Environmental Engineering Reference
In-Depth Information
8.4.3 Enzymatic Hydrolysis of Cellulose and Hemicellulose ........................................... 153
8.4.3.1 Cellulases ................................................................................................... 153
8.4.3.2 Hemicellulases ........................................................................................... 155
8.4.4 Fermentation of Biomass-Derived Sugars to Biofuels ............................................ 155
8.4.4.1 Overview of Fermentation-Based Conversions ......................................... 155
8.4.4.2 Fermentation of Six-Carbon Sugars ......................................................... 155
8.4.4.3 Fermentation of Five-Carbon Sugars ........................................................ 156
8.4.4.4 Fermentation of Five- and Six-Carbon Sugars to Ethanol and Other
Biofuels ...................................................................................................... 156
8.4.4.5 Fermentation of Five- and Six-Carbon Sugars to Hydrocarbons and
High-Value Products .................................................................................. 158
8.5 Comparison of Energy Efficiencies and Costs of Biomass Processing Technologies .......... 159
8.6 Conclusions ........................................................................................................................... 159
References ...................................................................................................................................... 161
8.1 IntroductIon
Although global interest in liquid biofuels for transportation has recently increased, these fuels
are not something new. Nearly a century ago automobiles were designed to run on gasoline,
ethanol, and blends of these fuels; these were the forerunners of modern flexible-fueled vehicles.
The motivation for biofuel use a century ago was different than today. Previously, biofuels were
one of a diversity of fuel sources whereas in the modern era of petroleum, biofuels are meant to
address a list of issues including global warming, rural economic development, domestic jobs,
energy security, and balance of trade. The list of potential transportation biofuels today is much
larger than in previous eras, mostly because of advances in biochemical and thermochemical
processing technologies, which have also increased the list of possible biomass feedstocks that
can be converted.
This chapter will first give a brief overview of the technologies for converting biomass
feedstocks into usable liquid transportation fuels. After that will be an introduction into the
main processing steps used in commercial practice and a review of current research and
development in this field. Afterward, a short summary will give the current status for global
biofuel production.
8.1.1 a
n
o
vErviEw
of
B
iomaSS
-
to
-B
iofuElS
p
rocESSing
t
EchnologiES
Processing routes for conversion of biomass to biofuels has traditionally been organized into two
categories depending on the agents for transformation and reaction conditions:
biochemical and
thermochemical. Biochemical conversion processes use biological catalysts (e.g., enzymes) at mild
temperatures to produce sugars from the original biomass and then microorganisms to ferment
sugars into oxygenated biofuels. The choice of microorganism should take into account the types
of sugars to be fermented and the desired fermentation products. For example, hydrolysis of woody
biomass will yield a mixture of five- and six-carbon sugars, but not many microorganisms are able
to readily ferment five-carbon sugars. Recent advances in metabolic engineering of microorganisms
have created unique metabolic pathways within microorganisms so that mixtures of sugars obtained
from lignocellulosic biomass can be fermented into oxygenated biofuels (e.g., ethanol and butanol)
but more recently into true hydrocarbon fuels. Thermochemical conversion processes use chemical
catalysts and are, for the most part, carried out at higher temperatures and pressures. These reactions
exhibit much shorter reaction times than biochemical conversion processes, but selectivity to a
particular biofuel product is not as specific as for biochemical conversions.
Figure 8.1 is an overview diagram showing the main conversion steps for conventional biofuels
(such as ethanol from starch crops and cane, biodiesel from triglycerides in soybeans) and for
