Combustion Heat

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Standard heat of combustion: The energy liberated when a substance X undergoes complete combustion, with excess of oxygen at standard conditions (25°C and 1 bar). In thermodynamical terms it is the negative of the enthalpy change for the combustion reaction.

nX + mO₂  →  xCO₂ (g) + yH₂O (l) + zZ + heat of combustion

where Z is any other products formed during the reaction and n, m, x, y and z are the number of moles of each molecule in the balanced equation.

The heat of combustion is utilised to quantify the performance of a fuel in a combustion system such as furnaces, motors and power generation turbines. It is the same as the gross heating value or energy content.

Generally, the heat of combustion can be expressed as the following:

ΔH_c° = -xΔH_f°(CO₂,g) - yΔH_f°(H₂O,l) - zΔH_f°(Z) + nΔH_f°(X) + mΔH_f°(O₂,g) 

where    ΔH_c° : heat of combustion at standard conditions (25°C and 1 bar)

             ΔH_f° : heat (enthalpy) of formation at standard conditions (25°C, 1 bar)

Then, the heat of combustion can be calculated from the standard enthalpy of formation (ΔH_f°) of the substances involved in the reaction, given as tabulated values.

See Thermodyamics key values internationally agreed, Standard state and enthalpy of formation, Gibbs free energy of formation, entropy and heat capacity and Standard enthalpy of formation, Gibbs energy of formation, entropy and molar heat capacity of organic substances

For compounds containing carbon, hydrogen and oxygen (as many organic compounds do), a general combustion reaction equation will be:

C_aH_bO_c  +  (a + ¼b - ½c) O₂  → aCO₂ (g) + ½bH₂O (l) + heat of combustion

and the heat of combustion can be calculated from the standard heat of formation of all compounds taking part in the reaction:

ΔH_c° = -aΔH_f°(CO₂,g) - ½b ΔH_f°(H₂O,l) + ΔH_f°(C_aH_bO_c) + (a + ¼b - ½c) ΔH_f°(O₂,g)  = -a(- 393.51) - ½b(-285.830) + ΔH_f°(C_aH_bO_c) + (a + ¼b - ½c)*0

        = a(393.51) + b(142.915) + ΔH_f°(C_aH_bO_c)

Example:

What is the heat of comustion of liquid ethanol, with the formula C₂H₅OH (=C₂H₆O)?

For ethanol, the constants a, b and c are 2, 6 and 1, respectively, and the chemical equation for the combustion of ethanol: 

C₂H₆O(l) + 3O₂(g) → 2CO₂(g)+ 3H₂O(l)

The standard heat of formation of liquid ethanol, ΔH_f°(C₂H₆O, l), is -277.6 kJ/mol.

The heat of combustion of ethanol, ΔH_c°(C₂H₆O, l) = 2*393.51 + 6*142.915 + (-277.6) = 1366.91 kJ/mol. This can be converted to kJ per mass units:

The molweight of ethanol is (2*12.01 + 6*1.01 + 1*16.00) = 46.08 g/mol

The heat of combustion of ethanol, ΔH_c°(C₂H₆O, l) = 1366.91[kJ/mol] *1000[g/kg] / 48.08 [g/mol] = 29664 kJ/kg ethanol  = 29.7 MJ/kg = 12754 BTU/lb = 7086 kcal/kg

The table below shows values of heat of combustion calculated after the above described method. For substances containing nitrogen, it is assumed that the nitrogen atoms ends up as N₂ gas with ΔH_f°(N₂) = 0 kJ/mol. In such cases the general equation applies to also these substances. If it is known that other substances is formed in the cumbustion reactions, the exact products must be known to be able to calculate the heat of combustion.

Conversion to other units.

See also Fuel Gases Heating Values and Fossil Fuels - Energy Content.

For full table - rotate the screen!

Unit conversion

• 1 kJ/kg = 1 J/g = 10^-3 GJ/tonne = 0.000278 kWh/kg = 0.4299 Btu/ lb_m = 0.23884 kcal/kg
• 1 Btu/lb_m = 2.326 kJ/kg = 0.55 kcal/kg
• 1 kcal/kg = 4.1868 kJ/kg = 1.8 Btu/lb_m

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