Study On Performance Of Biological/Chemical Coupling Pretreatment Of Straws For Biogas Production By Anaerobic Dry Fermentation

With the mechanization and large-scale development of agriculture and animal husbandry in China,straw and livestock manure are increasing year by year.Aiming to alleviate environmental pollution caused by improper handling of straw and livestock manure,and efficient use of straw and livestock manure,this thesis used different methods to pretreat corn stalk and mix with cow dung for anaerobic dry fermentation to produce biogas.Firstly,the effects of raw material ratio,fermentation temperature and total solid concentration on biogas production performance were investigated by using batch anaerobic dry fermentation.Secondly,the pretreatment conditions of corn straw were optimized by chemical and biological methods.In the final stage of the study,two pretreatment methods and the coupling pretreatment in different order were used to investigate the effects of pretreatment methods on the performance of the batch anaerobic dry fermentation of corn straw mixed with cow dung.The results are as follows:（1）The effects of raw material ratio,fermentation temperature and total solid concentration on biogas production performance by batch anaerobic dry fermentation of corn straw mixed with cow dung were investigated by full factor experiment.It is optimal fermentation conditions at ratio of cow dung to corn stalk of 1:2,with the fermentation temperature of 50℃,and at the concentration of total solids of 20%.Under these conditions,the gas production rates of raw materials are 141 mL·（g TS）^-1 and 182 mL·（g VS）^-1,respectively.The volumetric biogas yield is 1.09 m³·m^-3·d^-1.（2）In the optimization test of pretreatment conditions by chemical method,the optimal pretreatment conditions were obtained by single factor test firstly.It is optimal pretreatment concentration at sulfuric acid of 2%,with pretreatment time of 30 min,at pretreatment temperature of 120℃.Secondly,the conditions obtained in the single factor test are further optimized by response surface method,and the optimal conditions are predicted as follows:the optimal pretreatment concentration of sulfuric acid of 2.7%,the optimal pretreatment time of38.4min,and the optimal pretreatment temperature of 122.2℃.The actual test results under these conditions are that the reducing sugar content,the cellulose content and the hemicellulose content are 22.45%,51.16%and 7.52%respectively,which is basically in line with the predicted values.（3）In the optimization test of biological pretreatment conditions,a group of optimal compound flora,namely NF-3（Bacillus siamensis KCTC 13613（T））and NF-4（Bacillus subtilis subsp.Stercoris NCIB3610（T））,were obtained through primary screening,re-screening,Antagonism test between strains,and CMC enzyme activity curve.The CMC enzyme activity of the compound flora reaches the highest at 21.1 U·mL^-1 on the 3rd day.Finally,the pretreatment conditions of the compound flora are optimized,and the optimal pretreatment condition is pretreatment with enzyme liquid with static condition at 2 days,which reduced the cellulose content to 22.0%.（4）The effects of chemical pretreatment,biological pretreatment and two ways of coupling pretreatment materials in different sequences on the performance of batch anaerobic dry fermentation for biogas production were investigated under the optimal fermentation conditions in Chapter 2 and optimal pretreatment conditions in Chapter 3.The results show that the pre-biological post-chemical method pretreatment of raw materials has the highest improvement in biogas production performance:raw gas production rate are 201.3 mL·（g TS）^-1and 259.6 mL·（g VS）^-1,respectively,which·increased by 38.8%,25.0%and 27.9%compared with the untreated group respectively.This study provides some reference value into the resource utilization of agricultural wastes and animal husbandry wastes.