Pretreatment Of Forestry Residues For Enhancing Enzymatic Digestibility And Biogas Production

Due to the growing scarcity of fossil fuels and serious damage of natural environment caused by burning fossil fuels, research on utilizing renewable lignocellulosic biomass as raw fuel materials is becoming more and more important. Biofuel production from lignocellulosic biomass is one of the most promising directions. Forestry residues are a kind of abundant renewable feedstock for biofuels production with several advantages, such as high production, easy availability and high cellulose content. The raw biomass without any pretreatment is difficult to be used for efficient biofuel production due to different native structures and chemical compositions, so, the biomass should be pretreated with suitable methods. Choosing the suitable biomass materials and corresponding pretreatment methods is necessary for effective production of biofuels. In this study, high-pressure homogenization (HPH) pretreatment, thermo-chemical pretreatment and microwave assisted pretreatment (MAP) were used to pretreatment forestry residues; the characteristics of pretreated biomass and enzymatic hydrolysis efficiency of those pretreated materials were analyzed; and the operating conditions of these pretreatment methods were optimized.HPH pretreatment of grass clipping, corn straw, catalpa sawdust and pine sawdust was effective to improve the reducing sugar yield in enzymatic hydrolysis. Comparing with traditional methods, HPH pretreatment on grass clipping achieved better effectiveness under mild condition without chemical addition or heating. After HPH pretreatment, the average particle size of lignocellulosic biomass decreased, and the accessible surface area expanded because of internal explosion. The highest reducing sugar yield achieved to 267.39 mg/g (30 MPa) for grass clipping. The degradation of hemicellulose and the reduction of the average particle size caused by HPH pretreatment were the major causes for enhancing enzymatic hydrolysis. HPH pretreatment remarkably enhanced the biodegradation of lignocellulosic biomass. Moreover, grass clipping was the more suitable material for HPH pretreatment.The catalpa sawdust was a promising lignocellulosic material for biofuel production, and the thermo-Ca(OH)2 pretreatment was effective to improve enzymatic hydrolysis of catalpa sawdust. The thermo-alkaline pretreatments significantly improved the crystallinity and enzymatic hydrolysis of catalpa sawdust. Ca(OH)2 was superior to NaOH for the enzymatic hydrolysis of catalpa sawdust. Thermo-alkaline pretreatment could improve the enzymatic hydrolysis of catalpa sawdust by the removal of hemicellulose and lignin; thermo-Ca(OH)2 pretreatment could retain more raw material, thus had better treatment effect than thermo-NaOH pretreatment. The maximum reducing sugar yield of 518.14 mg/g was achieved under the optimal conditions, particle size of-40 mesh, solid content of 9%, Ca(OH)2 dosage of 1.75%, pretreatment temperature of 160℃, pretreatment time of 3 min, enzyme loading of 150 FPU/g, and enzymatic hydrolysis time of 96 h.In order to further improve conversion efficiency of thermal energy for energy saving, microwave method with high heat transfer efficiency was chosen for heating. The study showed that the microwave assisted Ca(OH)2 pretreatment was effective to improve catalpa sawdust enzymatic hydrolysis. Both water and alkaline pretreatments caused composition and structure changes of catalpa sawdust, but only microwave-alkaline pretreatment significantly improved its crystallinity and enzymatic digestibility. Microwave-alkaline pretreatment removed part of the hemicellulose in the raw materials, which lead to the destruction of cellulose-hemicelluloses-lignin network. That destruction was beneficial to enhance the enzymatic hydrolysis. The Ca(OH)2 was superior to the NaOH for MAP of catalpa sawdust. The maximum reducing sugar yield of 402.73 mg/g was achieved under the optimal conditions:particle size of-40 mesh, Ca(OH)2 dosage of 2.25%(w/v), microwave power of 400 W, pretreatment time of 6 min, hydrolysis time of 96 h, and enzyme loading of 175 FPU/g.HPH, thermo-Ca(OH)2, and microwave-Ca(OH)2 were used to pretreat grass clipping and catalpa sawdust to improve the efficiency of enzymatic hydrolysis and biogas production. HPH pretreated grass clipping produced 418 ml biogas in anaerobic digestion, around 2.5 times higher than that of the control. Thermo-Ca(OH)2, and microwave-Ca(OH)2 pretreated catalpa sawdust produced 721 and 706 ml biogas, around 1 times higher than that of the control. Pretreated sample could produce more biogas in a shorter time. Furthermore, both pretreated samples produced biogas immediately while the control had a delay.