Environmental Engineering Reference
In-Depth Information
and coal are mixed together and then burned in a boiler. In the second type, bio-
mass is processed separately and then introduced into the boiler through a separate
ingress point to that used for coal (both fuels are still burned simultaneously). In
the final category, which is also known as indirect co-firing, biomass is gasified
before the co-firing process is initiated by the addition of coal [53].
7.5 Chemical Processes
Many existing energy products that are used as fuels in the transport sector are
made from either petroleum-based materials or from biomass by some kind of
chemical process. The most common biofuels used in transport today are DME,
methanol, ethanol, butanol, and biodiesel.
7.5.1
Dimethyl Ether (DME)
Dimethyl ether (DME) is the simplest ether that remains in the gas phase at typi-
cal room temperatures and pressures. DME is non-carcinogenic but highly flam-
mable, and has the chemical formula CH 3 OCH 3 . It can be used as a fuel for
diesel engines, either by itself or as a blend with diesel because of its limited
tendency to self-ignite. DME has a high cetane number of 55-60 (diesel has a
cetane number of 40-55) and therefore has a shorter ignition time and better
combustion properties than diesel. DME produced for use as a fuel is usually
handled under pressure in liquid form. Because it can be produced from synthe-
sis gas, it can ultimately be generated from many raw materials (including
biomass) via gasification.
However, most of the DME produced today is derived from fossil raw material.
One common method involves the dehydration of methanol to DME. A drawback
of DME is that its use as a fuel requires the installation of pressure vessels and a
new distribution system; this would significantly increase fuel distribution costs if
adopted on a large scale.
DME has also been used as an additive for methanol to reduce its ignition time.
Moreover, it has found applications as a residential fuel for cooking and heating,
power generation, and hydrogen-rich fuel cells [56].
Other ethers such as methyl-tert-butyl ether (MTBE) and ethyl-tert-butyl-ether
(ETBE) have been used as fuel additives that can replace fossil-derived aromatic
compounds to increase the fuel's octane number. One of the advantages of MTBE
and ETBE is that they can also be used as fuels themselves in automotive motors.
7.5.2
Biodiesel
Biodiesel is a fuel made from natural (biological) renewable resources that can be
used directly in conventional diesel motors (engines). Biodiesel has several advan-
tages compared to diesel produced from fossil precursors. It is degradable,
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