Research On The Effect Of Urea-assisted Hydrothermal Pretreatment On Enzymatic Hydrolysis And Anaerobic Dark Fermentation For Hydrogen Production Of Corn Stover

Corn stover is one of the most abundant agricultural residues,whose annual availability is about 0.84 billion tons in China.At present,most of corn stover are burned directly.The direct combustion of corn stover causes serious environmental problems,such as resource waste,atmosphere pollution and frequent accidents.There are four primary utilization ways of corn stover including feedstuff,base material,fertilizer and energy.Biorefinery of lignocellulosic resource to obtain biofuels and biochemicals is considered as a promising approach for the large-scale utilization of corn stover.However,since the recalcitrant chemical structure of lignocellulosic biomass which consisted of cellulose,hemicellulose and lignin,the greatest challenge for biorefinery of corn stover is enzymatic saccharification.Therefore,it is of great significance to strengthen the research on the key technologies of pretreatment of corn stover raw materials and hydrogen production by stover anaerobic dark fermentation to improve the comprehensive economic benefits of corn stover anaerobic fermentation engineering,improving the ecological environment and promote the development of low-carbon circular agriculture.Several pretreatments including Mechanical crushing,steam explosion,acid,alkaline,hydrothermal and biological have been confirmed to be effective for enhancing the enzymatic saccharification of corn stover,via partially removing hemicellulose or lignin,decreasing the crystallinity of cellulose,or increase the surface area.Nevertheless,there is few report can be found about urea-assisted hydrothermal pretreatment of corn stover to enhance its enzymatic saccharification.In this work,A series of experiments were carried out respectively to study the effectiveness of urea-assisted hydrothermal pretreatment of corn stover to enhance lignin removal and enzymatic saccharification through the hydrothermal temperature?120-180??,holding time?0-60min?,urea content?0-4%?and solid-liquid ratio?5%-20%?.In addition,the main components of corn stover such as cellulose,hemicellulose and lignin were determined according to the standard protocol of National Renewable Energy Laboratory?NREL?.Physicochemical characteristics of corn stover before and after pretreatment were analyzed by Scanning Electron Microscope?SEM?,X-ray Diffraction?XRD?and Fourier Transform Infrared Spectroscopy?FTIR?.Finally,a dark fermentation for hydrogen production was carry out.And detect the volume of H₂ in optimum enzymatic saccharification conditions and analysis.It was found that urea-assisted hydrothermal pretreatment can effectively improve the lignin removal and enzymatic hydrolysis?EH?sugar recovery.The pH of urea-assisted hydrothermal pretreatment?9.08?was obviously different with hydrothermal pretreatment?4.60?and urea pretreatment?5.71?.The largest EH sugar recovery and lignin removal of urea-assisted hydrothermal pretreatment was 41%and69%respectively,which are obviously enhanced in comparison with hydrothermal pretreatment?29%and 44%?.Urea-assisted hydrothermal pretreatment can effectively remove ligninand retain cellulose and hemicellulose from corn stover.XRD analysis showed that the crystallinity of cellulose decreased,indicating that urea-assisted hydrothermal pretreatment could effectively break the structure of corn stalk and make it more looser.Through the analysis of FTIR,found that after urea-asisted hydrothermal pretreatment,on behalf of the functional group absorption peaks of the lignin decreased or disappeared,some representative of hemicellulose absorption peak rising,on behalf of cellulose functional decline.It was suggest that lignin structure has been broken or removed,most of hemicellulose is retained,and cellulose intermolecular force decreased significantly.Through the experiment of hydrogen production by fermentation,it was found that the hydrogen production of corn straw pretreated by urea-assisted hydrothermal treatment was significantly higher than that of other groups.The results of this study indicated that urea-assisted hydrothermal pretreatment is an effective method to enhance the delignification and enzymatic saccharification of corn stover.Based on this investigation,the optimal condition for urea-assisted hydrothermal pretreatment of corn stover has also obtained by Response Surface Methodology?RSM?method,that is pretreatment temperature of 178.2?,holding time of 27.2 min,urea content of 2.01%and solid-liquid ratio as 7.64%,respectively.The specific energy consumption?energy consumption/reduction sugar recovery rate?is 7.17.In conclusion,urea-assisted hydrothermal pretreatment is a potential option to enhance the enzymatic saccharification of corn stover and improve the biorefinery of lignin removal.