Calorific Value of a Fuel: Definition, Units and How to Calculate It

#Units

Fuel type	Common unit	SI unit
Solid	kJ/kg	J/kg
Liquid	kJ/kg	J/kg
Gas	kJ/m³	J/m³

Where older or regional units appear:

• cal/g or kcal/kg — 1 cal = 4.184 J, so 1 kcal/kg = 4.184 kJ/kg
• MJ/kg — 1 MJ/kg = 1,000 kJ/kg
• BTU/lb — common in North American engineering; 1 BTU/lb ≈ 2.326 kJ/kg
• kWh/kg — used in energy costing; 1 kWh = 3,600 kJ

The exam-standard answer to "calorific value is expressed in" is kJ/kg.

#Typical values

Fuel	Calorific value (kJ/kg)
Hydrogen	~150,000
Methane / natural gas	~55,000
LPG	~55,000
Petrol	~45,000
Kerosene	~43,000
Diesel	~45,000
Biogas	~35,000–40,000
Anthracite coal	~30,000–35,000
Charcoal	~30,000
Bituminous coal	~25,000–33,000
Dry wood	~17,000–22,000
Cow dung cake	~6,000–8,000

Two things stand out. Hydrogen is far ahead of everything else by mass, which is why it is the standard answer to "which fuel has the highest calorific value". And the gap between LPG and cow dung is about sevenfold, which is the entire practical argument for cleaner domestic fuels.

#How it is measured

The laboratory instrument is a bomb calorimeter:

1. A weighed sample of fuel is sealed in a strong steel vessel — the "bomb" — filled with oxygen at high pressure.
2. The bomb is immersed in a known mass of water.
3. The sample is ignited electrically.
4. The temperature rise of the water is measured precisely.

The heat released is then:

Q = (m_water x c_water + C_calorimeter) x ΔT

and the calorific value is:

CV = Q / mass of fuel burned

Worked example. Burning 2 g of a fuel raises the temperature of 2 kg of water by 5.4 °C. Taking the specific heat capacity of water as 4.2 kJ/kg°C and ignoring the calorimeter's own capacity:

Q = 2 x 4.2 x 5.4 = 45.36 kJ
CV = 45.36 kJ / 0.002 kg = 22,680 kJ/kg

That is roughly the value of a good bituminous coal.

#Gross versus net calorific value

Burning any hydrogen-containing fuel produces water. Whether you count the energy released when that water condenses gives two different figures:

• Gross (higher) calorific value — GCV/HCV. Includes the latent heat of condensation. This is what a bomb calorimeter measures, because the vessel cools and the water condenses inside it.
• Net (lower) calorific value — NCV/LCV. Excludes it, because in most real appliances the water leaves up the flue as vapour and the latent heat goes with it.

The difference is typically 5–10%, and larger for hydrogen-rich fuels. Condensing boilers are named for exactly this: they cool the flue gas enough to condense the water and recover that latent heat, which is how they achieve efficiencies quoted above 90% on a net basis.

#What affects a fuel's calorific value

• Carbon and hydrogen content. Hydrogen releases far more heat per kilogram than carbon, so hydrogen-rich fuels rate higher.
• Moisture. Water absorbs heat to evaporate and contributes none, which is why green wood performs so much worse than seasoned wood.
• Ash and mineral matter. Incombustible, and so pure dead weight — the reason high-ash coal delivers less heat per tonne.
• Oxygen already in the fuel. Partially oxidised fuels such as ethanol and wood have less energy left to release.

#Why it matters practically

Calorific value converts a price into a cost of heat:

cost per unit of heat = fuel price per kg ÷ calorific value

A fuel at half the price per kilogram but one-third the calorific value is more expensive to use, not less. This is the arithmetic behind every honest heating-fuel comparison, and it is why cost per kilowatt-hour, not cost per litre or per cylinder, is the number worth comparing.

Two follow-ons worth reading: which fuel has the highest calorific value, and why moisture in green wood destroys so much of the value the table promises.

#The compact answer

The calorific value of a fuel is the amount of heat energy produced by the complete combustion of one kilogram of the fuel. It is expressed in kilojoules per kilogram (kJ/kg) for solid and liquid fuels and kilojoules per cubic metre (kJ/m³) for gaseous fuels. Its SI unit is J/kg. A good fuel has a high calorific value.