Heat of Combustion Calculator
Calculate the higher heating value (HHV) of a fuel from its lower heating value (LHV), or compute any related combustion energy parameter.
🔥 Heat of Combustion (HHV) Calculator
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What Is Heat of Combustion?
The heat of combustion is the total amount of energy released as heat when a substance undergoes complete combustion with oxygen under standard conditions. It is a key thermodynamic property that determines how much useful energy a fuel can provide.
There are two important measures of combustion energy:
- Higher Heating Value (HHV) — Also called the gross calorific value. This includes the latent heat of vaporization of water in the combustion products. It assumes all water produced is condensed to liquid form.
- Lower Heating Value (LHV) — Also called the net calorific value. This excludes the latent heat of water vaporization, assuming all water exits as vapor.
Heat of Combustion Formula
The relationship between HHV and LHV is:
Where:
- HHV — Higher heating value (kJ/mol or other energy units)
- LHV — Lower heating value
- nH₂O — Moles of water produced per balanced equation
- nfuel — Moles of fuel in the balanced equation
- Hvap — Heat of vaporization of water = 44.01 kJ/mol = 2.257 MJ/kg
How to Calculate Heat of Combustion
- Write the balanced combustion equation for your fuel.
- Count the moles of water produced per mole of fuel burned.
- Determine the LHV of the fuel from reference data.
- Calculate the water energy contribution: (moles H2O / moles fuel) × 44.01 kJ/mol.
- Add to LHV to get the HHV.
Combustion: C8H18 + 12.5 O2 → 8 CO2 + 9 H2O
Given: LHV = 5,104.1 kJ/mol, Hvap = 44.01 kJ/mol
Step 1: Water ratio = 9 mol H2O / 1 mol fuel = 9
Step 2: Water energy = 9 × 44.01 = 396.09 kJ/mol
Step 3: HHV = 5,104.1 + 396.09 = 5,500.2 kJ/mol
HHV and LHV of Common Fuels
| Fuel | Formula | LHV (kJ/mol) | HHV (kJ/mol) |
|---|---|---|---|
| Hydrogen | H2 | 241.8 | 285.8 |
| Methane | CH4 | 802.3 | 890.3 |
| Propane | C3H8 | 2,043.5 | 2,219.5 |
| Methanol | CH3OH | 638.5 | 726.5 |
| Ethanol | C2H5OH | 1,234.8 | 1,366.8 |
| Octane | C8H18 | 5,104.1 | 5,500.2 |
What Is Heat of Vaporization?
The heat of vaporization (or enthalpy of vaporization) of water is the energy required to convert one mole of liquid water to steam at the boiling point. For water:
- 44.01 kJ/mol (per mole basis)
- 2.257 MJ/kg (per mass basis)
- 2,257 kJ/kg (same, in different units)
- 970.3 BTU/lb (imperial units)
This energy represents the difference between HHV and LHV. In most real-world combustion systems (engines, furnaces), the water exits as vapor, so the LHV better represents the actual usable energy. Condensing boilers, however, can recover some of this latent heat, approaching the HHV.
Why Does HHV vs. LHV Matter?
- Efficiency reporting: European standards typically report boiler efficiency based on LHV (so efficiencies can exceed 100% with condensing systems). North American standards often use HHV.
- Fuel comparison: When comparing fuels, you must use the same basis (HHV-to-HHV or LHV-to-LHV) for a fair comparison.
- Engineering design: Power plant and engine designers must account for whether they can recover the latent heat of water vapor.
Frequently Asked Questions
What is the difference between HHV and LHV?
HHV (Higher Heating Value) includes the latent heat released when water vapor condenses. LHV (Lower Heating Value) excludes it. The difference equals the amount of energy locked in the water vapor produced during combustion. HHV is always greater than or equal to LHV.
Why is the heat of vaporization of water important in combustion?
When fuels containing hydrogen burn, they produce water vapor. This vapor carries significant latent energy (44.01 kJ/mol). Whether this energy is recovered depends on whether the exhaust cools enough to condense the water. This directly impacts the efficiency calculation of any combustion system.
How do I find the moles of water produced?
Write the balanced combustion equation. Count the coefficient in front of H2O. For hydrocarbons CxHy, the moles of water produced per mole of fuel is y/2.