Hydrocarbon combustion

Hydrocarbon combustion refers to the type of reaction where a hydrocarbon reacts with oxygen to create carbon dioxide, water, and heat. Hydrocarbons are molecules consisting of both hydrogen and carbon. They are most famous for being the primary constituent of fossil fuels, namely natural gas, petroleum, and coal. For this reason, fossil fuel resources are often referred to as hydrocarbon resources.[1] Energy is obtained from fossil fuels through combustion (burning) of the fuel. Although impurities exist in fossil fuels, hydrocarbon combustion is the primary process in the burning of fossil fuel. An example of hydrocarbon combustion is illustrated in Figure 1. See simulation at the bottom of the page for more examples.

Figure 1. Methane combining with 2 oxygen to form carbon dioxide, water and heat.[2]











Description

Regardless of the type of hydrocarbon, combustion with oxygen produces 3 products: carbon dioxide, water and heat, as shown in the general reaction below. The energy required to break the bonds in the hydrocarbon molecules is substantially less than the energy released in the formation of the bonds in the [math]\ce{co2}[/math] and H2O molecules. For this reason, the process releases significant amounts of thermal energy (heat). This thermal energy can be used directly (perhaps to heat a home) or else it can be converted to mechanical energy, using a heat engine, although this is subject to efficiency losses governed by the second law of thermodynamics. If the thermal energy is converted into mechanical energy, as in an automobile engine, there will necessarily be significant energy losses (as waste heat) in the conversion process. The resulting useful mechanical energy will be a lot less than the initial thermal energy provided by the hydrocarbon combustion.

General Reaction Equation:

CxHy + N(O2) x(CO2) + 0.5y+(H2O)

  • x refers to the number of carbon atoms in the hydrocarbon
  • y refers to the number of hydrogen atoms in the hydrocarbon
  • N refers to the number of oxygen atoms required in the hydrocarbon combustion reaction

Hydrocarbon Combustion and Fossil Fuels

Note that CO2 is always produced in hydrocarbon combustion; it doesn't matter what type of hydrocarbon molecule. Producing CO2 and H2O is actually how useful energy is obtained from fossil fuels. For this reason, it is important to distinguish between carbon dioxide and other "waste" products that arise from impurities in the fuel such as sulfur and nitrogen compounds.[1] Wastes that arise from impurities can be eliminated with the right technology; CO2 cannot be eliminated unless the fossil fuels are not burned (used) in the first place.

Not all fossil fuels have the same composition. Natural gas is composed of over 90% methane (CH4) which is the smallest hydrocarbon molecule. Oil tends to be composed of medium sized molecules, although composition varies greatly from one grade of crude to the next. In general, the denser the oil, the longer the carbon chains in the molecules. Finally coal contains the largest and most complex hydrocarbon molecules.[1]

Since different hydrocarbons have different ratios of hydrogen to carbon, they therefore produce different ratios of water to carbon dioxide. In general, the longer and more complex the molecule, the greater the ratio of carbon to hydrogen. For this reason, combustion of equal amounts of different hydrocarbons will yield different quantities of carbon dioxide, depending on the ratio of carbon to hydrogen in molecules of each. Since coal contains the longest and most complex hydrocarbon molecules, burning coal releases more CO2 than burning the same mass of oil or natural gas. This also changes the energy density of each of these fuels.

Carbon dioxide emissions

Below is a chart of the CO2 emitted from the production of 293.1 kWh (1,000,000 BTUs) of energy from various hydrocarbon fuels.[3]

[[kg of CO2 emissions
Anthracite Coal 104
Bituminous Coal 93.5
Lignite Coal 97.9
Subbituminous Coal 97.4
Diesel 73.2
Gasoline 71.5
Propane 63.2
Natural gas 53.2

Combustion Animation

Choose a fuel from the drop down menu to see the net reaction that occurs during combustion.

References

  1. 1.0 1.1 1.2 R.D. Botts, D.M. Carson, and D.Coglon. "Petroleum in our live" in Our petroleum challenge, 8th ed. Calgary:Canadian Center for Energy Development, 2013, pp. 7-15.
  2. American Chemical Society. "Methane and oxygen react". Internet: http://www.middleschoolchemistry.com/multimedia/chapter6/lesson1, [October 25,2013]
  3. http://www.eia.gov/tools/faqs/faq.cfm?id=73&t=11