Page 58 - Advances in Eco-Fuels for a Sustainable Environment

Ecofuel feedstocks and their prospects 35

Cellulose, their main constituent, is a b-D-glucopyranose polymer; hemi-cellulose, the
second component, is like cellulose, though a less complex and easily hydrolyzed
polysaccharide. In the feedstock, soluble materials or extractives consist of those com-
ponents that are soluble in neutral organic solvents. Cellulose sugars are mostly glu-
cose. However, hemi-cellulose is made up of a mixture of different types of sugars: C6
(glucose, mannose, and galactose) and C5 (xylose, arabinose, and rhamnose) sugars.
Lignin acts as a cementing material, binding together all the other constituents and
providing the structural rigidity to cellulosic feedstock. Chemical composition of lig-
nocellulosic materials is a key factor for the efficiency of biofuel production. Structure
and chemical composition of lignocellulosic materials are highly variable because of
genetic and environmental influences: their typical composition is 48% C (weight),
6% H, and 45% O, with inorganic matter being a minor component. Cellulose and
hemicellulose contents are greater in hardwoods (78.8%) than softwoods (70.3%),
but lignin is more in softwoods (29.2%) than hardwoods (21.7%). Conversion of lig-
nocellulosic materials into ethanol can occur via different pathways: the sugar plat-
form employs enzymes to convert pretreated lignocellulosic biomass materials into
sugars, which are then fermented into ethanol; the syngas platform gasifies feedstock
to produce syngas, which is then converted into ethanol by a catalyzed chemical reac-
tion or a biological process [39].
Conversion of lignocellulosic materials into ethanol is much more difficult than
that of sugar- or starch-rich materials. The sugar platform with biochemical conver-
sion consists of three main process steps: pretreatment, enzymatic hydrolysis, and fer-
mentation. Pretreatment consists of milling the biomass to achieve small and
homogeneous particles, polymer fractionation, separation of the solid lignin compo-
nent, and end product recovery. Detoxification and fermentation of pentoses released
during pretreatment can also be carried out. Cellulose undergoes enzymatic hydrolysis
to produce hexoses, such as glucose. Pretreatment options can be biological, physical,
chemical, or a combination thereof, where temperatures and reaction times are the var-
iables, and is a major cost component of the overall process. Pretreatment with dilute
acid and intermediate temperatures is generally considered the most cost effective and
acts by loosening the cell wall matrix through degradation of hemicelluloses. Lignin is
unaffected by this process. Accessibility to cellulose microfibrils is sufficiently
increased to provide a high yield of monomeric sugars for fermentation. Acid treat-
ment will result in other high-value products such as furfural, hydroxyl-methyl furfu-
ral (HMF), phenolics, aldehydes, and aliphatic compounds that have to be removed
before subjecting the residue to further biochemical processes. Steam explosion, com-
pared to other methods, offers the potential for lower capital investment, significantly
lower environmental impact, greater energy efficiency potential, less hazardous pro-
cess conditions, and total sugar recovery. In the treatment, the substrate will be sub-
jected to high pressure and temperature for short retention times, followed by rapid
release to atmospheric pressure, breaking polymeric bonds in the substrate. To achieve
the same objectives, mechanical methods require 70% more energy than steam explo-
sion. Acid catalysis was also studied within steam explosion processes and was found
to reduce their temperature and retention time needs, with a decrease of unwanted
product formation and complete hemicellulose hydrolysis [40].