| This study developed a new method using alkali to pretreat wheat straw to obviously improve the biodegradability of wheat straw, then use the pretreated wheat straw to produce biogas. Both experimental and mechanism studies were conducted and a demonstration project was also built.1. Effects of different alkali pretreatment agents on the performances of anaerobic digestion of wheat straw. Four alkalis, which were Potassium hydroxide (KOH), Sodium hydroxide (NaOH), Calcium hydroxide (Ca(OH)2), and Urea (CO(NH2)3) in four addition amounts of 1%,2%,4%,6%, respectively, were used as chemical agents for pretreatment. Four organic loading rates (OLR) of 35,50,65 and 80 g·L-1 were used for anaerobic digestion tests for the pretreated wheat straw. Based on their effects on the performances of anaerobic digestion of wheat straw, NaOH was finally selected as optimal agent at the OLR of 65 g-L-1.2. Pretreatment parameters investigation and optimization. Pretreatment parameters for NaOH agent were further investigated. The parameters included NaOH addition amount, temperature, time, moisture content, straw size, and inocula. The results showed that:(1) Compared to untreated wheat straw, all pretreated ones obtained better performances.2% and 4% NaOH amounts achieved the highest biogas yields, increased to 44.0%, 48.0%, respectively, but the differences were not significant. Hence,2% addition amount was recommended. (2) There was no obvious difference in biogas yields, TS/VS reductions and average methane content at the temperatures of 10℃and 30℃, but the digestion time (dT80) of 10℃were 4 days shorter than 30℃, and 10℃is more easily achieved in practical conditions and therefore is chosed. (3) pH values were observed after NaOH were added to wheat straw when pretreated, it was found that pH values were decreased to almost neutural in three days. The wheat straws pretreated by 3 days and 6 days were anaerobically digested. The results showed that there was no obvious difference in the performances of anaerobic digestion. Therefore,3-day pretreatment time was determined. (4) Compared to 600% moisture content, the moisture content of 900% could increase biogas yield and TS/VS reductions by 6.1%,5.1%/4.9%; shorten digestion time by 2 days; implying that this moisture content was favorable to the destruction and degradation of wheat straw. (5) The biogas yield and TS and VS reductions were increased as particle size reduced, but the relationship was not linear. The milled straw got the highest biogas yield, but needed to consume more electric energy than grinding and kneading. Although the biogas yield of the kneaded straw was lower than milled and ground ones, the difference was acceptable, thus, kneading was considered to be more cost effective and more practical. (6) Compared to the digestates from swine manure and municipal actived sludge, the inoculum from stalk digester achieved the best anaerobic digestion performance. Another adavantage is the easier availability and lower cost. Hence, the digestate from stalk digester is recommended for the inoculation of wheat straw digestion.Base on the results and findings in this section, the optimal pretreatment parameters were determined as follows:2% NaOH addition amount,10℃pretreatment temperature,3-day pretreatment time,65g·L-1 organic loading rate, kneading, and use the digestate from stalk digester as inoculum.3. Investigation on the mechanisms of NaOH pretreatment of wheat straw. FTIR, SEM, and XRD were used to explore the mechanisms of NaOH pretreatment. It was found after pretreatment that:(1) The micro-physical structures were changed greatly, The surface of wheat straw was damaged, and the fiber structure was loosened remarkedly. The striping phenonium was also observed. (2) Some bondings inside cellulose were broken or damaged. It was also observed that ester bond between lignin and carbonhydrate was broken, making more cellulose exposed and become available to micro-organisms. (3) The contents of cellulose, hemicellulose, lignin, and lignicellulose were decresed from 30.6%,28.1%,12.4%, and 71.1% to 25.6%,21.9%,10.8%, and 58.3%, respectively; The percentages of cold-water, hot-water, and 1%NaOH extractives were increased by 92.3%,83.1%, and 20.0%; respectively, while the percentage of beneze-ethanol extractive decreased by 37.8%; The total sugar content was increased by 106.7%. It can be seen that the improvement on biodegradability and biogas yield of wheat straw after pretreatment were not resulted in by one reason but should be contributed to the comprehensive roles of the changes of micro-physical structure, chemical structure, and chemical compositions.4. Engineering application. The pretreatment method developed in laboratory was applied in a practical project in Dezhou city of Shandong province. The project was built through incorperating NaOH pretreatment method with new type of anaerobic digester with green house heating system. The biogas yield, temperature, methane content ect. were monitered and tested. The results showed that the technology was applicable and reliable in large-scale biogas plant of wheat straw.The NaOH pretreatment technology developed in this study solves the key technic problem with the anaerobic digestion of wheat straw, providing one practical way for the clean bioenergy production from wheat straw. It is a new model for clean energy production and pollution mitigation with wheat straw.
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