Anthracite, Lignite and Coke: What Is the Difference?

Coal is not a single substance. It forms as plant remains are transformed under pressure and heat over a very long time; the further this transformation has gone, the higher the coal's carbon and heating value generally are, and the lower its moisture and volatile matter. This degree of transformation is the coal's rank. This article compares the three names most often confused, lignite, anthracite and coke, without forgetting bituminous coal in between.

The Order of Coalification

From lowest to highest, the ranks run as follows. The carbon ranges given by the U.S. Energy Information Administration (EIA):

• Lignite: 25-35 percent carbon; the lowest energy content of all coal ranks.
• Subbituminous coal: 35-45 percent carbon.
• Bituminous coal (hard coal): 45-86 percent carbon.
• Anthracite: 86-97 percent carbon; generally the highest heating value.

These figures are for coal “as received”, including moisture and mineral matter. Because lignite is very wet, its carbon rises to 60-70 percent on a dry, ash-free basis.

Lignite

Lignite is the youngest and wettest coal. Its heating value is 10-20 MJ/kg on a moist, mineral-matter-free basis, roughly 2,400-4,800 kcal/kg, and its ash content varies widely. Worldwide, most lignite is burned in power stations. The quality of lignite used for heating varies greatly from basin to basin, so the calorific value and ash on the bag are what count. Our domestic lignite coal belongs to this rank, as does Soma coal, which its bag declares as class A1 lignite.

Bituminous Coal (Hard Coal)

It sits between lignite and anthracite, and its volatile matter varies over a wide range. It burns with a smoky flame and softens and swells as it burns. Bituminous coals low in sulphur and phosphorus are used in steelmaking as coking (metallurgical) coal; those that do not meet these requirements are used as thermal coal for power generation and heating.

Anthracite

Anthracite is the highest-rank coal: 86-97 percent carbon, with volatile matter of at most around 10 percent. Its heating value is 26-33 MJ/kg on a moist, mineral-matter-free basis, roughly 6,200-7,900 kcal/kg. It is hard to ignite, but once lit it burns with a short, blue, smokeless flame; that is why in some countries it is used as a household fuel in hand-fired stoves and automatic stoker boilers. Anthracite is not in our product range.

Coke

Coke is not a rank of coal dug from the ground but a processed fuel. It is usually made by heating low-ash, low-sulphur bituminous coal in an airless oven at about 1,000-1,100 °C; water, gas and tar are driven off, leaving a hard, porous, carbon-rich mass. Coke's main use is smelting iron ore in blast furnaces, where it acts as both fuel and reducing agent. Because it burns with little smoke, it was also used for home heating in some countries in the past. We do not sell coke; the kök coal in our range has a similar name in Turkish but is not coke, it is a domestic heating coal.

Calorific Value, Volatile Matter and Ash: What They Mean in a Stove

• Calorific value: higher calorific value means less coal for the same heat.
• Volatile matter: high-volatile coal lights easily and burns with a long, smokier flame; low-volatile coal is hard to light but burns cleaner.
• Ash: the non-combustible mineral part of coal; more ash means less heat per kilo and more cleaning.

How these values are printed on the bag is explained in our article on the values on a coal bag.

The Choice for Home Heating

For home heating the real choice is usually between domestic lignite and high-calorie imported coal. If budget comes first and storage is plentiful, domestic lignite; if you want it to take up little space and the stove to need filling less often, high-calorie imported coals such as Siberian coal come to the fore. A detailed comparison is in imported or domestic coal; all products are on our coal page.

Sources: U.S. Energy Information Administration (EIA), “Coal explained” (carbon ranges of the coal ranks); Wikipedia, “Lignite”, “Bituminous coal”, “Anthracite” and “Coke (fuel)” (heating value, volatile matter, burning behaviour, how coke is made and used). MJ/kg values converted at 1 MJ ≈ 239 kcal and rounded.