Page 174 - Lignocellulosic Biomass to Liquid Biofuels

142 Lignocellulosic Biomass to Liquid Biofuels

Figure 4.4 Simplified diagram of lignocellulose hydrolysis showing synergism and
limiting factors. Cellulose is symbolized straight lines. (1) Product inhibition of BG
and CBH by glucose and cellobiose. (2) CBH hydrolyzing from the end of a cellulose
chain. (3 and 4) Hemicelluloses and lignin associated with or covering the microfibrils
prevent the cellulases from accessing the cellulose surface. (5) Enzymes (both cellu-
lases and hemicellulases) can be unspecifically adsorbed onto lignin particles or sur-
faces. (6) Denaturation or loss of enzyme activity due to mechanical shear,
proteolytic activity or low thermostability [88]. BG, β-Glucosidase; CBH,
cellobiohydrolase.

reactors [90]. The increase of the enzyme/substrate ratio is a possible solu-
tion to overcome this problem, though it is in contrast with the objective
to minimizing the operating costs. Alternative solutions to reduce inhibi-
tion phenomena are the addition of surfactants to change surface proper-
ties of cellulose. The economical balance of the process can be further on
improved by adopting suitable techniques to recover the enzymes, once
immobilized, through recycling mechanisms [91]. Unfortunately, all of
these techniques for recycling and reducing enzyme adsorption have been
so far only tested at laboratory scale. Furthermore, most of the studies do
not include the costs associated to the recycling of the enzymes and to the
reduction of the enzyme. The ability to scale-up the techniques, as well as
their robustness and feasibility, still need to be demonstrated [88].
Finally, in order to reduce operating costs and the enzyme dosage,
innovative methodologies are being developed to carry out the cellulose
hydrolysis and the subsequent fermentation in a single reactor. This type
of process is commonly called simultaneous saccharification and fermentation