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木质纤维素生物质废弃物干法消化预处理与过程调控研究
张志凯
学位类型博士
导师许光文
2016-07
学位授予单位中国科学院研究生院
学位授予地点北京
学位专业化学工程
关键词木质纤维素 生物质废弃物 干法消化 沼气 过程调控
摘要

木质纤维素是最主要的一类生物质资源,富含纤维素、半纤维素,其碳、氢含量高,可转化为清洁的可燃气能源。我国木质纤维素生物质废弃物(Lignocellulosic biomass waste, LCBW)年产量大,农作物废弃物约7.29亿吨,可供能源化利用的林业剩余物和能源植物每年约3.5亿吨。但目前我国的LCBW主要以田间堆弃、直接焚烧等方式处理,资源化利用率不高,不仅浪费了其蕴藏的巨大生物质能,而且造成严重环境污染。厌氧消化可在温和的条件下将LCBW转化为清洁甲烷,副产消化残渣可用为农业肥料,尤其是干法消化,无需消耗大量水资源。在实现LCBW资源化利用的同时减少废弃物排放及其造成的环境污染,但目前国内尚未形成成熟的干法消化技术。本研究针对干法消化过程中存在的底物水解缓慢、消化停留时间长、传质困难等问题开展研究,通过调控干法消化关键技术参数而解决前述问题。选取高粱秆、杨树叶、青草为研究对象,首先对其进行不同方式预处理,找到适合干法消化的预处理方法,进而考察了初始固含量、接种率、共消化对干法消化的影响,以提高干法消化的降解效率和沼气产量并缩短消化停留时间,最后建立10 L干法消化装置,开展干法消化传质强化的研究。具体研究内容和主要结论如下:1、预处理研究。对LCBW分别进行水热预处理、碱预处理、酸预处理,考察了预处理前后底物的纤维素、半纤维素含量和结晶度等结构组成的变化,并对沼气产量较高的预处理方式进行经济性评价,从而得出适合干法消化的预处理工艺。结果表明:由于高粱杆、杨树叶、青草中的木质纤维素含量、以及结晶度等结构存在较大差异,其适宜的预处理方式不同。高粱杆的最佳预处理方式为4%的碱预处理,其累积沼气产量达535 mL/g VS;杨树叶的最佳预处理方式为4%的碱预处理,累积沼气产量为322 mL/g VS;青草的最佳预处理方式为180 oC水热预处理,累积沼气产量为320 mL/g VS。预处理后,底物的木质素含量及结晶度发生变化。对高结晶度底物,纤维素的晶型结构是影响干法消化效果的主要因素;而低结晶度底物,木质素是阻碍干法消化的主要因素。对包括预处理的干法消化过程进行经济性评价,表明:碱预处理的能耗最低并且经济效益较好,在工业化应用中具有可行性和优势,并具较好环境效应。2、干法消化影响因素研究。系统研究了不同初始固含量、接种率对杨树叶、高粱秆的干法消化沼气产量、pH值、COD、氨氮浓度、VFA以及产甲烷菌群落的影响。在保证干法消化效果的同时提高处理能力,最大化经济效益。初始固含量、接种率对高粱秆、杨树叶的干法消化过程影响类似。低初始固含量、高接种率能够缩短干法消化的启动时间,提高初始阶段沼气中的甲烷含量以及最终的累积沼气产量。干法消化稳定阶段及末期,初始固含量和接种率对体系中的微生物群落、甲烷含量影响不大。此外,在接种率低时,纤维素含量较高、结晶度较低的高粱秆在发酵过程中易积累大量乙酸,导致干法消化停滞,并且不容易从酸化状态恢复;而纤维素含量相对较低的杨树叶在出现酸化现象时,可通过调节pH值恢复产气。3、牛粪共消化研究。通过高粱秆、杨树叶、青草分别与牛粪混合进行共消化,得到了与牛粪混合的最佳碳氮比。高粱秆、杨树叶、青草单独消化时沼气产量分别为327 mL/g VS、312 mL/g VS、257 mL/g VS;当高粱秆、杨树叶、青草与牛粪混合的碳氮比分别为25、26、20时,累积沼气产量分别达到478 mL/g VS、431 mL/g VS、331 mL/g VS。由于高粱秆、杨树叶、青草中可生物降解组分含量不同,共消化的碳氮比存在较大差异。共消化的沼气产量受底物的碳元素含量、纤维素含量、木质素含量及结晶度的影响。与木质纤维素生物质废弃物单独进行的干法消化相比,加入牛粪实施共消化大大提高了底物的沼气产量,增强了干法消化过程系统的稳定性。比较牛粪、高粱秆共消化和利用尿素调节高粱秆碳氮比的干法消化,结果表明碳氮比仅在一定程度上解释了共消化对干法消化的促进作用,牛粪中丰富的微量元素可能是共消化取得较好发酵效果的重要原因。4、放大实验研究。综合基础研究结果,将反应器放大至10 L(内径200 mm,高400 mm),考察了机械搅拌传质和液体搅拌传质两种方式对厌氧消化的沼气产量、系统稳定性的影响。在直叶形双层搅拌桨反应器中(搅拌桨尺寸长140 mm×宽40 mm×厚度5 mm,搅拌轴直径为15 mm),搅拌桨端位最大搅拌线速度88 cm/s显著强化了传质,反应器的累积沼气产量、甲烷产量达到最高。对直叶形桨的双层搅拌进行CFD数值模拟表明,匀速转动过程中搅拌轴附近的剪切力最大。搅拌促进反应器内传质,从而提高水解阶段的反应速率。搅拌也影响污泥颗粒的状态进而影响产甲烷阶段。干法消化过程中,启动阶段需要降低搅拌强度;而在正常运行阶段可适当提高搅拌强度,以促进底物水解,提高干法消化速率。在渗滤液回流促进传质的反应器中,物料堆积高度与反应器直径比为3:2时得到了较高的沼气产量和甲烷产量。结合机械搅拌与液体搅拌可极大地促进反应器内传质,显著提高沼气产量。

其他摘要

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 avoid 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 thses 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 is hydrothermal method at 180 oC, 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 I/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. For stable biogas production period and the end of AD, the initial TS and I/S little affected methane content and methanogen community. At low I/S, AD of sorghum stem with high cellulose content and crystalinity was easy to become acidified due to the accumulation of acetic acid and its AD can hardly be 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 by co-digestion was studied. As the typical lignocellulosic biomass, sorghum stem, poplar leaves and grass were respectively co-digested with cow manure to obtain the performance 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 matters for 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 having its C/N 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 mm?40 mm ?5 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 slow 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. 

语种中文
文献类型学位论文
条目标识符http://ir.ipe.ac.cn/handle/122111/22913
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张志凯. 木质纤维素生物质废弃物干法消化预处理与过程调控研究[D]. 北京. 中国科学院研究生院,2016.
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