Ash — the non-combustible fraction
Ash is the mineral matter left after coal burns. It carries no energy, so higher ash means less usable heat per tonne — and more residue to handle and dispose of. Indian domestic coal generally runs higher in ash than imported grades, which is a key reason the two price differently. See the typical ash ranges by grade.
Moisture — inherent and surface
Coal carries inherent moisture (locked in the structure) and surface moisture (from washing, rain or transport). Vaporising it consumes energy during combustion, which is exactly the gap between gross and net calorific value — see GCV vs NCV.
How they interact with GCV
A headline GCV means little without the ash and moisture for the same sample. High ash or moisture pulls the usable figure down, so always read all three together — and on the same basis (ARB/ADB).
Reading a spec sheet
On any coal spec, check GCV, ash, moisture and sizing as a set, and confirm the moisture basis. A documented certificate for the actual consignment — not a typical range — is what protects the buyer.
Frequently asked
- What is a good ash content for coal?
- It depends on the process. Ash-tolerant users (cement, some sponge iron) accept higher ash; boilers and ash-sensitive processes want lower. Lower ash means more usable energy and less disposal cost, but usually a higher price.
- Does moisture reduce coal's calorific value?
- Yes. Energy is spent vaporising moisture during combustion, lowering the net (usable) calorific value. That's the difference between GCV and NCV.
- Why does high-ash coal cost more to use?
- Ash carries no energy and must be handled and disposed of, so you pay to haul and remove inert material and get less heat per tonne — even if the price per tonne looks low.