Research On Combustion And Pollutant Emissions Mechanism Of Dimethyl Ether And Alcohol Fuel

As the renewable and clean alternative fuels,dimethyl ether(DME)and alcohol fuel were developed to replace petroleum fuels for solving the increasingly severe energy crisis and environmental pollution problems.However,the emissions of aldehydes were existed during DME and alcohol fuel combustion.Therefore,in this paper,the gas chromatography and spectrum technology were combined to detect the pollutants and reactive intermediates produced from the combustion of DME and alcohol fuels.The pyrolysis,oxidation and combustion of DME,methanol and ethanol were investigated.The mechanism of reaction processes and pollutants production were explored by controlling temperature,equivalence ratio,fuel composition and catalyst addition.The reaction conditions were optimized to achieve the controlling and reducing pollutant emissions.The main contents are as follows:(1)The DME pyrolysis in the middle and low temperature region was experimentally studied and the mechanism of DME pyrolysis to methanol was investigated.The pyrolysis of DME in the temperature range of 700?1220 K was studied in a flow reactor,and gas chromatography was employed for the detection of pyrolysis products.Methanol was found in the products from DME pyrolysis for the first time,and the methanol concentration as a function of temperature was analyzed.Based on the reaction kinetic mechanism of DME pyrolysis,the formation mechanism and reaction path of methanol were established.It was suggested that methanol was mainly produced by the hydrogen abstraction of methoxy(CH3O·)and hydroxymethyl(CH2OH·)from other molecule and radicals.Among them,the hydrogen abstraction of CH3O·from DME molecule to produce methanol was the major path for methanol production.(2)The effects of catalyst on DME reaction process and pollutants production were investigated.The addition of catalyst not only improved the reaction rate of DME,but also promoted DME catalytic hydrogen production.Due to the large contact area between the reactants and catalyst and the sufficient oxygen environment,hydrogen was produced from DME catalytic oxidation starting at 450 K.The starting temperature of DME catalytic hydrogen production was greatly reduced.(3)The effects of equivalence ratio(?)on DME premixed combustion flame and pollutant production were investigated in a self-designed burner.For ?<1,double flames phenomenon were observed.The luminescence of double flames at different equivalence ratio were measured on-line by spectra technique.The result indicated that the chemiluminescence of OH*and very weak CH*were obtained from the upstream flame,while all bands of OH*,CH*and C2*chemiluminescence were obtained from downstream flame.Furthermore,the light intensity of CH*and C2*declined with the reducing of equivalence ratio.For ?>1,the emissions of methanol,formaldehyde and aldehyde were detected from DME rich combustion by gas chromatogram.With the reducing of equivalence ratio,the concentration of methanol and acetaldehyde decreased,while the concentration of formaldehyde first increased and then decreased.Based on the DME combustion reaction kinetics,the formation mechanism of three pollutants was analyzed and the effect of equivalence ratio on their generation path was explored.(4)The emission spectral of excited formaldehyde in the 350?500 nm wavelength range were captured from the premixed flame of DME enriched combustion.It was proposed that the excited formaldehyde might be directly generated by the reaction of excited OH*with CH3O·,without intermediates and transition states.The feasibility of this path has been verified by the experimental results and the potential energy surface analysis of reaction path.(5)The oxidation of ethanol gasoline and the production of pollutant were studied,and the source of aldehyde products was explored.The oxidation behaviour of ethanol gasoline,a mixture of iso-octane and n-heptane,was investigated in a flow reactor.The NTC effect was found during the low-temperature oxidation of iso-octane and n-heptane,with three pollutants such as methanol,formaldehyde and acetaldehyde in the oxidation products.The effects of oxygen concentration and ethanol content on the distribution of NTC and the emissions of three pollutants were explored.The multiple production paths of formaldehyde and acetaldehyde in each temperature region were clarified:at low temperature,acetaldehyde was partially derived from ethanol oxidation and partially from n-heptane oxidation;formaldehyde emission was mainly contributed by ethanol oxidation;at high temperature,acetaldehyde was mainly produced from ethanol oxidation;formaldehyde was partially formed from iso-octane oxidation and partially from the consumption of methanol.Both DME and alcohol fuels were good alternative fuels,but aldehyde pollutants was produced during their combustion.As a fuel,the ratio of fuel to air needed attention avoiding rich-fuel combustion.If blended with gasoline,it was necessary to control the blending content so as not to produce excessive aldehyde pollutants.