HSC Chemistry: Comparing Biofuel and Fossil Fuel

This is part of the HSC Chemistry course under the topic Alcohols.

HSC Chemistry Syllabus

compare and contrast fuels from organic sources to biofuels, including ethanol

Biofuel vs Fossil Fuel

This video introduces the concept of biomass and biofuel. The video also provides a discussion of the advantages and disadvantages of biofuels versus fossil fuels.

What is Fossil Fuel?

A fossil fuel is a fuel which has been formed through natural processes such as anaerobic decomposition of dead organisms. The fossil fuel can be reformed but it is often described as being a non-renewable energy source since the rate of its consumption is much faster than its reformation. These fossil fuels contain high percentages of carbon, of which the energy has been derived from photosynthesis in the dead organisms from which it is formed when that organism was still alive.

Ultimately the term fossil fuel can range from a variety of materials which have low carbon-hydrogen ratios such as methane, to liquids like petroleum, and anthracite coal which is almost purely carbon.

Fossil fuels have been a large target of debate because of the production of CO₂in the atmosphere which poses as a major contributor to global warming and climate change.

Some effects of this large anthrogenic production of carbon dioxide include:

Great increases in overall atmospheric carbon dioxide content

While photosynthesis from plants consumes carbon dioxide from the atmosphere, the rate of production of carbon dioxide exceeds the rate of its consumption. In the image we can see that human emissions equate for 9 billion tonnes of carbon dioxide as shown by the red number, and only 3 million tonnes of that is utilised in photosynthesis.

Photosynthesis equation:

$$6CO_2(g) + 6H_2O(l) \rightarrow C_6H_{12}O_6(s) + 6O_2(g)$$

Increase in global temperatures due to the absorption of infrared radiation from the greenhouse gas

In turn causes melting of ice caps, rises in sea levels, and loss of land

Increase in [CO₂(aq)] in water bodies.

CO₂ reacts with water to form carbonic acid which reduces the pH of water.
Many aquatic organisms cannot tolerate drastic changes in pH - ocean acidification

$$CO_2(g) \rightleftharpoons CO_2(aq)$$

$$H_2O(l) + CO_2(g) \rightleftharpoons H_2CO_3(aq)$$

Whilst climate change does pose the greatest danger resulting from large scale fossil fuel consumption, the use of fossil fuel is very cost-effective and energy effective with electricity production still predominantly relying on coal combustion.

What is Biofuel?

Biofuels are derived from biomass, which is material that has been derived from living organisms. This could either be animal or plant derived.

Common examples of biofuels are ethanol and biodiesel. Ethanol is an example of a biofuel because it has been created by fermenting biomass including carbohydrates like sugar, starch, sucrose, and glucose.

Fermentation of glucose is given by

$$C_6H_{12}O_6(aq) \rightarrow 2C_2H_6O(aq) + 2CO_2(g)$$

Biodiesel is an example of a biofuel because it is derived from vegetable oils, animal fats and recycled greases. Biodiesel is the most common type of biofuel that is found in Europe.

Vegetable oil can be converted to biodiesel by reacting with ethanol

Advantages of Biofuels

Biofuel is produced from renewable resources while petrol is produced from non-renewable crude oil reserves. Thus, bioethanol and biodiesel are sustainable fuels, which may be continually produced while petrol is unsustainable.

Bioethanol and biodiesel are more biodegradable. Thus, spills pose less of an environmental threat than spills of non-biodegradable petrol, which can cause long-term contamination of soil and water bodies.

Biofuel usage is carbon neutral

While the production and combustion of biofuel produces carbon dioxide, the use of biofuel is described as carbon neutral because the carbon dioxide may be removed by plants during photosynthesis.

Combustion of bioethanol produces less particulates, which can be responsible for significant lung disease and may even be associated with the formation of cancer. Reducing airborne pollutants also has the added benefit of reducing the cost of health care in a community.

Bioethanol and biodiesel are both partially oxidised (containing oxygen), therefore complete combustion of these biofuels requires less oxygen supply. This means that they are less likely to produce soot and carbon monoxide via incomplete combustion.

Disadvantages of Biofuels

Biofuels are less energy efficient than fossil fuel alternatives.

Ethanol has a lower heat of combustion (∆_cH) than petrol
Biodiesel has a lower heat of combustion (∆_cH) than diesel
Likely to be unappealing to consumers due to higher prices and lower efficiency.

Production process of bioethanol is not industrially and economically feasible.

Fermentation has a slow reaction rate and requires specific conditions (yeast, 37ºC and dilute solutions). These requirements mean ethanol is produced at a very slow rate from glucose.
After ethanol is produced, it needs to be separated from water using distillation which requires a significant amount of energy. If this energy is produced from burning fossil fuels, then use of bioethanol would not be carbon neutral.

Production of biofuels including both bioethanol and biodiesel requires food crops to be grown

Increased demand for biofuel to replaced fossil fuel may cause crops to be grown for fuel rather than food. Food shortages or increased food prices may result.
As more crops are grown to produce biofuels, more fertiliser is used. The excess use of fertilisers can result in soil erosion and can lead to land and water pollution.
Growing crops requires land.

Implementing ethanol as a main source of fuel is difficult because most vehicles and machines are incompatible with 100% ethanol.

Many countries including Australia only offers E10 fuel (petrol with 10% ethanol) which reduces greenhouse gases emission by up to 4%
E100 or any fuel blends containing high percentage of ethanol are only compatible in few countries e.g. Brazil