Experimental And Kinetic Study Of Gaseous Fuel Combustion Characteristics Under Elevated Pressures Based On Heat Flux Method

In the process of gasification of fossil fuels,due to different raw materials and differences in gasification methods,there is a wide variation range of gas fuel composition.For example,the main components of coal and biomass gasification gas are hydrogen and carbon monoxide,methane and also contain diluent gases such as nitrogen and carbon dioxide.Uncertainty of gas composition puts forward higher requirements for flame stability of combustion chamber and efficiently clean combustion.At the same time,the development of a variety of efficiently combustion methods such as oxy-enriched combustion combined with CO₂ capture and storage,lean-premixed combustion technology is of great significance to reduce carbon emissions and control pollutant generation.Fossil fuel gasification gas can be further synthesized into a variety of clean alternative energy sources.Among them,the development of oxygenated fuels like alcohol,ether and carbon-free fuels ammonia is of great significance to sea,land and air transportation and power production.The actual industrial combustion equipment includes gas turbines,internal combustion engines and supercharged boilers,which are mostly high-temperature and high-pressure combustion environments.Combustion stability related to changes of fuel composition,flashback,blow-off,self-ignition and flammability limits under high pressure are closely related to the inherent parameter of the premixed flame,laminar flame velocity.In order to further increase the understanding of the combustion characteristics of different fuels,carrying out laboratory-scale basic laminar combustion characteristics research under high pressure can provide an experimental basis for mechanism development and burner design.At the same time,measuring the emission characteristics of pollutants is helpful to select a suitable operating range and provides theoretical guidance for the application of new alternative fuels in industrial combustion equipment.In this paper,a high-pressure laminar combustion rig was built.Combining optical measurement methods,flue gas measurement methods and numerical simulation methods,the laminar combustion characteristics of different fuels and combustion methods under pressure were systematically studied.Firstly,a high-temperature and high-pressure combustion test bench based on a heat flux burner was built to obtain a one-dimensional adiabatic unstretched flat flame under high pressure.The oxy-enriched combustion characteristics of methane under high pressure were studied.The laminar burning velocity of CH₄/O₂/N₂ and CH₄/O₂/CO₂ at 0.5 MPa was measured considering the effects of the pressure,equivalence ratio,oxygen content and carbon dioxide dilution systematically.The current experimental measurement results are in good agreement with literature values and simulation results,verifying the reliability of the high-pressure chamber.Then,the one-dimensional flame model was used to analyze the effects of thermal-diffusion and chemical reaction of CO₂ dilution.Under normal pressure and high pressure,the reduction of laminar flame velocity caused by thermal diffusion effect of CO₂ dilution plays a major role.Then the flame speed obtained from the experiment was fitted to obtain the pressure power exponentsβ,and the flame speed at higher pressure can be predicted.The results show thatβincreases with the increase of the oxygen mole fraction.Moreover,the pressure power exponent increases first and then decreases in the rich conditions and the super-adiabatic flame temperature are observed,indicating that the reaction path changes at the rich conditions.Secondly,the laminar combustion characteristics and fluorescence measurement of high-pressure syngas were studied.In order to suppress the cell flame,the laminar flame velocity of various fuel blending ratios of the lean-premixed H₂-CO and H₂-N₂at 1.1 MPa was measured in O₂/He.Based on the obtained experimental results,five high-temperature and high-pressure reaction mechanism for syngas were tested.The same reaction path for different mechanisms and different rate constants are the reasons for different sensitivities and prediction results,especially the HO₂ chain growth reaction.As the pressure increases,the laminar flame speed decreases.For syngas with a higher hydrogen content in the fuel or a higher diluent content in the oxidizer,the mass combustion rate first increases and decreases.This indicates that the relatively low adiabatic flame temperature is the cause of the negative pressure dependence of the mass burning rate,and reduces the overall reaction order.The overall reaction order will continue to increase with the pressure for the syngas with a lower flame temperature.In addition,the effects of CO₂ dilution and methane addition on the laminar combustion characteristics of biomass-derived syngas H₂/CO/CH₄/O₂/diluent gas were studied.The measurement results of OH*chemiluminescence show that with the increase of pressure,the height of the flame front decreases and then increases,which corresponds to the non-monotonic change of the mass burning rate,and can be used as one of the targets of mechanism verification.Finally,the laminar combustion characteristics of ammonia,dimethyl ether and pollutant emission were studied.The laminar burning velocity of methane/dimethyl ether/hydrogen/air at atmospheric pressure was measured,and the influence of hydrogen addition on the oxidation path of dimethyl ether was compared.Analysis of the reaction path indicates that CH₃ is an important active radical in the C2 route and the DME decomposition path.It is concluded that the laminar flame velocity is linearly related to the peak mole fraction of[CH₃+H+OH].For the new carbon-free fuel,ammonia,through the heat flux method and flue gas analyzer,laminar burning velocity of ammonia/methane,ammonia/hydrogen,ammonia/carbon monoxide and ammonia/syngas at 0.5 MPa and different equivalence ratios,different ammonia contents were obtained.At the same time,detailed NO_x emission data in flue gas under different working conditions including NO,NO₂,N₂O,NH₃ and HCN were obtained.The prediction deviation increases gradually from the ammonia/carbon monoxide flame to the ammonia/hydrogen flame and is the largest at the medium ammonia content.This is because the increased ammonia mass burning rate introduces greater prediction uncertainty.Then,the pressure power exponentsβwas fitted,and the minimum value ofβunder the condition of moderate ammonia content caused the flame speed to be very sensitive to pressure changes within this range.The pressure exponent can be used as an independent indicator to verify and develop ammonia chemistry.Sensitivity and reaction path analysis show that ammonia chemistry is more important under rich conditions than under lean conditions,especially the path of forming N₂H_ithrough the recombination reactions affects the flame speed.The N₂H_i reaction path and the H₂NO,N₂O reaction path are the reasons that determine the difference in the prediction of the flame speed of the lean and rich sides,and point out the future direction of the mechanism optimization.Ammonia and syngas blends with different compositions have similar NO_x emission characteristics although their flame velocities differ greatly.Detailed NO_x emission measurement results for ammonia/hydrogen and ammonia/methane under high pressure show that NH₃,HCN and NO_x production are promoted at rich and lean sides,high-and low-ammonia content,respectively.Increasing pressure reduces NO_x emissions level,and this paper also gives the recommended blending ratio and equivalence ratio of practical application of ammonia fuel.