Chapter 11 – Biochemical Fuels
Fossil Fuels
• Most of our energy needs are met by burning
fossil fuels such as coal, petroleum and natural
gas.
• Coal is used to generate electricity and products
derived from crude oil, such as petrol and diesel
are used to transport fuels.
• Other products derived from crude oil are used to
manufacture a range of products, from plastics to
pharmaceuticals.
• There are large reserves of coal, however,
petroleum deposits are limited.
Peak Oil
• Is the time when worldwide oil extraction can no
longer keep up with increasing demand.
• The greatest amount of oil discovered was in
1964, and since then new reserves found have
gone down in size each year.
• However, demand for oil has increased.
• The situation provides motivation to devise
renewable and sustainable sources of the carbon
compounds, both for fuels and to provide
feedback for the organic chemical industry.
Biochemical Fuels
• Biochemical fuels, biofuels, are fuels derived from plant materials
such as grains, sugarcane or vegetable waste and vegetable oils.
• The two main biofuels are ethanol and boidiesel.
• They are used either alone or blended with fossil fuels such as
petrol or diesel.
• Biochemical fuels are not considered to contribute to an increase in
atmospheric carbon dioxide.
• The plant materials used in the generation of biochemical fuels are
produced by photosynthesis, which removes carbon dioxide from
the atmosphere and produces glucose.
• The plant converts the glucose into cellulose and starch.
• The carbon dioxide is then released when the biochemical fuel is
burned.
• Biochemical fuels are considered as carbon neutral.
Ethanol
• Enzymes are used to catalyse the breakdown of the polysaccharide
starch in grains to glucose and the the fermentation process uses
other enzymes from yeast organisms to convert small sugar
molecules such as glucose to ethanol and carbon dioxide.
• This is an anaerobic process, meaning it takes place without oxygen.
• The fermentation stops when the ethanol content is between 10
and 20%, at which point the yeasts and their enzymes can no longer
function.
• The fermented mixture is pumped to an evaporation plant where
steam is added to cause the ethanol to evaporate off.
• When cooled, the resulting liquid contains 95% ethanol and 5%
water.
• This is dehydrated leaving ethanol that is 99.7% pure.
• In Australia, the ethanol is denature (poisoned) by adding up to 5%
petrol to make it unsuitable for consumption as drinking alcohol.
Ethanol – minimising waste
• The carbon dioxide produced in the
fermentation step can be sold to
manufacturers of carbonated drinks.
• The waste water and cooling water can be
used for irrigating crops.
• Protein rich remains from the fermentation
can be sold as animal feed.
Ethanol cont…
• The yeasts are used to ferment material such as molasses
and grains contain enzymes that act on glucose.
• However, the fibrous waste material from sugar production,
called bagasse, is made of a different type of sugar
molecule, xylose.
• This is a pentose – a sugar that contains five carbons per
molecule, and is not recognised by usual yeasts.
• An Australian development has been to selectively breed
yeasts whose enzymes are able to digest the bagasse by
product.
• The development of these yeasts will greatly increase the
amount of ethanol produced from a given quantity of
sugarcane.
Ethanol cont…
• The cellulose found in biomass such as woody
plants can also serve as a raw material for
ethanol production.
• Genetically engineered E. coli bacteria have
been developed that convert the
polysaccharides in materials like corn stalks
and sugarcane residue into ethanol.
• It also reduces the need to dispose of these
waste materials in landfill or by burning.
Ethanol cont…
• A process developed
in Canada to convert
plant cellulose to
glucose.
Biodiesel
• Biodiesel is a mix of esters produced by a chemical reaction
between vegetable oil and an alcohol such as methanol.
• The chemical and physical properties of the esters in biodiesel
are similar to those of diesel.
• The usual raw material is fresh vegetable oil from sources
such as soybeans, canola or palm oil, but recycled vegetable
oil or animal fats can also be used.
• Fats and oils are triglycerides with a molecular structure
consisting of three hydrocarbon chains attached by ester
functional groups to a backbone of three carbon atoms.
Biodiesel cont…
• The triglyceride is hydrolysed by warming it with methanol
and potassium hydroxide solution.
• The potassium hydroxide solution acts as a catalyst in this
hydrolysis reaction so that overall the triglyceride breaks
down to three molecules of fatty acid esters, plus glycerol.
Biodiesel cont…
• Ethanol and sodium hydroxide can be used as
an alternative reagents but there are som
practical issues involved that make them less
useful for large-scale production.
• The esters produced are less viscous than
vegetable oil, when burnt they release a
similar amount of energy to petrodiesel.
• Biodiesel can also be used in a mixture with
petrodiesel; 5% and 10% mixtures of biodiesel
in petrodiesel are known as B5 and B10.
Biodiesel cont…
• Biodiesel is biodegradable, non-toxic and
produces fewer pollutants in the vehicle
emissions.
• It is not adding to the overall amount on
carbon dioxide in the atmosphere as it is
simply recycling the carbon dioxide already
present.
Biogas
• Biogas consists mainly of carbon dioxide and
methane in roughly equal quantities and is
generated when organic material decays in
the absence of oxygen.
• Biogas generators are particularly useful for
farms as the waste from the generators can be
used as a rich fertiliser.
Developing Biochemical Fuels
• Our current use of fossil fuels is releasing carbon into the
environment mainly in the form of atmospheric carbon
dioxide.
• Gases such as water vapour and carbon dioxide absorb
heat energy from the Sun an act as a blanket to trap this
heat in the atmosphere.
• Life on Earth would not survive without this process,
however, with the increased amount of carbon dioxide
there are further temperature increases causing a shift in
the climate.
• Biochemical fuels are seen as a useful alternative to the
use of fossil fuels.
Developing Biochemical Fuels
cont..
• Although their combustion does produce
carbon dioxide, they are produced from plant
materials made by photosynthesis, which
consumes an equal amount of carbon dioxide.