Study Of Mechanism And Flame Characteristics Of Methane And Hydrogen Assisted N-heptane Premixed Combustion

In the face of increasingly prominent energy and environmental problems,two practical ideas and solutions have been widely accepted.The first one is to adjust the energy structure of modern society and constantly develop and use new renewable and pollution-free alternative energy sources.The second is to optimize the current energy use mode and precisely control the combustion process of fossil fuels,so as to improve energy conversion efficiency and reduce pollutant emissions.N-heptane,as a representative of long-chain alkanes,has mature production process,higher energy density and less emission of combustion pollutants,which can be used as a new alternative energy source.However,n-heptane,as a kind of liquid fuel,has complex combustion process,combustion instability,low combustion conversion rate and more emission of incomplete combustion products.Therefore,based on the in-depth study of the combustion characteristics of n-heptane,this paper proposes that methane and hydrogen can be used as auxiliary fuel to improve the combustion stability and combustion conversion efficiency of n-heptane.At present,for n-heptane/methane and n-heptane/hydrogen mixed fuel combustion characteristic research mainly focused on analysis of methane and hydrogen on the influence of n-heptane ignition delay time.However,n-heptane/air,n-heptane/methane/air and n-heptane/hydrogen/air premixed combustion flame propagation speed,the flame temperature and combustion reaction kinetics characteristics and combustion mechanism research is very lack.The research in this paper is conducive to deepening the understanding of the process and combustion path of n-heptane premixed combustion,revealing the influence characteristics and mechanism of the addition of methane and hydrogen on n-heptane premixed combustion,and laying the experimental and theoretical foundation for the study of mixed fuel.The main innovative achievements obtained in this paper are as follows:（1）N-heptane premixed combustion flame characteristics have been got,and the premixed combustion path of n-heptane have been simplified.The n-heptane premixed combustion can be divided into three main stages,and C₂H₄ is the main intermediate in n-heptane combustion process,it has a key effect on the conversion efficiency of n-heptane combustion.The color of n-heptane premixed flames is light blue,with the increase of n-heptane equivalent ratio,incomplete combustion of continuously increase,and incomplete combustion products form diffused combustion with air in the environment,then incomplete combustion products increased,upper flame appears yellow flame.N-heptane and oxygen as initial reactant of combustion reaction,only the consumption rate in the combustion process,without generating rate,reaction of n-heptane only in the initial stage of the whole reaction,and then be consumed completely,while the consumption of oxygen reaction throughout the combustion process.The n-heptane combustion process can be divided into three stages: first,n-heptane carries out dehydrogenation reaction with H and OH groups,and it generates C7H15 isomers;Second,macromolecular hydrocarbon intermediates carry out cracking reactions,which generates C₂H₄ by one or more steps of cracking;In the end,C₂H₄ carries out oxidizing reaction combined with O and OH groups,and it finally generates CO₂ and H₂ O.In the process of n-heptane combustion,C7H16 dehydrogenation reaction is the beginning and important step of the reaction,the cracking and oxidation reactions related to C₂H₄ are the core steps of the whole combustion process.（2）CO and H₂ will be generated in n-heptane premixed combustion as combustion intermediate product,and these two products can carry out oxidation reactions combine with OH groups,the reactions will generate CO₂ and H₂ O as the final products of combustion.The two intermediates compete with each other to react with OH groups in the oxidation process,and H₂ dominates the competition.In the process of n-heptane combustion,the generation peak rate and consumption peak rate of H₂ in the flame position than the location more CO corresponding peak near the burner outlet.It means that n-heptane first decompose generate H₂,then generate CO in the combustion process of heptane.And H₂ first combine with OH groups to generate H₂ O,and CO to combine with OH groups to generate CO₂,in the combustion process of heptane namely in the later stages of combustion reaction.So,CO and H₂ compete with each other to combine with OH groups to produce CO₂ and H₂ O,but CO is at a disadvantage in the competition.Therefore,in the case of lack of oxygen,CO content in the products of combustion are increased,but the content of H₂ is very few,and the CO₂ content will be reduced.（3）The effects of methane addition on flame morphology,flame propagation speed and temperature,content of medium and small molecular groups and combustion products of n-heptane/methane premixed combustion were found,and the influencing mechanism of methane on n-heptane premixed combustion was obtained.The n-heptane/methane premixed flame is similar to n-heptane flame,which is pale blue.With the increase of methane content in the mixed fuel,the flame temperature decreases and the content of OH group in the flame decreases.The flame propagation velocity of n-heptane/methane-mixed premixed laminar combustion is between n-heptane and methane premixed laminar flame propagation velocity,close to n-heptane flame propagation velocity.When the methane content in the mixed fuel is small,the addition of methane has little effect on the content of the intermediate products of the combustion flame,while when the methane content increases to 50%,the content of some intermediate products and the location of the peak value of the mixed fuel combustion flame changes significantly.In the process of n-heptane/methane premixed combustion,with the increase of methane content in the mixed fuel,the reaction of C₂H₄ and CH3 is inhibited to form the stable compound CH4,so that more C₂H₄ can be converted into other unstable intermediate product groups,and the conversion and decomposition of hydrocarbons are promoted.At the same time,the increase of methane content in the mixed fuel helps to reduce the reaction path of CH3 to CH4,and CH4 to CH3,which optimizes the reaction path and increases the conversion rate of the mixed fuel.（4）The effects of hydrogen addition on flame morphology,flame propagation speed and temperature,medium and small molecular groups and combustion products of n-heptane/hydrogen premixed combustion were revealed,and the influencing mechanism of hydrogen on n-heptane premixed combustion was obtained.The n-heptane/hydrogen premixed flame is similar to n-heptane flame,which is pale blue.As the hydrogen content in the mixture increases,the content of OH group at the bottom of the flame decreases.When the hydrogen content exceeds 25%,a bright yellow dot will appear at the inner flame,and this phenomenon became more obvious with the increase of hydrogen content.Different from n-heptane and n-heptane/methane combustion,the equivalent ratio is in the range of 0.8-1.2,and the flame propagation speed of n-heptane/hydrogen increases with the increase of equivalent ratio.When hydrogen content in premixed fuel reaches 50%,it still does not reach the peak of hydrogen content in premixed flame.In the process of premixed combustion,some n-heptane still generates part of hydrogen through combustion or cracking reaction.At the end of the reaction,with the increase of hydrogen content in the premixed fuel,the reaction rate of hydrogen oxidation reaction keeps increasing,which can effectively promote the conversion of the incomplete combustion products at the top of the flame.The addition of hydrogen has little effect on the path of n-heptane reaction,and hydrogen can improve the combustion stability and conversion efficiency of n-heptane mainly by generating a large number of H and OH groups during the reaction process.