Study On Combustion Characteristics And Pollution Control Of Biodiesel Surrogates C7 Esters Blended With Alcohols

With the energy crisis intensifying and environment problem becoming increasingly prominent, biodiesel, as a kind of promising renewable energy, is getting more and more attention. Because the composition of biodiesel is very complex, domestic and foreign researchers usually use biodiesel surrogates or surrogate mixtures instead of biodiesel for the primary experiment, mechanism and chemical kinetic research. As biodiesel surrogates, two isomers of C7 esters (methyl hexanoate and ethyl pentanoate) can be used in the primary study of biodiesel. This paper mainly studied the combustion characteristics of the mixture of biodiesel surrogates two C7 isomeric esters and alcohols, and discussed the feasible methods of pollution control.Firstly, by means of making chemical kinetic analysis of the low-pressure premixed lean combustion flame of methyl hexanoate blended with different proportions of ethanol, the influence of ethanol addition on the combustion characteristic of methyl hexanoate has been studied. It was discussed that the effects of ethanol addition on flame temperature, main products and intermediate products, especially the potential pollutants in methyl hexanoate combustion. And the chemical effects of ethanol have been identified. The impact of ethanol addition on the combustion reaction path of methyl hexanoate was also analyzed. Results showed that when adding ethanol to methyl hexanoate flame, the concentrations of most potential pollutants (acetylene, propargyl, formaldehyde and ketene) are reduced, mainly attribute to the dilution and thermal effects of ethanol, but primarily because of the chemical effects of ethanol, the concentration of acetaldehyde is greatly increased. And the higher the concentration of ethanol blending is, the stronger the effects of ethanol on the concentrations of potential pollutants become. After methyl hexanoate diluted with ethanol, the most obvious change of the combustion reaction path of methyl hexanoate is that ethanol can greatly promote the generation of acetaldehyde.Secondly, due to the shortage of the basic combustion experiments of blended fuel of biodiesel surrogates and butanol, the combustion experiments of two kinds of mixture of methyl hexanoate/n-butanol and ethyl pentanoate/n-butanol with different blending ratios has been conducted in an atmospheric opposed-flow diffusion flame experiment platform. The effects of n-butanol addition and isomerization of esters was discussed on the main products and intermediate products, especially the potential pollutants in combustion of mixed fuels. The experimental results expressed that when adding n-butanol to methyl hexanoate and ethyl pentanoate, the changing trends of reactants consumption, concentrations of CO and intermediate products C1 to C5 hydrocarbons (containing potential pollutants acetylene and 1-butyne) in two esters flames are the opposite. However, it can be found that the concentrations of acetaldehyde are both on the rise after combustion of two esters diluted with n-butanol. Furthermore, due to the isomerization of esters, it was found that main products, intermediate products (except for methane and ethane) and potential pollutants in ethyl pentanoate flame all have higher concentrations than those in methyl hexanoate flame.Exhaust gas recirculation (EGR) is a promising technology for pollution control. In order to study the influences of two main components (CO2 and H2O) of exhaust gases in EGR additions on generation of pollutants in the combustion of biodiesel surrogates, firstly the chemical kinetic analysis has been carried out about adding CO2 and H2O individually or simultaneously to the low-pressure premixed lean methyl hexanoate flame, especially emphasizing the chemical effects of CO2 and H2O. The results showed whether adding CO2 and H2O to the flame of MH individually or simultaneously, the concentrations of the most potential pollutants are decreased. The chemical effects of CO2 can suppress the generation of acetylene, propargyl and acetaldehyde, and the chemical effects of H2O can cause the increase of concentrations of acetylene and propargyl, the coupling chemical effects of CO2 and H2O is similar to the chemical effects of CO2. Secondly, the effects of CO2 and H2O with different concentrations simultaneously additions on generation of pollutants in the methyl hexanoate combustion and the interactions of the chemical effects of two additives have been analyzed by using the same analysis method. It was found that adding different concentrations of CO2 and H2O simultaneously to the methyl hexanoate flame can all result in the decrease of concentrations of potential pollutants, and the interactions of the chemical effects of two additives are different to different potential pollutants in different conditions.Finally, for developing the research concerning the effects of the presence of ethanol on pollution control of EGR, it has been discussed about the influences of CO2 and H2O individually and simultaneously additions on pollutant formation in the low-pressure premixed lean combustion flame of mixtures of methyl hexanoate and ethanol with different mixing proportions. It was found that after ethanol addition, the single or coupling effects of CO2 and H2O are all very little for the peak concentrations of acetylene, propargyl, formaldehyde and ketene four kinds of potential pollutants, but for the peak concentration of acetaldehyde, the single or coupling effects of CO2 and H2O all have significant changes with ethanol addition.