How much of energy can be extracted from fossil fuels?

The fossil fuels have been extensively utilized in our society, because they are extremely rich in energy (see Why we rely on fossil fuels?). This post tries to address, then, how much of energy do the fossil fuels possess?

The type of energy that fossil fuels have is "chemical energy", which is stored in chemical bonds. Roughly speaking, the difference in the below two can be extracted by combusting the fossil fuels:

• Bonding involved in fossil fuels (such as methane)
• Bonding involved in the products of fossil fuels combustion (such as carbon dioxide)

Extraction of energy - consideration based on chemical equations

Let me try to consider chemical equations to discuss the extraction of energy. Because the fossil fuel comprises mostly carbon, hydrogen, and oxygen atoms, the chemical formula of C_XH_YO_Z well represents the fossil fuel. Burning the fossil fuel for energy extraction (=thermal power plant) an be described by the chemical reaction below.
C_XH_YO_Z + {(2x+y-z)/2} O₂ --> xCO₂ + yH₂O ...(1)

Roughly estimating the amount energy in each molecule requires knowledge of bonding energies. (There is another way that requires extensive calculation of thermodynamics and would fall outside of this post...). The representative values of bonding energies are listed below:
C-C bonding = 347 [kJ mol^-1]
C-H bonding = 410 [kJ mol^-1]
H-O bonding = 460 [kJ mol^-1]
O=O bonding = 494 [kJ mol^-1]
C=O bonding = 799 [kJ mol^-1]
*These values are representative ones, and exact values vary depending on the molecular formula.

Let's see below the actual amount of energy that some representative fossil fuels contain!

Example 1: Methane CH₄ (the major component of natural gas NG)
Equations: CH₄ + 2 O₂ --> CO₂ + 2 H₂O ...(2)
We need to count the number of bonds involved in the reactants and products.
Rectants＝C-H *4 + O=O *2
Products =C=O *2 + H-O *4
With these quantities, we obtain a value of 810 [kJ mol^-1], as a amount of energy that a methan molecule has (799 * 2 + 460 * 4) - (410 * 4 + 494 * 2) = 810 [kJ mol^-1]). This quantity tells us that, as an example, 200 [g] of methan has an energy value of approximately 10,000 [kJ] (= 810 [kJ mol^-1] * (200 [g] / 16 [g mol^-1]). With the unit of not joule but calorie, because 1 [cal] = 4.2 [J], this energy value corresponds to approximately 2,400 [kcal]. As a comparison; this quantity rougly equals to the total ammount of energy 2,660 [kcal] that one adult man needs in one day.^[1]

Example 2: Butane C4H10 (the mojor component of liquified petroleum gas LPG)
Equation: C₄H₁₀ + 13/2 O₂ --> 4 CO₂ + 5 H₂O ...(3)
By counting the number of bonds in the reactants and products;
Reactants ＝C-C *3 + C-H *10 + O=O * (13/2)
Products =C=O *8 + H-O *10
By doing similar calculations performed in the aforemention example, we obtaine 2640 [kJ mol^-1] (=(799 * 8 + 460 * 10) - (347 * 3 + 410 * 10 + 494 * 13/2))

Comparison of energy that can be extracted from NG and LPG

By comparing methane and butane??
methane CH₄ = 810 [kJ mol^-1]
butane C₄H₁₀ = 2640 [kJ mol^-1]
This comparison reveals that one mole of butane contains three (3) times greater ammount of energy than one mole of metane.

How about the energy per unit amount of carbon dioxide?
Equation (2) tells us that combustion of methane produces the same mole of carbon dioxide as methane, while Equation (3) shows that combustion of butane produces four (4) times greater moles of carbon dioxide than butanes. With these quantities, let's see then energy per cabon dioxide emission:
Methane combustion energy per produced CO₂ = 810 [kJ mol^-1]（= 810 [kJ mol^-1] / 1）
Butane combustion energy per produced CO₂ = 660 [kJ mol^-1]（= 2640 [kJ mol^-1] / 4）
Therefore, extraction of energy from methane accompanies less CO₂ emission than butane at a given quantity of energy. This fact relfects the difference in energy between C-C and C-H bonds. Based on this comparision, one can claim that we better choose natural gas than other fossil fuels as an energy source (More detail discussion on this point shall be given in another post).

References

1. 日本医師会 あなたの健康を応援する健康の森,"健康になる! 1日に必要なカロリー「推定エネルギー必要量」", https://www.med.or.jp/forest/health/eat/01.html (accessed on 2022/02/02)
2. 一般社団法人 日本ガス協会, "都市ガスとLPガスの違い", https://www.gas.or.jp/chigai/ (accessed on 2022/02/22)