Summary

The heat of combustion is the energy liberated when a substance undergoes complete combustion, at constant pressure usually in an environment with excess Oxygen. The heat of combustion is utilised to quantify the performance of a fuel in a combustion system such as furnaces, power generation turbines and motors. This article describes the heat of combustion and provides a list of heats of combustion for commons fuels and fuel components.

Definitions


$c_{p}$	:	Specific heat capacity
$h_{vap,water}$	:	Heat of vaporisation of water $(h_{vap,water} = 40.68 \; kJ/mol)$
$m$	:	Mass
$n$	:	Stoichiometric coefficient
$Q$	:	Heat transfer duty
$T$	:	Temperature
$\Delta H_{c}$	:	Heat of combustion

Heating Value

The heat of combustion is typically presented in the form of a heating value. The heating value is the amount of energy released during combustion and can be referenced as a higher or lower heating value.

Higher Heating Value

The higher heating value (HHV) accounts for the heat of combustion and any energy released to bring the combustion products back their pre-combustion temperatures (typically 25°C). By bringing the combustion products back to pre-combustion temperatures, the water component of the combustion products condenses and therefore the latent heat of vaporisation of water is incorporated in the higher heating value. The higher heating value is most useful in circumstances where condensation of combustion products is practical.

Lower Heating Value

The lower heating value (LHV) assumes that the combustion products are not brought back to pre-combustion temperatures and is therefore essentially the higher heating value minus the latent heat of vaporisation of the water product. The lower heating value may be approximated from the higher heating value as follows:

$LHV = HHV - h_{vap,water} \times \frac{n_{H_2O}}{n_{fuel}}$

The accuracy of the conversion between LHV and HHV may be improved by accounting for the change in sensible heat of the combustion products.

Molar Heats of Combustion

The molar heat of combustion (HHV) for a selection of compounds are presented below.

