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# Combustion Heat

## Heat of combustion (energy content) for som common substances - with examples how to calculate heat of combustion.

Go to tabulated values

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.

n X + m O2 x CO2(g) + y H2O (l) + z Z + 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 ° = - x ΔHf °(CO2,g) - y ΔHf °(H2O,l) - z ΔHf °(Z) + n ΔHf °(X) + m ΔHf °(O2,g)

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

ΔHf ° : 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 (ΔHf °) of the substances involved in the reaction, given as tabulated values.

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

C a H b O 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 ° = -a ΔHf °(CO2,g) - ½b ΔHf °(H2O,l) + ΔHf °(C a H b O c ) + (a + ¼b - ½c) ΔHf °(O2,g) = -a(- 393.51) - ½b(-285.830) + ΔHf °(C a H b O c ) + (a + ¼b - ½c)*0

= a(393.51) + b(142.915) + ΔHf °(C a H b O c )

Example:

What is the heat of comustion of liquid ethanol, with the formula C2H5 OH (=C2H6 O)?

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

C2H6 O(l) + 3O2(g) → 2CO2(g)+ 3H2O(l)

The standard heat of formation of liquid ethanol , ΔHf °(C2H6 O, l), is -277.6 kJ/mol.

The heat of combustion of ethanol, ΔH c °(C2H6 O, 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 °(C2H6 O, 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 N2gas with ΔHf °(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.

For full table - rotate the screen!

