摘要：
在能源问题日益紧张的时局下,寻求可再生清洁能源是亟待解决的关键问题.由农林废弃物转化获得新能源、新材料已经成为重要的发展趋势,其中生物乙醇作为环保、可持续的新型能源得到了广泛关注.预处理作为生物乙醇制备的第一个重要环节备受重视,传统化学预处理技术能量消耗大、对设备要求高、半纤维素降解严重且对环境造成污染,没有充分考虑半纤维素和木质素的高值化回收利用,单一化降解纤维素使得经济利用价值很低;生物预处理作为一种环境友好和低成本的预处理技术,也存在着转化效率低、作用周期长和碳水化合物损失严重的缺点.热水预处理通过条件参数优化可在尽量保留天然大分子原位结构的基础上,一定程度地破坏植物细胞壁的致密结构,且仅利用水作为反应试剂,具有无化学药剂使用、对环境友好、操作成本低等优点,其对生物质细胞壁的主要作用为使木素-碳水化合物复合体(LCC)连接键断裂并除去部分半纤维素,使木质素性质发生改变并进行再分配,且在一定程度上降低了纤维素的聚合度.热水预处理过程中生成的糠醛、5-羟甲基糠醛、甲酸、乙酸等产物,会对生物降解产生抑制作用,可以通过优化条件来控制其含量.酶解过程是指利用纤维素酶及其辅助酶将预处理后的纤维素降解为可发酵单糖,若直接将预处理后的产物进行发酵则需要较长时间且仅能获得极低浓度的乙醇.酶水解过程中由于半纤维素和木质素的保护作用,阻碍了纤维素酶与纤维素底物的接触,而预处理过程则会削弱或完全破坏这种阻碍作用,增大酶与纤维素的接触面积使酶解效率提高.提高预处理温度会使乙醇发酵得率提高,但是预处理温度过高会导致纤维素降解从而使乙醇得率降低.本文对热水预处理过程中纤维素、半纤维素、木质素物理化学性质的改变和处理过程中抑制物的转化生成进行总结,分析比较在不同预处理条件下生物质中各主要组分和降解产物不同程度的变化及其对后续酶水解、酵母发酵的影响.%With the energy problem increasingly serious,to seek renewable clean energy has become the key problem to be solved. Transformation of agricultural and forestry wastes into new materials,calorific value of energy,chemical materials are becoming an important new tendency. Bioethanol is attacking more and more attentions since it is a sustained development model of environmental protection type. Pretreatment is an initial step in ethanol preparation process and traditional chemical-pretreatment methods consume a lot of energy, require highly on equipment, seriously degrade hemicellulose and pollute environment seriously. Besides, the traditional pretreatment methods were not focused on hemicellulose and lignin for high value applications, facing the low-coefficient utilization of whole biomass and the financial upside of single product model. As one of the pretreatment methods,biological process is an eco-friendly and low-cost process, but it also has disadvantages of hydrothermal pretreatment tries to keep the structure of natural macromolecules,and break plant cells at a certain degree,in which water is the only reagent. It has such advantages as lower chemical dosage,environmental friendly,low operation costs and the major effects of hydrothermal pretreatment on biomass cell wall are breaking the bonds of lignin-carbohydrate connection ( LCC ) ,removing parts of hemicellulose, modifying lignin and reducing cellulose crystallinity. Inhibitors ( furfural,5-hydroxymethyl furfural,formic acid,acetic acid) are generated in the hydrothermal pretreatment process and restrain the biological degradation process. The quantity of inhibitors varies indifferent pretreatment conditions and can be controlled by changing the conditions. Enzyme hydrolysis is a bioprocess transforming cellulose into monosaccharide,more time and lower ethanol content could be obtained by direct fermentation of pretreated substrate. Hemicellulose and lignin hinder enzyme contacting with cellulose. When hemicellulose and lignin are broken by pretreatment,the contacting efficiency of enzyme and cellulose could be increased. The yield of ethanol increased with the increasing temperature,however,the yield decreased when the temperature was too high. The chemical and structural changes of cellulose,lignin and hemicelluloses are summarized,in this paper,as well as the generated inhibition in the hydrothermal process. The effects of various pretreatment conditions on enzyme hydrolysis and ethanol fermentation are also discussed in detail.
