Coal briquettes are compressed blocks of coal dust, fine coal particles, or low-grade coal waste, bound together into a uniform shape that burns more predictably than the raw material it comes from. They have been used for heating and cooking across much of the world for well over a century, and they remain a significant fuel source in parts of Asia, Africa, and Eastern Europe. What makes briquettes interesting is not just that they are a way to use coal, but that their engineered design allows manufacturers to manipulate burn rate, emissions, and energy output in ways that loose coal simply cannot match. A life-cycle comparison found that switching from raw coal to a formulated boiler briquette could cut sulfur dioxide emissions by about 63% and carbon dioxide by roughly a quarter, while boosting thermal efficiency by more than a third.
What Goes Into a Coal Briquette
At its simplest, a coal briquette is coal fines pressed under high pressure into a consistent shape. The coal used is often the stuff that would otherwise go to waste: screenings, slack, dust from coal preparation plants, and other fine particles too small to burn efficiently in a grate or stove on their own. But turning loose powder into a solid block that holds together under handling, transport, and combustion usually requires a binder of some kind.
The choice of binder is one of the most consequential decisions in briquette manufacturing. Common options include coal-tar pitch, starch, molasses, and various polymeric or mineral binders. Each affects not just how well the briquette holds together, but how it burns and what it releases. Research on composite fuel briquettes made from coal fines and torrefied biomass found that combining pitch and molasses as co-binders produced briquettes with superior compressive strength compared to using either binder alone, though the molasses-bonded versions had lower impact resistance and were more prone to crumbling during handling.1Heliyon. Densification of coal fines and mildly torrefied biomass into composite fuel using different organic binders This matters practically because a briquette that shatters when dropped off a truck is of little use, no matter how well it performs in a lab furnace.
Not all briquettes need binders, though. Some low-rank coals with high inherent moisture can be compressed into mechanically strong blocks without any additive at all, relying on the natural bonding properties of the coal’s own surface chemistry. In these cases, residual moisture content is the critical variable: there is an optimum moisture level at which the briquette reaches maximum strength, and drying too much or too little weakens the product.2Chemical Engineering & Technology. An Experimental Study on Binderless Briquetting of Low‐Rank Coals The hydrophobicity and chemical structure of the coal surface determine how well binderless briquettes hold together, which means this approach works only for specific coal types.
How Briquette Shape Affects Combustion
One of the more counterintuitive aspects of briquette design is that geometry matters as much as chemistry. The familiar honeycomb briquette, common across East and Southeast Asia, has a distinctive pattern of vertical holes running through it. These holes are not decorative. They control airflow through the burning fuel, and their number and size directly influence how fast the briquette burns, how hot it gets, and how efficiently it converts fuel into heat.
A study on honeycomb charcoal briquettes made from corn waste tested configurations with four, eight, and twelve holes. The eight-hole version brought water to a boil in 25 minutes, ten minutes faster than either the four-hole or twelve-hole versions. The four-hole briquette reached the highest peak temperature at about 589°C, while the twelve-hole version topped out at only 427°C. Meanwhile, having more holes increased fuel consumption without a proportional increase in useful heat.3G-Tech: Jurnal Teknologi Terapan. Optimization of Hole Variations in Honeycomb Charcoal Briquettes from Corn Waste for Enhanced Combustion Efficiency Too few holes starve the fire of oxygen and slow combustion. Too many let air rush through too fast, wasting fuel. The sweet spot sits somewhere in between, and finding it is essentially an engineering optimization problem that manufacturers solve for specific stove designs and cooking needs.
Chemical additives can also reshape how briquettes burn. Adding an alkaline promoter like sodium hydroxide to coal briquettes lowers the activation energy needed to sustain combustion and shifts the peak burning temperatures upward, essentially making the briquette catch fire more easily and then burn hotter. The catalytic effect comes from surface oxides that form during coal decomposition and act as oxygen carriers, accelerating the reaction. In one study, the activation energy dropped by more than half with the addition of sodium hydroxide.4South African Journal of Chemical Engineering. The role of the alkaline promoter on the formation of strength and burning of coal briquettes This is a useful trick for low-quality coals that are otherwise difficult to ignite or prone to incomplete combustion.
Environmental Gains Over Raw Coal
The strongest case for coal briquettes, compared to burning raw coal, is environmental. A comprehensive energy, greenhouse gas, and air quality analysis comparing a formulated boiler briquette coal (BB-coal) to the raw coal from which it was made found striking improvements across nearly every metric. Boiler thermal efficiency jumped 37%, meaning more of the coal’s energy actually became useful heat rather than escaping up the chimney. Fuel demand dropped 25% as a direct consequence. Carbon dioxide emissions fell 26%, carbon monoxide dropped 17%, and sulfur dioxide plummeted 63%. Fly ash and its carbon content were reduced by 97%, fine particulate matter (PM2.5) by 22%, and emissions of five toxic heavy metals in larger particles by 30%.5PubMed. Boiler Briquette Coal versus Raw Coal: Part II-Energy, Greenhouse Gas, and Air Quality Implications
Those numbers are dramatic, and they come from the same coal. The difference is entirely in how the fuel is prepared. Briquetting allows manufacturers to control the size, density, and porosity of each piece, which means more uniform airflow during combustion and fewer of the incomplete-burning conditions that generate soot, carbon monoxide, and unburned volatiles. Adding sulfur-fixing agents to the briquette mix locks some of the coal’s sulfur into the ash rather than releasing it as SO₂ gas. These are not marginal improvements; they represent the kind of emission reductions that, scaled across millions of household stoves or industrial boilers, could meaningfully change regional air quality.