Type	Fuel	Formula	Molecular Weight	Combustion Reaction (25 °C)	Heat of Combustion $(\Delta H_c)$ $kJ/mol (@25°C)$	Energy Density (MJ/kg)	Rel. to Octane (=100%)
Alkane	Hydrogen	$\mathrm{H_2}$	2.0	$\mathrm{2H_2 + O_2 \rightarrow 2H_2O}$	-286	141.00	298.5
Alkane	Methane	$\mathrm{CH_4}$	16.0	$\mathrm{CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O}$	-890	55.6	116.0
Alkane	Ethane	$\mathrm{C_2H_6}$	30.1	$\mathrm{2C_2H_6 + 7O_2 \rightarrow 4CO_2 + 6H_2O}$	-1560	51.8	108.1
Alkane	Propane	$\mathrm{C_3H_8}$	44.1	$\mathrm{C_3H_8 + 5O_2 \rightarrow 3CO_2 + 4H_2O}$	-2220	50.3	105.0
Alkane	Butane	$\mathrm{C_4H_{10}}$	58.1	$\mathrm{2C_4H_{10} + 13O_2 \rightarrow 8CO_2 + 10H_2O}$	-2877	49.5	103.3
Alkane	Pentane	$\mathrm{C_5H_{12}}$	72.1	$\mathrm{C_5H_{12} + 8O_2 \rightarrow 5CO_2 + 6H_2O}$	-3509	48.7	101.7
Alkane	Hexane	$\mathrm{C_6H_{14}}$	86.2	$\mathrm{2C_6H_{14} + 19O_2 \rightarrow 12CO_2 + 14H_2O}$	-4163	48.3	100.8
Alkane	Heptane	$\mathrm{C_7H_{16}}$	100.2	$\mathrm{C_7H_{16} + 11O_2 \rightarrow 7CO_2 + 8H_2O}$	-4817	48.1	100.4
Alkane	Octane	$\mathrm{C_8H_{18}}$	114.2	$\mathrm{2C_8H_{18} + 25O_2 \rightarrow 16CO_2 + 18H_2O}$	-5471	47.91	100.0
Alkane	Nonane	$\mathrm{C_9H_{20}}$	128.3	$C_9H_{20}(l) + 14O_2(g) → 9CO_2(g) + 10H_2O(l)$	-6125	47.7	99.6
Alkane	Decane	$\mathrm{C_{10}H_{22}}$	142.3	$2C_{10}H_{22}(l) + 31O_2(g) → 20CO_2(g) + 22H_2O(l)$	-6778	47.6	99.4
Alkane	Undecane	$\mathrm{C_{11}H_{24}}$	156.3	$C_{11}H_{24}(l) + 16O_2(g) → 11CO_2(g) + 12H_2O(l)$	-7431	47.5	99.2
Alkane	Dodecane	$\mathrm{C_{12}H_{26}}$	170.3	$2C_{12}H_{26}(l) + 37O_2(g) → 24CO_2(g) + 26H_2O(l)$	-8087	47.5	99.1
Alkane	Hexadecane	$\mathrm{CH_3(CH_2)_{14}CH_3}$	226.4	$2CH_3(CH_2)_{14}CH_3(l) + 49O_2(g) → 32CO_2(g) + 34H_2O(l)$	-10699	47.3	98.8
Alkene	Ethylene	$\mathrm{C_2H_4}$	28.1	$\mathrm{C_2H_4 + 3O_2 \rightarrow 2CO_2 + 2H_2O}$	-1411	50.21	104.8
Alkene	Propylene	$\mathrm{C_3H_6}$	42.1	$\mathrm{2C_3H_6 + 9O_2 \rightarrow 6CO_2 + 6H_2O}$	-2058	48.88	102.0
Alcohol	Methanol	$\mathrm{CH_3OH}$	32.0	$\mathrm{2CH_3OH + 3O_2 \rightarrow 2CO_2 + 4H_2O}$	-726	22.69	47.4
Alcohol	Ethanol	$\mathrm{C_2H_5OH}$	46.1	$\mathrm{C_2H_5OH + 3O_2 \rightarrow 2CO_2 + 3H_2O}$	-1367	29.65	61.9
Alcohol	1-Propanol	$\mathrm{C_3H_7OH}$	60.1	$\mathrm{2C_3H_7OH + 9O_2 \rightarrow 6CO_2 + 8H_2O}$	-2021	33.63	70.2
Alcohol	2-Propanol	$\mathrm{CH_3CH(OH)CH_3}$	60.1	$\mathrm{2C_3H_7OH + 9O_2 \rightarrow 6CO_2 + 8H_2O}$	-2006	33.38	69.7
Alcohol	1-Butanol	$\mathrm{C_4H_9OH}$	74.1	$\mathrm{C_4H_9OH + 6O_2 \rightarrow 4CO_2 + 5H_2O}$	-2676	36.11	75.4
Alcohol	1-Pentanol	$\mathrm{C_5H_{11}OH}$	88.2	$\mathrm{2C_5H_{11}OH + 15O_2 \rightarrow 10CO_2 + 12H_2O}$	-3331	37.77	78.8
Alcohol	1-Hexanol	$\mathrm{C_6H_{13}OH}$	102.2	$\mathrm{C_6H_{13}OH + 9O_2 \rightarrow 6CO_2 + 7H_2O}$	-3984	38.98	81.4
Alcohol	1-Heptanol	$\mathrm{C_7H_{15}OH}$	116.2	$\mathrm{2C_7H_{15}OH + 21O_2 \rightarrow 14CO_2 + 16H_2O}$	-4638	39.91	83.3
Alcohol	1-Octanol	$\mathrm{C_8H_{17}OH}$	130.2	$\mathrm{C_8H_{17}OH + 12O_2 \rightarrow 8CO_2 + 9H_2O}$	-5294	40.66	84.9
Cyclic	Benzene	$\mathrm{C_6H_6}$	78.1	$\mathrm{2C_6H_6 + 15O_2 \rightarrow 12CO_2 + 6H_2O}$	-3267.5	41.84	87.3
Cyclic	Toluene	$\mathrm{C_7H_8}$	92.1	$\mathrm{C_7H_8 + 9O_2 \rightarrow 7CO_2 + 4H_2O}$	-3910	42.45	88.6
Cyclic	Xylene	$\mathrm{C_8H_{10}}$	106.2	$\mathrm{C_8H_{10} + 10.5O_2 \rightarrow 8CO_2 + 5H_2O}$	-4348	40.94	85.5
Cyclic	Cyclohexane	$\mathrm{C_6H_{12}}$	84.2	$\mathrm{C_6H_{12} + 9O_2 \rightarrow 6CO_2 + 6H_2O}$	-3920	46.56	97.2
Inorganic	Carbon	graphite	12.0	$\mathrm{C + O_2 \rightarrow CO_2}$	-394	32.83	68.5
Inorganic	Carbon Monoxide	$\mathrm{CO}$	28.0	$\mathrm{2CO + O_2 \rightarrow 2CO_2}$	-283	10.11	21.1
Ketone	Acetone	$\mathrm{C_3H_6O}$	58.1	$\mathrm{2C_3H_6O + 8O_2 \rightarrow 6CO_2 + 6H_2O}$	-1790	30.81	64.3

