Experimental Study On Ethanol Production By Wheat Straw Fermentation Combined With Hydrothermal Treatment

Biomass energy is a kind of renewable energy with huge reserves,which can be used to alleviate the energy crisis of the world.Lignocellulose is the cheapest and most abundant biomass,which can be used as a raw material for the production of fuel ethanol by fermentation.Enzymatically hydrolysis via fermentation is a potential method for converting lignocellulose to bioethanol.The conversion from lignocellulose to biofuel has low biofuel productivity,due to the dense structure of lignocellulose.Therefore,a pretreatment process is requied to destroy the compact structure of lignocellulose,promoting the efficiency of enzymatic hydrolysis.Hydrothermal pretreatment can effectively degrade hemicellulose,while fungal pretreatment has great potential for lignin degradation.The combination of hydrothermal pretreatment and fungal can expose more cellulose,promoting the enzymatic hydrolysis efficiency.However,this new technology has not been clearly investigated,the pretreatment mechanism is not clear.Therefore,the processes of hydrothermal pretreatment via fungal pretreatment,enzymolysis,and fermentation fro converting wheat straw to bioethanol were investigated in this study.The varaition of composition in pretreatment process,the reaction kinetics of the hydrothermal pretreatment,the efficiency of enzymatic hydrolysis and ethanol yield in fermentation were analyzed.The wheat straw was used as the research object,the variation of composition and reaction kinetics of lignocellulose in hydrothermal pretreatment process was experimentally studied.It was found that hemicellulose was degraded into xylooligosaccharide,xylose,and furfural,which was the main degradation process in the hydrothermal pretreatment process.In addition,the concentration of xylooligosaccharide and xylose in the hydrolysate decreased with the increase of pretreatment temperature,while the concentration of furfural increased.Besides,the concentration of xylooligosaccharide and xylose in the hydrolysate first increased and then decreased with the incubation time,and the concentration of furfural tended to be stable with the incubation time.On the basis of experimental results,a new reaction path was proposed,and the kinetic rate constant and the activation energy of the reaction were obtained.The kinetic model clearly reflected variations in the concentration of the reactants,which agrees well with the experimental data.This kinetic model can provide guidance for the design of wheat straw hydrolysis pretreatment system.Finally,the enzymatic hydrolysis for converting wheat straw hydrothermal residues to produce sugar was investigated.When the pretreatment temperature was between 180 and 190°C,the enzymatic hydrolysis efficiency increased with the increase of pretreatment temperature and incubation time.When the pretreatment temperature was higher than 200°C,the effect of pretreatment conditions on the enzymatic conversion rate was negligible,and the enzymatic conversion rate of straw residue was close to 70%.The wheat straw residues after hydrothermal treatment was used as the experimental object,the activity of lignin degrading enzymes during the pretreatment of fungi and the changes of the composition of the wheat straw residues were studied.The extracellular lignin-degrading enzymes secreted by P.xanthomonas were mainly manganese peroxidase.They content of manganese peroxidase firstly increased and then decreased with pretreatment time.The content of manganese peroxidase was low at the end of the pretreatment process,indicating that the cellulose is the main energy source of fungus for growth and development in this stage.After fungal pretreatment,the lignin content was significantly reduced.Meanwhile,the fungal pretreatment process will secrete cellulase to degrade cellulose and reduce carbohydrate.Therefore,during the fungus pretreatment process,a part of the cellulose in the wheat straw residue is degraded.It was found that the hydrothermal pretreatment in 220°C combined with fungal pretreatment has a greater lignin removal rate and less cellulose loss than the the hydrothermal pretreatment in 200°C via fungal pretreatment.The reason is that the content of lignin increased and the content of cellulose decreased after the hydrothermal pretreatment in 220°,promoting the utilization of lignin in fungal treatment.The wheat straw pretreated with fungi was subjected to enzymolysis,and it was found that the enzymatic conversion rate was significantly improved after fungal pretreatment,which indicated that fungi combined with hydrothermal pretreatment can effectively treat lignocellulose and increase the enzymatic hydrolysis efficiency of cellulose.Fermentation experiments were performed on the hydrolyzed pretreatment residues of the enzymatic hydrolysate.It was found that the ethanol fermentation yield increased with the increase of pretreatment temperature and incubation time at the pretreatment temperature of 180°C and 190°C.Besides,the variation of pretreatment conditions had no significant effect on ethanol fermentation yieldwhen the pretreatment temperature was higher than 200°C.Meanwhile,the glucose conversion rate of the fermentation substrate is between 82% and 88%,which was independent on the pretreatment conditions.Fermentation of the residual enzymatic hydrolysate after fungal pretreatment,it was found that the ethanol fermentation yield was reduced after hydrothermal pretreatment in 200°C via fungal pretreatment.It indicated that the fungus has a large reduce of cellulose in hydrothermal after pretreatment at 200°C.Therefore,although the cellulose enzymatic conversion rate increases,the glucose content in the enzymatic hydrolysis solution decreases,causing less ethanol production.During the hydrothermal pretreatment in 220°C via fungal pretreatment,the ethanol fermentation yield was increased.It indicated that hydrothermal pretreatment combined with fungal pretreatment can enhance the efficience of lignocellulose treating ethanol fermentation yield.