How to match coal to your boiler: three questions
Does it have a draught fan or a flue gas extractor? If it does, go for anthracite: it gives maximum heat and the longest burn. If not, look at the chimney: with a height of 6-7 m or more and a smooth, straight flue, anthracite will burn too; with a low chimney or one with several bends, choose long-flame coal.
How is the fuel fed? Hand-fed: any fraction from 13 mm up, i.e. AO, AM or DG 50-100. Auger or retort: only the small fraction specified in the burner manual, usually AS 6-13 mm. Larger lumps jam the auger, and slack below 6 mm cakes and puts the burner out.
How many times a day are you prepared to tend the boiler? Anthracite AO in a fan-assisted boiler: once or twice. DG in a simple boiler: three or four times, but it quickly brings the temperature back up after a break. In long-burning (shaft-type) boilers on natural draught, use anthracite only if the manufacturer recommends it; otherwise it smoulders and never reaches operating temperature.
Anthracite or long-flame coal: the figures
| Parameter | Anthracite AO/AM/AS | Long-flame DG |
|---|---|---|
| Calorific value | 7,000-8,000 kcal/kg, ≈ 8 kWh/kg | 5,500-6,500 kcal/kg, ≈ 6.5-7 kWh/kg |
| Volatile matter | 2-8% | 35-45% |
| Lighting | On a bed of wood or DG, needs strong draught | From kindling and paper |
| Burn time per load | 8-12 hours | 4-6 hours |
| Smoke, soot | Almost none | Yes, clean the chimney every 1-2 months |
| Ash content | 10-15% | 10-20% |
| Suitable boilers | Fan-assisted, with a tall chimney, automatic (AS) | Any, including boilers without a fan, and stoves |
| Consumption per season, 100-150 m² house | 3-5 t | 4-6 t |
Anthracite usually costs more per tonne, but in terms of the price of a kilowatt-hour of heat it is closer to DG than the price list suggests. A comparison of all fuel types is in the article "Anthracite or long-flame coal: which suits your boiler".
How to fire a boiler with coal so it neither "won't burn" nor "smokes"
Anthracite is lit on a 10-15 cm bed of glowing wood or DG: add 15-20 cm of anthracite and give it full draught until the bed glows right through, then reduce the draught to the working setting. Do not close the damper completely: on weak draught anthracite smoulders and gives off carbon monoxide. Long-flame coal is added in small portions onto a glowing bed without filling the firebox to the top, and the ash-pit door is never shut completely: the volatiles need to burn off, otherwise you get black smoke and soot in the chimney. Shake the clinker off the grate before each load.
What else matters for a boiler
A test report for the batch. Ash content and moisture determine how much real heat you get from a tonne: the difference between a batch with 8% ash and one with 18% is a tenth of your money. We send the test report on request before payment. Graded coal without slack. Fines do not let air through and smother the fire bed; in graded coal they make up no more than 10%. Storage. Under a canopy, not in a puddle: wet coal is harder to light, and bags freeze together in winter.
Related pages: anthracite AS for automatic boilers, coal for stoves and pot-belly stoves, coal in bags.