Abstract: 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.

Key words: hydrothermal pretreatment, lignocellulosic biomass, inhibitors, enzyme hydrolysis, bioethanol