Study And Optimization Of Light Olefins-rich Syngas Preparation From Biomass

Biomass is the only current sustainable resource that can be converted into liquid fuels and chemicals.China is an agricultural country and has plenty of lignocellulosic biomass resources such as crop straw.Utilization of lignocellulosic biomass is crucial to the long-term supply of liquid hydrocarbon transportation fuels for China.Well known technologies can be used to convert the highly reactive light olefins into a wide range of hydrocarbons fuels,including diesel,gasoline,and aviation bio-fuels.Therefore,it is an awfully important and promising route for producing high-grade biomass-based liquid fuels via converting lignocellulosic biomass into light olefins.Here,we developed a direct and effective technology to convert lignocellulosic biomass into light olefins via catalytic pyrolysis.It is a pretty potential technology that can both enhance the resource utilization of biomass and expand the production diversities of light olefins in China.However,there is rare published research on light olefins preparation from lignocellulosic biomass via directly catalytic pyrolysis.Based on National Basic Research Program of China(2013CB228100)and National Natural Science Foundation of China(Major Program:51336008),this study conducted experimental and theoretical researches on converting lignocellulosic biomass into light olefins via catalytic pyrolysis from three aspects as follows:fundamental study,technology study and process optimization.Firstly,various zeolite supports,including USY,SAPO-34 and HZSM-5,were screened and the zeolite support with high performance of bio-oil catalytic cracking was determined.Then,the effects of different reaction conditions,including zeolite silica-alumina ratio(SAR),reaction temperature(T),oil content and weight hourly space velocity(WHSV),on bio-oil catalytic cracking were tested.Finally,the optimal conditions were obtained via a series of orthogonal experiments.Results illustrated that the highest mass yield of light olefins from bio-oil catalytic cracking was 27.81 wt.%when T=550℃,SAR=25,oil content=35%and WHSV=0.5 h-1.The reaction mechanisms of main compounds of bio-oil were explored using methanol,acetic acid,furfuraldehyde,hydroxyacetone,guaiacol,phenol and methyl acetate as characteristic compounds.According to the experimental results,the order of light olefins mass yield of different compunds was obtained as follows:methanol>acetic acid>hydroxyacetone>furfuraldehyde>guaiacol>phenol>methyl acetate.It was found that acids,aldehydes,ketones and esters,which were obstacles for bio-oil upgrading,were more effectively converted into light olefins.A new route that converting the water-soluble boi-oil that was rich in acids,aldehydes,ketones and esters into light olefins was put forward,both widening the preparation routes of bio-olefins and relieving the pressure of bio-oil upgrading.To break through the barriers such as low light olefins yield and severe catalyst surface coking,various Ce-modified HZSM-5 catalysts were prepared and tested in the catalytic cracking of water-soluble bio-oil.Results showed that Ce modification could effectively decline the amount of surface acids and adjust the acids distribution by increasing the L/B ratio.A higher mass yield of light olefins(31.49 wt.%)was obtained when T=550℃ and WHSV=0.5 h-1 on the 6Ce/HZSM-5 catalyst.In order to prevent the catalyst failure from dealuminzation and acidity loss during catalyst preparation and utilization processes,Ce-P modified HZSM-5 was prepared and tested.Results showed that Ce-P modification could effectively remain medium strong acidity,though the total amount of strong acidity was reduced.Ce-P modified HZSM-5 showed good performance and well stability during bio-oil catalytic cracking for light olefins production,even though it was used under harsh hydrothermal condition.Experiments suggested that 3Ce-1P/HZSM-5 owned a rather high L/B ratio and acidity retention,resulting into a high mass yield of 33.97 wt.%.According to the results obtained from the above work,experimental research on producing light olefins via directly catalytic pyrolysis of lignocellulosic biomass(pine sawdust)was conducted.A dual-steps device of biomass catalytic pyrolysis was designed and established,where thermal pyrolysis and catalytic pyrolysis were physically isolated,resuting in that the thermal coke and ash from thermal pyrolysis were retained in the first reactor,which could effectively ease the catalyst coking in the second reactor.Results indicated that a higher light olefins yiled(11.2 wt.%)could be obtained via dual-steps catalytic pyrolysis than that via single-step catalytic pyrolysis(6.7 wt.%).The optimal condition was ascertained as follows:thermal pyrolysis temperture=500℃,catalytic pyrolysis temperature=550℃ catalyst/feed=4 with 3Ce-1P/HZSM-5 catalyst.Based on the above research,a system with feed rate of 100 kg biomass per hour for light olefins production via biomass dual-steps catalytic pyrolysis was simulated using Aspen Plus software.It was found that the total energetic and exergetic efficiencies were 28.61%and 24.31%,respectively.And there were tremendous external exergy losses that accounted for about 74%of the total exergy losses.Therefore,an energy recovery and utilization unit was added to reduce these external losses.Except the heat supply for reactors,about 90 MJ electricity could be generated,which could be supplied to the pump and refrigerating circulator.Considering both light olefins and electricity as the final products,the optimized exergetic efficiency of entire system was enhanced by 11.91%.To investigate the productive potential of light olefins production via biomass catalytic pyrolysis,a comparison between biomass catalytic pyrolysis and biomass gasification-MTO was performed.It was found that both the energetic and exergetic efficiencies of biomass catalytic pyrolysis were much lower than those of biomass gasification-MTO route,which were 54.66%and 47.65%,respectively.Though the light olefins yield of biomass catalytic pyrolysis in this study remains at a fairly high level among similar published reseraches,there is still a gap between the experimental yield and the ideal one.Further effort should be made to increase the light olefin yield from biomass catalytic pyrolysis.