Biocoal is a solid fuel made by thermally upgrading plant matter until its energy content, grindability, and water resistance approach those of fossil coal. Depending on the feedstock and process used, the finished product typically delivers about 25 to 28 megajoules per kilogram, which puts it in the same energy bracket as bituminous coal. The appeal is straightforward: existing coal-burning infrastructure can accept biocoal with minimal modification, while the carbon released during combustion comes from recently captured atmospheric CO₂ rather than ancient geological reserves. That combination of compatibility and lower net emissions has made biocoal one of the more practical near-term routes for decarbonizing coal-dependent power generation.
How Biocoal Is Made
Two main thermal processes produce biocoal, and they work under very different conditions. Torrefaction is a dry process carried out at relatively mild temperatures, roughly 200 to 300 °C, in the absence of oxygen. During torrefaction the moisture bakes off first, followed by volatile organic compounds and light acids. Research on the individual components of wood shows that hemicellulose breaks down earliest, releasing acetic acid and furfural, while cellulose remains largely intact until temperatures push above 300 °C thanks to the thermal stability of its crystalline structure. Lignin, the rigid scaffolding in wood, barely reacts at 250 °C but begins yielding aromatic compounds under more severe treatment at 300 °C.1Fuel. Torrefaction, pyrolysis and two-stage thermodegradation of hemicellulose, cellulose and lignin The result is a dry, darkened solid that has shed much of its oxygen and hydrogen content while retaining most of its carbon and energy.
Hydrothermal carbonization, or HTC, takes the opposite approach to moisture. Instead of drying the feedstock first, HTC submerges it in water under pressure at temperatures between roughly 180 and 260 °C. The pressurized water acts as both solvent and reaction medium, breaking down the biomass into a carbon-rich slurry that can be filtered and dried into a solid sometimes called hydrochar. HTC is especially useful for feedstocks that arrive wet, like sewage sludge, food waste, or freshly harvested agricultural residues, because it skips the energy-intensive drying step that torrefaction would require. Research has demonstrated that HTC can handle material with 75 to 90 percent moisture content and still yield a high-calorie solid fuel.2Biofuels, Bioproducts and Biorefining. Production of biocoal from wastewater sludge and sugarcane bagasse using hydrothermal carbonization
Why Not Just Burn the Biomass Directly
Raw biomass has several properties that make it a poor substitute for coal in power plants. It holds a lot of moisture, has relatively low energy per kilogram, resists grinding, and absorbs water during outdoor storage. Torrefaction and HTC address all of these problems simultaneously. As moisture, CO₂, and light volatile compounds leave the biomass during heating, what remains is a hydrophobic solid with noticeably higher energy density on a mass basis and dramatically improved grindability. In fact, studies have found that energy consumption for grinding torrefied wood drops by roughly 50 to 85 percent compared to fresh wood.3Fuel. Grindability determination of torrefied biomass materials using the Hybrid Work Index That matters enormously in a power plant, where coal needs to be pulverized to a fine powder before injection into the boiler. If the fuel resists milling, it slows the whole operation down and costs more to process.
The water resistance of biocoal is another practical advantage. Untreated wood pellets swell and disintegrate when exposed to rain, which limits how they can be shipped and stored. Torrefied material sheds water rather than soaking it up. Research on pellets made from meadow hay found that torrefaction significantly improved resistance to water absorption, especially when binders like lignosulfonate were included during pelletizing.4Renewable Energy. Biomass pellets with organic binders – before and after torrefaction The trade-off is that torrefaction makes the pellets more brittle and harder to hold together mechanically, a tension that the pelletizing industry is still working to resolve.
