The Study On Difference In Substrate Characteristics And Enzymatic Hydrolysis Performance Of Different Plant Biomassunder Two Kinds Of Oxidation Pretreatment

In order to alleviate the pressure of energy shortage and environmental pollution, the world pays more attention to the exploitation of renewable energy, particularly the development and utilization of biomass energy. Lignocellulose is one of the most abundant renewable resources on the earth. Lignocellulose is difficult to be directly enzymatically hydrolysed to reducing sugars, due to its complexity of chemical composition and structural characteristics. The lignin wraps on the surface of cellulose and hemicellulose, forming a physical barrier, resulting in the low efficiency of enzymatic hydrolysis and low conversion of the lignocellulose. Pretreatment process is indispensable to energy utilization of lignocellulose, for improving the efficiency of enzymatic hydrolysis. Pretreatment process is to change the structural and chemical composition of lignocellulose for improving accessible the surface area, thereby increasing sugar yield. Nevertheless, the enzymatic hydrolysis efficiency of aquatic and terrestrial lignocellulose may differ greatly even under exactly the same pretreatment process, due to the great differences in the chemical compositions and structural characteristics of aquatic and terrestrial lignocellulose. Therefore, selection of the appropriate pretreatment method for different lignocellulose based on the chemical compositions and structural characteristics, could improve the utilization of lignocellulose as well as the reducing sugar yield and thereby reducing the cost of bioethanol production.In the present study, three kinds of lignocellulose were selected, namely Eichhornia crassipes(aquatic lignocellulose), sugarcane bagasse(terrestrial herbaceous lignocellulose) and metasequoia(terrestrial woody lignocellulose). Two kinds of oxidation pretreatment methods, peroxide acetic acid and Fenton reagent, were conducted to pretreat the three kinds of plant biomass, followed by enzymatic hydrolysis. The effects of different pretreatment methods on the chemical compositions and structural characteristics of different plant biomass were compared, and its relationship with the reducing sugar yield was analyzed. The main results are as follows.(1) Among the three kinds of plant biomass, the contents of cellulose and lignin in Eichhornia crassipes(aquatic plants) are the lowest, the contents of its hemicellulose and ash are the highest; the contents of cellulose and lignin in metasequoia(terrestrial plants) are the highest. Carbohydrate(cellulose and hemicellulose) contents of three kinds of biomass are more than 70%, thus it is suitable to use Eichhornia crassipes, sugarcane bagasse and metasequoia as biomass feedstock for bioethanol production.(2) Pretreatment conditions of Eichhornia crassipes, sugarcane bagasse and metasequoia with peracetic acid(PAA) are temperature of 25 oC, time of 24 h, PAA concentration of 15%(w/w), biomass concentration of 1g/10 m L. The recovery of Eichhornia crassipes, sugarcane bagasse and metasequoia pretreated with peroxide acetic acid were 67.17%, 55.71%, 76.17%, respectively. Reducing sugar yields of 9.25%, 45.87% and 55.52% were obtained from pretreated Eichhornia crassipes, sugarcane bagasse and metasequoia by PAA, respectively, when three samples were hydrolysed at cellulase loading of 8.23 FPU/g biomass after enzymatic hydrolysis of 72 h. Compare to unpretreated samples, the reducing sugar yields of pretreated samples showed significant improvement. The role of PAA is to change the substrate structures and to oxidate the lignin in lignocellulose for exposing the carbohydrate, to increase accessible surface area and water retention value(WRV), and finally, to improve the efficiency of enzymatic hydrolysis. The reducing sugar yields of the pretreated Eichhornia crassipes, sugarcane bagasse and metasequoia biomass reached 17.51%, 60.00% and 69.07% at enzymatic hydrolysis time of 72 h, respectively, when cellulase loading increased to 30 FPU/g biomass.(3) The optimal pretreatment conditions of Eichhornia crassipes with Fenton are p H of 3.6, temperature of 25 oC, time of 24 h, Fe2+: 30% H2O2=1: 25(w/w), biomass concentration of 1g/20 m L. The recovery of Eichhornia crassipes, sugarcane bagasse and metasequoia pretreated with Fenton reagent were 41.92%, 70.93%, 91.33%, respectively. The reducing sugar yields of Fenton pretreated Eichhornia crassipes, sugarcane bagasse and metasequoia were 27.49%, 9.61% and 1.93%, respectively. Where, the reducing sugar yield of pretreated Eichhornia crassipes improved remarkably compared with that before pretreatment. However, the reducing sugar yields of sugarcane bagasse and metasequoia before and after pretreatment had no significant differences. The reducing sugar yields of pretreated Eichhornia crassipes, sugarcane bagasse and metasequoia achieved 34.28%, 11.85% and 3.41% at enzymatic hydrolysis time 72 h, respectively, when cellulase loading increased to 30 FPU/g biomass.(4) Comparison of the effects of two kinds pretreatment methods on different biomass shows that the lignin removal effect of terrestrial lignocellulose by peracetic acid is better than that of by Fenton reagent. PAA pretreatment is suitable for terrestrial plants with high lignin content, and Fenton pretreatment is suitable for aquatic plants with low lignin content. The reducing sugar yield of pretreated lignocellulose is inversely proportional to its lignin content and is directly proportional to WRV of the substrate.