Alkaline Pretreatment of Biomass for Ethanol Production and Understanding the Factors Influencing the Cellulose Hydrolysis

Abstract

Alkaline pretreatments were investigated in connection with bioconversion of lignocellulosic biomass into ethanol. Corn stover and two different batches of hybrid poplar (High Lignin (HL) and Low Lignin (LL)) were the primary substrates of this study. Two different alkaline reagents, aqueous ammonia and dilute NaOH, were used in pretreatment of biomass for delignification and enhancement of digestibility. Two pretreatment processes with aqueous ammonia were used: Soaking in aqueous ammonia (SAA) and Ammonia recycle percolation (ARP). SAA is a batch process whereas ARP is a flow-through semi-batch process. Hybrid poplar is found to be more recalcitrant than the corn stover because of higher lignin content. More than 70%, and 90% of hemicellulose were retained in solids after ARP and SAA pretreatment process, respectively. Presence of hemicellulose is a significant resistance for cellulose hydrolysis. Additional external xylanase supplementation has significantly enhanced enzymatic hydrolysis of treated hybrid poplar. Overall sugar yields of 90%/60% and 90%/77% were obtained from LL/HL hybrid poplar after ARP and SAA treatments respectively. Modification in SAA process was done by adding H2O2 at lower temperature. Different H2O2 feeding strategy and temperature profiles in treatment were attempted in the modified SAA treatment with HL hybrid poplar. Above 60% of delignification was attained for hybrid poplar by stepwise increase of temperature (60°C for 4hrs and then 120°C for rest of the treatment). Glucan digestibility of 86% was achieved from the HL hybrid poplar treated under these conditions. NaOH was used as an additional pretreatment reagent (without and with H2O2) because of its alkalinity much higher than ammonia. Maximum overall sugar yield obtained from HL hybrid poplar was 80% with 5%NaOH + 5% H2O2 at 80°C. To understand the reaction resistances other than the lignin and hemicellulose, the mechanism of cellulase reaction was investigated using pure cellulosic substrates including Avicel, filter paper, a-cellulose, cotton and NCC (Non-crystalline cellulose). NCC is highly amorphous and has Degree of Polymerization (DP) of 100-150. It was found that exo-glucanase (Exo-G) also contributes in the generation of COS. Initial hydrolysis rate of cellulose is mainly controlled by endo-glucanase (Endo-G) whose activity is strongly influenced by the crystallinity. DP of NCC affects the reactivity of Exo-G and its terminal hydrolysis rate. Surface characteristics of substrate such as adsorptivity and surface area affect the initial hydrolysis rate but DP and crystallinity of cellulosic substrate determine maximum conversion. On the basis of the unique properties of NCC, an analytical procedure was developed that can simultaneously measure relative activities of Endo-G and Exo-G in different cellulases using NCC.