Study On Heterogeneous Reduction Of No During Coal And Biomass Combustion

At present,carbon-based solid fuels still account for a very important proportion in China’s energy structure.The combustion of coal and other fuels provides sufficient guarantee for China’s power energy demand.On the other hand,China’s efforts to control air pollution are increasing,and the nitrogen oxides（NO_x）emission limits of thermal power plants are becoming more and more strict.This puts forward higher requirements for in-depth understanding of NO_x formation/reduction behavior characteristics in the combustion process and mastering more advanced and mature NO_x emission reduction technologies.Among the existing NO_x emission reduction technologies,controlling the conversion of fuel nitrogen（fuel-N）to NO in the process of combustion has attracted continuous attention because of its good economic applicability.The forms of energy utilization under the policy of“peak carbon dioxide emissions before 2030 and achieve carbon neutrality before 2060”include combining some biomass energy for mixed combustion with traditional pulverized coal fuel,so as to reduce carbon emission,which also puts forward new requirements for the research on the characteristics of fuel-N conversion in the combustion process of related fuels.Therefore,how to further explore and improve the NO_x reduction capacity of relevant carbon-based solid fuels in the combustion process based on existing research,and further reduce NO_x emission to a more ideal level through the regulation of the combustion process is a problem that still needs to be studied at present.For different types of coal and biomass fuels,this thesis studies the fuel-N conversion characteristics in the combustion process by means of experimental research and computational research.The NO reduction reactions of coal/coal char and coal char/biomass char were carried out by using experimental devices including micro fluidized bed reaction analyzer（MFBRA）and drop tube furnace（DTF）.The fuel-N conversion of different fuels under different reaction conditions was studied,especially the characteristics of NO_x formation,reduction and intrinsic reaction kinetic parameters in the heterogeneous char-NO reaction.On this basis,combined with the char structural characterization,the evolution characteristics of char physiochemical structure under different reaction conditions and their relationships with the characteristics of fuel-N conversion to NO_x were deeply analyzed,and the interaction mechanism of different factors in the heterogeneous reaction process was discussed.Based on the existing chemical reaction mechanism of fuel combustion,the solution of fuel pyrolysis process is improved to optimize the fuel combustion calculation model.Based on the existing chemical reaction mechanism of fuel combustion,the solution of fuel pyrolysis process is improved to optimize the fuel combustion calculation model.Taking pulverized coal combustion as an example,the characteristics of fuel-N conversion to NO_x in three different fuel combustion patterns are predicted,including non-staging combustion,fuel-staging combustion and air-staging combustion.The accuracy and applicability of the combustion calculation model are analyzed.In order to solve the problems of stable combustion and NO_x emission of ultra-low volatile carbon-based fuels,the mixed combustion of Shenmu coal/coal char is studied,and the experimental tests are carried out in MFBRA and DTF.The results show that if the O₂ concentration is high,the oxidation rate of fuel-N will be increased and the NO reduction efficiency will be reduced.If the O₂ concentration is low,it may be beneficial to NO reduction.The DTF results also show that CO₂ in the reaction atmosphere reacts with pulverized coal to produce more reducing gas components,which promotes the efficiency of NO reduction by pulverized coal.In the coal/coal char mixtures,the NO reduction efficiency of the studied char is lower than that of pulverized coal,which is related to the gas-phase pyrolysis components produced by coal with the ability to reduce NO during the reaction.Based on the study of coal/coal char mixed combustion,the reaction characteristics of heterogeneous reduction of NO by char were further studied.Shenmu coal char,two kinds of biomass chars and their mixed chars in different proportions were used.The effects of various influencing factors on the heterogeneous reduction process were investigated in detail by MFBRA,and the intrinsic reaction kinetic parameters of heterogeneous reduction of NO by different chars were calculated,the effect of char mixing ratio on intrinsic reaction kinetic parameters was obtained.The results show that the increase of O₂ concentration,CO₂ concentration and inlet NO concentration will reduce the NO reduction efficiency,and the inlet NO concentration has the weakest effect;Reaction temperature plays a key role in heterogeneous reduction reaction.For all coal char and biomass chars,the influence of temperature on reduction reactivity becomes more obvious when the temperature reaches 1173 K and higher.In addition,the types of biomass also have a significant impact on the NO reduction process.The testing results show sawdust char has significantly better reduction activity than rice husk char.With the change of coal char/biomass chars mixing ratio,the intrinsic reaction kinetic activation energy E changes linearly,while the pre-exponential factor A changes exponentially.The above research found that there are obvious differences between the reactivity of different chars,and there are many factors affecting the reactivity difference.Based on the detailed analysis of the effects of various factors on the reactivity,the effects of different factors on the evolution characteristics of char physiochemical structures in the heterogeneous reaction process were further explored.With the progress of the reaction,the pore structure of all chars has developed,and the degree of carbon structure ordering has gradually increased.The pore structure of sawdust char is more developed than that of coal char,and it also has better NO reduction reactivity.The conversion characteristics of nitrogen-containing functional groups on different chars’surfaces are related to the reaction process.For sawdust char,when there is no O₂ in the reaction atmosphere,it is inferred that the existence of AAEM element contributes more to the NO reduction reactivity than the change of char physical structure caused by acid soaking treatment.When O₂ exists in the reaction atmosphere,the importance of AAEM elements decreases,and the influence of char physical structure change becomes more important.For coal char,the physical structure change of char plays a relatively more important role in the char’s reactivity,but the effect is weakened after introducing O₂ into the reaction atmosphere.In addition,the reaction atmosphere has a great influence on the development of char physical structure,especially CO₂ and O₂.In the presence of O₂,the char surface is etched and the fragmentation becomes serious,resulting in the increase of BET surface area and pore volume.The effect of reaction atmosphere on the chemical structure of char is relatively limited.Finally,a fuel-N conversion calculation model in coal pyrolysis combustion process is proposed,which combines the sub model based on CPD pyrolysis network and the sub model based on detailed reaction mechanism with nitrogen chemistry.In the improved calculation model,CPD pyrolysis network model is used to deal with the coal pyrolysis process,which avoids the deviation caused by using experimental results in the literatures or relatively simple defined methods to deal with the coal pyrolysis process in the original model.The results show that the model can reasonably predict the NO emission in the coal combustion process,but at a lower reaction temperature（≤1423 K）and at a smaller air ratio in the primary combustion zone（λ1≤0.75）,the calculation results are biased.The smaller air ratio value is basically not within the optimal air ratio range of air staging combustion,which makes the current model have certain practical value.Even so,the reaction process and related parameters in the model still need to be further optimized and improved to solve this problem in future research.