The bomb calorimeter experiment determines the calorific value of a solid or liquid fuel by burning a known mass in oxygen inside a sealed steel “bomb” and measuring the heat released. It is the standard laboratory method for rating fuels such as coal, diesel, and biofuels, and a core thermodynamics practical for mechanical and chemical engineering students.
Aim of the experiment
To determine the higher (gross) and lower (net) calorific value of a given fuel using a bomb calorimeter.
Theory
Calorific value is the heat released by the complete combustion of unit mass of a fuel. The higher calorific value (HCV) includes the latent heat recovered when the water vapour formed condenses; the lower calorific value (LCV) excludes it. A bomb calorimeter measures HCV at constant volume.
The heat released is found by an energy balance — the heat from the burning fuel raises the temperature of the surrounding water:
HCV = (W + w) × c × (t₂ − t₁) / mf
where W is the mass of water in the calorimeter, w is the water equivalent of the apparatus, c is the specific heat of water (4.187 kJ/kg·K), t₁ and t₂ are the initial and final temperatures, and mf is the mass of fuel burnt. A cooling (radiation) correction is applied to the temperature rise.
The lower calorific value is then:
LCV = HCV − 9 × H × hfg
where H is the mass fraction of hydrogen in the fuel and hfg ≈ 2454 kJ/kg is the latent heat of steam (each kg of hydrogen forms 9 kg of water).
Apparatus required
- Bomb calorimeter (stainless-steel bomb, crucible, fuse wire, oxygen valve)
- Copper calorimeter with a measured mass of water and a stirrer
- Beckmann or precision thermometer
- Oxygen cylinder, fuel pellet press, and a precision balance
Procedure
- Weigh a small pellet of fuel and place it in the crucible, connected by the fuse wire.
- Assemble the bomb and charge it with oxygen to about 25 atmospheres.
- Immerse the bomb in a known mass of water in the calorimeter; start the stirrer.
- Record the water temperature at regular intervals, then fire the charge electrically.
- Note the maximum temperature reached and apply the cooling correction.
- Calculate HCV from the energy balance, then LCV using the hydrogen content.
Precautions
- Ensure complete combustion and a leak-free bomb charged with sufficient oxygen.
- Weigh the fuel and water accurately.
- Apply the radiation/cooling correction to the temperature rise for accuracy.
Applications
Calorific value governs the rating of coal, petroleum fuels, and biofuels, and feeds directly into boiler and engine design. The value measured here is exactly the CV used to compute brake thermal efficiency in the IC engine performance test. Related thermodynamics practicals include the refrigeration cycle experiment and the compressor test rig. See the key terms in our engineering lab glossary.
Frequently asked questions
What is the difference between HCV and LCV?
HCV (higher calorific value) includes the latent heat released when combustion water vapour condenses; LCV (lower calorific value) excludes it, so LCV is always less than HCV.
Why is the fuel burned in oxygen at high pressure?
High-pressure oxygen (about 25 atm) ensures rapid and complete combustion of the fuel, which is essential for an accurate calorific value.
Why does a bomb calorimeter measure constant-volume calorific value?
The fuel burns inside a sealed, rigid bomb of fixed volume, so the heat released is measured at constant volume rather than constant pressure.
Need a bomb calorimeter for your thermodynamics or fuels lab? Request a quote from Scientico India — ISO 9001:2015 certified, CE marked, exporting to 60+ countries, reply within 24 hours.
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