Coal grades AO, AM, AS, AK, DG explained: what the letters mean and which to choose
A coal price list is full of letters: AO, AM, AS, DG. The first letter tells you what kind of coal it is, the second the size of the lumps. Once you have worked this out, you will never again overpay for a grade your boiler does not need.
Two labelling systems in one price list
Ukrainian coal labelling is inherited from DSTU 3472 and has two parts. The first is the grade by rank of coalification, that is, by how much volatile matter and carbon the coal contains: B (Б, brown coal), D (Д), DG (ДГ), G (Г), Zh (Ж), K (К), P (П), A (А, anthracite). The second is the size class of the lumps: P (П, slab), K (К, large), O (О, nut), M (М, small), S (С, pea), Sh (Ш, slack), R (Р, run-of-mine). Only a few combinations matter to household buyers, and those are exactly the ones you see in price lists.
Anthracite: AK, AKO, AO, AM, AS, ASh
The letter A stands for anthracite: over 90% carbon, 2-8% volatile matter, a calorific value of 7,000-8,000 kcal/kg. Chemically all anthracite grades are the same; the only difference is the lump size, and therefore how the fuel gets into the firebox and how it burns.
| Grade | Size class | Size, mm | Used for |
|---|---|---|---|
| AK (АК) | large | 50-100 | large boiler house fireboxes, hand feeding |
| AKO (АКО) | large with nut | 25-100 | hand feeding, a mix of two sizes |
| AO (АО) | nut | 25-50 | all-purpose grade for hand-fed domestic boilers |
| AM (АМ) | small | 13-25 | small fireboxes, automatic boilers with moving grates |
| AS (АС) | pea | 6-13 | automatic boilers with auger and retort feed |
| ASh (АШ) | slack | 0-6 | industrial pulverised coal firing; not suitable for homes |
The rule is simple: hand feeding means AO or AM, an auger or retort means AS. The larger the lump, the better air passes through the bed and the longer a load burns; the smaller the lump, the more tightly it packs and the more even the heat, but the higher the risk that the bed "suffocates" without forced draught.
Bituminous coal: D, DG, G
This is coal with a high volatile matter yield (35-45%). Plenty of volatiles means it lights easily and burns with a long flame, which is why grade D is called long-flame coal. Its calorific value is lower than anthracite: 5,500-6,500 kcal/kg.
| Grade | Meaning | Volatiles | Characteristics |
|---|---|---|---|
| D (Д) | long-flame | 35-45% | burns easily, lots of smoke, for stoves and simple boilers |
| DG (ДГ) | long-flame gas coal | 35-42% | the most common household grade, slightly more calorific than D |
| G (Г) | gas coal | 33-40% | closer to coking coal, mostly goes to industry |
A size class is added to the grade just as with anthracite: DG 50-100 (large, for hand feeding), DG 25-50, DG 13-100 (run-of-mine, a mix of sizes, cheaper). For households, DG 50-100 is most often sold in bags and DG 13-100 in bulk for boiler houses.
Figures in the certificate: what they mean
Ash content (A) is the share of ash by mass. Sized anthracite has 10-15%, DG 10-20%. Every extra per cent means another kilogram of ash from each 100 kg and slightly less heat.
Moisture (W) is up to 8% for anthracite and up to 12-14% for bituminous coal. Wet coal is heavier, burns worse and spends part of its heat evaporating water.
Volatile matter yield (V) is what determines the grade. Low volatiles: anthracite burns without flame or smoke and needs draught. High volatiles: bituminous coal burns with a flame, smokes and needs air.
Sulphur (S) is usually 1-2%. The higher it is, the stronger the smell when burning and the faster the heat exchanger corrodes.
Net calorific value (Q) is how much heat a kilogram gives in real combustion, taking moisture into account. This is the figure to compare between grades and to use when calculating your supply for the season.
Which grade to buy: by boiler type
| Boiler or stove | Grade | Explanation |
|---|---|---|
| Solid fuel boiler with a forced-draught fan | AO, AM | the fan provides the draught anthracite needs; longest burn |
| Solid fuel boiler without forced draught, tall chimney | AO or DG 50-100 | anthracite if the draught really is good, otherwise DG |
| Simple boiler, stove, pot-belly stove, masonry heater | DG 50-100 | burns on natural draught, lights easily |
| Automatic boiler with an auger | AS, sometimes AM | size as per the boiler manual; DG is not suitable for an auger |
| Top-burning long-burn boiler | AO, AM or DG 25-50 | as per the manufacturer's instructions; often both types are allowed |
| Commercial boiler house | AK, AKO, DG 13-100 in bulk | lowest price per tonne, large firebox |
How not to overpay for a grade
The most common mistake is buying AO "just in case" for a stove, where it will smoulder and give off fumes, or DG for a boiler with forced draught, where it will burn out twice as fast as anthracite. The second mistake is buying a large size for an auger: AO or DG 50-100 jams in an auger burner. The third is buying run-of-mine DG 13-100 for the home: it is cheaper, but the slack in it blocks the air in a small firebox, and half the coal ends up in the ash.
Before buying, open the boiler manual and find the line about recommended fuel: it will state the grade and the maximum lump size. That is your filter for the price list. If you do not have the manual, give us a call and we will advise based on the boiler model.
Questions and answers
What is the difference between AO and AM coal?
Chemically it is the same coal, anthracite; the only difference is the lump size: AO (nut) is 25-50 mm, AM (small) is 13-25 mm. AO is the all-purpose grade for hand-fed domestic boilers. AM suits small fireboxes and automatic boilers with moving grates, but it packs more tightly and can "suffocate" without forced draught.
What coal do I need for an auger-fed boiler?
Automatic boilers with auger and retort feed use anthracite AS (pea, 6-13 mm), sometimes AM; the exact size is given in the boiler manual. Large sizes such as AO or DG 50-100 jam in the auger, and long-flame coal is not suitable for an auger at all.
What does the coal grade DG mean?
DG is long-flame gas bituminous coal with 35-42% volatile matter. It lights easily, burns with a long flame and gives 5,500-6,500 kcal/kg: less than anthracite, but natural draught is enough for it. It is the most common household grade for stoves and simple boilers; a comparison with anthracite is in the article anthracite or long-flame coal.
Is run-of-mine DG 13-100 coal worth buying for a home?
Run-of-mine DG 13-100 is a mix of sizes; it is cheaper, but it contains slack. In a small domestic firebox the fines block the air, and a large part of the coal ends up in the ash. For a home, DG 50-100 is better, while run-of-mine coal makes sense in boiler houses with a large firebox.
What are normal ash and moisture levels for coal?
Normal ash content is 10-15% for sized anthracite and 10-20% for DG; every extra per cent means another kilogram of ash from each 100 kg. Moisture is up to 8% for anthracite and up to 12-14% for bituminous coal. Wet coal is heavier and spends part of its heat evaporating water.
Can I heat a house with ASh anthracite?
No. ASh is slack with a particle size of 0-6 mm, used for industrial pulverised coal firing. In a domestic boiler it packs tightly, blocks the air and does not burn properly. For a home, choose AO, AM or AS depending on the boiler; you will find them in the coal section.
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Prices per tonne and per bag, stock and delivery across Ukraine are on the category page. For wholesale: special terms.