GCV vs NCV: Gross and Net Calorific Value of Fuels
A fuel’s calorific value is the heat it releases when burned completely. The gross value counts the latent heat of the water vapor in the flue gas as if it were recovered; the net value does not. In a boiler that vapor goes up the stack, so the two bases give different efficiencies for the same plant.
You'll learn
- Define gross and net calorific value
- Calculate NCV from GCV, hydrogen and moisture
- See why wet and hydrogen-rich fuels differ most
- Convert an efficiency from one basis to the other
Before this: Latent heat, Heat balance and boiler efficiency
Two ways to count a fuel’s heat
Burning a fuel turns its hydrogen into water, and any moisture already in the fuel boils off. Both leave the furnace as water vapor, carrying their latent heat with them. A laboratory bomb calorimeter cools the products right down, the vapor condenses and gives its latent heat back, and the result is the gross calorific value (GCV), also called the higher heating value.
A boiler’s stack is far too hot for that vapor to condense, so its latent heat is never recovered. Subtracting it gives the net calorific value (NCV), also called the lower heating value: the heat a real boiler could at most use.
Indian boiler practice, BEE and most Indian fuel suppliers quote GCV. Much European equipment data uses NCV. Always ask which.
From GCV to NCV
| Symbol | Meaning | Unit |
|---|---|---|
| Hydrogen in the fuel, % by mass | % | |
| Moisture in the fuel, % by mass | % | |
| 584 | Latent heat of water vapor used by BEE | kcal/kg |
| Each kilogram of hydrogen burns to 9 kg of water | — |
- Coal: GCV 3,501 kcal/kg, hydrogen 2.0413%, moisture 31.6%.
- Furnace oil: GCV 10,000 kcal/kg, hydrogen 12%, moisture 0.5%.
- Coal:
- Oil:
Why the basis changes efficiency
Efficiency is output divided by fuel heat input. On an NCV basis the input is smaller, so the same boiler shows a higher number. BEE’s coal example is 77.77% on GCV; on NCV it is 84.8%. Nothing about the boiler changed.
This is how a condensing gas boiler can be advertised at over 100% efficiency “on NCV”: by condensing the flue gas it recovers latent heat that the NCV definition said was not available. On a GCV basis it is still below 100%, as it must be.
Key takeaways
- GCV counts the latent heat of the water vapor in flue gas; NCV does not.
- NCV = GCV − 584 × (9H + M) / 100, in kcal/kg.
- Moisture and hydrogen widen the gap: wet coal and biomass, and gas, differ most.
- An efficiency means nothing without its basis: η_NCV = η_GCV × GCV/NCV.