Study On Influences Of Extractives On Hydrothermal Pretreatment Of Corn Stover

Ethanol production by enzymatic hydrolysis and the fermentation of abundant, low-cost, and renewable lignocellulose resources is considered as an efficient way to solve the energy crisis and environmental pollution. Cellulose, hemicelluloses, and lignin, which are the primary components of lignocellulose, are closely linked to each other through covalent and hydrogenic bonds that make their structure highly robust and resistant to enzymes. Utilisation of lignocellulose materials in their original form results in low cellulose digestibility. In order to obtain high cellulose digestibility, it is therefore necessary that the hemicelluloses and(or) lignin are removed with a pretreatment that makes the cellulose more accessible to cellulase. Hydrothermal pretreatment is regarded as a promising pretreatment method because it is environmentally friendly and does not requrie anti-corrosive equipment. However, from the industrial perspectives, energy consumption from hydrothermal pretreatment needs to be reduced. After hydrothermal pretreatment, cellulose digestibility and the concentration of ethanol during fermentation need to be further improved. Extractives are the main components of herbaceous lignocellulose(the content: 10%-25%). In this study, corn stover, which is an important agriculture waste product in China, was chosen as substrate. The study focuses on the effects of extractives on the process and effects of hydrothermal pretreatment and its mechanism. The object was to decrease the activation energy of hydrothermal pretreatment, to improve cellulose digestibility and ethanol concentration during fermentation, and to establish an improved hydrothermal pretreatment.First, the corn stover was pretreated with hydrothermal method. The pretreatment parameter was optimized, and structure characteristics of pretreated corn stover were studied. The cellulose digestibility of corn stover and extractivesfree corn stover after hydrothermal pretreatment were compared. Within the parameters of the experiment(pretreatment temperature: 180-220 °C, pretreatment time: 10-25 min), maximum cellulose digestibility(76.2%) was achieved at a hydrothermal pretreatment of 210 °C and duration of 20 min. The structural analysis results of the hydrothermally pretreated corn stover were as follows. The linkages between the main chain and the oligomers, comprised of arabinose, mannose, and galactose in side chains of hemicellulose, were easily cleaved. The acetyl group in the hemicellulose was hydrolysed as the pretreatment temperature increased during hydrothermal pretreatment. H+ cannot obviously degrade the crystalline structure of the cellulose. The formation of pseudo-lignin occurred during the hydrothermal pretreatment process. It was found that extractives could inhibit the cellulose digestibility of hydrothermally pretreated corn stover.Second, the effect of extractives on structure of pretreated corn stover and cellulose digestibility was studied. It was found that some substances in extractives could form pseudo-lignin on the surface of corn stover after hydrothermal pretreatment by comparing the surface morphology of hydrothermally pretreated corn stover and extractives-free corn stover. The sources of pseudo-lignin were glucose and some substances which are the same or similar to the degradation products of lignin. Results from molecular weight analyse, the solid-state 13 C NMR and infrared spectroscopy analysis indicated that pseudo-lignin formed from extractives consist of aromatic, aliphatic(alkane), and carboxylic structures. The pseudo-lignin could be formed by polycondensation and/or polymerization of free glucose, and the polymerisation of substances that are the same or similar to the degradation product of lignin. The pseudo-lignin could lower cellulose digestibility. The mechanisms include the space obstacle and the inhibition of cellulase activity. Cellulase could remove some of the pseudo-lignin during enzymatic hydrolysis, so the space obstacle gradually decreased as enzymatic time extended. After removing the pseudo-lignin and repolymerised lignin, the corn stover pretreated at 210 °C, 20 min reached 92.4% cellulose digestibility.Finally, the effect of extractives on the process of hydrothermal pretreatment and cellulose digestibility and fermentation was studied. It was found that extractives have a buffering capability and could inhibit H+ from catalysing hemicellulose hydrolysis during hydrothermal pretreatment. Based on the mechanism of the effects of the extractive effect on hydrothermal pretreatment, an improved method of removing substances, with a buffering capacity and ability to form pseudo-lignin in extractives, and hydrothermal pretreatment was constructed. The optimal process of removing these substances in extractives was as follows: tap water as solvent, atmospheric pressure, room temperature(20-30 °C), liquid-solid 20, 120 r/min, and 60 min. After removing these substances, the activation energy of the hemicellulose hydrolysis was reduced by 30.4%, from 59.6 k J/mol to 41.5 kJ/mol. After these substances were removed, the corn stover was pretreated by the hydrothermal method. The optimization of process variables(temperature and time) was addressed by the response surface methodology. When the optimisation processes(206.9 °C, 38.2 min) were performed, 89.1% of cellulose digestibility was obtained. After simultaneous saccharification and fermentation, a concentration of 42.1 g/L of ethanol was achieved, which is 30.7% higher than that of the corn stover pretreated at 210 °C for 20 min and higher than the ethanol concentration for distillation efficiency of 40 g/L.