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.