【作者】 赵岩；

【导师】 王洪涛；

【作者基本信息】 清华大学 ， 环境科学与工程， 2009， 博士

【摘要】 秸秆是最具开发潜力的生物质能源之一,其物质组成中木质素的缠绕作用和纤维素的结晶结构,决定了其必须经过预处理和水解,才能为其制取乙醇铺平道路。利用超临界水溶剂化能力强等性质,可使秸秆中的纤维素与木质素分离并快速溶解和水解,但同时水解产物六碳糖迅速即分解为不能进行乙醇发酵的物质。而在亚临界条件下,六碳糖的分解速率明显下降,有利于六碳糖的积累。由此本论文采用超临界亚临界组合工艺,秸秆首先在超临界水中预处理和水解,使其中的纤维素水解为低聚糖,再经亚临界条件进一步水解为六碳糖,突破了木质纤维素单独超临界水解和单独亚临界水解的技术瓶颈,实现了木质纤维废物超临界亚临界组合预处理与水解资源化。论文研究表明,固液比、反应温度和时间是影响秸秆中主要组分纤维素超临界水解的关键因素,高于而接近临界点的温度和相对较短的时间有利于低聚糖的积累。在组合预处理与水解试验研究中,选定380℃、16s为纤维素水解的最佳超临界条件,水解产物再在280℃的亚临界条件下继续水解44s可获得最大的六碳糖产率,约为40%。论文揭示了亚临界条件下低聚糖水解与六碳糖分解的反应动力学规律,在280℃至360℃内低聚糖水解反应速率常数比六碳糖分解反应速率常数更大,这是六碳糖得以积累的根本原因。利用动力学研究结果,确定了任一适用温度下低聚糖水解和六碳糖分解反应速率常数,获得了六碳糖最大产率及相应最佳反应条件的理论计算方法,这一成果对秸秆等实际木质纤维废物组合预处理与水解过程的参数选择和产率预期具有理论指导意义。经过破碎的玉米秸秆与小麦秸秆等实际木质纤维废物通过超临界亚临界组合预处理与水解,其天然木质纤维结构能够得到有效破坏,并能够在超临界阶段发生溶解和水解生成低聚糖,进而在亚临界阶段继续水解并获得可发酵糖。本研究中,在384℃的超临界条件下玉米秸秆反应17s的水解产物与淋洗液的混合液,和小麦秸秆超临界条件下反应19s的水解产物,分别于280℃下反应27s和54s,可分别获得占秸秆可水解组分66.7%和30.7%的可发酵糖,成功实现了木质纤维废物超临界亚临界组合预处理与水解资源化。更多还原

【Abstract】 Ethanol production from lignocellulosic waste, one of the most important sources of bio-energy, is receiving much attention due to its feasibility and valuable products. Pretreatment and hydrolysis are the key processes of ethanol production from plant stalks because of the special structure of lignocellulose. Due to its high dissolving and catalyzing capacity, supercritical water can separate lignin from cellulose and rapidly hydrolyze the cellulose into water-soluble oligosaccharides and then to hexoses, primarily glucose. However, glucose decomposes rapidly into unfermentable products in supercritical water. In contrast, glucose decomposition rate is much lower in subcritical water. Therefore, a combined supercritical and subcritical process was applied in this study, in which lignocelluloses are pretreated and hydrolyzed in supercritical water to remove the lignin and to produce oligosaccharides from cellulose. This is followed by a second step using subcritical water that further converts the oligosaccharides into fermentable hexoses. This may prove to be a promissing strategy for utilizing lignocellulosic waste.The investigation of cellulose hydrolysis showed that solid/liquid ratio, temperature, and reaction time were the key parameters in the supercritical process. A temperature slightly above the critical point of water and a relatively short reaction time favored oligosaccharide production. In the combined supercritical and subcritical experiments, an approximately 40% yield of hexoses was obtained in the combined process under the conditions of 380°C, 16s (supercritical) and 280°C, 44s (subcritical). Kinetics of oligosaccharide hydrolysis and hexose decomposition under subcritical conditions revealed that the smaller reaction rate constants for hexoses decomposition compared to those for oligosaccharides hydrolysis allowed hexoses to accumulate. Based on the kinetic analysis, the theoretical value for the maximum yield of hexoses and the corresponding optimum reaction time can be both calculated, which can then be used to provide key parameters for the combined process of lignocellulosic waste conversion. Combined supercritical and subcritical experiments with either corn stalks or wheat straw as a starting maerial were proven to be efficient for lignin separation and cellulose hydrolysis. Cellulose in these plant materials could be converted into oligosaccharides in supercritical water and then to fermentable hexoses in subcritical water. In the combined processes, the mixture of the supercritical hydrolyzate from corn stalks (obtained at 384°C, 17s) and the water extraction containing soluble sugars was treated at 280°C for 27s to obtain a high yield of fermentable hexoses (66.7% of the available compositions). In wheat straw, 30.7% of the available components were converted into fermentable hexoses when a supercritcal hydrolyzate from wheat straw (obtained at 384°C, 19s) was further hydrolyzed at 280°C for 54s. This research demonstrated the successful pretreatment and hydrolysis of lignocellulosic waste using a combined supercritical and subcritical technology.更多还原