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  Combustion: energy production from carbon based fuels – the science

Image: burning coals ona barbecueDespite increased use of nuclear and alternative energy sources for electricity generation, over two thirds (2004 data) of fuels used are fossil and therefore carbon based; mainly coal, gas and oil, but also including wood, peat and even bitumen. Whether the fuel is used directly e.g. in transport or heating, or indirectly for electrical power generation, the same chemistry i.e. oxidation or combustion of the fuel releases the energy, predominantly in the form of heat and with the emission of CO2. What matters is the efficiency with which this energy is extracted and used AND whether the carbon comes from fossil sources. This enables us to entertain use of allegedly “greener” carbon based fuels such as bio-ethanol, bio-diesel etc. Similarly whether the fuel is used in a car or a power station makes little difference to the basic chemistry BUT may make a difference in terms of efficiency and emissions e.g. catalytic converters on cars, carbon capture and storage in power stations. We make use of the law of conservation of energy to transform the heat energy produced to other forms of energy that we require, most frequently via kinetic energy in transport or in generating electricity.

Image: burning oil pipe on industrial complex, by nightThe process of combustion is a typical exothermic oxidation reaction, and would be thought to be simple. However, the precise conditions alter the chemistry for example depending on the availability of oxygen which determines whether the combustion is complete or incomplete and therefore what the products are and how much energy is released, and depending on the other substances present – air for example contains mainly nitrogen and coal, wood, oil and gas have many impurities including sulphur.

From the above we see that the use of fossil fuels can be improved in two related ways: improving the quality of emissions, increasing efficiency of energy output per unit mass of fuel. Carbon fuels which are not fossil fuels also offer an advantage (carbon neutral) because we are not adding to the current carbon by releasing carbon from millions of years ago i.e. in principle the carbon is “captured” by e.g. wheat, made into fuel which then releases back the same carbon. It does not however reduce the carbon level.

The chemistry of combustion is well understood and developing methods of reducing emissions and improving efficiency are now a major focus for science and technology. Combustion is referenced in the links but in essence it follows the same basic equations as respiration. The physics however e.g. fluid mechanics of combustion is not covered in this section since it is the carbon chemistry which relates to the Stern Review more directly.

Since the energy output from the combustion of a fuel reflects the net effect of making and breaking chemical bonds, it follows that different fuels provide different amounts of energy per unit even when fully combusted. These enthalpies of combustion are available in tables or can be calculated using Hess’s Law. For practical purposes of course, other factors come into play e.g. ease of production or extraction, cost, safety etc.
  Combustion: wikipedia entry
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Biofuels: Green energy or grim reaper
What is Biodiesel's Energy Balance?
World Coal Institute
Fossil Fuels
Calculating Enthalpies of combustion
Key World energy Statistics 2006 (PDF)

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