What is calorific value, and how are gross and net calorific values different?

In one line

Calorific value is the total quantity of heat liberated when a unit mass (or unit volume) of a fuel is burnt completely in air or oxygen.

Explained simply

Calorific value is the "energy score" of a fuel, like the calorie count on a food packet. The higher the score, the more heat you get from each kilogram.

Most fuels contain hydrogen, which burns to form water. In the laboratory the water condenses and gives back its latent heat, so we measure more heat. In a real furnace the water escapes as steam and that latent heat is lost. So we define two values:

• Higher (gross) calorific value, HCV or GCV: heat liberated when unit mass of fuel is burnt completely and the products are cooled to room temperature (about 15 °C), so the steam condenses.
• Lower (net) calorific value, LCV or NCV: heat liberated when the products are allowed to escape, so the latent heat of steam is not recovered. LCV = HCV − latent heat of steam formed.

How it works

1. H₂ + ½O₂ → H₂O. By mass, 2 parts of H give 18 parts of water, so 1 part H gives 9 parts water.
2. If the fuel has H% hydrogen, 1 g of fuel forms \( \frac{9H}{100} = 0.09H \) g of water.
3. Each gram of steam carries 587 cal of latent heat (at room temperature conditions used in textbooks).
4. Subtract this from HCV to get LCV.

Key formula

\( \text{LCV} = \text{HCV} - 0.09\,H \times 587 \)

• HCV, LCV in cal/g (same number as kcal/kg).
• \( H \) = percentage of hydrogen in the fuel.
• 587 = latent heat of condensation of steam in cal/g.

Units: cal/g, kcal/kg, J/kg or kJ/kg for solids and liquids; kcal/m³ or kJ/m³ for gases; BTU/lb in the British system. 1 kcal/kg = 1.8 BTU/lb, and 1 cal = 4.184 J.

Quick example

A coal has HCV = 8000 kcal/kg and 5% hydrogen. LCV = 8000 − 0.09 × 5 × 587 = 8000 − 264.15 = 7735.85 kcal/kg.

Common mistakes

• Using 9H instead of 0.09H (forgetting H is a percentage).
• Saying LCV is higher. HCV is always ≥ LCV; they are equal only for a fuel with no hydrogen.
• Mixing units: kcal/kg and cal/g are numerically equal, but kJ/kg is 4.184 times larger.

How MU asks it

"Define calorific value. Differentiate between HCV and LCV." (3 marks), often followed by a small numerical. For full marks give both definitions, the relation with the reason for 0.09H, the units, and state which value is used in practice (LCV, since steam escapes in real furnaces).