Page 266 - Synthetic Fuels Handbook

252 CHAPTER EIGHT

as a route to biodegradable plastics. An alternative is to employ thermochemical conversion
processes which use pyrolysis or gasification of biomass (Chaps. 9 and 10) to produce
a hydrogen-rich synthesis gas which can be used in a wide range of chemical processes
(Chap. 7).
Thus, a biorefinery is a facility that integrates biomass conversion processes and equip-
ment to produce fuels, power, and chemicals from biomass (Fig. 8.6) (Ruth, 2004). The
biorefinery concept is analogous to the petroleum refinery, which produce multiple fuels
and products from petroleum (Chap. 3).
A biorefinery can have two or more options for the production of biofuels from wood
and other biomass materials (Mabee and Saddler, 2006). There is the (a) bioconversion, (b)
thermal conversion, and (c) thermochemical conversion. Each of these options has merits
but is selected depending on the feedstock and the desired product slate.

8.6.1 Bioconversion
The bioconversion option uses biologic agents to carry out a structured deconstruction of
lignocellulose components. This platform combines process elements of pretreatment with
enzymatic hydrolysis to release carbohydrates and lignin from the wood (Fig. 8.9).
The first step is a pretreatment stage which is based on existing pulping processes, how-
ever, traditional pulping parameters are defined by resulting paper properties and desired
yields, while optimum bioconversion pretreatment is defined by the accessibility of the
resulting pulp to enzymatic hydrolysis. This function of this step is to optimize the biomass
feedstock for further processing and is designed to expose cellulose and hemicellulose for
subsequent enzymatic hydrolysis, increasing the surface area of the substrate for enzymatic
action to take place. The lignin is either softened or removed, and individual cellulosic
fibers are released creating pulp.
In order to improve the ability of the pretreatment stage to optimize biomass for enzy-
matic hydrolysis, a number of nontraditional pulping techniques have been suggested
(Mabee and Saddler, 2006) and include: (a) water-based systems, such as steam-explosion
pulping, (b) acid treatment using concentrated or dilute sulfuric acid, (c) alkali treatment
using recirculated ammonia, and (d) organic solvent pulping systems using acetic acid or
ethanol. As with traditional pulping, pretreatment tends to work best with a homogenous
batch of wood chips but the pretreatment option may have to be selected according to the
type of lignocellulosic feedstock (Mabee et al., 2006a).
Once pretreated, the cellulose and hemicellulose components of wood can be hydro-
lyzed (in this option) using enzymes to facilitate bioconversion of the wood. Enzymatic
hydrolysis of lignocellulose materials uses cellulase enzymes to break down the cellulosic
microfibril structure into the various carbohydrate components.
The enzymatic hydrolysis step may be completely separated from the other stages of
the bioconversion process, or it may be combined with the fermentation of carbohydrate
intermediates to end products. Separate hydrolysis and fermentation (SHF) stages may
offer this option more flexibility insofar as process adaptation to feedstock type and product
slate is available. Simultaneous saccharification and fermentation (SSF) has been found to
be highly effective in the production of specific end products, such as bioethanol (Mabee
et al., 2006a).
The benefit of the bioconversion platform is that it provides a range of intermediate
products, including glucose, galactose, mannose, xylose, and arabinose, which can be rela-
tively easily processed into value-added bioproducts. The process also generates a quantity
of lignin or lignin components; depending upon the pretreatment, lignin components may
be found in the hydrolysate after enzymatic hydrolysis, or in the wash from the pretreat-
ment stage. The chemical characteristics of the lignin are therefore heavily influenced by