A coal or peat boiler for a village loop needs a grate with air fed from below, a firebox lined with cast iron or firebrick, and ash room, because at 120 kW and 80% efficiency it burns 18.0 to 22.5 kg of bituminous coal an hour, 27 to 36 kg of sub-bituminous coal, or 41.9 kg of peat at 30% moisture, and the coal leaves 5 to 15% of its weight as ash. Judge the fuel by weighed test burns, never by a single heating value, and keep the flue hot enough that sulphur in the fuel does not condense and corrode it.
Part of District Heating
Using coal or peat in a village boiler: fuel rates, what changes in the firebox, and how to handle ash and sulphur.
Fuel Rates At 120 kW
Heating values: coal ranges from Coal Types & Quality (lignite 10 to 15 GJ/t, sub-bituminous 15 to 20, bituminous 24 to 30, anthracite 30 to 35); peat at 30% moisture, 12.9 MJ/kg, from BTU Content Compared to Wood. Plant: 120 kW is 432 MJ per hour at 80% efficiency, so the fuel is 432 ÷ (heating value × 0.8).
| Fuel | Heating value | Fuel per hour | Ash at 5 to 15% |
|---|---|---|---|
| Bituminous coal | 24 to 30 MJ/kg | 22.5 to 18.0 kg | 0.9 to 3.4 kg |
| Sub-bituminous coal | 15 to 20 MJ/kg | 36 to 27 kg | 1.4 to 5.4 kg |
| Lignite | 10 to 15 MJ/kg | 54 to 36 kg | 1.8 to 8.1 kg |
| Peat, 30% moisture | 12.9 MJ/kg | 41.9 kg | not given in the corpus |
The ash column applies the corpus’s 5 to 15% coal ash figure (from the coal-stove comparison page) to the fuel rate; ash content varies with the seam. A fuel that is wetter than its test figure delivers less. BTU Content Compared to Wood gives the moisture correction: net heat = dry value × (1 − w) − 2.44 × w, in MJ/kg.
What Changes In The Firebox
Coal Stove vs Wood Stove Differences and Burning Coal Effectively give the rules:
- Air from below, through a grate. Coal burns as a glowing bed; without a grate and an ash pan the ash packs and smothers it.
- Hotter, more corrosive. Use cast iron or firebrick for the firebox; plain steel plate can warp or burn through.
- Large ash room. Empty the pan often.
- Start with a hot wood fire, feed small batches. A deep single load or a fire damped too low makes smoke and carbon monoxide.
Peat burns slowly with a long ember bed and more smoke than seasoned wood (Burning & Performance); give it a grate and strong draught, and dry it first (Peat Drying & Curing). The moisture stakes are large: fresh peat at 80 to 90% water has almost no net heat.
Sulphur And The Flue
Coal sulphur makes acidic flue gas that condenses in a cool flue and corrodes metal. Keep the flue gas hot enough to carry the vapour out, line or choose masonry flues, and clean them often. A cold network return into a coal boiler invites condensation on the heat-exchange surfaces; the mixing-valve return protection in Wood-Fired Boiler Design applies here too.
Handling And Storage
- Store coal dry and apart from the firebox and from wood.
- Remove ash into a metal container and keep it away from combustible material; ash retains hot coals for hours.
- Check the boiler room for carbon monoxide whenever the fire is damped or the draught is poor.
Common Mistakes
- Using one heating value for all “coal”. Rank changes the fuel rate by a factor of three, from 18 to 54 kg/h.
- Burning a wood boiler’s fuel mix in a coal firebox. The air path is different.
- Letting the return run cold. Sulphur acid forms on cold metal.
Safety
Carbon monoxide and ash
A coal or peat fire starved of air makes carbon monoxide, which is odourless and kills in a closed boiler house. Keep the air control open enough that the fire does not smoulder, ventilate at floor and ceiling, never sleep in the boiler house, and treat all ash as hot for a full day. See Coal Mine Safety for the mining side, which is a separate hazard.
What’s Next
- Learn next: Biogas Boiler Integration — a small supplementary source
- Combine with: Fuel Consumption Estimates — tonnes per season by fuel