| As a very potential pollutant emission reduction technology,the coupled combustion of biomass and coal has attracted worldwide attention.This thesis focuses on studying the combustion characteristics and pollutant emission characteristics in the coupled combustion process of biomass and coal through a combination of experiments and simulations,focusing on the migration and transformation laws of nitrogencontaining substances.There are various ways of coupling combustion of biomass and coal,and there are also many pretreatment methods for biomass.Therefore,this thesis not only focuses on direct coupling,but also indirect coupling.On this basis,one of the important pretreatment methods for biomass is torrefaction that the author did in-depth research in order to make a complete,comprehensive and detailed understanding and understanding of the coupled combustion of biomass and coal.First,a lot of experiments were done on the direct coupled combustion and pollutant emission characteristics of biomass and coal on a tube furnace.The experimental data show that biomass blending is beneficial to increase the burning rate and shorten the burning time.Although increasing the biomass mixing ratio resulted in a decrease in NO concentration,the synergistic effect of coupled combustion on NO was found to be strongest when the biomass mixing ratio was 50 wt%.This indicates that the synergistic effect on NO inhibition reaches a maximum.The inhibitory effect of the coupled combustion of pine sawdust and Huadian coal on NO emissions is not only the reduction of NO by the special pyrolysis components of biomass in the volatile combustion stage,but also due to the faster than expected NO release rate in the stable carbon combustion stage,resulting in coupled combustion.The NO conversion rate decreases during the process.Since the nitrogen release rate is not consistent with the carbon combustion rate during the combustion process,the formation of NO is more sensitive to temperature changes than the coke combustion rate.The opposite is true for O2,where the combustion rate is more affected by changes in O2 concentration.Based on the tube furnace,this chapter further explores the combustion characteristics and pollutant emission characteristics of biomass and coal coupled combustion by using a 20KW circulating fluidized bed.With the increase of the mixing ratio,NO showed a trend of first decreasing and then increasing.The minimum value is 121.56 mg/m3 at 20%;it shows that the coupling effect of pollutant emission occurs in the mixed combustion,which inhibits the emission of NO and promotes the emission of N2O.The reason for the low emission of nitrogen oxides in the raw material with a blending ratio of 20%biomass is caused by the secondary air outlet and below.The specific reason may be that biomass plays the role of re-gas,and coke has a positive effect on the reduction of NO.With the increase of the secondary air rate,the NO emission concentration decreased,but when the secondary air rate increased to a certain level,the NO emission level gradually increased with the increase of the secondary air ratio.As temperature increases,NO emissions increase and N2O emissions decrease.The increase of temperature inhibits the reduction and decomposition of NO already generated in the axial direction of the riser to a certain extent,and the amount of N2O after this NO is inhibited and decomposed is also significantly lower than that of N2O under low temperature conditions.After adding limestone,the emission of NO increased,and the emission of N2O decreased.The main reason for the increase in NO is because of the increased NO generation in the area of the secondary tuyere.At the same time,the decomposition of N2O is further enhanced.Then,further simulation studies were done on the coupled combustion of biomass and coal indirect gasification.Through the simulation,it is found that with the increase of the furnace height,the average NO concentration of the furnace section increases first,then decreases,then increases and finally stabilizes.The different stages reflect the different behaviors of combustion and the progression of chemical reactions.After the biomass gasification gas was introduced,the regional average temperature level after the SOFA wind was significantly improved compared with the non-gasification gasification condition.The introduction of biomass gasification gas makes the coke nitrogen release in advance and the reduction effect of biomass gasification gas,resulting in the final NO emission higher than the working condition of feeding biomass gasification gas.As the boiler load decreases,the gas jet velocity decreases,the airflow stiffness decreases,and the diameter of the tangent circle increases gradually.In the sofa wind area,the higher excess air coefficient in the burner area at lower load results in less unburned coke reaching the sofa wind area,and at the same time due to the lower flue gas flow under low load conditions,the combustible gas The flow rate does not change,so the reduction effect of combustible gas on NO is more obvious under low load.Eventually,the NO emission concentration gradually decreases with the decrease of load.The increase of biomass blending ratio has little effect on the furnace temperature curve.The condition with higher blending ratio of biomass gasification gas resulted in lower final NO emission due to its strong NO homogeneous reduction reaction.The change of the nozzle position of biomass gasification gas basically does not affect the temperature distribution.The high nozzle position of biomass gasification gas will cause the coke combustion process to extend to the upper part of the furnace,which will lead to the release of coke nitrogen behind,which will lead to NO.Release increases.It can be seen that the change of the inclination angle of the burner has a relatively large influence on the boiler temperature.Under the condition of upward 15°inclination of the burner,due to the upward movement of the overall combustion,the proportion of coke burning in the sofa wind area and above increases,and the final NO amount at the furnace outlet is also significantly higher than that in the 0° condition.For the-15° operating condition,the NO emission is also between 15° and 0°.Finally,the changes in the physicochemical properties of biomass after torrefaction were explored,and the coupled combustion characteristics of biomass and coal after torrefaction were further studied.The physicochemical properties of the torrefied biomass have changed greatly,the water content is reduced,the grindability is improved,the energy density is improved,and the properties are closer to coal.The torrefaction affects the transformation and distribution of nitrogen-containing functional groups,and has a relatively large impact on the emission of nitrogen oxides from the coupled combustion of biomass and coal.After roasting,as the roasting temperature increased,the samples roasted at higher temperatures released more NO due to the increased fuel nitrogen content in the samples.Therefore,simply from the coupled combustion of biomass and coal after torrefaction,at 250℃,no matter it is an inert atmosphere or an oxidizing atmosphere,the final NO emission concentration is the same.Furthermore,the interaction of primary biomass/torrefied biomass and coal on NO emission reduction during coupled combustion turned from positive to negative with increasing torrefaction temperature,i.e.,biomass and biomass torrefied at lower temperatures.The coupled combustion of biomass and coal has a synergistic effect on NO reduction,while the coupled combustion of torrefied biomass and coal at a higher temperature has an inhibitory effect on NO reduction.It is further illustrated that baking at low temperature is the optimal solution for the reduction of pollutants. |
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