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The energy content of biofuel is the chemical energy contained in a given biofuel, measured per unit mass of that fuel, as specific energy, or per unit of volume of the fuel, as energy density. A biofuel is a fuel produced from recently living organisms. Biofuels include bioethanol, an alcohol made by fermentation—often used as a gasoline additive, and biodiesel, which is usually used as a diesel additive. Specific energy is energy per unit mass, which is used to describe the chemical energy content of a fuel, expressed in SI units as joule per kilogram (J/kg) or equivalent units. Energy density is the amount of chemical energy per unit volume of the fuel, expressed in SI units as joule per litre (J/L) or equivalent units.

Energy and CO₂ output of common biofuels

The table below includes entries for popular substances already used for their energy, or being discussed for such use.

The second column shows specific energy, the energy content in megajoules per unit of mass in kilograms, useful in understanding the energy that can be extracted from the fuel.

The third column in the table lists energy density, the energy content per liter of volume, which is useful for understanding the space needed for storing the fuel.

The final two columns deal with the carbon footprint of the fuel. The fourth column contains the proportion of CO₂ released when the fuel is converted for energy, with respect to its starting mass, and the fifth column lists the energy produced per kilogram of CO₂ produced. As a guideline, a higher number in this column is better for the environment. But these numbers do not account for other green house gases released during burning, production, storage, or shipping. For example, methane may have hidden environmental costs that are not reflected in the table.

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Notes

1. While all CO₂ gas output ratios are calculated to within a less than 1% margin of error(assuming total oxidation of the carbon content of fuel), ratios preceded by a Tilde (~) indicate a margin of error of up to (but no greater than) 9%. Ratios listed do not include emissions from fuel plant cultivation/Mining, purification/refining and transportation. Fuel availability is typically 74–84.3% NET from source Energy Balance.
2. While Uranium-235 (U) fission produces no CO₂ gas directly, the indirect fossil fuel burning processes of Mining, Milling, Refining, Moving & Radioactive waste disposal, etc. of intermediate to low-grade uranium ore concentrations produces some amount of carbon dioxide. Studies vary as to how much carbon dioxide is emitted. The United Nations Intergovernmental Panel on Climate Change reports that nuclear produces approximately 40 g of CO₂ per kilowatt hour (11 g/MJ, equivalent to 90 MJ/kg CO₂e). A meta-analysis of a number of studies of nuclear CO₂ lifecycle emissions by academic Benjamin K. Sovacool finds nuclear on average produces 66 g of CO₂ per kilowatt hour (18.3 g/MJ, equivalent to 55 MJ/kg CO₂e). One Australian professor claims that nuclear power produces the equivalent CO₂ gas emissions per MJ of net-output-energy of a Natural Gas fired power station. Prof. Mark Diesendorf, Inst. of Environmental Studies, UNSW.

Yields of common crops associated with biofuels production

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Notes

1. Typical oil extraction from 100 kg of oil seeds
2. Chinese Tallow (Sapium sebiferum, or Tradica Sebifera) is also known as the "Popcorn Tree"

See also

• Eichhornia crassipes#Bioenergy
• Syngas
• Conversion of units
• Energy density
• Heat of combustion

References

1. Kenneth E. Heselton (2004), "Boiler Operator's Handbook". Fairmont Press, 405 pages. ISBN 0881734357
2. "The Two cap of SI Units and the SI Prefixes". NIST Guide to the SI. Retrieved 2012-01-25.
3. ^ Intergovernmental Panel on Climate Change (2007). "4.3.2 Nuclear energy". IPCC Fourth Assessment Report: Climate Change 2007, Working Group III Mitigation of Climate Change. Retrieved 2011-02-07.
4. ^ Benjamin K. Sovacool.Valuing the greenhouse gas emissions from nuclear power: A critical survey. Energy Policy, Vol. 36, 2008, p. 2950.
5. Used with permission from The Global Petroleum Club.

• Biofuels technology