Environmental Engineering Reference
In-Depth Information
Use of crops like corn and cane in the bioethanol industry in the developed world is well
documented. However, the cost-effectiveness of this approach has always been a concern
because of the enormous use of water and heat (Somerville 2007). In one interesting report, Lal
(2008) summarized an update on plants that can be used for biofuel production that is based on
their efficient growth in tropical conditions and cost-effectiveness. These include, according to
the list from Lal (2008), biomass from the following groups of plants with examples given in
parentheses:
• Warm season grasses
: Switchgrass (
Panicum virgatum
L
.
), big bluestem (
Andropogan
gerardi
Vitman), big bluestem (
Andropogan gerardi
Vitman), Indian grass (
Sorghastrum
nutans
(L.) Nas), giant reed (
Arundo donax
), bluejoint grass (
Calamagrostis canadensis
(Michx.) Beau. L.), bluejoint grass (
Calamagrostis canadensis
(Michx.) Beau. L.), cord
grass (
Spartina pectinata
Link), kallar grass (
Leptochloa fusca
), guinea grass (
Panicum
maximum
), setaria (
Setaria sphcelate
), molasses grass (
Melinis minutiflora),
), and elephant
grass (
Pennisetum purpureum
Sch m.);
• Legumes
: Alfalfa (
Medicago sativa
), mucana (
Mucuna utilis
), kudzu (
Pueraria phaseoloi-
des
), and stylo (
Stylosanthes guianensis
);
• Broad leaf species
: Cup plant (
Silphium perfoliatum
L.);
• Short rotation woody perennials
: Poplar (
Populus
spp.), willow (
Salix
spp.), black locust
(
Robinia pseudoacacia
L.), mesquite (
Prosopis juleflora),
), birch (
Onopordum nervosum
),
and eucalyptus (
Eucalyptus
spp.); a nd
• Herbaceous
spp.: Miscanthus (
Miscanthus
spp.), reed canary grass (
Phalaris arundinacea
L.), and cynara (
Cynara cardunculus
).
5.1.3 c
hallEngES
in
B
iofuEl
p
roduction
from
p
lant
c
Ell
w
allS
During the process of evolution, plants have developed efficient mechanisms for resisting attack
on their cell walls from the microbial and animal kingdoms. This intrinsic property underlies
what has been termed “recalcitrance” (the innate resistance of plant cell walls to deconstruction).
This recalcitrance creates technical barriers to the cost-effective transformation of lingo-cellulosic
biomass into fermentable sugars. According to Himmel et al. (2007), there are several factors
that cause the recalcitrance of lingo-cellulosic feedstocks against their deconstruction. Most of
these factors are directly associated with properties of plant cells and cell walls, such as pres-
ence of cuticle and epicuticular waxes on the epidermal tissue of the plant body, the vascular
bundle arrangement and density, the relative amount
of thick-walled tissues (sclerenchymatous),
lignification, and the structural heterogeneity and complexity
of cell wall components such as
microfibrils and matrix polymers. Recalcitrant biomass with these structural and chemical prop-
erties often exhibits reduced liquid permeability
and/or enzyme accessibility and activity pos-
ing higher conversion
costs (Himmel et al. 2007). Cell wall microfibrils that contain a core of
crystalline cellulose exhibit elevated resistance to chemical
and biological hydrolysis (Nishiyama
et al. 2002), likely because of biophysical qualities such as the formation
of a dense layer of water
near the hydrated cellulose surface, which contributes by the hydrophobic face of cellulose sheets
(Matthews et al. 2006), and the presence of strong interchain hydrogen-bonding networks in them
(Nishiyama et al. 2002). In contrast, hemicellulose
and amorphous cellulose lack similar features
and are easily digestible. Another contributory factor to plant cell wall recalcitrance is its highly
complex structure: the crystalline cellulose core of microfibrils is protected by coatings of hemi-
celluloses and amorphous cellulose, making the microbial accessibility to the crystalline cellulose
difficult (Himmel et al. 2007). Overall, of all the aforementioned factors, the most salient contrib-
uting to plant cell wall recalcitrance are the occurrence of lignin, waxes, and phenolic compounds
as well as the multilayered complex structure.
