The biogas yield of rice straw during anaerobic digestion can be substantially increased through solid-state sodium hydroxide (NaOH) pretreatment. This study was conducted to explore the mechanisms of biogas yield enhancement. The chemical compositions of the pretreated rice straw were first analyzed. Fourier transform infrared (FTIR), hydrogen-1 nuclear magnetic resonance spectroscopy ( 1 H NMR), X-ray diffraction (XRD), and gas permeation chromatography (GPC) were then used to investigate the changes of chemical structures and physical characteristics of lignin, hemicellulose, and cellulose. The results showed that the biogas yield of 6% NaOH-treated rice straw was increased by 27.3-64.5%. The enhancement of the biogas yield was attributed to the improvement of biodegradability of the rice straw through NaOH pretreatment. Degradation of 16.4% cellulose, 36.8% hemicellulose, and 28.4% lignin was observed, while water-soluble substances were increased by 122.5%. The ester bond of lignin-carbohydrate complexes (LCCs) was destroyed through the hydrolysis reaction, releasing more cellulose for biogas production. The linkages of interunits and the functional groups of lignin, cellulose, and hemicellulose were either broken down or destroyed, leading to significant changes of chemical structures. The original lignin with a large molecular weight and three-dimensional network structure became one with a small molecular weight and linear structure after NaOH pretreatment. The cellulosic crystal style was not obviously changed, but the crystallinity of cellulose increased. The changes of chemical compositions, chemical structures, and physical characteristics made rice straw become more available and biodegradable and thus were responsible for the enhancement of the biogas yield.

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As a lignocellulose-based substrate for anaerobic digestion, rice straw is characterized by low density, high water absorbability, and poor fluidity. Its mixing performances in digestion are completely different from traditional substrates such as animal manures. Computational fluid dynamics (CFD) simulation was employed to investigate mixing performances and determine suitable stirring parameters for efficient biogas production from rice straw. The results from CFD simulation were applied in the anaerobic digestion tests to further investigate their reliability. The results indicated that the mixing performances could be improved by triple impellers with pitched blade, and complete mixing was easily achieved at the stirring rate of 80 rpm, as compared to 20-60 rpm. However, mixing could not be significantly improved when the stirring rate was further increased from 80 to 160 rpm. The simulation results agreed well with the experimental results. The determined mixing parameters could achieve the highest biogas yield of 370 mL (g TS)(-1) (729 mL (g TS(digested))(-1)) and 431 mL (g TS)(-1) (632 mL (g TS(digested))(-1)) with the shortest technical digestion time (T 80) of 46 days. The results obtained in this work could provide useful guides for the design and operation of biogas plants using rice straw as substrates.

This study was conducted to investigate the changes of main compositions and extractives and their effects on biogas yield enhancement. Four NaOH doses (4%, 6%, 8%, and 10%) and four loading rates (35, 50, 65, and 80 g/L) were used. The rice straw was first pretreated by NaOH in solid-state conditions and anaerobically digested. The main compositions and extractives were then analyzed. The results showed that, compared to the untreated rice straw, 3.2%−58.1% more biogas yields were obtained with 4%−10% NaOH-treated rice straws. Hemicellulose, cellulose, and lignin were decomposed by 35.2%−54.2%, 14.2%−16.4%, and 8.0%−44.5%, respectively, for 4%, 6%, 8%, and 10% NaOH-treated rice straws. Considerable fractions of them were converted to relatively readily biodegradable substances, as indicated by increases of 80.3%−173.6% cold-water extractives and 80.4%−152.8% hot-water extractives. Some irresistible substances were removed, as represented by a 30.9%−51.8% decrease of benzene−ethanol extractives. The chemical structures of hot-water and benzene−ethanol extractives were also changed obviously. It was also found that the soluble sugar contents in the 6% NaOH-treated rice straw were twice that of the untreated one. The results specified that NaOH pretreatment was one of efficient methods to enhance biogas production of rice straw, and the changes of main compositions and extractives made important contributions to the enhancement.

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