Environmental Engineering Reference
In-Depth Information
technologies can be grouped in 4 main categories: thermochemical, biochemical, mechanical /
physical and chemical processes.
3.2.1. Thermochemical processes
There are two main thermochemical processes for converting biomass into energy and
chemical products. The first is gasification, which consists in keeping biomass at high
temperature (> 700°C) with low oxygen levels to produce syngas, a mixture of H
2
, CO, CO
2
and CH
4
(Paisley et al., 1998; Spath and Dayton, 2003). Syngas can be used directly as a fuel
or can be a chemical intermediate (platform) for production of fuels (FT-fuels, Dimethyl
ether, ethanol, isobutene…) or chemicals (alcohols, organic acids, ammonia, methanol and so
on). The second most common thermochemical pathway is pyrolysis, which uses high
temperatures (300 - 600°C) in absence of oxygen to convert the feedstock into a liquid
pyrolytic oil (or bio-oil), solid charcoal and light gases similar to syngas (Bridgwater and
Peacocke, 2000; Guo et al., 2001). Their yields vary with process conditions and for
biorefinery purposes the treatment which maximizes the production of liquid bio-oil is the
most desirable (flash pyrolysis). The application of bio-oil as a transportation fuel is
problematic (see following section) and its use as a chemical source of phenols or
levoglucosan is still under development (Helle et al., 2007; Meister, 2002; Zhuang et al.,
2001). Together with charcoal, it is generally best suited as a fuel for stationary electric power
or thermal energy plants. In addition to gasification and pyrolysis, direct combustion is also
included among the thermochemical processes (Gani and Naruse, 2007; Senneca, 2007). This
is the most common and oldest form of biomass conversion that involves burning biomass in
an oxygen-rich environment mainly for the production of heat. Another less widespread
thermochemical process is the biomass hydrothermal upgrading (HTU), which can be
conducted at different conditions of temperature and pressure and with or without a catalytic
mean (Karagöz et al., 2005; Zhang et al., 2002). The purpose of the HTU process is to convert
biomass into the so-called “biocrude”, a liquid fuel with an energy density approaching that
of fossil fuels which requires additional treatment before to be used as a transportation
biofuel. This technology is still at a research and development stage.
3.2.2. Biochemical processes
Unlike thermochemical processes, biochemical processes occur at lower temperatures
and have lower reaction rates. The most common types of biochemical processes are
fermentation and anaerobic digestion. The fermentation uses microorganisms and/or enzymes
to convert a fermentable substrate into ethanol, the most common fermentation product, but
the production of many other chemical compounds (e.g. hydrogen, methanol, succinic acid,
among others) is nowadays object of many activities of research and development. Hexoses,
mainly glucose, are the most frequent fermentation substrates, while pentoses (sugars from
hemicellulose), glycerol and other hydrocarbons have required the development of
customized fermentation organisms to enable their conversion to ethanol (Hamelinck et al.,
2005; Lynd, 1996). Anaerobic digestion involves bacterial breakdown of biodegradable
organic material in the absence of oxygen over a temperature range of about 30 - 65 °C. The
main end product of this process is biogas (a gas mixture made of methane, CO
2
and other
impurities), which can be upgraded up to > 97% methane and used as a surrogate of natural
gas (Berglund and Börjesson, 2006; Romano and Zhang, 2008).