Energy Content and How It Compares to Fossil Coal
The central selling point of biocoal is that its heating value can match commercial coal grades. A study that produced biocoal from several types of lignocellulosic biomass reported heating values between 25.4 and 28.2 megajoules per kilogram, a range the authors describe as comparable to commercial coals.5PubMed Central. Bio-coal: A renewable and massively producible fuel from lignocellulosic biomass Separately, bio-coal derived from palm shell via HTC at 250 °C reached 27.9 megajoules per kilogram, which the researchers noted is comparable to high-volatile bituminous coal.6Applied Thermal Engineering. Energy efficiency of bio-coal derived from hydrothermal carbonized biomass: Assessment as sustainable solid fuel for municipal biopower plant
Those numbers deserve some context. Thermal coals used for power generation span a wide range themselves, from low-grade lignite at around 15 megajoules per kilogram to high-grade anthracite above 30. Biocoal sits comfortably in the middle of that spectrum, overlapping with the sub-bituminous and bituminous grades that most coal-fired plants are actually designed to burn. That overlap is what makes drop-in substitution feasible rather than aspirational.
Co-Firing in Existing Power Plants
The most immediate application for biocoal is blending it with fossil coal in power stations that are already running. This approach, called co-firing, avoids the enormous capital cost of building new infrastructure. Research on pulverized-coal boilers has shown that it is possible to co-fire fairly large proportions of biofuel without any measurable impact on boiler performance, combustion quality, or the pollution-control systems downstream. The practical bottleneck turns out to be the coal mills themselves: their drying capacity and ability to grind the blend to the required fineness set the ceiling on how much biofuel can be mixed in.7Applied Energy. Co-firing of biomass in coal-fired utility boilers
Combustion tests with oil palm trunk biocoal blended at 20 and 50 percent with sub-bituminous coal found average combustion efficiencies between 87 and 92 percent, suggesting that temperature had little effect on how well the blended fuel burned. The 20 percent biocoal blend emerged as the most suitable for optimization.8Energy Conversion and Management: X. Co-combustion of oil palm trunk biocoal/sub-bituminous coal fuel blends A 20 percent blend rate is a common starting point in the industry: high enough to make a meaningful dent in fossil fuel use, low enough to avoid pushing the plant’s existing equipment beyond its design envelope.
One persistent headache with burning biomass in boilers designed for coal is the alkali metals, especially potassium and sodium, that many plant tissues contain. These metals lower ash melting temperatures and promote slagging, fouling, and corrosion on boiler tubes. HTC processing helps with this. Work on Miscanthus grass showed that hydrothermal carbonization significantly reduces alkali metal content, raises safe combustion temperatures, and lowers the theoretical tendency of the resulting fuel to slag, foul, and corrode.9Fuel. The potential for production of high quality bio-coal from early harvested Miscanthus by hydrothermal carbonisation Washing the raw feedstock before thermal treatment can remove even more of these problematic elements. Research on rice husk found that washing at 65 °C removes over 80 percent of both potassium and chlorine, with most of the chlorine leaving within the first ten minutes.10PubMed Central. Parametric and kinetic study of washing pretreatment for K and Cl removal from rice husk
The Feedstock Question
One of biocoal’s most attractive features is the breadth of raw materials that can feed the process. Almost any plant-derived material works in principle: wood chips, agricultural residues like corn stover and rice husks, dedicated energy crops like Miscanthus, and even wetter waste streams like brewery spent grain and municipal sewage sludge. Each feedstock brings a different starting composition and moisture level, which influences whether torrefaction or HTC makes more sense.
Dry, woody materials are natural candidates for torrefaction. Wet wastes suit HTC, which can take wastewater sludge and sugarcane bagasse and turn them into a viable solid fuel without the penalty of pre-drying.11Biofuels, Bioproducts and Biorefining. Production of biocoal from wastewater sludge and sugarcane bagasse using hydrothermal carbonization The ability to use genuine waste streams rather than purpose-grown biomass is important for the economics and the environmental case alike. An energy crop competes for land and water; a waste stream that would otherwise go to landfill or incineration does not.
