NO Emission Characteristics Of Biomass Fuel Combustion

Biomass has the characteristics of high volatile content,high carbon activity,low N and S content,low ash content,and zero carbon emission.It is particularly suitable for combustion and conversion utilization and is a high-quality fuel.Comparing with coal and other fossil fuels,the nitrogen content in biomass is relatively low.Considering its lower calorific value,the nitrogen oxide emission per unit calorific value of biomass is even higher.At the same time,my country has issued stricter standards for the NO_x emissions of biomass boiler power generation.Therefore,mastering the mechanism of NO formation in the combustion process is of great significance to the efficient and clean utilization of biomass energy.This paper firstly studies the NO production law of biomass volatile matter and coke burning alone,and then compares with biomass direct combustion to explore the influence of the interaction between biomass coke and volatile matter on NO production,and finally based on the volatile matter combustion law.It simulates the production rules and paths of NO from rice straw with or without secondary air,and proposes suggestions for controlling NO emissions,which provides a certain theoretical basis for reducing NO emissions from biomass combustion.Firstly,the volatile matter of biomass and the separate combustion of coke were experimentally studied on the horizontal tube furnace test bench,and the effects of biomass types,combustion reaction temperature and oxygen concentration on the production of NO were explored.The results show that the higher the nitrogen content of the biomass,the lower the conversion rate of NO.The conversion rate of volatile NO increased first and then decreased with the rise of temperature,reaching a peak at 800℃,and the conversion rate of NO of coke decreased with the rise of temperature.The conversion rate of volatile NO increased with the increase of oxygen concentration,while the conversion rate of coke NO did not change significantly with the increase of oxygen concentration.Secondly,the direct combustion of biomass was experimentally studied on the horizontal tube furnace test bench,and compared with the combustion of volatile matter and coke,the effect of the interaction between biomass coke and volatile matter on NO production was explored.The results show that the interaction between coke and volatile matter changes the conversion path of fuel nitrogen.Among them,the interaction between pine sawdust coke and volatile matter has stronger inhibitory effect on NO than rice straw.The inhibitory effect of the interaction between coke and volatile matter on NO increased with temperature and oxygen concentration increasing.Finally,based on the law of biomass volatile matter combustion to produce NO,the production law and path of NO from rice straw volatile matter with or without secondary air were simulated.In the CHEMKIN software,a biomass combustion NO emission calculation model was constructed and a biomass volatile matter calculation model containing N-containing precursors was added.The results show that when there is no secondary air,lower excess air coefficient and temperature can effectively reduce NO emissions.Staged air combustion can effectively reduce the concentration of NO,and the optimal primary air excess coefficient of rice straw is about 0.9.The higher the temperature,the more obvious the effect of air staging combustion in reducing NO emissions,but the NO emission concentration is still lower at low temperature combustion.It is proposed that in the actual biomass fluidized bed boiler,the combustion temperature should be controlled at about 750°C,and fuel grading should be used.Feeding ports are set at the bottom of the combustion furnace and the dilute phase zone,and two are added above the feed port for the dilute phase zone.In the secondary air,the excess air coefficient of the primary air is controlled at about 0.9,so that the fuel staying in oxygen-lean combustion is prolonged under the condition of ensuring the combustion efficiency,the height of the secondary air is increased as much as possible,and the conversion rate of fuel nitrogen to N₂ is improved.