Environmental Engineering Reference
In-Depth Information
7.6 Primary Alcohols
Methanol, ethanol, and butanol are liquid biofuels that can be synthesized from
biomass and used in both four-stroke gasoline (Otto engines) and diesel engines.
These alcohols can be prepared from sugarcane, sugar beet, wheat, barley, corn,
switch grass, agricultural residues, wood, and many other industrial wastes. The
most important characteristic of alcohols that makes them suitable as fuels for
Otto engines is their high octane number. Fuels with high octane numbers can be
burned at high compression ratios in engine cylinders, increasing the efficiency of
the combustion process and reducing fuel consumption without presenting any
risk of uncontrolled self-ignition. However, ethanol and methanol have low cetane
numbers (8 and 5 respectively), which means that they can only be used in con-
junction with an ignition improver of some kind (e.g. di-tert-butyl-peroxides).
Such additives are usually costly. Fuels with excessively low cetane number ignite
slowly and reduce engine performance.
Conventional gasoline engines can operate using gasoline-ethanol blends con-
taining up to 5% ethanol without requiring any modification. Because ethanol is a
renewable fuel, this can significantly reduce the net emissions of CO
2
into the
atmosphere. An unfortunate drawback of ethanol and methanol is that they are
highly flammable and burn with flames that can be difficult to see (i.e. low levels
of low smoke).
As described in Chapter 4, ethanol can also be used to produce other related
chemicals such as ethyl acetate, acetic acid, and acetaldehyde via chemical pro-
cesses such as oxidation and esterification.
7.6.1
Methanol
Methanol is a colourless liquid with no particular smell at room temperature. It
can be used as a fuel in both Otto and diesel engines. Methanol is highly toxic,
corrosive, and flammable and, because it burns with a flame that is hard to see,
methanol fires are not always immediately apparent to the eye. However, spillages
of methanol into bodies of water or the soil degrade relatively quickly and do
comparatively little environmental harm.
Methanol can be produced from fossil fuels and from biomass. It is usually
produced from natural gas but can also be produced through gasification of bio-
mass followed by conversion of the synthetic gas produced to biomethanol in
the presence of catalysts at high pressures and temperatures (e.g. 220-275°C,
50-100 bar, and Cu/Zn/Al as catalysts). The product is generally contaminated
with DME and water, which are removed by distillation.
Methanol can be blended with gasoline in small quantities to act as an oxy-
genate (i.e. a high-octane oxygen-containing compound). In addition, various
methanol-heavy blends have been used as vehicle fuels, the most common of
which is known as M85 (85% methanol and 15% gasoline). Neat methanol (M100)
can also be used as a vehicle fuel.
