7.2.1 Lignocellulosic Biomass—Raw Material for Second Generation Biofuel

Lignocellulosic feedstocks consist of mainly cellulose, hemicellulose, and lignin and can be found in the cell walls of almost all plant-derived materials, such as wood and grass, agricultural residues, and municipal solid wastes. The relative composition of the lignocellulosic material, however, varies greatly, depending on source (Chandel and Singh 2011; Garrote et al. 1999; Mosier et al. 2005) and for an overview, the weight percentage of dry biomass of representative lignocellu — losic materials are listed (Table 7.1).

Cellulose (b-1-4-glucan), a linear polymer of glucose units, is the major component of the lignocellulose (accounting up to 50 % of the total plant dry weight), the most abundant form of biologically fixed carbon in the biosphere, and a primary target for biofuels that are metabolites from microbial conversions (as in bioethanol production). It is hence a material of high interest to utilize well, but also a very recalcitrant material, making its utilization difficult. A major challenge is still to manage to convert lignocellulose in high yields to fermentable sugars

(see also Sect. 7.3. Lignocellulosics requires pretreatment for degradation) and to follow this with an efficient process that reduces the oxygenated carbohydrates to fuel molecules (Chundawat et al. 2011). In the process to obtain fermentable sugars, microbial GHs are used as catalysts to obtain saccharification (hydrolysis) of different polysaccharides in the biomass (explained more in the sections below). The microbial GHs are catalysts designed to degrade complex carbohydrate polymers into mono or oligosaccharides, that allow uptake and metabolism by the microorganism selected as cell-factory for the conversion into the desired biofuel, even if the microorganism on its own is not capable to degrade the polymeric carbohydrate forms.

It has been predicted that based on available land, the energy potential of lignocellulosics worldwide allows an energy outtake of approximately 100 EJ/ annum (1 EJ = 1 x 1018 J, covering woody biomass, straw, and energy crops) (Parikka 2004), which is to be compared to the global energy demand (425 EJ in

2001) (Lewis and Nocera 2006) showing that approximately one-quarter of the current demand can be obtained, and thus additional resources are needed to cover a shift from fossil to renewable resources. A means to increase the possible overall energy outtake is to also turn to biomasses from marine environments.