The heat of combustion is exothermic, that is, energy is liberated through the combustion reaction. To calculate the total heat generation for the fuel listed above simply multiply the number of moles of fuel burnt by the molar heat of combustion listed above.

Heats of Combustion for Commercial Fuels

Below the heat of combustion (HHV) for several common commercial fuels is presented in order of decreasing heating value.

Commercial Fuels	Typical Heat of Combustion $(ΔH_c)$ MJ/kg
Natural Gas	-54
Gasoline	-47.3
Kerosene	-46.2
Diesel Fuel	-44.8
Ethanol	-29.7
Coal (Anthracite)	-27.0
Methanol	-22.7
Wood	-15.0
Coal (Lignite)	-15.0

Experimental Calculation of Heat of Combustion

In the absence of published data the heat of combustion (LHV) can be experimentally determined using the following procedure:

Measure a known quantity of water into a flask and stand this on top of a tripod situated above a fuel source.
Measure the initial temperature of the water and keep a thermometer submersed in the water to measure any temperature changes.
Measure the initial mass of the fuel being used. For example methanol in a controlled burning manner e.g. through a wick.
When the temperature of the water has increase by at least 10 °C then the experiment can be stopped by extinguishing the flame.
Record the final mass of the fuel.
Record the final temperature of the water and calculate the temperature change.
Calculate the heat of combustion using the methodology demonstrated below.

An example of the experimental calculation process is shown as follows. Please note that these numbers are not obtained from an actual experiment, they are purely for demonstrating the experimental and calculation method to apply if performing the heat of combustion determination experiment please refer to literature for actual heat of combustion values for different fuels and do not undertake any experiments without completely understanding and controlling the hazards associated with the experiment.

Calculation Parameter	Value
Initial temperature of water	20°C
Final temperature of water	30°C
Change in temperature of water	10°C
Mass of water	100g
Initial mass of methanol	20.00g
Final mass of methanol	19.79g
Fuel consumed	0.21g
Molecular Weight of Methanol	32 g/mol

Step 1: Calculate mols of fuel consumed

$\begin{aligned} n &= \frac{0.21 g}{32 g/mol} \\ &= 6.55 \times 10^{-3} mols\\ \end{aligned}$

Step 2: Calculate enthalpy increase of water

$\begin{aligned} Q &= m \times c_p \times \Delta T \\ &= 100g \times 4.18 J/g.K \times \left( 30\degree C - 20\degree C \right) \\ &= 100g \times 4.18 J/g.K \times 10 K \\ &= 4180 J \\ &= 4.18 kJ \\ \end{aligned}$

Step 3: Calculate the heat of combustion assuming heat not lost to surroundings

$\begin{aligned} \Delta H_c &= \frac{Q}{n} \\ &= \frac{4.18 kJ}{6.55 \times 10^{-3} mols} \\ &= 638.6 kJ/mol \end{aligned}$

Therefore, from the experiment we can deduct the heat of combustion of methanol is 638.6 kJ/mol.