Blog · 5 September 2026

What is Green Coal? Calorific Value, Properties and Uses

In brief

Summary

Green coal is a solid, coal-like fuel made by torrefying biomass or waste — roasting it at 200–350 °C in a low-oxygen chamber until it is dry, dense and brittle enough to store, handle and grind like coal. Its calorific value depends on the feedstock, from 3,500–3,800 Kcal/kg for undigested STP sludge to 3,700–6,000 Kcal/kg for woody biomass. The main use is co-firing in coal-fired power stations, where Indian rules now require 7% biomass from FY 2025–26.

What green coal is

Green coal is a solid fuel made by heat-treating biomass or waste until it behaves like coal. The raw material — crop residue, wood, municipal solid waste, refuse derived fuel or sewage sludge — is roasted in a low-oxygen chamber, which drives off moisture and part of the volatile content and leaves a dark, brittle, energy-dense solid. It is also called bio-coal, torrefied fuel or torrefied charcoal.

The point of the material is substitution. It ships, stores and grinds closely enough to coal that a thermal power plant or an industrial boiler can burn it in equipment built for coal, while the carbon in it comes from plants or waste rather than a mine.

How it is made

The process is torrefaction: heating the feedstock to 200–350 °C in the near-absence of oxygen so that it chars rather than burns. Moisture leaves first, then the lighter volatile compounds, and what remains is a friable solid with a higher share of fixed carbon.

KeyTor, Keystone Energy's continuous torrefaction technology, runs this as a continuous process rather than in batches, and it is the subject of Indian patent applications 202211023704 and 202211023710. Plant configurations run from 50 to 1,000 tonnes per day of input and are engineered for 6,000+ operating hours per year.

Properties

Torrefaction changes four things that matter to a fuel handler:

Property	Raw biomass	Green coal (torrefied)
Moisture	Absorbs water, needs covered storage	Largely hydrophobic, tolerates open storage
Grindability	Fibrous, clogs coal mills	Brittle, grinds in existing coal mills
Energy density	Lower, bulky to transport	Higher, closer to coal per tonne
Biological decay	Rots and self-heats in a pile	Resists decay

What torrefaction does not do is turn every feedstock into the same fuel. Energy content still depends heavily on what went in.

Calorific value by feedstock

These are the ranges measured across Keystone Energy's plant configurations. The figure that matters is the one for your feedstock, not the overall span:

Feedstock	Calorific value (Kcal/kg)	Typical source
Woody biomass	3,700 – 6,000	Forestry and wood residue
Rice straw biomass	4,000 – 4,300	Agricultural residue, Punjab and Haryana
Refuse Derived Fuel (RDF)	3,800 – 4,000	Segregated combustible municipal fraction
Municipal Solid Waste (MSW)	3,700 – 4,000	Mixed municipal waste
Undigested STP sludge	3,500 – 3,800	Sewage treatment plant residue

Across these five feedstocks the span runs from 3,500 Kcal/kg for undigested STP sludge at the low end to 6,000 Kcal/kg for woody biomass at the high end. Woody biomass carries the widest band because species and moisture vary considerably. Undigested STP sludge sits lowest because it arrives wet and a larger share of the input energy goes into drying it.

What green coal is used for

Co-firing in coal-fired power stations is the main near-term use in India. The Ministry of Power revised its co-firing policy in June 2023, requiring 5% biomass co-firing from FY 2024–25 and 7% from FY 2025–26 across all coal-based thermal power plants. Untreated residue pellets can be co-fired at roughly 7 to 10 percent without modifying plant equipment; green coal behaves closer to coal in mills and burners, and NTPC has demonstrated co-firing of up to 20 percent using torrefied biomass.

Beyond power generation, green coal substitutes for coal and lignite in industrial boilers and process kilns, and it can be used as a feedstock for gasification. Torrefied pellets have sold in the range of Rs 14,000–18,000 per tonne. On the supply side, the CPCB capital subsidy for such plants is Rs 58 lakh per TPH or 40% of capital cost, capped at Rs 2.8 crore.

The raw material is not scarce. India generates about 70 million tonnes of municipal solid waste a year, a figure projected to reach 165 million tonnes by 2030, alongside large volumes of crop residue burned in the field each winter.

Where the fuel is made

Green coal only displaces mined coal if there are plants making it. Keystone Energy designs, engineers, procures and commissions continuous torrefaction plants using the KeyTor system, with projects in four Indian states.

The 100 TPD legacy-waste, RDF and biomass plant in Warangal, Telangana, built for Keystone Greentech, is fully commissioned. A 500 TPD MSW plant for NTPC Ltd. (via NVVN) in Gorakhpur, Uttar Pradesh and a 200 TPD rice straw biomass plant for TransPacific Engineers in Sangrur, Punjab are both in commissioning. A 50 TPD STP sludge pilot for Delhi Jal Board in Delhi has been completed.

How KeyTor works · See the projects

Questions

What is green coal?

Green coal is a solid, coal-like fuel produced by torrefaction, a process that roasts biomass or waste at 200 to 350 degrees Celsius in a low-oxygen chamber. It is drier, denser and more brittle than the raw feedstock, and it can be stored, handled and ground in equipment built for coal. It is also called bio-coal, torrefied fuel or torrefied charcoal.

Is green coal the same as torrefied biomass or bio-coal?

In practice, yes. Green coal, bio-coal and torrefied fuel all describe the same category of material: a coal-like solid made by torrefying plant matter or waste. Keystone Energy labels its output by feedstock, as torrefied MSW, torrefied biomass or torrefied charcoal.

What is the calorific value of green coal?

It depends on the feedstock. Keystone Energy's measured ranges are 3,700 to 6,000 Kcal/kg for woody biomass, 4,000 to 4,300 Kcal/kg for rice straw biomass, 3,800 to 4,000 Kcal/kg for refuse derived fuel, 3,700 to 4,000 Kcal/kg for municipal solid waste, and 3,500 to 3,800 Kcal/kg for undigested STP sludge.

What is green coal used for?

The main use in India is co-firing with coal in thermal power plants, where a Ministry of Power policy revised in June 2023 requires 7 percent biomass co-firing from FY 2025-26, up from 5 percent the year before. Green coal is also burned in place of coal or lignite in industrial boilers and kilns, and it can serve as a gasification feedstock. NTPC has demonstrated co-firing of up to 20 percent using torrefied biomass.

Who makes green coal in India?

Keystone Energy designs, engineers and commissions continuous torrefaction plants in India using KeyTor, Keystone Energy's continuous torrefaction technology. Its 100 TPD legacy-waste, RDF and biomass plant in Warangal, Telangana is fully commissioned. A 500 TPD MSW plant for NTPC Ltd. in Gorakhpur and a 200 TPD rice straw biomass plant for TransPacific Engineers in Sangrur are in commissioning, and a 50 TPD STP sludge pilot for Delhi Jal Board has been completed.