Bioconversion of Lignocellulosic Material into Ethanol: Pretreatment, Enzymatic Hydrolysis, and Ethanol Fermentation

Abstract

Four different novel processes, ammonia recycle percolation (ARP), low-liquid ARP, two-stage (hot water-ARP) percolation, and soaking in aqueous ammonia (SAA) at room/moderate temperature, were investigated for pretreatment of lignocellulosic material (corn stover). These pretreatment methods were aimed at improving enzymatic hydrolysis and increasing the fermentability of the biomass. The ARP process feeds aqueous ammonia into a flow-through reactor at high temperatures and high pressures. This method is highly effective in delignifying biomass, reducing the lignin content by 70?85%. FTIR (Fourier transform infrared spectroscopy) and lignin staining results verify the lignin removal by ARP process. The SEM (scanning electron microscope) pictures indicate that the biomass structure is deformed and its fibers are exposed by the pretreatment. ARP pretreatment increases the crystallinity index as the amorphous portion of biomass is removed. The crystalline structure of the biomass cellulose, however, is not changed by the ARP treatment. Low-liquid ammonia treatment method reduced the liquid throughput to the level of 3.3 mL of liquid per gram of corn stover, leading to a shorter residence time and lower energy requirements. A high degree of delignification is not necessary to attain high enzymatic digestibility or high ethanol yield. An ethanol yield of 85% of the theoretical maximum was achieved using the low-liquid ARP treatment with SSF (simultaneous saccharification and fermentation) process. The two-stage process combines the hot water and ARP treatments, using a flow-through (percolation) reactor. The first stage hot water processing removes hemicellulose and the second stage ARP performs the delignification. A high fractionation of the biomass was achieved using the two-stage treatment; resulting in 92–95% xylan hydrolysis and xylose yield of 83–86% with 75–81% lignin removal. The solid residue after two-stage treatment contained 78–85% cellulose. A simpler alternative pretreatment process was also investigated. In this process, corn stover was soaked in 30% aqueous ammonia for 10 days at room temperature (SAA at room temperature) or in 15% for 12 hours at 60?C (SAA at moderate temperature), with no agitation under atmospheric pressure in a closed vessel. This process retains 85% of the xylan and removed 55–67% of the lignin. The treated corn stover was fermented to ethanol by the simultaneous saccharification and co-fermentation process using a recombinant E.coli. This organism utilized both glucan and xylan in the biomass, producing ethanol yield of 77.0–77.3% based on total glucan and xylan, and an ethanol concentration of 19.2–19.8 g/L . The advantage of this method is that the process is simple and yet provides high fermentability. The ethanol yield based on glucan alone was 113–116%, a clear indication that most of xylan is converted to ethanol during the SSCF.