Different combinations of the first three bioconversion steps have been investigated in order to reduce production costs, increase end-product yield and reduce time required for bioconversion. Sequential hydrolysis and fermentation provides the opportunity of optimizing each process separately, although it can result in the use of large amounts of enzymes such as b-glucosidase to overcome end-product inhibition during the hydrolysis making this a costly process (Blanch, 2012; Dashtban et al., 2009). Simultaneous saccharification and fermenta­tion (SSF) combines both steps into one reaction, which in theory allows direct fermentation of hydrolysates into bioethanol with a reduction in enzyme costs. However, involved both reactions and end-product yields can be compromised in SSF (Dashtban et al., 2009; Ong, 2004). Another method termed consolidated bioprocessing can be used to combine all three steps into one with the use of one or many microorganisms (Hasunuma et al., 2013; Matano et al., 2013; Amore and Faraco, 2012; Blanch, 2012; Hasunuma and Kondo, 2012; Girio et al., 2010; Dashtban et al., 2009). This particular process possesses the potential to reduce bioethanol production costs to competitive fuel levels. Although significant advances have been made with regard to CBP (Hansunuma et al., 2013; Hyeon et al., 2013; Matano et al., 2013; Olson et al., 2012), more research into the microbial cell fac­tories, enzymes and physicochemical and catalytic condi­tions (pH, temperature, and synergies) is required (Olson et al., 2012; Menon and Rao, 2012; Van Dyck and Pletschke, 2012).

However, key technologies are available to convert a variety of biomass into electricity, gas, or different liquid fuels (Table 2.3). These technologies use various types of feedstocks, and are produced in different ways (Farine et al., 2011).