Page 59 - Advances in Eco-Fuels for a Sustainable Environment

36 Advances in Eco-Fuels for a Sustainable Environment

Sulfite pretreatment to overcome the recalcitrance of lignocellulose (SPORL) is a
recently developed method used for woody biomass. In the process, wood chips first
react with a solution of sodium bisulfite (or Ca, Mg, or other bisulfite) at 160–190°C,
low pH (2–5) for 10–30min; the wood chips are then mechanically fiberized to gen-
erate a fibrous substrate for saccharification and fermentation. The removal of the
woody biomass’ strong recalcitrance by SPORL is obtained by the combined effects
of hemicellulose dissolution, cellulose depolymerization, delignification, sulfonation
of lignin, and increased surface area by fiberization. SPORL pretreatment was shown
to increase sugar yields from 57% to 88% and reduce inhibitors up to 65%, compared
to dilute sulfuric acid pretreatment [32]. Many other pretreatment processes were
developed in the last decade and are being continuously improved. Pretreatment
has important effects on all following steps and ultimately has a great influence on
overall biofuel yield and cost.
Hydrolysis, involving breakdown of polysaccharides into simple sugar, can be
achieved by two common methods: (1) concentrated acid (H 2 SO 4 30%–70%, 40°C,
few hours) to achieve >90% glucose yields, used in Japan; (2) enzymatic hydrolysis
(cellulase mixture, 50°C, several days) to reach 75%–95% glucose yields, which is
currently favored to avoid costly recovery and treatment due to the use of acid,
and because it produces better yields than the acid-catalyzed one. Enzymatic hydro-
lysis of lignocellulosic materials involves enzymatic reactions converting cellulose
into glucose by cellulase, and hemicellulose into pentoses (xylose and arabinose)
and hexoses (glucose, galactose, and mannose) by highly specific hemi-cellulase
enzymes. The process is usually carried out at pH 4.8 and 45–50°C. The economic
production of enzymes and the reduction of the enzyme-to-biomass ratio in hydrolysis
are important issues for the commercialization of biofuels.
Fermentation of sugars derived from biomass enzymatic hydrolysis is another
step, where a great deal of technical innovation is needed to make lignocellulosic
ethanol technology feasible. As tested natural organisms fall short in this area,
development of a modified organism satisfying specific requirements such as high
yield of product, broad substrate utilization range, resistance to process-generated
inhibitory compounds, ability to withstand high substrate and alcohol concentra-
tions, higher temperatures, lower pH, and with minimal byproduct formation
would be needed. Saccharomyces cereviseae and Zymomonas mobilis,the com-
monly used organisms for alcohol fermentation, lack the ability to ferment
hemicellulose derived pentose (C5) sugars, which is critical for any bioethanol pro-
ject. While there are organisms that can ferment these sugars (e.g., Pichiastipitis,
Pachysolentannophilus, Candida shehatae), their efficiencies are lower, they need
microaerophilic conditions, and are sensitive to inhibitors (high concentrations of
ethanol, low pH) [41]. The most frequently used microorganism for industrial fer-
mentation of bioethanol is S. cerevisiae, which has proved robust and well suited to
the fermentation of lignocellulosic hydrolysates. Its strains have been metaboli-
cally modified by introducing heterologous xylose utilization pathways to convert
xylose to ethanol. However, no amount of genetic engineering has been successful
so far in generating a strain of S. cerevisiae capable of efficiently fermenting both
xylose and arabinose.