Enhancing Biogas Production From Dry Digestion Of Lignocelluloses And Its Mechanism

Rice straw is one of main agricultural residues in country and smooth cordgrass is main salt-mash plant at the coast of China Sea. Rice straw and smooth cordgrass all belong to lignocelluloses. Recently open-field burning of rice straw and excessive spread of smooth cordgrass has lead to seriously environmental pollution, economical loss and ecological problem. Effective utilization of rice straw and smooth cordgrass has become a new problem. Dry digestion of rice straw or smooth cordgrass for biogas production may not only solve the pollution question, but also may bring economical and environmental benefits to local inhabitants. This helps to further control and lessen their negative impacts on environment.In this work, an investigation on changes of lignocellulosic composition and structure was performed during lime pretreatment and dry digestion of rice straw; changes of complex lignocelluloses structure was discussed by X-ray, FTIR and TG analysis. Result showed that lignocellulosic structure was main limited factor of low biogas yield. Enhancing measures for biogas production, including thermophilic conditions, simultaneous lime treatment and dry digestion process, and two-phase process was carried out. Main contents and results are as follows:In chapter2, structure changes and biogas production characteristics of rice straw were investigated during lime pretreatment and dry digestion. Low hydrolysis rate limits utilization of lignocelluloses for microorganism. Cumulative biogas yield reached278.1mL/g VS at60days dry digestion of rice straw, and the relative content of cellulose and hemicellulose decreased23.4%and13.1%, respectively; and whereas lignin content relative increased59%. Lime pretreatment resulted in the lignin and polysaccharides removal by the cleavage of ester linkage and rearrangement of hydrogen bonding in the crystalline region. The decomposition of polysaccharides was carried out by the cleavage of β,1-4glycosidic bonds linkages during dry digestion. Biogas conversation rate was only39.9%and measures need be adopted for the improvement of biogas production for dry digestion of lignocelluloses. Pretreatment and dry digestion had obvious influence on pyrolysis characteristics of rice straw. The part of main weight loss for pyrolysis rice straw very fit first-order kinetic model, and the value of activate energy ranged from42.27to47.82KJ/mol.In chapter3, enhancing biogas production was discussed by thermophilic conditions. Thermophilic conditions enhanced biogas production and lignocellulosic bioconversion. During60days dry digestion, biogas yield reached283.9mL/g VS and181.2mL/g VS at55℃and35℃, respectively. Compared with32.2%biogas conversion rate and57.3%VS reduction at mesophilic condition (35℃), thermophilic condition (55℃) obtained50.4%biogas conversion rate and69%VS reduction.In chapter3, enhancing biogas production was also discussed by simultaneous lime treatment and dry digestion (SLTTD) process. SLTTD process was achieved in one reactor with highest initial biogas production constant (k=0.023d-1) and VS removal rate (71.9%). Biogas yield with192.7mL/g VS was obtained at the beginning of40days during60days dry digestion, but inhibition of anaerobic microorganisms occurred at SLTTD process during dry digestion. Sodium, potassium and calcium ions were not the cause of inhibition for SLTTD process.In chapter4, the breakage of lignocellusic structure was investigated at mesophilic and thermophilic conditions. Thermophilic conditions brought a greater rupture of lignocellulosic structure. Highest breakage of lignocelluloses and higher composition remove was carried out in SLTTD experiment. Lignin removal occurred at lime pretreatment phase by the cleavage of ester linkage and the reducing carboxyl, aryl and phenolic C functional group. Polysaccharides decomposition happened at dry digestion by the cleavage of β,1-4glycosidic bonds linkages, and the reduction of carbohydrate and Di-O-alkl C functional group. The part of main weight loss very fit a first-order kinetic model for smooth cordgrass pyrolysis. The increase in activate energy appeared at pretreatment phase, and whereas the decrease occurred at dry digestion phase.In chapter5, Two-phase digestion of smooth cordgrass was investigated for improving hydrolysis rate of lignocelluloses. Free ammonia (FA) inhibition occurred at two-phase process, when the C/N rate of digester was adjusted by adding urea. Adding chemical for adjusting C/N rate should be very cautious and co-digestion with organic wastes containing high nitrogen would be better choice for adjusting C/N rate. When FA concentration was over55mg/L, biogas production rate obviously decreased or stopped. Biogas yield was only98.6mL/g VS with17.5%bioconversion rate during53days two-phase digestion of smooth cordgrass. Hydrolysis of smooth cordgrass was a limited step during anaerobic digestion and lignocellulose pretreatment need be a great scale improved for higher biogas production.