Sub-bituminous coal is a sedimentary fossil fuel formed from partially decomposed plant matter subjected to moderate heat and pressure over millions of years. In the coal rank hierarchy, it lies between lignite and bituminous coal.
It offers a balanced combination of energy content, combustion efficiency, and environmental performance, making it a preferred fuel for thermal power plants, industrial boilers, cement kilns, and steam-intensive industries.
Coal Rank Classification
| Coal Rank | Moisture | Carbon | Calorific Value |
|---|---|---|---|
| Lignite | Very High | Low | Low |
| Sub-Bituminous | Moderate | Medium | Medium |
| Bituminous | Low | High | High |
| Anthracite | Very Low | Very High | Very High |
Geological Formation & Origin
Formed during early coalification stages under sedimentary pressure.
Retains higher moisture and oxygen content than bituminous coal.
Less compact and easier to grind.
Major producing regions include Indonesia, Australia, USA (Powder River Basin), South Africa, and Russia.
Key Characteristics
Moisture Content (15%–30%): Improves combustion stability and reduces dust.
Carbon Content (35%–45%): Balanced fixed carbon for steady heat release.
Calorific Value (17–23 MJ/kg): Ideal for base-load power and continuous heating.
Low Sulfur (<1%): Reduced SO₂ emissions and corrosion.
Moderate Ash: Lower slagging and easier ash handling.
Volatile Matter: Easy ignition and stable flame.
Physical Properties
| Property | Typical Range | Industrial Significance |
|---|---|---|
| Density | 1.2 – 1.5 g/cm³ | Storage & transport |
| Hardness | Low | Easy crushing |
| HGI | 40 – 55 | Efficient pulverization |
| Porosity | High | Better combustion |
| Ignition Temperature | 350 – 400°C | Quick flame stability |
Chemical Properties
| Parameter | Typical Value | Impact |
|---|---|---|
| Fixed Carbon | Moderate | Controlled burn rate |
| Hydrogen | 5% – 6% | Flame stability |
| Oxygen | 20% – 30% | Cleaner combustion |
| Sulfur | < 1% | Low emissions |
| Ash Fusion Temp | Low–Medium | Reduced clinker |
Combustion & Performance
Burns with a long, stable flame
Lower soot and smoke formation
Reduced slagging and clinker formation
Compatible with pulverized coal boilers, FBC systems, and chain-grate furnaces
Processing & Handling
Crushing, screening, washing, and size grading available.
Requires controlled stockpiling due to spontaneous combustion risk.
Proper ventilation and moisture management recommended.
Available Commercial Forms
ROM coal
Sized lump coal
Crushed coal (6–25 mm, 0–50 mm)
Pulverized coal
Washed / beneficiated coal
GCV-based graded coal
Packaging & Supply
Bulk truck supply
Rail & barge transport
Containerized export
Customized sizing and grading on request
Applications
Power Generation: Thermal power plants, CHP units
Industrial Boilers: Textile, food, chemical, paper industries
Cement Industry: Kilns and preheaters
Brick & Ceramics: Continuous firing kilns
Gasification: Syngas, IGCC, CTL projects
Environmental Advantages
Lower sulfur → reduced SO₂
Lower NOx formation
Easier emission compliance
Reduced flue-gas treatment requirements
Sub-Bituminous vs Bituminous Coal
| Parameter | Sub-Bituminous | Bituminous |
|---|---|---|
| Sulfur | Lower | Higher |
| Cost | Lower | Higher |
| Emissions | Cleaner | Higher |
| Moisture | Higher | Lower |
| Grindability | Easier | Harder |
Quality Standards & Testing
Proximate & ultimate analysis
GCV / NCV testing
Ash fusion temperature
HGI testing
ISO / ASTM / IS standards
Sub-bituminous coal provides a cost-effective, cleaner-burning, and operationally stable fuel solution for industries requiring consistent heat and steam generation. Its low sulfur content, moderate energy value, and excellent combustion behavior make it a preferred choice for power plants, cement kilns, boilers, and gasification projects worldwide.