Study On The Mechanism And Pretreatment Technology Of Rice Straw Pretreated With Naoh In Solid State For Enhancing Biogas Production

China is one of the largest agricultural countries in the world with0.7billion tons of stalks being generated annually. Most of stalks cannot be fully and economically utilized, so it arouses environmental pollution problems subsequently. Anaerobic digestion (AD) is one of the feasibility technologies to utilize stalks, which can convert them into biogas, a clean fuel for energy generation. However, the high percentage of lignocellulose in stalks limits them resistant to attack by anaerobic microorganisms, leading to lower digestion rate and biogas production, and thus prevent stalks from large-scale utilization. Pretreatment prior to AD has been proven to be one of simple and effective methods to improve biodegradability and biogas production of stalks. In our study, NaOH was applied as chemical reagent for rice straw pretreatment in solid state and at ambient temperature. Our studies have proven that NaOH pretreatment could improve biodegradability and enhance biogas production significantly. However, the mechanisms for such improvement are yet unclear. There have no correlative reports about this at home and abroad. The objective of this thesis was to (1) investigate the effect of NaOH pretreatment in solid state at ambient temperature on the biodegradability and biogas production of rice straw,(2) explore the reaction mechanisms of biogas yield enhancement of rice straw pretreated with NaOH, and (3) study the effect of NaOH and Na2S catalystic pretreatment on the biodegradability and biogas production of rice straw. The results could provide new ideas and theories for improving pretreatment methods and enhancing the biodegradability of rice straw.AD of untreated and6%NaOH-treated rice straw at four loading rates of35,50,65, and80g-L-1at mesophilic temperature was first studied to analyze the biogas capability. The results showed that, as compared to the untreated rice straw, the6%NaOH-treated one achieved64.5%,44.4%,27.3%, and42.9%more biogas production per gram VS digested (B/VS),9.2-28.5%more total solids (TSs) reduction, and14.8-29.7%more volatile solids (VSs) reduction at the four loading rates, respectively. The results proved that NaOH pretreatment was one of efficient approaches to enhance biogas production from rice straw.Second, this study was conducted to explore the mechanisms of biogas yield enhancement of rice straw. Fourier transform infrared spectroscopy (FTIR), hydrogen-1nuclear magnetic resonance spectroscopy (’HNMR), gas permeation chromatography (GPC), X-ray diffraction (XRD), and scanning electron microscopes (SEM) were used to investigate the changes of chemical compositions, chemical structures and physical characteristics of lignin, hemicellulose, and cellulose. The results are shown in the followings.â‘ After NaOH pretreatment, degradation of16.4%cellulose,36.8%hemicellulose, and28.4%lignin was observed. The cold-water, hot-water, and1%NaOH extractives of NaOH-treated rice straw were increased by124.0%,122.0%, and4.5%, respectively, while benzene-ethanol extractives were decreased by51.0%. The increased amount of the water extractives and the decreased amount of the benzene-ethanol extractives would generally help improve the biodegradability and contribute to the increase of biogas production from rice straw.â‘¡The internal structures of lignin and lignin-carbohydrate complex (LCC) were greatly altered after NaOH pretreatment, resulting in the separation of cellulose from lignin and the decomposition of lignin from large molecules with three-dimensional network structures to small molecules with linear structures. The changes made the compositions of the pretreated rice straw more easily to be digested by anaerobic microorganisms as compared to raw rice straw, leading to the improved biodegradability and the increased biogas yield of the pretreated rice straw.â‘¢Significant changes such as a partial disruption of β3-(1â†'4)-glycosidic linkages and the hydrogen bonds occurred in the morphology of the NaOH-treated cellulose fibers. The crystallinity and the microcrystalline size at002lattice plane of cellulose increased as well. The intramolecular and intermolecular degradation of hemicellulose occurred during the process of NaOH pretreatment. The removal of hemicellulose and the structural changes of cellulose caused extensive changes in the accessibility of cellulose that became more accessible and more open to swelling on contact with microorganisms, and led to the increase of biogas yield of rice straw.â‘£After AD, the degradation rates of cellulose, hemicellulose and total lignocellulose of the NaOH-treated rice straw increased more5.3%-44.0%than that of the untreated rice straw. The intramolecular and intermolecular changes of chemical structures which were caused by NaOH pretreatment made the infrared crystallization index (ICI) of NaOH-treated rice straw after AD smaller than that of untreated one after AD. Such changes made rice straw become more available and biodegradable and thus were responsible for the enhancement of biogas yield.Finally, according to the chemical reaction of NaOH with rice straw and the theory of catalysis, pretreatment of rice straw with6%NaOH and6%Na2S in solid state was studied. The untreated, NaOH and Na2S-cotreated rice straw were then anaerobically digested at four loading rates of35,50,65, and80g·L-1, respectively. The biodegradability and the changes of the main compositions of the rice straws were analyzed. The results showed that NaOH and Na2S pretreatment could effectively improve the biodegradability of rice straw. In comparison with the untreated rice straw, the B/VS of NaOH and Na2S-cotreated one was increased by12.3%～19.9%, the TS reduction, and VS reduction was increased by0.2%-9.2,7.9%-19.3%at the loading rate of35,65,80g·L-1, respectively. As compared to the NaOH-treated rice straw, the NaOH and Na2S-cotreated rice straw achieved higher B/VS at the loading rate of80g·L-1, and the increase rate was7.5%. TS reduction and VS reduction of NaOH and Na2S-cotreated one were increased by26.8%and23.7%, respectively. As a whole, pretreatment of rice straw with NaOH and Na2S could improve more biodegradability of rice straw than that of pretreated with NaOH, but the optimal conditions needed to be optimized in the later experiments.Above all, not only NaOH pretreatment in solid state, but also NaOH catalystic pretreatment in solid state can effectively enhance the biodegradability and biogas yield of rice straw. The obvious changes of chemical compositions, chemical structures, and physical characteristics during the process of pretreatment made rice straw become more available and biodegradable and thus were responsible for the enhancement of biogas yield.