Fuels and Chemicals from Hemicellulose Sugars - Biotechnology in China: Biofuels and Bioenergy - page 200

1 Introduction

In the 21st century, human society will face great obstacles of resource shortages
and environmental pollution. After 28 years of rapid development, since 1993
China has become a huge oil-consuming and net oil-importing country [ 1 , 2 ], and
is now facing a very serious energy shortage problem. Thus, the Chinese
government is encouraging ethanol use as an alternative transportation fuel by
introducing fuel ethanol production and distribution in several provinces [ 3 - 5 ]. By
2020, the total utilization of fuel ethanol (based on grain or non-grain) will reach
10 million metric tonnes, according to 'A Long and Mid-Term Planning for
Renewable Energy Plan' issued by the Chinese National Development and Reform
Commission [ 6 , 7 ]. Although the current emphasis is on ethanol production from
corn and other grains, China has huge quantities of low-cost lignocellulosic bio-
mass that could significantly expand ethanol production volume and reduce
feedstock costs. The key now is to successfully commercialize current technolo-
gies to reduce the cost of lignocellulosic biomass processing to ethanol. However,
three obstacles still hinder the development of lignocellulosic ethanol: inefficient
pretreatment measures, high cost of cellulase, and lack of good methods for uti-
lization of hemicellulose sugars [ 8 - 10 ]. In this chapter, we focus on different
methods for hemicellulose sugar utilization.
Industrial processes of lignocellulosic material have made use of only the
hexose component of the cellulose fraction. Pentoses and some minor hexoses
present in the hemicellulose fraction, which may comprise as much as 40% of the
lignocellulosic material (Table 1 ), have therefore in most cases been wasted.
Without a profitable use for the hemicellulose fraction, bioethanol is too expensive
to compete in commercial markets [ 12 ]. Therefore, to foster the commercial
production of lignocellulosic ethanol, conversion of the hemicelluloses into fer-
mentable sugars and then to fuels or chemicals is essential. This chapter deals with
the structure and availability of these hemicellulose sugars, and with the bio-
chemistry and process technology involved in their conversion to fuels and
chemicals.

2 Hemicellulose Structure and Hydrolysis

2.1 Hemicellulose Structure

Hemicellulose is often described as plant cell-wall polysaccharides that are
associated with cellulose in lignified tissues. The close association of hemicellu-
lose with cellulose and lignin contributes to cell-wall rigidity and flexibility. The
majority of the hemicellulose polysaccharides are derived from cell-wall middle
lamella. Some of the non-starch, non-cellulose polysaccharides, excluding pectic
materials, which are known as pentosans, are sometimes also considered