Combustion is defined as rapid oxidation accompanied by heat and usually light. In a combustion reaction some "fuel" will react with air and release chemical energy and produce CO2 and H2O as by-products. During the process of combustion air typically oxidizes with fuel, but in essence pure oxygen or any oxidizing agent may also contribute. When examining combustion reactions it is essential to know what is oxidizing with the fuel so it is important to recognize the constituents that air is composed of. For the most part air is composed of 21% oxygen, 78% nitrogen, and 1% of other constituents by volume. For calculations we can say 21% oxygen and 79% nitrogen. Since we have a ratio of 79 moles nitrogen to 21 moles oxygen we use (O2+3.76N2) for air. The internal combustion engine became popular during the industrial revolution (1860's) and has been the main contributor of atmospheric CO2. Anthropogenic carbon dioxide emissions has brought atmospheric concentrations of CO2 to about 390ppm. Because carbon dioxide is one of the leading green house gases there has been an emergent need of more sustainable energy sources. It may be necessary to weigh our different options of fuel use while we transition into clean sustainable energy because it is not currently economically viable to retrofit our current energy infrastructure while we are so dependent on fossil fuels. This said, we can measure amounts carbon dioxide released into the atmosphere for the combustion of different fuels and the amount of energy it would create to help us reduce our emissions of CO2 during this transition.
Commonly used combustible fuels
Fuel | Chemical formula |
---|---|
Gasoline(Octane) | C8H18 |
Diesel(average formula) | C12H23 |
Ethanol | C2H6O |
Propane | C3H8 |
Butane | C4H10 |
Wood | C6H12O6 |
Coal(simple) | C5H4O |
Natural Gas(average formula) | C1.2H4 |
Balanced combustion equations for common fuels
(All balanced equations assume no leftover oxygen and combustion did not result in the formation of NOx)
Gasoline(Octane) C8H18 + 12.5(O23.76N2) -> 8CO2 + 9H2O + 47N2
Diesel(average formula) C12H23 + 17.75(O23.76N2) -> 12CO2 + 11.5H2O + 66.74N2
Ethanol C2H6O + 3(O2 + 3.76N2) -> 2CO2 + 3H2O + 11.28N2
Propane C3H8 + 5(O2 + 3.76N2) -> 3CO2 + 4H2O + 18.8N2
Air Fuel Ratio (AF)
The Air Fuel Ratio or AF is the ratio of mass of air to mass of fuel:
AF= mair/mfuel
This is found by finding the product of the molar weight of components and their coefficient in the balanced chemical formula for combustion, and summing these components to find the molar wight of air. The components in air are usually O2 and N2.
The same process is done to find the molar weight of the fuel.
Example:
C2H4 + 5(O2 + 3.76N2) -> 2CO2 + 2H2O + 18.8 N2
AF = mair/mfuel = (5*32g/mol + 5*3.76*28g/mol)/ (2*12g/mol+4*1g/mol) = 300 g /28 g = 10.7
Notes and references
See also
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Interwiki links
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External links
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