Biofuels from Lignocellulosic Biomass - Sustainable Biotechnology: Sources of Renewable Energy - page 35

may be used to convert syngas into liquid fuels (methanol, ethanol, gasoline, and
FT diesel). The catalytic conversion of syngas to ethanol can occur under high-
temperature and high-pressure conditions ( 250 ◦ C, 60-100 atm) with a molar

ratio of H 2 to CO at 2-3:1. However, most syngas (Table 3) does not contain
such a high H 2 /CO ratio. Also, the catalysis reaction is not specific, resulting in a
final mixture of methanol, ethanol, some other higher alcohols, and reactant gases.
Considerable technical progress is required to generate ethanol from syngas at a
viable commercial scale and various projects continue to explore possible options.
For example, Range Fuels in Georgia (Table 1), is in the process of building a
20 million gallon pilot plant to evaluate using this approach for lignocellulose to
ethanol conversion. Syngas can also be converted into gasoline or diesel through
the so called MTG (methanol-to-gasoline) or the more common FT process. While
these methods have been utilized for many years in the fossil fuel industry (coal or
natural gas feedstocks), the utilization of lignocellulosic biomass is not yet viewed
as being commercial [23]. Two DOE-funded companies (Table 1) are in the pro-
cess of building demonstration scale plants to further explore the feasibility of the
gasification-FT process for biofuel production.
In the microbial fermentation process, anaerobic bacteria such as Clostridium
ljungdahlii are used to convert cleaned syngas into ethanol [24]. Reactions involved
in the biological conversion process are as the follows:

∼

CO + 3H 2 O → C 2 H 5 OH + 4CO 2

6H 2 + 2CO 2 → C 2 H 5 OH + 3H 2 O

In general, conditions for microbial conversion of syngas to ethanol are mild
and specific, and the H 2 :CO ratio is not critical. However, microbial toler-
ance to ethanol concentration in the fermentation broth is currently a limitation.
Several public and private R&D projects are underway to address the issue (e.g.
http://www.coskata.com ; http://www.ineosbio.com ).

6 Biochemical Conversion of Lignocellulosic Biomass

6.1 Overview

Theoretically, the basic process for biochemical conversion of lignocellulosic
biomass into ethanol or other biofuels is relatively straightforward. First, the lig-
nocellulosic matrix must be treated to gain access to and/or separate the main
components: lignin, cellulose, hemicellulose, and pectin. The polysaccharides (cel-
lulose and hemicelluloses) are then hydrolyzed to sugars, which are fermented to
ethanol. This hydrolytic conversion process for lignocellulosic biomass contributes
to the technical barriers that currently limit commercial operations. The fermen-
tation process for ethanol production from lignocellulosic biomass is also more