Modelling Of NO_x Emissions From Biomass Fluidized Bed Combustion

Biomass as a renewable energy source can be used as a substitute for fossil fuel in modern power plants.China's 13th Five-Year Plan sets clear targets for biomass power generation.At the same time,China has issued stricter environmental regulations for NO_x emissions from biomass boilers,which also imposes higher requirements on NO_x control technologies to be able to stably control emissions and be cost-competitive.However,there is still a lack of connection between the operational parameters and N conversion inside the boilers.The purpose of this thesis are to predict the NO_x emissions of different biomass under different operating conditions by developing a NO_x prediction model of biomass combustion in fluidized bed boilers and to analyze the chemical mechanism and provide guiding suggestions.In this thesis,the model is constructed in Chemkin-Pro software.The ideal reactor network is built up based on a gas-phase chemical calculation to simulate fluidized-bed combustion.Besides,a model of biomass particle devolatilization is added to predict the input components of volatile.A 20KW fluidized-bed boiler burning wheat straw and a 0.5MW fluidized-bed boiler burning sugarcane leaves were used as validations to investigate the effects of temperature and excess air ratio on NO_x emissions.The results show that the preliminary oxidation of N-containing precursors HCN and NH₃ is affected by temperature and air,which in turn affects the oxidation and reduction pathways of the reaction intermediates NCO,NHi,and NNH,thus presenting different results.Secondly,models of the above two biomass fluidized-bed boilers were modified to simulate conditions with secondary air,and the effects of secondary air rate and excess air ratio on NO_x were investigated.The results show that the positions of fuel inlet and secondary air inlet as well as secondary air rate influence NO_x by affecting the mixing of volatile and air in the boiler.With a low fuel inlet,NO_x emission decreases first and then increases with the total excess air ratio.When feeding at the bottom,the excess air ratio of the primary air??₁?has a significant effect on NO_x emissions.When?₁ is around 0.9,both NO and N₂O have a minimum value.When feeding in the free-board zone,the NO_x can be reduced by about 20-40%with secondary air rate of 30%.Finally,a 75t/h biomass circulating fluidized-bed boiler in a biomass power plant was researched and the actual operation conditions were used to simulate the effects of fuel N content and moisture on NO_x.Both total N content and NH₃/HCN ratio in the precursor will affect the amount of NO and N₂O.The addition of secondary air can effectively reduce NO and N₂O.Moisture inhibits NO in a certain range,but excessive amounts are counterproductive.According to the actual situation,the air volume ratio was optimized to 55:15:30,and the calculated NO_x emission is significantly reduced after optimization.