Numerous studies for developing large-scale production of ethanol from lignocel — lulosic biomass have been carried out in the world. One of the advantages of the use of lignocellulosic biomass is that this feedstock is not directly related to food production, which would implement the extra production of bioethanol without the need of employing vast extensions of cultivable land for cane or corn produc­tion. In addition, lignocellulosics is a resource that can be processed in different ways for the production of many other products, such as synthesis gas, methanol, hydrogen, and electricity (Chum and Overend 2001). However, the main limiting factor is the higher degree of complexity inherent to the processing of this feed­stock. This complexity is related to the nature and composition of lignocellulosic biomass (see Chapter 3, Section 3.3.1). Two of the main biomass polymers need to be broken down into fermentable sugars in order to be converted into ethanol or other valuable products. But this degradation process is complicated, energy-con­suming, and not completely developed. Consequently, the involved technologies are more complex leading to higher ethanol production costs compared to cane, beet, or corn. However, the fact that many lignocellulosic materials are by-prod­ucts of agricultural activities, industrial residues, or domestic wastes offers huge possibilities for the production of fuel ethanol at a large scale as well as its global consumption as a renewable fuel. It is thought that lignocellulosic biomass will become the main feedstock for ethanol production in the near future (Cardona and Sanchez, 2007). According to Berg (2001), the output/input ratio of energy for the production of lignocellulosic ethanol reaches a value of 6, indicating a bet­ter energy efficiency than in the case of corn ethanol.

The classic configuration employed for converting lignocellulosic biomass into ethanol involves a sequential process in which the hydrolysis of cellulose and the fermentation are carried out in different units. As mentioned in Chapter 7, Section 7.1.4.1, the main feature of this configuration known as separate hydrolysis and fermentation (SHF) is that optimum conditions of pH and temperature for both processes can be ensured in an independent way. The general flowsheet of this technology is illustrated in Figure 7.6. Depending on the pretreatment method, the lignin can be recovered in this step or remain in the stillage from which it can be burned to generate steam. The solid fraction obtained in the pretreatment reactor is sent to the hydrolysis bioreactor where it comes in contact with microbial cel — lulases. This scheme involves the fermentation of the hemicellulose hydrolyzate contained in the liquid fraction exiting the pretreatment reactor using pentose — assimilating yeasts (like Candida shehatae or Pichia stipitis) in a parallel way to the glucose fermentation carried out with S. cerevisiae. In the alternative variant involving the simultaneous saccharification and fermentation (SSF), the hydrolysis and fermentation are performed in a single unit as discussed in Chapter 9, Section 9.2.2.3. The most employed microorganism for fermenting the hydrolyzates of the lignocellulosic biomass is S. cerevisiae, which ferments the hexoses contained in the hydrolyzate, but not the pentoses. This configuration is depicted in Figure 9.4.