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166 P. Champagne
include its poor thermal stability and its corrosivity. Upgrading bio-oils by lowering
the oxygen content and removing alkalis by means of hydrogenation and
catalytic cracking of the oil may be required for certain applications [26]. The
chemical decomposition through pyrolysis is essentially the same process used
to refine crude fossil-fuel oil and coal. Biomass conversion by pyrolysis has
many environmental and economic advantages over fossil fuels [4]. A significant
feature of producing pyrolysis bio-oil is that it can be produced at a separate
location from where it is eventually used, using transportation and storage
infrastructure similar to that used for conventional fuels.
4.2.4. Liquefaction
Liquefaction is a thermochemical conversion process of biomass into a stable
liquid hydrocarbon using low temperatures and high hydrogen pressures in the
presence of a catalyst. The interest in liquefaction is low because the reactors and
fuel-feeding system are more complex and more expensive than for pyrolysis
processes. With respect to the catalytic effects of alkaline hydroxides and carbonates,
with a few exceptions, there have been few explanations regarding the roles
of the catalyst in the liquefaction process [4]. One other process that produces
bio-oil is hydrothermal upgrading (HTU). In this process, biomass is converted,
in a wet environment at high pressure, to partly oxygenated hydrocarbons [26].
4.3. Mechanical conversion processes
4.3.1. Extraction
Oilseed crops which contain a high oil fraction can be physically crushed, and the
oils extracted and converted to esters. These can then be used directly to replace
diesel or as heating oil. There is a wide range of oilseed crops that can be used for
biodiesel production, but the most commonly used crop is rape-seed. Rape-seed
production and subsequent esterification (using methanol to produce rape-seed
methyl ether, RME) and distribution are well-established technologies in some
European countries [14]. Other feedstocks employed include cotton, groundnuts,
palm oil, sunflower oil, soya bean oil and recycled frying oils. The cost of the
raw material is the most important factor influencing the overall cost of production
[4]. The energy content of vegetable oils is of the order of 39.3–40.6 MJ�kg�1 [33]. Three tonnes of rape-seed are required per tonne of rape-seed oil produced
[26]. There are a number of benefits associated with biodiesel, in comparison
with conventional fossil fuels, including a reduction in greenhouse gas emissions
of at least 3.2 kg of carbon dioxide equivalents per kilogram of biodiesel, a 99%
reduction of sulfur oxide emissions, a 39% reduction in particulate matter emission
and a high degree of biodegradability (this compared vehicles operated on
biodiesel vs vehicles operated on gasoline) [34].
5. Bioeconomics
The bioenergy and biofuel industry will continue to grow as long as petroleum
and gas prices continue to rise. Biomass feedstock costs can be a major component