One of the problems of using enzymes to degrade lignocellulose is the cost of the enzymes. Recent reduction in the cost of cellulose enzymes (Greer, 2005) has allowed lignocellulose to be considered as a viable alternative to sugar and starch as a substrate

for ethanol production. A number of countries have pilot plants in operation process­ing lignocellulose, although most are in the USA (Table 6.10) and larger commercial plants are being developed. Figure 6.13 shows the process developed by Iogen, a Canadian enzyme manufacturer, for the use of lignocellulose to produce ethanol. Wood chips or straw are treated with dilute acid steam explosion and the cellulose and hemi — cellulose hydrolysed with enzymes produced by Trichoderma sp. Initially the fermenta­tion used S. cerevisiae but later a recombinant Z. mobilis has been introduced which could ferment the pentoses produced from the hemicellulose. The lignin is separated and burnt in a combined heat and power unit to provide energy for the process.

Another process is that designed by the National Renewable Energy Laboratory (NREL) (Fig. 6.14). The biomass, maize stover, is pretreated with dilute acid followed by simultaneous saccharification and co-fermentation (SSCF) with a recombinant Z. mobilis capable of fermenting glucose, xylose and T. reesei. The cellulose hydroly­sis is carried out with T. reesei cellulases and the hemicellulase hydrolysate is detoxi­fied before adding to the fermenter.