Heat of combustion is essentially a measurement of the energy content of a fuel or substance. To convert between BTU/lb, kJ/kg and more, use our free heat of combustion calculator.
In the section below, we answer all the questions you have about heat of combustion - what is it, what units does it involve, and how can you convert?
Combustion is the chemical reaction of a given fuel and oxygen, which generates heat.
Heat of combustion is the amount of energy released when a specified amount of a substance burns up. The substance is usually a food or fuel.
A fuel is a chemical substance that uses oxygen while burning, to produce heat energy.
Heat of combustion is an equation used to measure the performance of a certain fuel in a furnace, motor, or power generation turbine.
It's the amount of heat produced where one mole of a substance is burnt in oxygen. One mole is the number of atoms in 12 thousandths of a kilogram of C-12.
There are two sorts of heat of combustion. These are higher and lower heating values. It depends on whether water compounds can condense and on how much products can cool.
You can calculate these with a bomb calorimeter. That's a sealed vessel able to withstand high pressure.
You can also calculate these as the difference between the heat of formation of products and reactants.
We can express the heat of combustion in HHV, LHV and gross heating value (GHV).
In terms of units, the heat of combustion is often expressed as kJ/kg, BTU/lb or kJ/mol depending on what's being measured. It's also common to see readings in kilocalories per kilogram.
Working with different elements and different forms means you'll need to convert the heat of combustion to and between units.
You'll multiply your starting kg/cal by the required factor above to convert.
To convert Btu/lbm, you'll multiply your starting amount by one of the numbers above.
Simply multiply your starting amount of kJ/kg by any of the ratios above. If you want a much quicker and easier way to convert to other units, try our heat of combustion converter.
To see some examples for heat of combustion, look at the heating values below of two different fuels at 25 degrees Celsius.
Heat of combustion can be with a higher or lower heating value. Let's look at the difference between them.
You can determine the high heating value (HHV) by bringing all end-products to the pre-combustion temperature. You need to concentrate on condensing any vapor produced in this process.
This higher heating value considers the latent heat of water vaporization in the products of combustion. This is helpful in calculating heating values of fuels where condensation of the reaction substances is useful.
Examples are gas-fired boilers making space heat.
HHV assumes that all the water components remain in the liquid state after combustion. It also presumes that heat generated below 302 degrees Fahrenheit is usable.
Lower Heating Value (LHV) is the result when you subtract the heat of vaporization of the water from the HHV.
This method treats any water formed as vapor. This approach finds that energy required to vaporize water does not generate heat.
LHV assumes that water elements of the combustion process end up as vapor. This contrasts with the HHV, which assumes all water in the process of combustion is in a liquid state after the combustion process ends.
LHV is useful for comparing fuels for which the condensation of fuels isn't practical or where heat less than 302 degrees Fahrenheit isn't of use.
In summary, we measure HHV with the product of water in liquid form while we measure LHV with the product of water in vapor form. The difference between the two can cause confusion when references don't state the convention in use.
LHV might be useful for benchmarking purposes, but overall efficiency calculations should use HHV.
Always state the value as LHV or HHV to avoid confusion.
Enthalpies of combustion are always negative as they are exothermic. Why?
Since it involves heat, the reaction is exothermic. Heat goes out to the environment.
The energy needed to break bonds is less than the energy let out when bonds form. That's why the heat of combustion is always negative.
The actual symbol for the heat of combustion assumes a negative value.
Some tables express it as a positive, which can be confusing. They do this because they assume the negative value from the measurement unit.
So while we express the number as a positive, we assume you will interpret it as a negative due to its negative unit of value.
Now that you have a better understanding of just what heat of combustion is all about, you should be in a much better position to complete your work. It can be a tricky concept to get your head around, especially with the Higher Heating Value and Lower Heating Value differentials.
But now you know what it is you are measuring with heat of combustion. Your next stumbling block will be converting units of measurement. With easy online calculators like ours, you won't need to worry about that either.