Hydrogen and carbon content in fossil fuels

All fossil fuels are composed of hydrogen and carbon (among other things that don't factor into the combustion/energy production phase), hence the term hydrocarbons to refer to these energy sources. The vast proportion of the useable energy comes from hydrogen, whereas the carbon generates the vast majority of the waste.

When carbon burns completely in an oxygen atmosphere (such as the earth's), the product is a lot of carbon dioxide (CO2), the gas blamed for global warming. Other byproducts of fossil fuel combustion are carbon monoxide (CO), nitrogen oxides (NOx), sulfur oxides (SOx), and various particulates like mercury that cause undue environmental harm.

Gasoline, heating oil, and propane are over 80 percent carbon by weight. Natural gas only contains 75 percent carbon, and this is one of the reasons it's favored by environmentalists. It also burns cleaner and more efficiently, with less extraneous byproducts. In general, the less carbon a molecule contains (in relation to the amount of hydrogen), the cleaner the burning process.

Because energy is derived from the hydrogen content of a particular fossil fuel and waste is related to the carbon content, it's of interest to look at the different fossil fuels and their relative proportions of hydrogen and carbon. Fuels with a high carbon-hydrogen rate are limited in how low a pollution may be obtained via combustion. Table 5-1 lists the percentages of carbon and hydrogen in common fuels. Note that, although wood is not a fossil fuel, I include it in this chart because its energy is also derived from combustion processes.

Table 5-1 The Hydrogen and Carbon Content of Common Fuels

Fuel

State

Atom % carbon

Atom % hydrogen

C/H ratio

Wood

Solid

90

10

9.0

Coal

Solid

62

38

1.63

Oil

Liquid

36

64

0.56

Octane

Liquid

31

69

0.44

Methane

Gas

20

80

0.25

Hydrogen

Gas

20

80

0.25

As Table 5-1 shows, hydrogen gas has no carbon. This is why there's so much interest in developing hydrogen fuels, which exhaust only water when combusted. This is clearly the wave of the future, and I get into more details in Chapter 15.

Wood, as Table 5-1 shows, has more carbon than any of the other sources listed, and it produces a lot of carbon dioxide when it burns. Here's the interesting thing, though: Wood left on the forest floor to rot releases just as much CO2 into the atmosphere due to natural decomposition as gets released if the wood were burned. That's why wood, despite its high carbon content, is relatively benign to the environment because at some point the carbon dioxide is going to be released, burn or rot. Add the fact that wood is renewable, sustainable, and local in production, and the above chart slanders wood more than is merited. (Head to Chapter 14 for more on wood as an alternative energy source.) Coal, on the other hand, does not decompose in the ground and so coal left alone does not naturally increase global warming.

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