Environmental Engineering Reference
In-Depth Information
The energy content of methanol on a per-liter basis is almost half that of petrol
but its octane number is much higher, making it an energy-efficient fuel. The
octane number of gasoline can be increased by adding aromatic compounds such
as benzene or toluene. However, these compounds are carcinogens. They can be
replaced with methanol, which has the added benefit of increasing the octane
number of gasoline while presenting such health hazards.
Methanol is a bulk chemical that is produced around the world and is used exten-
sively in industry as an important intermediate in the production of other chemicals
such as formaldehyde, dimethyl ether, methyl tert-butyl ether, and acetic acid.
7.6.2
Ethanol
Ethanol is a colorless liquid that has many applications in the medical and food
industries. It is soluble in water and can be used in pure form as a fuel or in blends
with gasoline or diesel. It is also less toxic than gasoline, diesel, or methanol.
Ethanol spills can be broken down by bacteria to carbon dioxide and water.
Blends of ethanol with gasoline containing up to 25% ethanol can be used in
unmodified Otto engines; a 75:25 blend of gasoline and ethanol has been widely
used in Brazil since 2002 and is known as gasohol. Diesel and ethanol are difficult
to blend and an emulsifying agent must be added to enable the formation of a
stable, homogenous solution. Pure ethanol has also been used in heavy vehicles
with diesel engines, and blends containing up to 85% ethanol with 15% gasoline
(E85) have been used in flexible fuel vehicles (FFV) such as the Ford Taurus.
Ethanol combustion produces much fewer emissions than that of fossil fuels.
For example, the emissions of particulate matter, NO
x
, CO, and other organic
compounds from ethanol-burning engines are much lower than those produced by
diesel combustion.
Ethanol can be produced by various processes and also from different raw mate-
rials including cellulose, waste from paper mills, excess wine in Europe, and other
biological materials. Its production has increased in several countries in recent
years including the USA, China, Brazil, India, and some European countries.
In contrast to methanol, ethanol can also be produced biochemically by the
fermentation of carbohydrates (sugar) obtained from many different raw materials
such as sugarcane, sugar beet, and corn. The three feedstock types used in ethanol
production are: sugar feedstocks such as sugarcane; starch feedstocks such as
cereal grains and potatoes; and cellulose feedstocks such as forest products and
agricultural residues. More details on the three feedstock types are provided in the
following sections.
7.6.2.1
From Sugar Feedstock
Ethanol production from sugarcane (
Saccharum
sp.) is one of the easiest and most
efficient fermentation processes because sugarcane contains about 15% sucrose.
The glycosidic bond in the disaccharide can be broken down to two sugar units
