Figure 21.2 shows a general schematic of the conversion of lignocellulosic biomass to bioethanol. The process consists of a pre-treatment step, a hydrolysis step and a fermentation step, followed by distillation and dehydration. In this process, lignin is discharged as a by-product and can be used to generate electricity to supply the process with energy or to export to the electricity grid.

Pre-treatment is necessary to break open the lignocellulosic structures and to facilitate the separation of the main carbohydrate fractions hemicellulose and cellulose from lignin, in order to make these better accessible for hydrolysis, the next step in the process (Mosier et al., 2005). Pre-treatment is considered by

many as the most costly step in lignocellulosic biomass conversion to ethanol. Pre-treatment may also significantly affect costs of subsequent steps in the process, including hydrolysis, fermentation as well as down-stream process steps (e. g. product separation). A variety of pre-treatment methods have been studied and some have been developed at pilot scale or demonstration scale. Current pre-treatment methods include: steam explosion, liquid hot water or dilute acid, lime, and ammonia pre-treatments (Maas, 2008). Pre-treatment methods using organic solvents such as ethanol or organic acids have been evaluated as well.

Hydrolysis is the process to convert the carbohydrate polymers cellulose and hemicellulose into fermentable sugars. Hydrolysis can be performed either chemically in a process involving the use of concentrated acids or enzymatically by using enzymes. Most pathways developed today are based on enzymatic hydrolysis by using cellulases and hemicellulases that are specifically developed for this purpose. Fermentation is the main process used to convert fermentable sugars, produced from the previous hydrolysis step, into ethanol. While in principal, the fermentation process is largely similar to that in the current ethanol production facilities, a major fraction of sugars produced from lignocellulosic are pentoses (5-carbon sugars such as xylose), which are difficult to ferment with standard industrial microorganisms. Therefore, a second important challenge in the conversion of lignocellulosic biomass to ethanol is the optimisation of ethanol-fermenting microorganisms that can convert all biomass-derived sugars, including xylose and arabinose. Furthermore, the efficient integration of various unit operations into one efficient facility is challenging. In some processes, the hydrolysis and fermentation steps are combined into one process which is often referred to as simultaneous saccharification and fermentation or SSF. Lignocellulosic biomass conversion to ethanol is currently in the pilot plant stage, with more than 30 pilot plants being operated or erected in both North America,
the EU and elsewhere (IEA Task 39, 2009). Furthermore, in the recent years two demonstration plants for lignocellulosic biomass conversion to ethanol were erected in Canada and in Spain. In addition, one demonstration plant for cellulosic ethanol was commissioned in Denmark, and further plants are in the planning phases. All three demonstration plants were designed to use wheat straw as primary feedstock.