Coal rank, from soft brown lignite through sub-bituminous and bituminous to hard anthracite, sets how much heat a kilogram gives: practical ranges run from about 10-15 GJ per tonne for lignite to 30-35 GJ per tonne for anthracite. Never carry a heating value from one rank to another; identify the rank by look, feel and a weighed test burn, and measure the ash left behind.
Part of Coal Surface Mining
How coal is ranked, what each rank looks and behaves like, and how to grade a local seam with improvised tests.
Rank Controls Energy, Smoke And Ignition More Than Anything Else
Coal rank rises with the age and burial of the original plant matter. Low-rank coal is brown, soft, wet and smoky. High-rank coal is black, hard, dry and clean-burning. The parent article’s table gives practical ranges as dug: lignite 10-15 GJ/t, sub-bituminous 15-20, bituminous 24-30, anthracite 30-35 (1 GJ/t is 1 MJ/kg).
The formal US classification (ASTM D388, quoted here from memory, so check it against the standard if you can) bounds the lower ranks by heating value on an ash-free basis, in Btu per pound (1 Btu/lb = 0.002326 MJ/kg): lignite is below 8,300 (under about 19.3 MJ/kg), and sub-bituminous runs from 8,300 up to 11,500 (about 19.3 to 26.7 MJ/kg). Bituminous starts near the top of that band, with caking behaviour also deciding the class, and anthracite is set by fixed carbon at 86% or more of the dry, mineral-free coal. These figures describe clean coal. Dug coal carries ash and surface water on top, which is why the parent table’s sub-bituminous range (15-20 GJ/t) starts below the formal class floor of about 19.3 MJ/kg.
| Rank | Look and feel | Burns |
|---|---|---|
| Lignite | Brown, woody texture, crumbles as it dries, wet | Smoky, low heat, ignites easily |
| Sub-bituminous | Dull black or brown-black, soft | Moderate heat, much smoke |
| Bituminous | Black, often banded bright and dull, breaks in blocks | Long yellow flame, hot; many types soften and swell |
| Anthracite | Hard, glassy black, breaks with a curved shell-like surface | Short blue flame, hot, almost smokeless; slow to light |
Method: Grade A Local Seam
Step 1: Look at fresh coal, not weathered. Compare against the table above.
Step 2: Burn a weighed sample for ash. Weigh 1 kg of dried coal, burn it fully in a hot grate or forge until no black remains, and weigh the ash. Parent figure: typical coal ash is 5-15% by weight. A coal that leaves far more has a high stone content and delivers proportionally less heat per kilogram.
Step 3: Look for sulphur. Brassy yellow specks or lumps (pyrite) and a sharp acrid smell on burning mean sulphur. Sulphur corrodes flues and, in a forge, makes hot iron crack when worked (red-short), so keep sulphurous coal out of the smithy.
Step 4: Record each batch. Rank, ash percentage, sulphur sign and the part of the seam it came from. Seams vary in quality along their length and by depth.
Worked Example: How Much Of Each Rank Replaces A Tonne Of Wood?
Inputs: air-dried hardwood at 20% moisture (wet basis) gives about 14.3 GJ/t (derived in the heating-value comparison article); coal ranges taken as the midpoints of the parent table.
- Lignite: 14.3 ÷ 12.5 = 1.14 t.
- Sub-bituminous: 14.3 ÷ 17.5 = 0.82 t.
- Bituminous: 14.3 ÷ 27 = 0.53 t.
- Anthracite: 14.3 ÷ 32.5 = 0.44 t.
Lignite is not “better than wood” by weight; it takes about 1.14 t to replace 1 t of air-dried hardwood. The comparison ignores differences in stove efficiency.
Common Mistakes
- Quoting a single “coal” figure. A bare “coal gives X MJ/kg” is wrong for most of the ranks.
- Judging by colour. Black does not mean bituminous.
- Ignoring moisture on lignite. Dry it before burning and before measuring ash.
Safety
Test burns make smoke and carbon monoxide
Do test burns outdoors or in a stove with a working chimney. Sulphurous and low-rank coals produce acrid smoke, and incomplete burning gives carbon monoxide, which is colourless and odourless (see Carbon Monoxide Risks).
What’s Next
- Learn next: Sorting & Grading — applying the grade at the stockpile
- Combine with: Burning Coal Effectively — what each rank needs from the stove