Pelletizing and the Brittleness Trade-Off
Biocoal in its raw form, whether torrefied chips or dried hydrochar, is often too bulky and dusty to ship economically. Pelletizing compresses it into dense, uniform cylinders that are easier to handle, meter into a boiler, and transport by the same logistics chains that move wood pellets or coal. Pilot-scale pelletizing of torrefied biomass has been demonstrated, though the researchers noted that no fine-tuning of production or steam additives were used, even though quality standards like ENplus allow additives up to a two percent level.12Renewable Energy. Pelleting torrefied biomass at pilot-scale – Quality and implications for co-firing
The underlying challenge is that torrefaction degrades the natural binders in biomass, especially hemicellulose, which acts as a glue during conventional pelletizing. Without that glue, the pellets are harder to form and more fragile once made. Adding organic binders can help. Work with meadow hay pellets found that lignosulfonate at 15 weight percent and wheat flour at 2 weight percent produced the best combination of hardness and mechanical durability, both before and after torrefaction. After torrefaction, hardness and durability still fell significantly compared to untreated pellets, but the binder-enhanced versions held together better than pellets without additives.13Renewable Energy. Biomass pellets with organic binders – before and after torrefaction Finding the right binder recipe for different feedstocks remains an active area of development, and getting it wrong means pellets that crumble into fines during shipping, creating dust-explosion risks and handling headaches at the receiving plant.
Greenhouse Gas Reductions from Co-Firing
The environmental case for biocoal rests on the idea that the carbon it releases during combustion was recently pulled from the atmosphere by the plants it was made from, rather than locked underground for millions of years. In practice, producing and transporting biocoal still generates some fossil emissions, from farm equipment, truck transport, and the heat needed to run the torrefaction or HTC reactor. Life-cycle analysis puts those emissions in perspective.
A detailed assessment of a corn-stover torrefaction plant found that the full life-cycle greenhouse gas footprint of producing biocoal, including harvesting and transporting the stover, came to about 11.4 grams of CO₂ equivalent per megajoule of biocoal. When that biocoal was co-fired at a coal plant, the reductions scaled roughly in proportion to the blend ratio: 10 percent biocoal cut life-cycle emissions by about 8.5 percent, 20 percent biocoal cut them by 17 percent, and 30 percent biocoal cut them by roughly a quarter compared to burning coal alone.14Biomass and Bioenergy. Life cycle assessment of a corn stover torrefaction plant integrated with a corn ethanol plant and a coal fired power plant Those are meaningful reductions achievable without retiring the coal plant or building anything new on the generation side.
The near-linear scaling is worth noting. It suggests that, at least up to a 30 percent blend, each additional increment of biocoal delivers roughly proportional climate benefit. Whether that linearity holds at even higher blend ratios likely depends on the specific plant’s milling and handling constraints more than on any combustion chemistry limit.
Gasification and Syngas Production
Co-firing is not the only route to extracting energy and value from biocoal. Gasification converts a solid fuel into synthesis gas, a mixture of carbon monoxide and hydrogen, by reacting it with a limited supply of oxygen at high temperatures. Syngas can be burned for power, but it can also serve as a chemical feedstock for producing liquid fuels, methanol, or ammonia.
Entrained-flow gasification tests of HTC-derived biocoal found carbon conversion rates of 84 percent at 1000 and 1200 °C, rising to 88 percent at 1400 °C. When the researchers ran the same tests with lignite under identical conditions, the lignite converted slightly less, indicating that biocoal is at least as reactive as a typical brown coal under gasification conditions.15Fuel. Entrained flow gasification of biocoal from hydrothermal carbonization Further work confirmed that biocoal’s high conversion rates and comparatively low tar loading make it well suited for entrained-flow gasifiers.16Chemical Engineering & Technology. Air‐Blown Entrained‐Flow Gasification of Biocoal from Hydrothermal Carbonization
The gasification pathway matters because it opens up applications beyond electricity. A power plant can only produce electrons; a gasifier can produce the building blocks for fuels and chemicals, potentially displacing fossil inputs in sectors that are harder to electrify, like long-haul shipping or industrial chemistry.
