Environmental Engineering Reference
In-Depth Information
cellulose. The four main processing steps are mechanical size reduction, chemical pretreatment,
enzymatic hydrolysis, and fermentation. Although mechanical size reduction has obvious effects
on downstream equipment and processes and may come before or after chemical pretreatment,
the focus of this chapter will be on the remaining three steps: pretreatment, enzymatic hydrolysis,
and fermentation.
8.4.1 o
vErviEw
of
B
iochEmical
c
onvErSion
p
rocESSES
The purpose of pretreatment and enzymatic hydrolysis is to fractionate and recover monomeric
sugars from lignocellulosic biomass, whereas fermentation takes those sugars and converts
them into valuable products. Cellulose is a linear homopolymer of the six-carbon sugar glucose.
Hemicellulose is a branched heteropolymer of five- and six-carbon sugars, primarily xylose,
arabinose, galactose, glucose, and mannose. Six-carbon (hexose) sugars are readily fermented
by many microorganisms, whereas the five-carbon (pentose sugars) are fermented by only a
few native strains (Mosier et al. 2005). Figure 8.6 shows the major unit operations found in a
biochemical conversion process for production of ethanol or other fermentation products from
woody biomass.
Pretreatment serves two basic purposes: it begins the process of breaking down the more-easily
hydrolyzable (mostly hemicellulose) sugars, and it opens up the structure of the lignocellulosic
biomass to make the cellulose more accessible for enzymatic hydrolysis. Pretreatment must also
preserve the hemicellulose sugars, limit the formation of degradation products, and make it easier
to reduce biomass particle size (Mosier et al. 2005).
Separate hydrolysis and fermentation (SHF) is when enzymatic hydrolysis takes place separately
from fermentation. However, certain processes can be consolidated in different configurations with
some advantages. Simultaneous saccharification and fermentation (SSF) is the term for hydrolysis
taking place in the presence of fermenting microorganisms. This reduces the effect of product
inhibition during hydrolysis as the fermenting organisms consume the products (sugars) as they
are produced If pentose and hexose sugars are being fermented in the same vessel as enzymatic
hydrolysis, it is known as simultaneous saccharification and co-fermentation (SSCF). This can be
done with a mixed culture of organisms that can ferment five- and six-carbon sugars, or with a
single organism that can utilize all sugars.
Ultimately, genetic engineering maybe able to produce custom organisms capable of converting
raw biomass into value-added product without pretreatment or enzymatic hydrolysis. This goal
is known as consolidated bioprocessing (CBP). SSF and SSCF are preferred to SHF, because
minimizing unit operations leads to lower costs. They are generally viewed as realistic near-term
possibilities, whereas CBP is a longer-term goal (Mosier et al. 2005; Lynd et al. 2002; Wright 1988).
8.4.2 p
rEtrEatmEnt
h
ydrolySiS
c
onvErSionS
Pretreatment is among the most costly steps in processing cellulosic biomass, and it has effects on
upstream (size reduction) and downstream (enzymatic hydrolysis) processes. For example, more
efficient pretreatment can lead to lower enzyme loading requirements for enzymatic hydrolysis.
Therefore, the interactions between pretreatment and enzymatic hydrolysis are of critical importance
(Mosier et al. 2005; Wyman et al. 2005b). Several pretreatment technologies are available; however,
their relative attributes differ. Table 8.6 shows several of the most promising biomass pretreatment
methods along with their advantages and disadvantages.
In general, low pH treatments give liquid fractions containing most of the hemicellulose sugars
and a solid residue containing most of the cellulose and lignin. High pH conditions remove lignin
while leaving a solid residue that contains most of the cellulose and hemicellulose. Although AFEX
(ammonia fiber explosion) is a high pH treatment, it does not generate a liquid stream, and therefore
essentially 100% of the feedstock is recovered as dry matter with a disrupted structure suitable for
