Page 21 - Lignocellulosic Biomass to Liquid Biofuels

2 Lignocellulosic Biomass to Liquid Biofuels

LCB is usually categorized into three types of waste: biomass, virgin
biomass, and energy crops. Trees, bushes, and sand grasses are placed into
virgin biomass class, whereas agricultural residue, stover, and bagasse are
placed in waste biomass class. Energy crops are raw materials used for the
production of second-generation biofuels as they offer high biomass
productivity.
LCB has a long history as an energy source: for many centuries, wood
has been the most widely used raw material to burn fire. During the
Industrial Revolution, due to the increase in energy needs, wood was
progressively replaced by fossil fuels. However, from the middle of the
20th century, problems rose from pollution and the exhaustion of
fossil fuels has increased the demand of biomass for the production of
energy [2].
The first biofuels to be developed have been bioethanol, initially
obtained from starch and sugars, and biodiesel, obtained from fats and oils.
However, the diffusion of these products, so-called first-generation bio-
fuels, has been limited as they cause direct competition between biofuel
and food production.
More recently, second-generation biofuels were developed, based on
the conversion of LCB components to liquid fuels. Second-generation
biofuels allow the utilization of the entire plants, such as woody crops,
agricultural residues, or waste, as well as dedicated nonfood energy
crops grown on marginal land, thus allowing a dramatic increase of the
productivity.
The production of biofuels and energy from LCB is based on two
main routes. Biochemical processes are typically carried out with LCB
having C/N ratio lower than 30 and humidity at collection higher than
30%. These processes are based on chemical reactions carried out thanks
to the exploitation of enzymes, mushrooms, and microorganisms. An
alternative is offered by thermochemical processes, used when the LCB
available has C/N ratio higher than 30 and humidity content below 30%.
In the last years, novel biofuels have been produced from LCB, such as
bio-H 2 , butanol, dimethylfuran, and gamma-valerolactone [3,4].
Some technological barriers still arise in the production of biofuels
from LCB, but robust research is going on to overcome those obstacles.
One of such problems is that LCB has evolved to resist deprivation and to
deliberate hydrolytic stability and structural toughness to the cell walls of
the plants. This robustness is attributable to the cross-linking between the
polysaccharides and the lignin via ether and ester linkages [5].