Optimization of pretreatment process for corn straw anaerobic digest

Abstract: Lignocellulose is an abundant renewable energy source in crop straws. Lignocellulose is mainly composed of cellulose, hemicellulose, lignin and ash. The cellulose is not only encapsulated by lignin and hemicellulose, but also has a very stable structure and is not easily degraded in the natural environment. Lignocellulose has a high degree of crystallinity and polymerization, if the crop straw is directly used for biogas production, there are some problems such as slow start, low efficiency, and low conversion and utilization may occur. Therefore, straw raw materials must be pretreated to destroy the crystallinity of cellulose and reduce the degree of polymerization, remove the covering effect of lignin and hemicellulose, increase the hydrolysis rate of cellulose and the gas production of biogas before the biogas fermentation. In order to improve the anaerobic digestion performance of corn straw and reduce the treatment cost, this study used alkaline treatment, acid treatment and biogas slurry treatment to pretreat corn straw, and studied the effects of different pretreatment methods on cellulose content and anaerobic digestion rate. The results showed that the degradation rate of lignin and hemicellulose in alkaline treatment were much higher than that in the acid treatment and the biogas slurry treatment, and the weight loss rate was lower, and the cellulose composition was well preserved. Because of the biogas yield and methane yield were the highest in all treatment groups, the alkali treatment with the best pretreatment effect was selected for the subsequent process optimization experiment. Based on Box-Behnken Design (BBD) test design, the quality percentage of alkali treatment, pretreatment temperature and pretreatment time were selected as the test factors. Combined with the results of single factor test, the response surface analysis method of three factors and three levels was designed, and the response surface methodology was used to establish the process mathematical model with the response value of biogas cumulative yield and reducing sugar yield as the response value. The results showed that the optimum pretreatment conditions of corn straw were: the solid-solid ratio was 22.4%, the pretreatment temperature was 37.9 ℃, and the pretreatment time was 39.7 h. Under this condition, the experimental value of reducing sugar yield was 738.6 mg/g, and the cumulative biogas production value was 661 mL/g. The difference between the test value and the predicted value was less than 0.5%, which proved the accuracy and feasibility of the response surface optimization method. Compared with other pretreatment methods, alkali pretreatment had advantages in improving the conversion rate of straw, increasing the gas production rate and reducing the cost. The pretreatment of straw biogas project should not only consider the increase of gas production rate, but also the subsequent treatment of biogas slurry. The alkaline pretreatment could neutralize various acidic products during the anaerobic fermentation process, and could also prevent the decrease of pH value during subsequent acidification. Alkaline pretreatment was a good additive to prevent acidification of fermentation broth. Alkali pretreatment is an ideal method for biogas fermentation of corn straw in the future.