| Bioethanol production from crop straw,which is a very important source of renewable energy,mainly includes two steps:enzymatic hydrolysis of cellulose and hemicellulose to monosaccharides and fermentation of monosaccharides to ethanol.The recalcitrance of crop straw by complex structural and chemical mechanisms leads to the low conversion efficiency and high cost of enzymatic hydrolysis.Thus,pretreatment of crop straw is essential to overcome the recalcitrance and improve the conversion efficiency.Mechanical fragmentation,usually considered as the first step for pretreatment,has become an essential part of bioconversion of crop straw.In this study,corncob,rice straw,wheat straw and corn stover were used to learn the impact and mechanism of mechanical fragmentation at different plant scales on the microstructure features,physical and chemical properties.Then the enzymatic hydrolysis of crop straw at different scales and energy requirement for mechanical fragmentation were measured.The relationship among microstructure features,enzymatic hydrolysis and energy requirement for mechanical fragmentation at different scales were investigated.At last,a combined pretreatment of mechanical fragmentation and liquid hot water was studied to improve the total sugar of corn stover.The main contents and results are listed as follows:(1)Corncob sample at a cellular scale exhibited some special properties compared to corncob samples either at a plant scale or tissue scale,i.e.,an increase in the special surface area(SSA)and pore volume(PV)with the inner meopores and macropores exposed to the surface;breakage of crystalline cellulose and linkages in polysaccharides;a higher proportion of polysaccharides and a lower proportion of lignin/extractives on the surface;an increase in density and poorer fluidity caused by increased interaction between particles;the decrease of water holding capacity,oil holding capacity and swelling capacity due to breakage of porous and long-chain structure of fiber;an increase in soluble sugar and decrease in degree of polymerization.The enzymatic hydrolysis of corncob sample at a cellular scale resulted in sugar yield of 90.9%of glucose and 82.2%of xylose.(2)Significant correlations were found among particle size(D50),crystallinity(CrI),crystalline phase amount(CP)and 72 h hydrolysis glucose yield(GY)for rice straw.The quantitative equations among the particle size,crystalline properties and glucose yield(mg/g substrate)are as follows:①CrI=44.14 ×[1-exp(-0.03658 × D50)](R2=0.80)和 CP=28.26-15.85 × exp(-0.03602 × D50)(R2=0.66);②GY=-5.636CrI+343.7(R2 = 0.98),GY=-14.62CP+512.1(R2=0.98);③GY=97.218+ 247.5×exp(-0.03824 × D50)(R2 = 0.70)。(3)Microstructure properties(D50,SSA,PV,CrI,and O/C)significantly correlated with the energy requirement(ER)and could be described by different regression equations.The order of importance of modifications of different microstructure parameters on the ER was as follows:CrI>O/C>PV>SSA>D50.Modification of the CrI required the most energy consumption,whereas reducing the MPZ showed the lowest demand for energy consumption.The ER showed a reciprocal relationship with the particle size,but a negative linear relationship with CrI.An exponential relationship was observed between the ER with a specific surface area and the O/C.The monosaccharide yield(MY)showed a clear,positive linear relationship with the ER,indicating that grinding patterns with a low ER did not significantly enhance the enzymatic hydrolysis yield for wheat and rice straw,due to plant cell wall recalcitrance.The energy efficiency(EE)declined with increasing ERs because the MY was markedly lower than the corresponding increase in the ER.Based on the considerations of both MY and EE,the balance value of mechanical energy consumption was approximately 0.59 kWh/kg with a corresponding MY of 165 g/kg and an EE of 0.29 kg/kWh for a sole mechanical fragmentation pretreatment.(4)The combination of mild ball milling(30 min)followed by mild liquid hot water pretreatment(190℃,15 min)solubilized 28.0%of xylan into soluble sugar and achieved both high glucose(83.0%)and xylose(72.0%)yields for a total sugar yield of 79.0%of theoretical.Thus,combining ball milling with liquid hot water pretreatment allows for milder conditions for both processes that lead to enhanced cellulose and hemicellulose conversion and reduces the energy requirement for mechanical fragmentation and enzyme dosage for enzymatic hydrolysis process.In conclusion,the combining pretreatment is an effective pretreatment for corn stover. |
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