| Agricultural residue is one of the most strategically important sustainable energy sources due to its large quantities and various forms. Fuel ethanol is one of the best alternative fuels with the potential to reduce the rising levels of carbon dioxide. Thus, cellulosic ethanol produced from agricultural residue has become a hot research point in energy field and has been widely studied in recent years. Pretreatment, enzymatic hydrolysis and fermentation are three main steps in the bioconversion of biomass to bioethanol. Efficient enzymatic hydrolysis requires pretreatment to alter or remove structural and compositional impediments to hydrolysis. This is known as biomass recalcitrance which is partially influenced by the presence of lignin and its complex interactions with carbohydrate. Consequently, the development of effective pretreatments to overcome biomass recalcitrance becomes one of major technological challenges for bioconversion process commercialization.Based on traditional AFEX (Ammonia fiber expansion) pretreatment, a new pretreatment named HP-LAT (Combined hydrogen peroxide and liquid ammonia treatment) was proposed in this paper. A comprehensive study of HP-LAT process for corn stover was conducted according to the research goal. This study could provide theory basis for development of new pretreatment for lignocelluloses, and it promotes the fundamental research for cellulosic ethanol production and its developing utilization.The main contents and conclusions were as follows:(1) The pretreatment of corn stover by hydrogen peroxide (HP) or liquid ammonia treatment (LAT) alone was conducted. The effects of various pretreatment conditions on solid recovery, composition changes and enzymatic hydrolysis were investigated.For HP pretreatment, temperature had a significant impact on solid recovery. Low temperature (60℃) HP pretreatment could effectively remove lignin, and the carbohydrates could be retained when the H2O2 loading was below 0.9. However, high temperature (120℃) HP pretreatment could hardly remove lignin. The effect of HP pretreatment was only limited to improving glucan conversion and this affected its application.For LAT pretreatment, temperature and water loading had a significant impact on solid recovery, ammonia loading and water loading had a significant impact on enzymatic hydrolysis. The weight loss of LAT pretreatment was little correlation with carbohydrates and lignin removal, and it was most from other soluble components. Compared with HP pretreatment, the lignin removal of LAT pretreatment was very limited. After treated at 130℃ for 10 min with 0.7 water loading and 1.0 ammonia loading, then hydrolyzed at 15 FPU/(g gulcan) cellulase loading (with a certain amount of beta-glucosidase and xylanase) for 72hr, the glucan and xylan conversions of the LAT-treated biomass were 82.1% and 79.4%, respectively, and 497.6 g fermentable monosaccharides was obtained per 1000 g of dry basis material. The LAT-treated substrates obtained a 1.9-fold higher total monosaccharide yield than untreated substrates.(2) The HP-LAT pretreatment of corn stover was completed. The differences in solid recovery, composition changes and enzymatic hydrolysis for HP, LAT and HP-LAT pretreatments of corn stover were comparatively investigated.After treated at 130℃ for 10 min with 0.7 water loading,1.0 ammonia loading and 0.5 H2O2 loading, and hydrolyzed at 15 FPU/(g gulcan) cellulase loading (with a certain amount of β-glucosidase and xylanase) for 72 h, the glucan and xylan conversions of the HP-LAT-treated biomass were 88.1% and 90.6%, respectively, and 527.4 g fermentable monosaccharides was obtained per 1000 g of dry basis material. Compared with untreated biomass, the results showed that a 2.0 fold higher increase of sugar yield. The solid recovery of HP-LAT pretreatment was lower than LAT, as a result, the monosaccharide yield of HP-LAT increased by 6% than LAT pretreatment. The enhancement of sugar yield for HP-LAT was owing to xylose yield. For biomass pretreatment, the fermentable sugar yield was determined both by solid recovery and polysaccharide conversion.Some results from the comparative study were as follows:Hydrogen peroxide had a significant effect on weight loss and lignin solubilization compared with ammonia. High ammonia loading mainly caused glucan and soluble sugars loss, while high hydrogen peroxide loading mainly caused xylan loss. Ammonia had a significant effect on polysaccharide conversions, while hydrogen peroxide only improved glucan conversion. The weight loss, polysaccharide removal and lignin solubilization could be promoted in the co-presence of ammonia and hydrogen peroxide. The ammonia and hydrogen peroxide had synergistic effects.(3) For LAT and HP-LAT pretreatments, the relationship between lignin removal or xylan degradation and sugar yield was discussed.For LAT and HP-LAT pretreatments, solubilization of 15%～20% of the lignin and degradation of 5%～10% of the xylan resulted in close to optimal enzymatic digestion. Therefore, an effective pretreatment did not need high level of lignin removal but break the connections in lignin-carbohydrate complexes. The removal (dissolved or remobilization) of a certain amount of lignin to form micro fiber porous surface structure was important to improve the accessibility of enzyme. A significant positive linear correlation (R2=0.97) between glucose yield and xylose yield was observed. Thus, increasing the recovery of xylan was more important than removal of xylan to improve glucose yield.(4) On the basis of effectiveness of HP-LAT pretreatment, the HP-LAT pretreatments of different parts of corn stover were comparatively investigated.With the same HP-LAT pretreatment conditions of 0.7 water loading,1.0 ammonia loading and 10 min residence time, the optimal pretreatments of corn stalk, corn leaf and corn cob were 130℃ and 0.7 H2O2 loading,130℃ and 0.4 H2O2 loading,110℃ and 0.4 H2O2 loading, respectively. After treated by the optimal pretreatment conditions and hydrolysis at 15 FPU/(g gulcan) cellulase loading (with a certain amount of β-glucosidase and xylanase), the different parts of corn stover (stalk, leaf, cob) could obtain 497.3 g, 411.6 g,593.2 g fermentable monosaccharides per 1000 g of dry basis material. Compared with untreated materials, the sugar yields from different parts of corn stover were increased by 3.0,2.1,3.5 times, respectively. The corn cob obtained the highest sugar yield, and the increase of sugar yield in corn cob was mainly contributed to xylose yield from mass balance.(5) Finally, the microscope morphology, crystallinity and chemical structure changes of HP-LAT-treated corn stover were analyzed using optical microscope, X-ray diffraction spectra and infrared spectrum technologies.Taking different tissues (corn leaf including leaf blade, leaf sheath, leaf midevein, corn stalk including rind, pith and vascular bundle) of untreated and HP-LAT-treated under a magnification of 1000 times using optical microscope system, the fiber and cell wall were subject to different levels of damage under chemical decomposition and machinery division. X-ray diffraction analysis showed that the crystalline region and amorphous region of corn stover synchronously decreased, the crystallinity changes depended on the interaction of the two effects. Enzymatic hydrolysis was not directly related with the crystallinity of absolute value, but was closely related with the damage degree of crystalline regions. Infrared spectrum analysis showed that HP-LAT pretreatment effectively broke down cellulose hydrogen bonds, and disrupted ester linkage in lignin-carbohydrate complexes. Meanwhile, lignin molecular structure was partly destructed, and the distribution of lignin produced certain changes. As a result, these changes effectively reduced the biomass recalcitrance of corn stover. |
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