Study On Combustion And Emission Performances And Soot Formation Characteristics Of Biomass-derived Long-chain Ether Oxygenated Fuels

As an important renewable energy source,biomass energy has been widely valued by countries around the world.The liquid fuels from biomass feedstocks is one of the important ways to utilize biomass energy,and can mitigate global warming and promote the diversification of the energy structure.However,the physical and chemical properties of bio-crude oil produced by rapid pyrolysis of biomass are unstable,and bio-crude oil cannot be directly applied to internal combustion engines.Moreover,the biofuels upgraded by catalytic hydrogenation,catalytic cracking and catalytic esterification still have many problems,including poor combustion performance and lower blending ratio with diesel.Therefore,biomass-derived long-chain ether oxygenated fuels can be produced by extending the carbon chain,which have high cetane number and high oxygen content,and have good combustion performance.These additives are more suitable as additives in diesel engines,and have broad prospects for development and application.In the experiment,four biomass-derived long-chain ether oxygenated fuels,including polymethoxy dimethyl ether（PODE）,diglyme（DGM）,dipropylene glycol dimethyl ether（DPGDE）and tripropylene glycol monomethyl ether（TPGME）,were selected as additives.The CO,NO_x,soot and PAHs emissions of oxygenated additives/diesel blended fuels were measured at the diesel engine combustion and emission test platform.And the soot precursors in the TPGME flame were detected at the opposed-flow diffusion flame test platform.Therefore,the influence of the molecular structure of these additives on the mechanism of pollutant reduction was analyzed,and the conclusions were obtained as follows:The regulated pollutants from biomass-derived long-chain ether oxygenated additive/diesel blended fuels were studied,and the changes of CO,NO_x and soot emissions under different operating conditions（speed,load and blending ratio）were investigated.The results showed that the four biomass-derived long-chain ether oxygenated additives could be applied in diesel engines with smooth operation,and the special structure in which the oxygen atoms were evenly distributed in the carbon chain exerted a good effect.With a blending ratio of 25 vol.%,these additives reduced soot emissions by 81%～93%,and reduced CO emissions by 28%～56%.However,the impact on NO_xemissions was more complex,and PODE and TPGME could still reduce NO_x emissions by 23%～28%,while DPGDE and DGM would cause NO_x to increase by 14%～27%.With the increase of TPGME blending ratio,the CO and soot emission reductions were enhanced,while the NO_x emission reduction became weaker.SEM characterization results indicated that long-chain ether oxygenated additives could reduce the particle size of soot particles in the exhaust and XPS results showed that the degree of graphitization of soot had increased.The decrease of the I₂₉₂₅/I₁₆₀₀ ratio in FTIR implied a tendency to decrease the polycyclic aromatic hydrocarbons（PAHs）generated during combustion.The PAHs pollutants from biomass-derived long-chain ether oxygenated additive/diesel blended fuels were studied,and particulate PAHs and gaseous PAHs in the exhaust were collected under the conditions of 1500 rpm and 5 k W load,and quantitatively analyzed by GC-MS.The effects of biomass-derived long-chain ether oxygenated additives on PAHs emission concentration,PAHs ring number distribution and PAHs toxicity equivalent were explored.The results showed that the concentrations of phenanthrene,naphthalene,and acenaphthylene were higher than other PAHs compounds in the exhaust.The addition of oxygenated fuels could reduce particulate PAHs emissions by 26%～62%and reduce gaseous PAHs emissions by 25%～44%,and the PAHs toxicity was also reduced by 24%～54%.The presence of oxygenated additives could promote the combustion in the cylinder,and the proportion of gaseous PAHs had increased,while there was a tendency for the conversion of 4-ring PAHs to 2-ring PAHs.Furthermore,with the increase of TPGME blending ratio,the reduction of PAHs emissions was further enhanced,and 50%TPGME could reduce the total PAHs emissions by 74%and reduce the toxic equivalent by 73%.In order to further study the soot reduction mechanism of TPGME,a TPGME/n-heptane blended fuel was used in the opposed-flow diffusion flame experiment to study the change of combustion intermediate products.The relevant results showed that TPGME reduced the concentration of soot precursors（acetylene）by 6.68%.This was due to the increase of oxygen radicals in the flame,which consumed part of the soot precursors.At the same time,the structure in which the oxygen atoms were evenly distributed in the carbon chain could make use of each oxygen atom as much as possible to suppress the generation of soot precursors,thereby ultimately reducing the soot.