgeneration and the principle of NO_xreduction by re-burning are studied,and the mathematical models of coal powder coupled biomass combustion are established to analyze the re-burning process of solid biomass and biomass gas and the main influencing factors of biomass re-burning.(2)Based on the design parameters and boiler structure of a 660MW coal-fired boiler,the thermal calculation of biomass(gas)re-burning was carried out,and the basic scheme for the reformation of the re-burning system was determined.Based on Fluent software,a re-burning combustion model was built to study the effects of substance types,re-burning nozzle inclination and excess air coefficient on combustion and NO_xgeneration in the furnace.(3)The effects of different types of solid biomass and biomass gas re-burning on furnace combustion characteristics and pollutant emissions were studied.Three kinds of solid biomass fuels,straw,pine wood,and food waste,and their corresponding biomass gas were used as re-burning fuels to simulate the re-burning process.The results show that the change of the re-burning fuel state has little effect on the velocity field and temperature field of the combustion flue gas in the furnace,but it can significantly reduce the pollutant emission.Among the biomass solid re-burning fuels,the reduction effect of pine wood re-burning is more prominent,and the reduction effect on NO_xcan reach 14.54%.The emission reduction effect can reach 13.87%,and the emission reduction effect on NO_xcan reach 24.13%.The three components of straw gas,biogas and methane and the gas with large difference in low calorific value were selected as the re-burning fuel to simulate the re-burning process.The reduction effect increases as the content of reducing components in the re-burning gas increases.When the straw gas is re-burning,the flue gas temperature at the furnace outlet is the highest,which is 1351.68K.The NO_xemission reduction rate of methane re-burning is the highest,which can reach 26.2%.(4)The influence of the excess air coefficient in the re-burning zone and the inclination angle of the re-burning nozzle on the re-burning characteristics and pollutant emissions was studied.Using solid straw as re-burning fuel,the numerical simulation of re-burning process was carried out by changing excess air coefficient in re-burning zone.The results show that if the excess air coefficient is too high,it can destroy the reducing atmosphere in the furnace and reduce the NO_xreduction effect.When the excess air coefficient is 0.8,the NO_xreduction effect is the best,and the emission reduction rate is 12.74%.The straw gas,biogas and methane were used as re-burning fuels,and the downward inclination angle of the nozzle was set as 0°,5°,15°and 25°to simulate the re-burning.The flue gas flow conditions can improve the combustion effect,and can increase the residence time of the re-burning fuel in the re-burning zone,and improve the re-burning reduction effect.When the hem angle of the nozzle is 15°,the NO_xemission reduction effect at the furnace outlet is the best.The emission reduction rate of straw gas,biogas and methane reached 31.73%,the emission reduction rate reached 33.85%,and the emission reduction rate reached 35.66%. |
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