Biogas from a household digester is best used for cooking, which takes about 1.5-2.0 m³ a day for a family of 4-5, and for lighting with a mantle lamp at 0.1-0.15 m³ an hour; it can also fuel an engine, but a day’s gas runs a 3 kW generator for under an hour. The gas replaces firewood and charcoal, the digestate returns the fertiliser, and every use needs a leak-free line, a water seal and ventilation.
Part of Biogas Digesters
Using biogas covers the burners, lamps and engines that burn the gas, and the digestate left behind.
How Much Each Use Takes
| Use | Gas needed | Basis | Page |
|---|---|---|---|
| Cooking, family of 4-5 | 1.5-2.0 m³ a day | parent | Cooking Burners |
| One meal | 0.3-0.4 m³ | parent | same |
| Mantle lamp | 0.1-0.15 m³ an hour | parent | Lighting |
| 3 kW engine | 1.7-2.4 m³ an hour | worked below | Engine Fuel |
| Fertiliser from the digestate | no gas | Digestate as Fertilizer |
The parent’s meal figure and its daily family figure do not match exactly: 0.3-0.4 m³ a meal over three meals is 0.9-1.2 m³, so 1.5-2.0 m³ a day covers about four to seven meals, or cooking plus other uses such as water heating.
Energy In A Cubic Metre
At 18-25 MJ per cubic metre (55-70% methane), one cubic metre holds 5-7 kWh of heat. An engine turning 25% of that into electricity (an assumed efficiency for a small spark-ignition engine) gives about 1.3-1.7 kWh per cubic metre. A 3 kW generator at 25% needs 12 kW of fuel, 43 MJ an hour, which is 43 ÷ 25 = 1.7 to 43 ÷ 18 = 2.4 m³ an hour. A day’s 1.5-2.0 m³ would run it 1.5 ÷ 1.73 = 0.87 hours up to 2.0 ÷ 2.4 = 0.83, under an hour, about 2.5 kWh. So a household plant is for cooking and light; engines need larger plants.
What The Gas Replaces
Firewood and charcoal for cooking, and the labour of gathering them. It does not replace food: the feed is manure and waste, and the digestate keeps the nutrients. If the feed is a crop grown for the digester, it takes land from food, and the corpus puts food first (N Ratio & Feedstock Selection).
First Gas Rules
A new plant produces a mixture of air and gas at first; vent it outdoors and do not light anything on it (Gas Collection & Storage).
Common Mistakes
- Planning an engine on a family digester.
- Using the gas before leak-testing.
- Leaving a burner or lamp unattended.
Gas indoors
A leak or a blown-out flame puts methane into the room, which explodes at 5-15% in air, and hydrogen sulfide, which can kill and numbs the nose. Close the valve at the appliance after every use, keep the room ventilated, light burners with the gas valve opened only after the flame is ready, and never use a flame to find a leak (Piping & Valves, Pressure Regulation). Burners that burn badly make carbon monoxide.
What’s Next
- Learn next: Cooking Burners
- Combine with: Digestate as Fertilizer