Biochemical conversion route makes use of biological/chemical agents, like microorganisms and enzymes, to break down the complex structure of the ligno — cellulose into its base polymers and further degrading them into sugar monomers (mainly glucose and xylose) (Pandey 2009). These sugar monomers can be sub­jected to microbial fermentation to produce bioalcohols (ethanol and butanol). The feedstocks that can be deconstructed using bioagents are mainly agricultural and for­est residues; however, they may also include industrial and municipal solid wastes.

The biochemical route mainly consists of four basic components: (1) feedstock pulverization, (2) pretreatment, (3) enzymatic hydrolysis, and (4) fermentation (Fig. 3). The complete process also includes feedstock harvesting, handling, recovery, and transportation; fractionation of the polymers; lignin separation; and recovery of end products (IEA 2008). The energy yield of liquid biofuels could be in the range of 2.3-5.7 GJ/tonnes of feedstock, considering 20 GJ/dry tonne of lignocellulose. The maximum energy efficiency that can be achieved is 35 % in the laboratory conditions;

however, under industrial conditions, it is yet to be known (Sims et al. 2010). As stated in the section above, other processes could be integrated such as combustion of lignin or conversion of some carbohydrates into other products of high value.

The downstream-processing step generates substantial amount of CO2, waste­water, and solid waste-containing lignin, residual carbohydrates, proteins, and cell mass. This represents about 1/3rd of the initial raw material (dry-weight basis) and can generate substantial heat and electricity upon combustion, thereby improving the overall process efficiency. The biochemical route seems to be quite promis­ing owing to its low-temperature requirements, cogeneration of heat and electricity from lignin combustion, and lower GHG emissions. At the moment, it is difficult to realize the full potential of biochemical route due to lack of data on its perfor­mance at demonstration or commercial scale units.