| Under the background of increasing depletion of fossil energy and environmental contamination worsening, development of renewable energy to cope with current energy and environmental degradation has become problems common concerned and to be solved in all countries in the world today. Anaerobic digestion from biomass has been attracting extensive attention of researchers as an environmentally friendly process that does not consume fossil fuels. And a lot of research results have been made. However, the utilization of lignocellulosic biomass for biogas production via anaerobic digestion has not been widely adopted because the complicated structure of the plant cell wall makes it resistant to microbial attack. Pretreatment of recalcitrant lignocellulosic biomass is essential to achieve high biogas yield in the anaerobic digestion process. In our study, soaking in aqueous ammonia (SAA) was investigated as a pretreatment method for biomass. The objective of this thesis was to investigate the effect of soaking in aqueous ammonia pretreatment in single stage process and multi stage process on the biodegradability and biogas production of corn stover, and explore the reaction mechanisms of delignification improvement of corn stover pretreated with aqueous ammonia. The results could provide new ideas and theories for improving pretreatment methods and enhancing the biodegradability of corn stover.Soaking in aqueous ammonia was investigated as a method of pretreatment for enzymatic as well as anaerobic digestion of corn stover. Batch experiment were carried out to determine the effect of aqueous ammonia concentration, reaction temperature, reaction time, ratio of solid to liquid on the composition of corn stover. The results show that the treated corn stover retained 80% cellulose and 70% of hemicellulose, but removed of 73%. The process conditions of soaking in aqueous ammonia pretreatment were optimized by response surface methodology. The optimum treatment conditions were found to be 15 wt.% of NH3,50℃,1:9 of solid-to-liquid ratio, and 29 h of treatment time. Under this condition, the enzymatic digestibility of the corn stover increased to 93.6%, which was 28.8% more than the before optimization.The delignification kinetic model based three first-order reactions was applied to describe the kinetic behavior of lignin removal for corn stover within SAA pretreatment. The activation energies for delignification reactions were estimated as 61.05 and 59.46 kJ/mol in the bulk and residual phases, respectively. In the model, compared the kinetic constant under different ammonia concentration, the residual rate of delignification is 100 times faster than the bulk rate. At low ammonia concentration, residual phase delignification dominated, whereas the delignification rate in the residual phase decreased with increase of ammonia concentration.This study was conducted to explore the mechanisms of delignification rate and enzymatic digestibility enhancement of corn stover. Fourier transform infrared spectroscopy (FTIR), Heteronuclear-single-quantum-cohercnce nuclear mangnetic resonance spectroscopy (HSQC NMR), and gas permeation chromatograph (GPC) were used to investigate the changes of chemical composition, chemical structures and physical characteristics of lignin. The results showed that this changes made the compositions of the prctrcatcd corn stover more large delignification rate, and more easily to be digested by anaerobic microorganisms as compared to corn stover, leading to the improved biodcgradability and enzymatic digestibility, and then the increased biogas yield of the prctrcatcd corn stover.The influences of different factors on volatile fatty acid (VFA) production during the process of anaerobic hydrolysis and acidogencsis of prctrcatcd corn stover were studied. The effects of organic loading rate, concentration of vaccinal sludge and pH have been determined in batch experiments. The change of VFA concentration, VFA composition, SCOD concentration and Volatile solids (VS) removal rate were investigated to find out the best acid process condition. The results showed that the optimum acidogencsis conditions were found to be 50 g/L organic loading rate,30% of inoculation quantity, and at pH 6.0. Under this conditions, VFA concentration, proportion of acetic acid, total solids (TS) and volatile solids removal rate were 4.86 g/L,72.2%,23.2% and 26.5%, respectively.The single-phase and two-phase anaerobic digestions were conducted with untreated and pretreated corn stover. In single-phase anaerobic digestion process, biogas productivity of pretreated corn stover was always higher than that of untreated one. The final biogas volume of pretreated corn stover reached 25409 mL, which was increased by 52% than untreated corn stover. Compared to the untreated corn stover, the final biogas productivity per gram VS and TS of pretreated corn stover reached 290.38 ml/g and 315.21 ml/g, which was increased by 53.6% and 42.3% than untreated corn stover, respectively. And the total solids reduction and volatile solids reduction were increased by 17.9%å'Œ14.8% than untreated corn stover, respectively. The results proved that soaking in aqueous ammonia pretreatment was one of efficient approaches to enhance biogas production from corn stover.From the cumulative biogas volume, there was little obvious difference between two phase anaerobic digestion and single-phase anaerobic digestion. However, due to the higher acetic acid content and stability of pH obtained from the acidification of system, the methane phase pH could be stable at 7.20, which would provide more stable ecological environment for microorganisms. This condition could make biogas methane content more high, which of average content reached 61%. Therefore, two phase system has the advantages of high stability and shorter hydraulic retention time.Above all, soaking in aqueous ammonia pretreatment can effectively enhance the biodegradability and biogas yield of corn stover. The obvious changes of chemical compositions, chemical structures, and physical characteristics during the process of pretreatment provided a theoretical basis for the lignin removal process and made corn stover become more available and biodegradable and thus were responsible for enhancement of biogas yield. These research results can lay a foundation for promoting the early industrialization of biogas production process, and it is of great scientific and practical importance. |
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