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In thermochemical conversion processes, bonds btw adjacent carbon, hydrogen, and oxygen molecules are broken down to release their chemical energy (the energy stored in the bonds of atoms & molecules) [EIA, 2023].

  

For any chemical reaction, the overall energy change, i.e., the enthalpy change (DH), is the difference btw i) the energy absorbed in bond-breaking (an endothermic process: absorbs energy from its surroundings) and ii) the energy released in bond-making (an exothermic process: releases energy to its surroundings).  Table 1 shows selected bond energies, i.e., the amount of energy required to break one mole (about 6×10^23) of such bond.  As shown in Figure 1, the enthalpy change for the reaction 2H2 + O2 => 2H2O is 470 kJ/mol.

  

Crude oil (petroleum) is made up of carbon (82–87%, by weight) & hydrogen (10–14%), and, to a lesser extent, oxygen & nitrogen (about 1.5% each), indicating that it contains mainly carbon-hydrogen & carbon-carbon bonds.  Table 2 shows the energy content of hydrogen & fossil fuels.