7.1.4.1 Fermentation of Cellulose Hydrolyzates

The classic configuration employed for fermenting biomass hydrolyzates involves a sequential process where the hydrolysis of cellulose and the fermentation are carried out in different units (Sanchez and Cardona, 2008). This configuration is known as separate hydrolysis and fermentation (SHF). When this sequential process is employed, the solid fraction of pretreated lignocellulosic material undergoes hydrolysis (saccharification). This fraction contains the cellulose in a form accessible to acids or enzymes. Once hydrolysis is completed, the resulting cellulose hydrolyzate is fermented and converted into ethanol. S. cerevisiae is the most employed microorganism for fermenting the hydrolyzates of lignocellulosic biomass. This yeast ferments the hexoses contained in the hydrolyzate, but not the pentoses. One of the main features of the SHF process is that each step can be per­formed at its optimal operating conditions (especially temperature and pH). The overall scheme of this process is presented in Figure 7.6. Depending on the type of biomass pretreatment, the lignin can be separated in this step (as in the case of the organosolv process [see Chapter 4]) or remain in the stillage. This scheme involves the fermentation of hemicellulose hydrolyzate by pentose-assimilating yeasts in a way parallel to the fermentation of glucose using S. cerevisiae.

BC International Corporation (Dedham, MA, USA) has operated a pilot plant in Louisiana for producing ethanol from biomass by SHF. This plant has the

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capacity to process about 500 ton/year of lignocellulosic materials. The University of Florida has granted the license to this company for worldwide commercializa­tion of this technology, including the patent of the microorganism employed in the process, a recombinant strain of Escherichia coli to which Zymomonas mobilis genes, ensuring the biosynthesis of ethanol, has been introduced (Ingram et al., 1991). The company has been seeking investors for a $90 million project for an ethanol production facility using cane bagasse with a capacity of 23.2 mill gal EtOH/year, but the construction has not yet started. In fact, SHF is the technol­ogy with the most possibilities of being implemented at a commercial scale. For instance, the company Abengoa Bioenergia (Spain) is constructing a demonstra­tion facility for ethanol production from lignocellulosic biomass with a capacity of 5 million L/year (Abengoa, 2008). The conversion of wheat and barley straw into ethanol will be done by an SHF scheme, as that shown in Figure 7.7, although this company plans to implement the pentose fermentation step at the midterm.

Some attempts to produce ethanol from municipal solid waste (MSW) have been done considering the cellulosic fraction of this type of materials (Sanchez and Cardona, 2008). Park et al. (2001) have studied the hydrolysis of waste paper contained in MSW obtaining significant sugars yield and evaluating the viscosity as an operating parameter. Bioethanol production from the cellulosic portion of

MSW has been already patented (Titmas, 1999) and some strategies for improv­ing the fermentability of acid hydrolyzates of MSW have been defined. Nguyen et al. (1999) employed a mixed solids waste (construction lumber waste, almond tree prunings, wheat straw, office waste paper, and newsprint) for producing ethanol by SHF using yeasts. In this process, the recycling of enzymes was implemented through microfiltration and ultrafiltration achieving 90% cellulose hydrolysis using a net enzyme loading of 10 filter paper units (FPU)/g cellulose.

S. cerevisiae has demonstrated its elevated resistance to the presence of inhibi­tors in the lignocellulosic hydrolyzate. In the case of the more productive continu­ous regime, one way to enhance this resistance is the increase in the cell retention to prevent washout and maintain high yeast cell density. Brandberg et al. (2005) employed a microfiltration unit to recirculate the cells under microaerobic condi­tions achieving sugar conversion up to 99% for undetoxified dilute-acid pretreated hydrolyzates of softwood (spruce) supplemented with mineral nutrients, although the productivity was low.