Combustion Heat

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 ΔH_f °(CO₂,g) - y ΔH_f °(H₂O,l) - z ΔH_f °(Z) + n ΔH_f °(X) + m ΔH_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_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 ΔH_f °(CO₂,g) - ½b ΔH_f °(H₂O,l) + ΔH_f °(C_a H_b O_c ) + (a + ¼b - ½c) ΔH_f °(O₂,g) = -a(- 393.51) - ½b(-285.830) + ΔH_f °(C_a H_b O_c ) + (a + ¼b - ½c)*0

= a(393.51) + b(142.915) + ΔH_f °(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, ΔH_f °(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 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.

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Standard Heat of Combustion

Name	Formula	State*	ΔHc ° kJ/mol	ΔHc ° kJ/g or MJ/kg	ΔHc ° kJ/kg	ΔHc ° BTU/lb	ΔHc ° kcal/kg
Acetaldehyde	C2H4 O	liq	1167	26.49	26487	11387	6326
Acetamide	C2H5 NO	cry	1185	20.06	20058	8623	4791
Acetic acid	C2H4 O2	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)	C2H2	gas	1300	49.92	49923	21463	11924
l-Alanine	C3 H7 NO2	cry	1577	17.70	17697	7608	4227
Ammonia	NH3	gas	383	22.48	22477	9663	5369
Aniline	C6 H7 N	liq	3393	36.43	36429	15662	8701
Anthracene	C14 H10	cry	7068	39.65	39654	17048	9471
Benzene	C6 H6	liq	3268	41.83	41833	17985	9992
Benzoic acid	C7 H6 O2	cry	3228.2	26.43	26432	11364	6313
1,3-Butadiene	C4 H6	gas	2542	46.99	46987	20201	11223
Butane	C4 H10	gas	2878	49.50	49501	21282	11823
1-Butanol	C4 H10	liq	2676	36.09	36092	15517	8621
2-Butanone	C4 H8 O	liq	2444	33.89	33888	14569	8094
1-Butene	C4 H8	gas	2718	48.43	48432	20822	11568
cis-2 Butene	C4 H8	gas	2710	48.29	48289	20761	11534
trans-2-Butene	C4 H8	gas	2706	48.22	48218	20730	11517
Butanoic acid	C4 H8 O2	liq	2183.6	38.91	38909	16728	9293
Butylbenzene	C10 H14	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	C4 H8	gas	2745.1	48.91	48914	21029	11683
Cyclobutene	C4 H6	gas	2588	47.84	47837	20566	11426
Cyclohexane	C6 H12	liq	3920	46.57	46567	20020	11122
Cyclopentane	C5 H10	liq	3291.6	46.92	46922	20173	11207
Cyclopropane	C3 H6	gas	2091	49.68	49679	21358	11866
Decane	C10 H22	liq	6778	47.63	47625	20475	11375
Diethyl ether (ethoxyethane)	C4 H10 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 O2	liq	1185	19.09	19088	8206	4559
Ethanol	C2H6 O	liq	1367	29.67	29666	12754	7086
Ethyl acetate	C4 H8 O2	liq	2238	25.40	25397	10919	6066
Ethylene (ethene)	C2H4	gas	1411	50.29	50285	21619	12010
Formaldehyde	CH2O	gas	571	19.01	19014	8175	4541
Formic acid	CH2O2	liq	254	5.52	5518	2372	1318
Glycerol	C3 H8 O3	liq	1654	17.96	17957	7720	4289
Heptane	C7 H16	liq	4817	48.06	48059	20662	11479
Heptanoic acid	C7 H14 O2	liq	4145.2	31.83	31835	13687	7604
Hexane	C6 H14	liq	4163	48.31	48307	20767	11538
Hexadecanoic acid	C16 H32 O2	liq	10031.3	39.11	39112	16815	9342
Hexanoic acid	C6 H12 O2	liq	3492.2	30.06	30059	12923	7179
Hydrazine	N2H4	liq	622	19.40	19401	8341	4634
Hydrogen	H2	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	CH4	gas	891	55.51	55514	23867	13259
Methanol	CH4 O	liq	726	22.65	22652	9739	5410
Methyl acetate	C3 H6 O2	liq	1592	21.49	21487	9238	5132
Methylamine	CH5 N	gas	1086	34.95	34953	15027	8348
Methylcyclohexane	C7 H14	liq	4565.3	46.48	46485	19985	11103
Methylcyclopentane	C6 H12	liq	3938.1	46.78	46782	20113	11174
Methyl formate	C2H4 O2	liq	973	16.20	16200	6965	3869
Methyl tert-butyl ether	C5 H12 O	liq	3369	38.21	38209	16427	9126
Naphthalene	C10 H8	cry	5157	40.23	40232	17294	9609
Nitric oxide	NO	gas	91	3.03	3032	1304	724
Nitrobenzene	C6 H5 NO2	liq	3088	25.08	25081	10783	5990
Nitromethane	CH3 NO2	liq	710	11.63	11630	5000	2778
Nitrous oxide	N2O	gas	82	1.86	1863	801	445
Nonane	C9 H20	liq	6125	47.74	47743	20526	11403
Octane	C8 H18	liq	5470	47.87	47873	20582	11434
1-Octanol	C8 H18 O	liq	5294	40.64	40642	17473	9707
Pentane	C5 H12	liq	3509	48.62	48621	20903	11613
Pentanoic acid	C5 H10 O2	liq	2837.3	27.78	27776	11942	6634
1-Pentanol	C5 H12 O	liq	3331	37.78	37779	16242	9023
Phenanthrene	C14 H10	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 H8	gas	2220	50.33	50329	21638	12021
Propanenitrile	C3 H5 N	liq	1911	34.69	34689	14914	8285
Propanoic acid	C3 H6 O2	liq	1527.3	20.61	20614	8862	4924
1-Propanol	C3 H8 O	liq	2021	33.62	33622	14455	8030
2-Propanol	C3 H8 O	liq	2006	33.37	33372	14347	7971
Propene	C3 H6	gas	2058	48.90	48895	21021	11678
Propyne	C3 H4	gas	185.0	4.62	4617	1985	1103
Pyridine	C5 H5 N	liq	2782	35.17	35166	15119	8399
Toluene (methylbenzene)	C7 H8	liq	3910	42.43	42431	18242	10134
Trimethylamine	C3 H9 N	gas	2443	41.32	41316	17763	9868
2,4,6-Trinitrotoluene	C7 H5 N3 O6	cry	3406	14.99	14994	6446	3581
Undecane	C11 H24	liq	7431.4	47.53	47530	20434	11352
Urea	CH4 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/ 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