Sulfur Fixation and Emission Additives
Sulfur is one of coal’s most problematic constituents. When coal burns, the sulfur it contains converts primarily to sulfur dioxide, a gas that contributes to acid rain, respiratory illness, and urban smog. One of the advantages briquettes hold over loose coal is that they can be formulated with sulfur-fixing agents mixed directly into the fuel before pressing.
The most common approach uses calcium-based sorbents, which react with sulfur during combustion to form stable calcium sulfate (essentially gypsum) that stays trapped in the ash. Research has shown that the combustion atmosphere matters considerably: calcium sulfate forms preferentially in the outer layer of the briquette ash ball where oxygen is plentiful, while calcium sulfide forms in the oxygen-starved interior.6Journal of Cleaner Production. Sulfur fixation characteristics of single coal briquette particle during the process of combustion in different atmospheres This radial distribution means that briquette designers need to think about internal airflow when they want to maximize sulfur capture.
Not all additives help equally. When researchers tested three metal oxide additives (aluminum oxide, iron oxide, and manganese dioxide) for their ability to enhance calcium-based sulfur retention, only aluminum oxide showed any meaningful improvement.7PubMed. The role of additives in bio-mass coal briquette on sulfur retention enhancement This kind of finding matters for manufacturers trying to balance cost against performance: not every additive that sounds chemically plausible actually works in practice.
An alternative approach to sulfur reduction is to process the coal before briquetting. Co-pyrolysis of low-rank coal with biomass at around 600°C can reduce both volatile matter and sulfur content simultaneously. The biomass acts as a hydrogen donor, which promotes the release of hydrogen sulfide during heating, effectively pulling sulfur out of the coal before it ever reaches the consumer’s stove.8Journal of Analytical and Applied Pyrolysis. Low-temperature co-pyrolysis of a low-rank coal and biomass to prepare smokeless fuel briquettes The resulting briquettes qualify as “smokeless fuel,” a classification that carries regulatory weight in countries like the UK and Ireland where certain fuels are banned in urban smoke control zones.
Indoor Air Quality and the Hidden Cost of “Cleaner” Fuel
The outdoor emissions story for briquettes is broadly positive compared to raw coal, but the indoor picture is more complicated. Millions of households, particularly in developing countries, burn coal briquettes in poorly ventilated spaces for cooking and heating. Even briquettes marketed as cleaner alternatives to traditional fuels release concerning levels of pollutants indoors.
Measurements of Pakistani coal briquettes burned under typical domestic conditions found that both traditional fuels and coal briquettes produced peak carbon monoxide levels between 100 and 250 parts per million, sulfur dioxide between 2 and 5 parts per million, and nitrogen oxides between 1 and 5 parts per million during the early volatile-burning phase. Emissions dropped substantially during the later char-burning phase, but those initial peaks can occur in enclosed kitchens with limited ventilation.9Environment International. Impact on indoor air quality during burning of Pakistani coal briquettes The practical implication is that the lighting and early burn period is the most dangerous time for indoor exposure, and keeping windows open or using a chimney during that phase matters far more than the choice of fuel type alone.
Ultrafine particles are another concern that does not always track with the “smokeless” label. A UK study monitoring particulate matter in homes using different stove fuels found that wood briquettes produced the highest concentrations of ultrafine particles, followed by smokeless coal. Both manufactured fuels released higher ultrafine particle levels than kiln-dried or seasoned wood.10PubMed Central. Particulate matter exposure from different heating stoves and fuels in UK homes Ultrafine particles are small enough to penetrate deep into lung tissue and even cross into the bloodstream, so a fuel that produces less visible smoke is not necessarily safer from this standpoint. The finding is a useful corrective to the assumption that “smokeless” means “harmless.”
Fluorosis From Coal Briquettes in China
One of the most severe health consequences linked to coal briquettes is not a respiratory disease but a skeletal one. In Guizhou Province and surrounding areas of southwestern China, more than 10 million people have suffered from dental and skeletal fluorosis caused by exposure to fluorine released from coal briquettes.11PubMed Central. Health impacts of domestic coal use in China The mechanism is specific and startling: local coals in the region are naturally high in fluorine, and the clay binders traditionally used in briquette production are also fluorine-rich. When these briquettes burn in open indoor ovens, the fluorine vaporizes and deposits on corn and other foods dried above the flames. People then eat the contaminated food over years and decades.
