Pretreatment And Process Control For Dry Anaerobic Digestion Of Lignocellulosic Biomass Waste

Lignocellulose is the major valuable biomass resource, and it is rich in cellulose and hemicellulose, has high contents of carbon and hydrogen, and can be converted into clean gas energy. A large amount of lignocellulosic biomass waste (LCBW) is generated in China every year, among which agricultural waste is about 725 million tons and the forest residues and energy crops are about 350 million tons. Nevertheless, the utilization rate of LCBW in China is very low and most of LCBW is discarded or directly combusted in field. This not only wastes biomass energy but also causes serious environmental pollution. Anaerobic digestion (AD) provides an efficient method to convert LCBW into biogas under mild conditions, while its residues can be used as fertilizer. Dry anaerobic digestion that has low demand for water resources not only reduces waste generation but also avoids environmental pollution. However, there is not mature dry anaerobic digestion technology in China. This type of digestion has slow hydrolysis rate and low mass transfer efficiency causing long digestion time, and many studies have been performed to solve these problems.In this thesis, sorghum stem, poplar leaves and grass were taken as the typical anaerobic digestion materials. Different pretreatment methods were tested to look for an appropriate one for dry anaerobic digestion. The effects of initial total solid content (TS), inoculum to substrate ratio (I/S) and co-digestion on dry anaerobic digestion were explored. All the experiments were conducted with the purpose to enhance degradation efficiency. increase biogas production and shorten retention time of dry digestion. At last, a 10 L reactor was designed and built to improve the mass transfer of dry anaerobic digestion.1. Pretreatment study. Hydrothermal, alkaline and acid pretreatments were performed for LCBW. The composition and structure of substrates such as cellulose and hemicellulose contents as well as crystallinity index were studied for the conditions with or without pretreatment. Economic assessment was carried out to evaluate the selected pretreatment method enabling the highest biogas yield and to identify its suitability for efficient dry anaerobic digestion. The realized effect of pretreatment was distinctive for sorghum stem, poplar leaves and grass due to the difference in their composition and structure such as lignocellulose content and crystallinity. The appropriate pretreatment method for sorghum stem was alkaline pretreatment at 4% ratio by giving a biogas yield of 535 mL/g VS. For poplar leaves, the highest biogas yield was 322 mL/g VS under alkaline pretreatment at 4% ratio. For grass, its suitable pretreatment was hydrothermal method at 180℃, and this gave a biogas yield of 320 mL/g VS. Pretreatment causes lignocellulosic waste to have great changes in lignin content and crystallinity. For substrate with high crystallinity the crystalline structure is the main factor affecting dry anaerobic digestion. In substrates with low crystallinity, their lignin is the main factor that hinders anaerobic digestion performance. When assessing the economic benefits of dry anaerobic digestion with pretreatment, the alkaline pretreatment was found to be feasible for industral application to allow low cost, good economic profits and big environmental benefits.2. Parametric investigation. The thesis systematically investigated the effect of initial total solid content (TS) and inoculum to substrate ratio (I/S) on biogas yield, pH value, COD, ammonia nitrogen concentration, VFA and methanogen community of substrate for the separate dry anaerobic digestion of sorghum stem and poplar leaves. Results showed that the effects of initial TS and 1/S on anaerobic digestion were similar for both sorghum stem and poplar leaves. Low initial TS and high I/S could shorten the start-up time of dry anaerobic digestion, while they also increased both the methane content in biogas at the start-up period and the final cumulative biogas yield at the end of digestion. In stable biogas production period and at the end of AD, the initial TS and I/S had little impact on methane content and methanogen community. At low I/S, AD of sorghum stem with high cellulose content and low crystalinity was easy to become acidified due to the accumulation of acetic acid and its AD can be hardly recovered from acidification, whereas the AD of poplar leaves which contains relatively low celllulose can return to normal by adjusting pH value of substrate.3. Co-digestion study. Co-digestion was conducted to enhance dry anaerobic digestion. As the typical lignocellulosic biomass, sorghum stem, poplar leaves and grass were respectively co-digested with cow manure to improve their performances under optimal carbon to nitrogen ratio (C/N) of lignocellulosic substrate mixed with cow manure. Separately digesting sorghum stem, poplar leaves and grass gave the cumulative biogas yield of 327 mL/g VS,312 mL/g VS and 257 mL/g VS, respectively. When they were mixed with cow manure to get the C/N ratio of 25,26 and 20, their biogas yield were 478 mL/g VS,431 mL/g VS and 331 mL/g VS, respectively. Due to different biodegradable matter contents in sorghum stem, poplar leaves and grass, their C/N ratios were not the same when they were separately co-digested with cow manure. In co-digestion, the biogas production was greatly affected by carbon content, lignocelllulose content and crystallinity. Comparing with the dry anaerobic digestion of each individual substrate, the co-digestion with cow manure greatly improved the biogas yield of lignocellulosic substrate and also stabilized the dry anaerobic digestion process. The co-digestion of sorghum stem and cow manure was further compared with the AD of which the C/N was adjusted by urea. It was shown that adjusting the C/N ratio facilitated dry anaerobic digestion, but it is not the dominant cause. The trace elements in cow manure might positively work on the co-digestion.4. Process study. On the basis of preceding research, the digester was scaled up to 10 L (diameter 200 mm and height 400 mm) to investigate the influence of mechanical agitation and liquid-promoted mass transfer on biogas yield and system stability of AD. In an agitating reactor with straight double paddle (paddle size 140 mmx40 mm x5 mm, axis diameter 15 mm), the digester produced high biogas yield and methane yield at the agitation speed of 88 cm/s, due to the sufficiently enhanced mass transfer in the digester. CFD simulation showed that the shear stress near the agitator axis is strongest in agitation at a constant rotation speed. Agitation could increase the hydrolysis rate by enhancing mass transfer inside the digester. Meantime, it remarkably influences sludge granules and thus the methanogenesis behavor. In the start-up period of dry anaerobic digestion, the digester should have a low agitation speed. In turn in the usual operation period, it was good to increase the agitation speed to enhance the hydrolysis stage and thus increase the digestion rate of AD. In the digester using leachate recirculation to promote mass transfer, good performance was obtained when the ratio of feed stock height to reactor diameter (H/D) was 3:2. The combination of mechanical agitation and liquid agitation could greatly enhance the mass transfer inside digester and consequently increase the biogas yield obviously.