A central boiler for a 30-house village must deliver 120 kW at peak, which takes 37.8 kg of seasoned wood an hour, 18.0 to 22.5 kg of bituminous coal, 41.9 kg of 30%-moisture peat or 25.7 m³ of biogas, in each case assuming an 80% efficient boiler. The 10 to 20 kg an hour sometimes quoted for wood matches the season-average load of about 48 kW, not the peak the boiler must be built for.
Part of District Heating
The fuel and heat source options for a village heating plant, with the fuel each needs per hour at the peak load.
Size The Boiler For The Peak, The Fuel Store For The Average
The boiler has to meet the coldest day: 30 houses at 4 kW each is 120 kW. Over a 200-day season the average load is lower (40% in the reference case, or 48 kW). The hourly fuel rate at any output is
fuel per hour = P × 3,600 kJ per kWh ÷ (heating value × boiler efficiency)
Fuel Rates At 120 kW And 80% Efficiency
Heating values: wood at 20% moisture 14.3 MJ/kg and peat at 30% moisture 12.9 MJ/kg from BTU Content Compared to Wood; coal from the practical ranges in Coal Types & Quality; biogas at about 21 MJ/m³ for 60% methane, the figure in Cooking Burners. The boiler efficiency of 80% is the lower end of the carrier page’s 80 to 90%.
| Fuel | Heating value | Fuel at 120 kW |
|---|---|---|
| Seasoned wood, 20% moisture | 14.3 MJ/kg | 37.8 kg/h |
| Peat, 30% moisture | 12.9 MJ/kg | 41.9 kg/h |
| Bituminous coal | 24 to 30 MJ/kg | 22.5 to 18.0 kg/h |
| Sub-bituminous coal | 15 to 20 MJ/kg | 36 to 27 kg/h |
| Lignite | 10 to 15 MJ/kg | 54 to 36 kg/h |
| Biogas, 60% methane | 21 MJ/m³ | 25.7 m³/h |
Working, wood: 120 kW is 432 MJ per hour; 14.3 × 0.80 = 11.44 MJ of useful heat per kg; 432 ÷ 11.44 = 37.8 kg/h. At the season-average 48 kW the same boiler burns 48 × 3.6 ÷ 11.44 = 15.1 kg/h.
The carrier page says 90 to 150 kW comes from 10 to 20 kg of wood an hour. At these values 10 to 20 kg an hour gives 10 × 11.44 ÷ 3.6 = 32 kW to 64 kW; 90 to 150 kW needs 28 to 47 kg/h.
Choosing The Source
- Wood. The default. A coppice that regrows on a cycle (see Coppicing & Fuel Management) keeps it renewable. Must be seasoned to 20% moisture. See Wood-Fired Boiler Design.
- Coal or peat. Where a deposit is near. More ash and sulphur. See Peat Boiler.
- Biogas. A supplement, not a source: the arithmetic is in Biogas Boiler Integration.
- Solar. A summer and preheating source: Solar Thermal Integration.
Boilers of this size are built on the principles in Boiler Construction and Fire-Tube Boiler; the hot-water boiler can run at atmospheric pressure, which makes it far less dangerous than a steam boiler.
Common Mistakes
- Taking an hourly fuel rate from the average load. The boiler and the stoker are sized for the peak.
- Forgetting the moisture. At 40% moisture wood gives only 10.1 MJ/kg (from the coppicing page), 29% less than at 20%.
- Mixing the boiler efficiency with the system efficiency. The network loss is on top of it.
Safety
A boiler house holds three hazards at once
Hot water or steam under pressure, a live fire and a fuel store. Keep the water open-vented where possible, fit a safety valve on any closed part (Safety Valve), ventilate for carbon monoxide, and keep wood stacks and coal piles at a distance from the firebox.
What’s Next
- Learn next: Wood-Fired Boiler Design — the default plant in detail
- Combine with: Fuel Consumption Estimates — what the season needs in tonnes