Economics and Commercial Viability
Whether biocoal can compete on price depends heavily on feedstock cost, which in turn depends on whether you are buying a crop or getting paid to take a waste stream. A techno-economic study of biocoal production from brewery spent grain, a genuine waste product, projected a total capital investment of about 7.6 million US dollars for a plant producing 69 tonnes per day. The production cost came out to roughly 191 dollars per tonne, and the researchers pegged the selling price at 200 dollars per tonne, slightly below the prevailing market price of fossil coal. The project showed a positive net present value with a payback period of about 4.6 years and a return on investment of nearly 22 percent.17South African Journal of Chemical Engineering. Techno-economic assessment for bio coal production from brewers spent grain
Those numbers look promising, but they are context-dependent. Brewery spent grain is available in large quantities at low or negative cost near breweries, and the study was set in South Africa, where labor and construction costs differ from those in Europe or North America. Feedstocks that need to be purchased and transported, like forestry residues or energy crops, would push production costs higher. Carbon pricing and renewable energy subsidies tilt the math in biocoal’s favor in jurisdictions where they exist, while regions without such policies leave biocoal competing on raw fuel cost alone, a tougher fight.
Self-Heating and Storage Safety
Biocoal shares a hazard with conventional coal that deserves more attention than it often gets: it can self-heat during storage. The material slowly oxidizes at ambient temperatures, releasing a trickle of heat. In a small pile, that heat dissipates harmlessly. In a large storage container or silo, the heat can accumulate faster than it escapes, and the temperature climbs until, in the worst case, the material spontaneously ignites.
Recent research has exposed a troubling gap in the standard classification tests used for shipping dangerous goods. The UN N.4 test, which is widely used to decide whether a material qualifies as “self-heating,” relies on a fixed assumption about the activation energy of the oxidation reaction. When researchers measured the actual activation energy of biocoal at temperatures relevant to self-heating, they found it was substantially lower than the 87 to 90 kilojoules per mole that the N.4 test assumes. As a result, the standard test classified the biocoal as non-self-heating, while more detailed thermal modeling predicted that the same material would undergo thermal runaway at container-scale storage volumes.18Fuel. Self‑heating in biocoal with focus on activation energy determination and comparison of standard test methods In plain terms, the official safety test can give a clean bill of health to material that is actually dangerous in bulk.
For anyone handling or storing biocoal, this means the regulatory classification alone may not be a reliable safety guide. Temperature monitoring in storage, adequate ventilation, and limits on pile size are practical precautions that the current testing framework does not always mandate. Industry awareness of this gap is growing, but regulatory standards have not yet caught up.
Where Biocoal Fits in the Broader Energy Transition
Biocoal occupies an unusual niche. It is not a next-generation technology that requires new infrastructure; it is designed to slot into the infrastructure we already have. That makes it especially relevant in countries where coal-fired power is deeply embedded and cannot realistically be retired on the timelines that climate targets demand. A coal plant that co-fires 20 to 30 percent biocoal keeps running, keeps its workforce employed, and cuts its carbon footprint by a meaningful fraction while longer-term alternatives like grid-scale storage and green hydrogen mature.
The flip side is that biocoal’s value proposition depends on coal plants continuing to operate. In markets where coal is being retired outright in favor of renewables or gas, the window for biocoal shrinks. And scaling up biomass supply raises its own land-use and sustainability questions, particularly if demand grows large enough to require purpose-grown crops rather than waste streams. The most defensible use cases are the ones where the feedstock is genuinely waste, the coal plant is genuinely needed for grid stability, and the co-firing displaces enough fossil carbon to make the logistics worthwhile. Those conditions exist in large parts of Asia, Africa, and Eastern Europe today, which is where much of the applied research and early commercialization is focused.