Coal and Lignite as Boiler Fuels
Coal is still the most common fuel for medium and large industrial boilers in India, and it varies more than any other. Two numbers on a coal’s analysis — its ash and its moisture — decide how it burns, how much of it is needed, and what it does to the boiler.
You'll learn
- Read a proximate and an ultimate analysis
- Understand how Indian coal is graded
- Compare high-ash Indian coal with imported coal and lignite
- Know what each property does in the boiler
Before this: GCV vs NCV
What is in a lump of coal
Coal is plant matter buried and compressed over millions of years. What burns is its carbon and hydrogen; what does not burn is its moisture and its ash, the mineral matter left behind. The older and more compressed the coal, the more carbon and the less moisture it has: lignite is the youngest and wettest, bituminous coal is in the middle, anthracite the oldest and hardest.
Indian steam coal is typically high in ash and moderate in heat content: published GCV figures for it run roughly 3,500–4,500 kcal/kg, and GyanTosh’s steam cost tool uses 4,000 kcal/kg as an indicative default. Imported coal, much of it from Indonesia, usually has less ash and often more moisture.
Reading a coal analysis
| Analysis | Reports | Tells you |
|---|---|---|
| Proximate | Moisture, volatile matter, ash, fixed carbon | How the coal will ignite and burn, and how much ash it leaves |
| Ultimate | Carbon, hydrogen, nitrogen, sulfur, oxygen, plus ash and moisture | Air needed for combustion, flue gas composition, SO₂, NCV |
| Calorific value | GCV, kcal/kg | Heat per kilogram: the basis of price and fuel consumption |
- Moisture takes heat to evaporate and adds nothing. It also matters for storage and handling.
- Volatile matter is the gas driven off on heating. High volatiles ignite easily; low volatiles burn slowly and need hot furnaces.
- Ash dilutes the heat, has to be removed and disposed of, erodes tubes and can slag.
- Fixed carbon is what burns on the grate or in the bed after the volatiles have gone.
Analyses are reported on different bases — as received, air-dried, dry, dry ash-free — and the same coal looks quite different on each. Compare like with like; the as-received basis is what the boiler actually burns.
Grades, and what ash and sulfur do
Indian non-coking coal is graded and priced by GCV, in bands of 300 kcal/kg from G1, the highest, down to G17. A boiler designed for one grade does not simply burn another: a lower grade means more coal, more ash and a different combustion behavior for the same steam.
- High ash raises unburnt carbon losses, erodes convection tubes and loads the ash handling and dust collection system. It is why Indian coal-fired boilers have generous furnace volumes and conservative gas velocities.
- Ash fusion temperature decides slagging. Ash that softens at furnace temperature sticks to grates and walls.
- Sulfur burns to SO₂, which governs stack emission compliance and the acid dew point, and so how far the flue gas can be cooled.
- Lignite has very high moisture, so its GCV as received is low and much of its heat goes into evaporating water. It suits boilers designed for it, often fluidized bed, near the mine.
Key takeaways
- Proximate analysis: moisture, volatiles, ash, fixed carbon. Ultimate: C, H, N, S, O.
- Indian non-coking coal is graded by GCV in 300 kcal/kg bands, G1 to G17.
- Indian coal is high in ash; imported coal is lower in ash, often wetter; lignite is wettest.
- Compare coals by cost per unit of heat delivered to the steam, not per tonne.