Heat of Combustion: LHV, HHV, and Fuel Energetics
Heat of combustion is the energy released when a fuel burns completely. Lower heating value (LHV) excludes water vapor condensation; higher heating value (HHV) includes it. Bomb calorimetry measures HHV at constant volume. Hess's law allows calculation from formation enthalpies.
Why This Chemistry Calculation Matters
Why: Heat of combustion determines fuel energy density for engines, boilers, and fuel cells. LHV is used when water stays vapor (e.g., exhaust); HHV when water condenses (e.g., condensing boilers).
How: Use Hc = LHV + Hv × (nH₂O/nfuel). Balance the combustion equation to get moles of water per mole of fuel. Hv ≈ 2.257 MJ/kg for water. Bomb calorimeters measure HHV directly.
- ●Hydrogen has the highest LHV (~120 MJ/kg) of common fuels.
- ●HHV - LHV ≈ 2.257 × (moles H₂O per mole fuel) MJ/kg.
- ●Bomb calorimeters operate at constant volume; coffee-cup at constant pressure.
Compact Examples
Inputs
⚠️For educational and informational purposes only. Verify with a qualified professional.
🔬 Chemistry Facts
H₂ LHV ≈ 120 MJ/kg; CH₄ ≈ 50 MJ/kg.
— NIST
Hv (water) ≈ 2.257 MJ/kg at 100°C.
— CRC
Bomb calorimeters measure ΔU (constant volume).
— ASTM
CH₄ + 2O₂ → CO₂ + 2H₂O: 2 mol H₂O per mol CH₄.
— Stoichiometry
📋 Key Takeaways
- • Hc = LHV + Hv × (nH₂O/nfuel) | HHV from LHV and water formed
- • LHV excludes water vapor condensation; used for engines
- • HHV includes condensation; used for boilers, fuel cells
- • Hv ≈ 2.257 MJ/kg | Latent heat of vaporization of water
Did You Know?
Hydrogen has the highest LHV (~120 MJ/kg) of common fuels—3× methane.
Source: NIST
HHV - LHV ≈ 2.257 × (moles H₂O per mole fuel) MJ/kg.
Source: IUPAC
Car engines use LHV since exhaust water stays vapor.
Source: Automotive
Fuel cells can recover HHV when water condenses.
Source: Electrochemistry
Bomb calorimeters measure HHV (constant volume).
Source: ASTM
CH₄ + 2O₂ → CO₂ + 2H₂O: 2 mol H₂O per mol CH₄.
Source: Stoichiometry
How the Heat of Combustion Calculator Works
Enter LHV, moles of water vaporized, and moles of fuel combusted. Or use masses with molar masses. The calculator adds the latent heat of vaporization contribution to get HHV.
From LHV
Hc = LHV + Hv × (nH₂O/nfuel)
From Masses
Convert mass to moles, then apply formula
Expert Tips
Stoichiometry
Balance combustion equation to get nH₂O/nfuel.
Fuel Database
Select fuel for pre-filled LHV values.
Hv Default
2.257 MJ/kg is standard for water at 100°C.
Complete Combustion
Assumes 100% combustion to CO₂ and H₂O.
Common Fuels LHV (MJ/kg)
| Fuel | Formula | LHV | Type |
|---|---|---|---|
| Hydrogen | H_{2} | 120 | Gas |
| Methane | ext{CH}₄ | 50 | Natural gas |
| Ethane | C_{2}H₆ | 47.5 | Natural gas |
| Propane | C_{3}H₈ | 46.4 | LPG |
| Butane | C₄H_{1}_{0} | 45.7 | LPG |
| Pentane | C₅H_{1}_{2} | 45.4 | Liquid fuel |
| Paraffin wax | C_{2}₅H₅_{2} | 42 | Solid fuel |
| Kerosene | C_{1}_{2}H_{2}₆ | 43 | Aviation fuel |
| Diesel | C_{1}₄H_{3}_{0} | 42.8 | Diesel fuel |
| Natural gas | ext{Mixed} | 38 | Natural gas |
Frequently Asked Questions
What is LHV vs HHV?
LHV excludes water vapor condensation energy; HHV includes it. HHV = LHV + Hv × (nH₂O/nfuel).
When to use LHV?
Internal combustion engines, gas turbines—water vapor exits in exhaust.
When to use HHV?
Boilers, fuel cells, condensing systems—water condenses and releases latent heat.
What is Hv?
Latent heat of vaporization of water ≈ 2.257 MJ/kg at 100°C.
How to find nH₂O/nfuel?
Balance the combustion equation. CH₄ + 2O₂ → CO₂ + 2H₂O gives 2/1 = 2.
Why does hydrogen have highest LHV?
Lightest fuel; energy per kg is highest. Per volume it is lower.
What units?
MJ/kg (megajoules per kilogram) for energy density.
Key Numbers
📚 Official Sources
⚠️ Disclaimer: This calculator uses NIST and IUPAC thermochemical conventions. For precise work, consult NIST Chemistry WebBook for combustion enthalpies and ASTM standards for bomb calorimetry.