Biogas can only supplement a village heating loop: 30 households whose manure yields the corpus figure of 1.5 to 2.0 m³ of gas per household per day make 45 to 60 m³ a day, which at 21 MJ/m³ and 80% burner efficiency is 8.75 to 11.7 kW, 18 to 24% of the 48 kW season-average load and only 7.3 to 9.7% of the 120 kW peak. Full output of 120 kW would burn 25.7 m³ an hour, 617 m³ a day, so the gas feeds a store and a modest burner while wood or coal carries the peak.
Part of District Heating
How much heat village biogas supplies to a boiler loop, how to feed it, and what to do about the gas’s hazards.
How Much Gas There Is
Biogas Digesters says a family of 4 to 5 needs about 1.5 to 2.0 m³ of biogas a day for cooking, from roughly 3 to 4 cows’ manure. Taking that as the gas available per household, 30 households yield 45 to 60 m³ a day.
Energy: biogas at 60% methane holds about 21 MJ/m³ (Cooking Burners). With an 80% efficient burner and boiler:
- 45 m³/day × 21 MJ × 0.80 = 756 MJ/day = 8.75 kW average.
- 60 m³/day × 21 × 0.80 = 1,008 MJ/day = 11.7 kW average.
Against the reference village (30 houses at 4 kW design, 40% average load): 8.75 ÷ 48 = 18% and 11.7 ÷ 48 = 24% of the season-average load; 8.75 ÷ 120 = 7.3% and 11.7 ÷ 120 = 9.7% of the 120 kW peak. This matches the carrier page’s “reduces wood by 15 to 25%”, provided every household’s gas goes to the boiler and none to the kitchen. The cooking use the corpus gives (1.5 to 2.0 m³/day) is the same gas: if it still cooks, none is left for the boiler.
For the boiler to carry 120 kW alone would need 120 kW × 3,600 ÷ (21 × 0.80) = 25.7 m³ an hour, or 617 m³ a day, 10 to 14 times the supply (617 ÷ 60 = 10.3, 617 ÷ 45 = 13.7). Biogas is a supplement.
Feeding The Boiler
- Store the gas. Heat demand peaks at times the gas does not. Size the store for the biggest draw between refills, as Gas Collection & Storage explains, not for the day’s total.
- Use a separate burner. A gas burner on the boiler or a small dedicated gas boiler feeding the loop; do not try to burn gas in a solid-fuel firebox.
- Run low pressure pipes with water traps and a ball valve at the appliance; see Piping & Valves.
- Scrub the hydrogen sulphide. The gas contains H₂S, which corrodes metal and is toxic; renew the scrubber on a calendar. See Anaerobic Digestion Science.
- Control by priority. Burn gas first when the store is full, wood when it is not.
Methane Rises
Methane is 16 g/mol against air’s 29, so it is lighter than air. A leak collects at the ceiling and at the top of any pit or shaft, not at the floor. Put gas detection and ventilation high, and keep ignition sources out of the roof space of the gas room.
Common Mistakes
- Counting the gas twice. The same gas cannot cook and heat.
- A burner without flame-failure protection. An unlit burner fills the room with gas.
- Sizing the burner for the average gas supply. Size it for the draw from the store.
Safety
Biogas is flammable, toxic and lighter than air
Methane burns explosively in air between roughly 5% and 15% (recalled value). Never test for a leak with a flame; paint soapy water on every joint and watch for bubbles. Ventilate the roof space of the gas room, keep the scrubber fresh because H₂S is toxic, and keep the gas store away from the boiler house.
What’s Next
- Learn next: Solar Thermal Integration — the other supplement
- Combine with: Digester Types — which digester suits a village