 Name Formula State* ΔH c ° kJ/mol ΔH c ° kJ/g or MJ/kg ΔH c ° kJ/kg ΔH c ° BTU/lb ΔH c ° kcal/kg Acetaldehyde C2H 4 O liq 1167 26.49 26487 11387 6326 Acetamide C2H5 NO cry 1185 20.06 20058 8623 4791 Acetic acid C2H 4 O 2 liq 874 14.55 14552 6256 3476 Acetone C3 H6 O liq 1790 30.81 30814 13248 7360 Acetonitrile C2H3 N liq 1256 30.59 30589 13151 7306 Acetylene (ethyne) C2H 2 gas 1300 49.92 49923 21463 11924 l-Alanine C3 H 7 NO 2 cry 1577 17.70 17697 7608 4227 Ammonia NH3 gas 383 22.48 22477 9663 5369 Aniline C6 H 7 N liq 3393 36.43 36429 15662 8701 Anthracene C 14 H 10 cry 7068 39.65 39654 17048 9471 Benzene C6 H6 liq 3268 41.83 41833 17985 9992 Benzoic acid C 7 H6 O 2 cry 3228.2 26.43 26432 11364 6313 1,3-Butadiene C 4 H6 gas 2542 46.99 46987 20201 11223 Butane C 4 H 10 gas 2878 49.50 49501 21282 11823 1-Butanol C 4 H 10 liq 2676 36.09 36092 15517 8621 2-Butanone C 4 H 8 O liq 2444 33.89 33888 14569 8094 1-Butene C 4 H 8 gas 2718 48.43 48432 20822 11568 cis-2 Butene C 4 H 8 gas 2710 48.29 48289 20761 11534 trans-2-Butene C 4 H 8 gas 2706 48.22 48218 20730 11517 Butanoic acid C 4 H 8 O 2 liq 2183.6 38.91 38909 16728 9293 Butylbenzene C 10 H 14 liq 5872.7 43.8 43754 18810 24994 Carbon (graphite) C cry 394 32.81 32806 14104 7836 Carbon monoxide CO gas 283 10.10 10104 4344 2413 Cyclobutane C 4 H 8 gas 2745.1 48.91 48914 21029 11683 Cyclobutene C 4 H6 gas 2588 47.84 47837 20566 11426 Cyclohexane C6 H 12 liq 3920 46.57 46567 20020 11122 Cyclopentane C5 H 10 liq 3291.6 46.92 46922 20173 11207 Cyclopropane C3 H6 gas 2091 49.68 49679 21358 11866 Decane C 10 H 22 liq 6778 47.63 47625 20475 11375 Diethyl ether (ethoxyethane) C 4 H 10 O liq 2724 36.74 36741 15796 8775 Dimethyl ether (methoxymethane) C2H6 O gas 1460 31.68 31684 13622 7568 Ethane C2H6 gas 1561 51.89 51895 22311 12395 1,2-Ethanediol C2H6 O 2 liq 1185 19.09 19088 8206 4559 Ethanol C2H6 O liq 1367 29.67 29666 12754 7086 Ethyl acetate C 4 H 8 O 2 liq 2238 25.40 25397 10919 6066 Ethylene (ethene) C2H 4 gas 1411 50.29 50285 21619 12010 Formaldehyde CH2O gas 571 19.01 19014 8175 4541 Formic acid CH2O 2 liq 254 5.52 5518 2372 1318 Glycerol C3 H 8 O3 liq 1654 17.96 17957 7720 4289 Heptane C 7 H 16 liq 4817 48.06 48059 20662 11479 Heptanoic acid C 7 H 14 O 2 liq 4145.2 31.83 31835 13687 7604 Hexane C6 H 14 liq 4163 48.31 48307 20767 11538 Hexadecanoic acid C 16 H 32 O 2 liq 10031.3 39.11 39112 16815 9342 Hexanoic acid C6 H 12 O 2 liq 3492.2 30.06 30059 12923 7179 Hydrazine N2H 4 liq 622 19.40 19401 8341 4634 Hydrogen H 2 gas 286 141.58 141584 60870 33817 Hydrogen cyanide CHN gas 672 24.86 24861 10688 5938 Ketene C2H2O gas 1025 24.38 24382 10482 5824 Methane CH 4 gas 891 55.51 55514 23867 13259 Methanol CH 4 O liq 726 22.65 22652 9739 5410 Methyl acetate C3 H6 O 2 liq 1592 21.49 21487 9238 5132 Methylamine CH5 N gas 1086 34.95 34953 15027 8348 Methylcyclohexane C 7 H 14 liq 4565.3 46.48 46485 19985 11103 Methylcyclopentane C6 H 12 liq 3938.1 46.78 46782 20113 11174 Methyl formate C2H 4 O 2 liq 973 16.20 16200 6965 3869 Methyl tert-butyl ether C5 H 12 O liq 3369 38.21 38209 16427 9126 Naphthalene C 10 H 8 cry 5157 40.23 40232 17294 9609 Nitric oxide NO gas 91 3.03 3032 1304 724 Nitrobenzene C6 H5 NO 2 liq 3088 25.08 25081 10783 5990 Nitromethane CH3 NO 2 liq 710 11.63 11630 5000 2778 Nitrous oxide N2O gas 82 1.86 1863 801 445 Nonane C 9 H 20 liq 6125 47.74 47743 20526 11403 Octane C 8 H 18 liq 5470 47.87 47873 20582 11434 1-Octanol C 8 H 18 O liq 5294 40.64 40642 17473 9707 Pentane C5 H 12 liq 3509 48.62 48621 20903 11613 Pentanoic acid C5 H 10 O 2 liq 2837.3 27.78 27776 11942 6634 1-Pentanol C5 H 12 O liq 3331 37.78 37779 16242 9023 Phenanthrene C 14 H 10 cry 7055 39.58 39581 17017 9454 Phenol C6 H6 O cry 3054 32.45 32448 13950 7750 Propanal C3 H6 O liq 1522 26.20 26201 11264 6258 Propane C3 H 8 gas 2220 50.33 50329 21638 12021 Propanenitrile C3 H5 N liq 1911 34.69 34689 14914 8285 Propanoic acid C3 H6 O 2 liq 1527.3 20.61 20614 8862 4924 1-Propanol C3 H 8 O liq 2021 33.62 33622 14455 8030 2-Propanol C3 H 8 O liq 2006 33.37 33372 14347 7971 Propene C3 H6 gas 2058 48.90 48895 21021 11678 Propyne C3 H 4 gas 185.0 4.62 4617 1985 1103 Pyridine C5 H5 N liq 2782 35.17 35166 15119 8399 Toluene (methylbenzene) C 7 H 8 liq 3910 42.43 42431 18242 10134 Trimethylamine C3 H 9 N gas 2443 41.32 41316 17763 9868 2,4,6-Trinitrotoluene C 7 H5 N3 O6 cry 3406 14.99 14994 6446 3581 Undecane C 11 H 24 liq 7431.4 47.53 47530 20434 11352 Urea CH 4 N2O cry 632.7 10.53 10533 4528 2516 *  gas = gas,  liq = liquid,  cry = crystalline (solid)

Unit conversion

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

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## Citation

• The Engineering ToolBox (2017). Combustion Heat. [online] Available at: https://www.engineeringtoolbox.com/standard-heat-of-combustion-energy-content-d_1987.html [Accessed Day Month Year].

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