Fluorosis at this scale is not a subtle condition. Dental fluorosis causes permanent discoloration and pitting of teeth. Skeletal fluorosis stiffens and deforms joints, causes chronic pain, and in severe cases leads to crippling disability. The problem persisted for generations because the link between briquette composition and fluorine exposure was not immediately obvious, and the drying of food over coal fires was deeply embedded in local food-preservation practices.
Arsenic is another contaminant that follows a similar pathway. Pilot experiments in Yunnan Province measured fluorine and arsenic emissions from briquettes and tested fixation agents that could trap these elements in the ash rather than releasing them into the air. The experimental briquettes achieved fluorine fixation rates more than four times higher than untreated controls, and arsenic fixation rates about 1.2 times higher.12PubMed. The pollution control of fluorine and arsenic in roasted corn in “coal-burning” fluorosis area Yunnan, China These fixation additives offer a practical solution that does not require people to abandon their traditional food-drying methods entirely, which makes adoption more likely than simply telling communities to stop using coal.
Blending Coal With Biomass
A growing area of research involves blending coal with agricultural waste, wood residues, or other biomass to create hybrid briquettes that sit somewhere between a fossil fuel and a renewable one. The logic is straightforward: biomass is carbon-neutral in theory (the CO₂ it releases when burned was recently absorbed from the atmosphere by growing plants), so mixing it with coal dilutes the net fossil carbon emissions per unit of heat.
Biomass also brings practical advantages to briquette formulation. As noted earlier, co-pyrolysis with biomass helps strip sulfur from coal. Biochar, a charcoal-like material produced from biomass pyrolysis, can serve as both a fuel component and a structural element in briquettes. The interaction between biochar and molasses as a binder was found to improve compressive strength in composite coal-biomass briquettes, though impact resistance remained lower than in pitch-bonded versions.13Heliyon. Densification of coal fines and mildly torrefied biomass into composite fuel using different organic binders
The trade-offs are real. Biomass generally has lower energy density than coal, so blending it in reduces the heat output per kilogram of briquette. It also introduces variability: agricultural waste differs from batch to batch in moisture, ash content, and composition in ways that coal from a single mine does not. For a manufacturer trying to produce a consistent product, this variability creates quality-control headaches. Still, in regions where biomass waste is abundant and coal is expensive or environmentally regulated, hybrid briquettes offer a pragmatic middle path.
What Happens to the Ash
Burning any solid fuel generates ash, and coal briquettes are no exception. But the ash from formulated briquettes is not the same as raw coal ash. When calcium-based sulfur fixatives are used, the ash contains calcium sulfate (gypsum), along with various calcium and mineral compounds. This turns out to have an unexpected second life.
Research in northeastern China tested the application of bio-briquette ash to salt-affected soils, the kind of degraded agricultural land where high sodium levels stunt crop growth. The gypsum in the ash reacted with sodium compounds in the soil, lowering pH and reducing the concentration of sodium carbonates and bicarbonates. Corn yields increased with higher ash application rates.14沙漠研究. Salt-affected Soil Amelioration with Low-quality Coal Bio-briquette Ash in Northeastern China The finding suggests that in areas where coal briquettes are widely burned, the ash need not be treated as pure waste. Applied to the right soils, it functions as a low-cost soil amendment that addresses a real agricultural problem.
This does not mean all briquette ash is safe to spread on fields. Ash from briquettes made with high-fluorine or high-arsenic coals, like those linked to fluorosis in Guizhou, could introduce the same contaminants to soil and water. The composition of the ash depends entirely on what went into the briquette and what additives were used. Any reuse strategy needs to be matched to the specific chemistry of the fuel, not applied as a blanket practice.
Briquettes in a Transitioning Energy Landscape
Coal briquettes occupy an awkward position in contemporary energy discussions. In wealthy countries with access to natural gas, heat pumps, and renewable electricity, they are largely a relic or a niche product for wood-burning stove enthusiasts. In much of the developing world, they remain a primary cooking and heating fuel for hundreds of millions of people who lack affordable alternatives. The global conversation about phasing out coal tends to focus on power plants and industrial boilers, but household coal use, much of it in briquette form, is a significant and stubborn source of both greenhouse gases and indoor air pollution.
The formulated briquette represents an intermediate technology: better than raw coal, worse than gas or electricity, and far more accessible than either in many contexts. The 37% efficiency gain and 26% CO₂ reduction documented in the boiler briquette study represent meaningful improvements that could be achieved quickly and cheaply across large populations, without waiting for infrastructure that may be decades away.15PubMed. Boiler Briquette Coal versus Raw Coal: Part II-Energy, Greenhouse Gas, and Air Quality Implications Whether that incremental improvement is worth pursuing, or whether it risks locking communities into coal dependence, is a question that energy policy researchers continue to wrestle with. The answer probably depends on which community you are asking about and how soon cleaner alternatives can realistically arrive.