Bioethanol production from steam-explosion pretreated straw

The complete bioethanol production process consists of at least five steps: Physical or chemical pretreatment of lignocellulosic material, enzymatic liquefication, enzymatic hydrolysis, bioethanol formation and downstream processing. In addition to this, ways of utilization for hemicelluloses and lignin have to be found to ensure an economic and cost effective process. Several pretreatment methods with diverse advantages and disadvantages have been developed and tested in the past. Physical pretreatment including milling, grinding, extrusion and expansion reduces the particle size and the cellulose cristallinity but consumes a lot of power. In physico-chemical pretreatment processes, the biomass is treated e.g. by hot water, steam or carbon dioxide at increased pressure. During an abrupt release of the pressure, some liquid evaporates immediately, thus, disintegrating the material and increasing the specific surface area. Lignin and some of the hemicelluloses are removed and cellulose becomes accessible for enzymatic hydrolysis. Chemical pretreatment e.g. by acid hydrolysis increases the internal surface area of cellulose because of swelling, decreases the degree of polymerization and results in the degradation of hemicellulose and lignin removal. Drawbacks are the formation of inhibitors affecting the fermentation, loss of sugars, corrosion problems and high cost of acid. In biological pretreatment e.g. with white soft fungi, lignin and hemicellulose are degraded. This process has a low energy demand but due to the low hydrolysis rate it is very time-consuming. The hydrolysis of hemicellulose and cellulose can be reached through enzymatic or acid hydrolysis. The pretreatment is crucial for this step. To obtain a maximum of C5 sugar (mainly xylose) und C6 sugar (mainly glucose) from hemicellulose and cellulose in the hydrolysis in the pretreatment, a highly digestible material has to be produced and at the same time no degradation of sugars should occur. The production of inhibitors like carboxylic acids, furan derivatives and phenolic compounds should also be minimized.