Absolutely Small - Michael D. Fayer [117]
C14H30 + 21.5O2 → 14CO2 + 15H2O.
Tetradecane has 30 H atoms, which make 15 water molecules, each with two H atoms. It also has 14 C atoms, which go into the 14 carbon dioxide molecules. To make the 14CO2 and 15H2O molecules requires 43 oxygen atoms, or 21.5 O2 molecules. That is the reason for the 21.5O2 on the left side of the chemical equation. Notice that in the chemical equation for burning methane, twice as many water molecules than carbon dioxide molecules are produced. In burning tetradecane, there are approximately the same number of water and carbon dioxide molecules produced. This will turn out to be important.
In addition to natural gas (methane) and oil (long chain hydrocarbons), the third common fossil fuel is coal. In its idealized form, coal is pure carbon. This is not actually true, but for right now we will accept this statement. Then, the chemical equation for burning coal is
C + O2 → CO2.
So, in contrast to burning hydrocarbons, burning coal does not produce any water, just CO2. In burning hydrocarbons, carbon-carbon and carbon-hydrogen bonds must be broken, which costs energy. Then, carbon-oxygen and oxygen-hydrogen bonds are formed to make carbon dioxide and water, which produces energy. Coal also has bonds that must be broken. These are carbon-carbon bonds. Initially, we will take coal to be graphite, which is pure carbon. We want to compare the amount of energy produced by burning each type of fossil fuel with how much of the greenhouse gas, CO2, is produced. Although graphite is not used as a fuel because it is difficult to ignite, its well-defined chemical structure makes it a useful example.
Energy Production and the Amount of Carbon Dioxide
First we will look at an idealized picture of energy production from burning fossil fuels. We ignore the fact that fuels are not pure and that a good deal of energy is lost in power plants. The actual energy production from real fuels will be discussed below. The three chemical equations for burning the fossil fuels are:
CH4 + 2O2 → 2H2O + CO2
C14H30 + 21.5O2 → 15H2O + 14CO2
C + O2 → CO2.
In the first and third equations for burning natural gas and coal, a single CO2 is produced in the reaction. For our model of heating oil (tetradecane), 14 CO2s are produced. We want to determine the amount of energy produced for one CO2. Using thermodynamics, it is possible to calculate the maximum usable energy (free energy) produced by each reaction. We assume that all of the reactions start at room temperature with methane (a gas), tetradecane (a liquid), and graphite (a solid). Of course, burning fuel will initially leave the products hot, but we will consider the situation after everything has cooled to room temperature. For natural gas, we just use the free energy produced by burning one molecule; for graphite, we use the energy produced by burning one carbon atom. For tetradecane, we divide the energy produced by burning one tetradecane by 14 to get the energy per one carbon dioxide molecule produced.
The results are:
methane (natural gas) 1.4 × 10-18 J free energy generated per CO2 produced
tetradecane (heating oil) 1.1 × 10-18 J free energy generated per CO2 produced
graphite (coal) 0.7 × 10-18 J free energy generated per CO2 produced
We see that getting the same amount of energy from coal generates twice as much carbon dioxide (greenhouse gas) as burning natural gas. Coal is also a factor of 1.6 worse in terms of greenhouse gas production for the same amount of energy produced than heating oil. Heating oil is a factor of 1.3 worse than natural gas.
Burning Real Fossil Fuels
The numbers given here are accurate except for coal. The different types of coal, that is, anthracite, bituminous, subbituminous, and lignite (brown coal), produce different amounts of energy per pound and also have different average carbon contents. Even for the same type of