Study On Pretreatment Of Sorghum Stalks By Reflowed Aqua Ammonia Explosion

Currently, cellulosic ethanol production from the abundant and renewablelignocellulosic biomass is a leading and dominant technology route in globalbioenergy industry. However, the plant cell walls naturally resist decompositionmicrobes and enzymes, so the breakthrough technologies have to be developed toovercome barriers, which can implement economical feasibility and efficiency ofbioconversion. Based on combination of advantages of the ammonia recycledpercolation (ARP) and the ammonia fiber explosion (AFEX), a new pretreatmentmethod named the reflowed aqua ammonia explosion (RAAE) pretreatment wasdeveloped. The sorghum stalk in circulated and fluidized state could adequatelycontact with aqueous ammonia, which enhanced the effect of mass and heat transfer,so as to eliminated local overheating of reaction and reduced the formation ofinhibitory compounds. In this circulated flowthrough system, lignin was dissolved andswept from the pretreated samples. The pretreatment effect was influenced by somerelated factors such as temperature, pretreatment time, moisture, the ammonia flowrate and time. A series of experiments were carried out to explore how these factorsaffect the composition and enzymatic digestibility of treated samples. The treatedsamples were then further examined and analyzed by SEM, X-Ray crystallography,and FTIR to explore changes of physical characteristics and chemical composition inpretreatment.With the increasing of temperature and pretreatment time, although the recoveryof glucan and xylan decreased, more than91.48%glucan and82.58%xylan based onthe raw sample were preserved, and the amount of lignin removal gradually increasedfrom18.66%to46.23%. The enzymatic digestibility of glucan and xylan bothincreased with temperature increasing. The maximum enzymatic digestibility ofglucan and xylan were respectively for87.1and84.1%, and the total sugar yield was77.31%, all of which were achieved at temperature of85℃. With pretreatment timeincreasing, the glucan enzymatic digestibility steadily increased. However, whenpretreatment time exceeded25min, the glucan enzymatic digestibility increased moreslowly, and the total sugar yield only increased from72.68%to75.17%whenpretreatment time increased from25min to35min. So the optimum pretreatment temperature and time were selected as85℃and25min, respectively.With moisture content increasing, the composition of the pretreated sorghumstalk was almost unchanged, and the recovery of glucan and xylan were more than96.36%and90%respectively. The maximum glucan enzymatic digestibility was79.46%at moisture content of60%, while the enzymatic digestibility of xylan and thetotal sugar yield decreased with the increase of moisture. Synthetically consideringthe glucan enzymatic digestibility and total sugar yield, the optimal moisture contentcondition was selected as60%.With flow rate increasing, the amount of lignin removal increased from23.01%to38.86%, and the glucan enzymatic digestibility also increased gradually, but thistrend slowed down when the flow rate increased over3L/min. The maximum xylanenzymatic digestibility and total sugar yield were respectively for91.03%and75.05%at flow rate of3L/min. So the optimal flow rate was selected as3L/min.With flow time increasing, the lignin removal increased, and the maximum valuewas39.86%. Meanwhile, the cellulose content increased from42.69%to45.67%, andthe enzymatic digestibility of glucan also increased. However, when flow time wasexceeded9min, the glucan enzymatic digestibility increased more slowly. Themaximum total sugar yield was80.14%, achieved at flow time of9min.The SEM analysis indicated that the rigid biomass structure was deformed andmore cellulosic fibers were exposed after pretreatment. The crystallinity indexincreased with pretreatment temperature and time increasing, because part of theamorphous portion (lignin and hemicellulose) of biomass was removed. Chemicalchange was further confirmed by FTIR analysis.A positive linear correlation was observed between lignin removal andglucan/xylan enzymatic digestibility. When PEG6000with concentration of0.05g/gsubstrate was added in enzymatic hydrolysis, the glucan enzymatic digestibility wassignificantly improved.