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

Biofuels vs Fossil Fuels

  

Fossil Fuel vs Biofuel

Fossil Fuel

  • Formed by natural process such as anerobic decomposition of dead organisms. Fossil fuel can be reformed but is described as non-renewable because the rate of its consumption is much faster than its reformation.
  • Fossil fuels contain high percentage of carbon, of which the energy is derived from photosynthesis in the dead organisms.
  • Fossil fuels range from materials that have low carbon-hydrogen ratios e.g. methane to liquids e.g. petroleum and non-volatile materials which consist of purely carbon e.g. anthracite coal.
  • Burning of fossil fuel produces CO2 which leads to
    • Overall increase in atmospheric CO2 because nature can only absorb a fraction of what’s produced via photosynthesis
    • Global warming which in turn causes melting of ice cap, rise in sea level, 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 and temperature
  • Use of fossil fuel is very cost-effective and energy efficient. Electricity production is still predominantly relies on coal combustion.

 

What are Biofuels?

  • Biofuels are derived from biomass, which is materials derived from living organisms. This could be either animal- or plant-derived material.

  • Ethanol and biodiesel are common biofuels.

  • Ethanol is made by fermenting the carbohydrates, including sugars e.g. starch, sucrose and glucose

Biodiesel is derived from vegetable oils, animal fats and recycled greases. Biodiesel is the most common biofuel in Europe

 

 

Vegetable oil can be converted to biodiesel by reacting with ethanol

 

Analysis of Biofuels

Advantages

  • 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 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.
  • Combustion of petrol increases greenhouse gas concentrations in the atmosphere. If green energy is used to produce biofuel, the carbon dioxide released during the combustion may be removed by plants during photosynthesis, resulting in a carbon neutral process.
  • 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 and is less likely to produce soot and carbon monoxide via incomplete combustion.

 

Disadvantages

  • 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.

 

  • 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