The yeast Saccharomyces cerevisiae and the bacterium Zymomonas mobilis are not able to directly utilize the cellulose for ethanol production. In general, these microorganisms have the highest probability to be used in an industrial process for conversion of lignocellulosic biomass into fuel ethanol. For this reason, a step for cellulose hydrolysis (saccharification) to obtain a fermentable solution of glucose is required in an analogous way to the starch saccharification. As in the case of starch, the cellulose can be hydrolyzed with the help of acids either dilute or concentrated. If dilute acids are used, temperatures of 200 to 240°C at 1.5% acid concentrations are needed in order to hydrolyze the crystalline cellulose, but the degradation of glucose into hydroxymethylfurfural (HMF) and other nonde­sired products is unavoidable under these conditions. In a similar way, xylose is degraded into furfural and other compounds (Wyman, 1994). H2SO4 and HCl have been historically used for these purposes (Jones and Semrau, 1984; Song and Lee, 1984). As mentioned in Chapter 4, the pretreatment and cellulose hydrolysis of lignocellulosic biomass can be carried out in two stages using dilute acids. Thus, a first stage under mild conditions (190°C, 0.7% acid, 3 min) is carried out to recover pentoses, while the remaining solids undergo harsher conditions (215°C, 0.4% acid, 3 min) to recover hexoses from cellulose hydrolysis in the second stage. In this way, 50% glucose yield is obtained (Hamelinck et al., 2005). One variant of the acid hydrolysis is the employment of extremely low acid and high tempera­ture conditions during batch processes (autohydrolysis approach) that have been applied to sawdust (Ojumu and Ogunkunle, 2005; Sanchez and Cardona, 2008). The degradation of cellulose with near-critical water has been proposed, although the obtained glucose yield is low (40%) and the fermentability of resulting hydro — lyzate is limited due to the formation of unknown inhibitors (Sakaki et al., 1996).

The hydrolysis of cellulose with concentrated acids allows achieving glucose yields near 90%, but in this case, the recovery of the acid is a key factor in the process economy (Hamelinck et al., 2005). Several configurations for separation of formed glucose and recovery of employed acid have been proposed. Among the early procedures suggested, the lime addition, ionic exclusion columns based on commercial resins (Neuman et al., 1987), or electrodialysis (Baltz et al., 1982) can be highlighted. Concentrated acid processes using 30 to 70% H2SO4 have a higher glucose yield (90%) and are relatively fast (10 to 12 h), although the amount of acid used is a critical economic factor. By continuous ionic exchange, it is pos­sible to recover over 97% of the acid (Hamelinck et al., 2005). In general, the acid hydrolysis of cellulose implies high energy costs and the construction of reactors resistant to the corrosion, which increase the capital costs.