Biomass Fuels: Husk, Wood, Briquettes and Bagasse
Biomass is the fuel of a large share of Indian process boilers: rice husk in rice mills, bagasse in sugar mills, wood and briquettes almost everywhere else. Its heat value is decided less by what it is than by how wet it is, and its ash decides how hard the boiler has to work to stay clean.
You'll learn
- Know the common biomass fuels and their typical heat values
- Calculate how moisture reduces GCV and NCV
- Understand biomass ash: silica, alkali and slagging
- Buy and store biomass on heat content, not weight
Before this: GCV vs NCV
The common biomass fuels
| Fuel | Indicative GCV, kcal/kg | Note |
|---|---|---|
| Rice husk | 3,200 | Published ranges roughly 3,000–3,500; high silica ash |
| Biomass briquette | 3,800 | Published ranges roughly 3,500–4,500, with moisture |
| Wood chips | 3,000 | Varies widely with species and seasoning |
| Bagasse | 2,300 | At about 50% moisture, as it leaves the mill |
Typical figures marked low confidence in the tool: use your own fuel’s laboratory analysis for any real calculation.
Biomass is renewable and, under India’s carbon market, its emissions are reported separately rather than counted against a plant. It is also bulky, variable and often wet, which is why biomass boilers are larger, carry more refractory and need more attention than oil or gas boilers of the same output.
What moisture does to heat value
Water in the fuel adds weight but no heat, and it takes heat to evaporate. Both effects pull the usable heat down as moisture rises:
| Moisture, % | GCV, kcal/kg | NCV, kcal/kg |
|---|---|---|
| 0 | 4,400 | 4,085 |
| 10 | 3,960 | 3,618 |
| 20 | 3,520 | 3,151 |
| 35 | 2,860 | 2,451 |
| 50 | 2,200 | 1,750 |
NCV from NCV = GCV − 584 × (9H + M)/100, with hydrogen scaled to the wet basis. Illustrative fuel; the trend is what matters.
Going from 10% to 35% moisture cuts this wood’s NCV by 32%. The boiler needs that much more fuel by weight for the same steam, and its stack carries more water vapor and more heat away.
Ash, slagging and buying on heat
Rice husk ash is mostly silica and is high in quantity, so husk boilers are designed for heavy, abrasive ash and continuous ash removal. Many agro-residues are high in potassium and other alkalis, which lower the ash’s melting point: the ash softens in the furnace and sticks to grates, walls and tubes as slag. Furnace temperature control, the grate type and sootblowing are designed around the fuel’s ash, which is why a boiler built for husk may struggle on mustard stalks or cotton stalks.
- Buy on heat: ask for GCV and moisture on an as-received basis, and price per thousand kcal, not per tonne.
- Store dry: covered storage pays for itself when monsoon moisture would otherwise cut heat value and cause feeding problems.
- Size matters: oversized or stringy fuel bridges in hoppers and feeders; fines blow through the furnace unburnt.
- Watch the mix: blended fuels change ash behavior; change the blend slowly and watch the grate.
Key takeaways
- Common biomass: rice husk, wood, briquettes, bagasse; all variable, often wet.
- Moisture cuts GCV in proportion and NCV even faster.
- Husk ash is abrasive silica; alkali-rich residues slag.
- Buy and compare biomass on heat delivered, as received, not on weight.