Gas collection is a holder (the dome, a floating drum, a bag or a tube), a pressure regulator and a pipe: the holder must store the gas produced between uses, so size it for the largest draw less what is made meanwhile. A household that draws 0.7-1.0 m³ in an evening while its digester makes 0.0625 m³ an hour needs a holder of about 0.4-0.7 m³, and the first gas from a new plant is a mix with air that must be vented, not burned.
Part of Biogas Digesters
Gas collection and storage cover holding the biogas between production and use, delivering it at a steady pressure and getting it safely to the appliance.
Production Is Steady, Use Is Not
A digester makes gas all day and night; people use it at meals and in the evening. The holder bridges the difference.
Worked Example: Sizing The Holder
Take a household with a digester making 1.5 m³ a day (the parent’s family cooking need lower bound) = 1.5 ÷ 24 = 0.0625 m³ an hour. The evening draw: one meal of 0.3-0.4 m³ (parent) plus a lamp at 0.1-0.15 m³ an hour for 4 hours = 0.4-0.6 m³ (see Lighting), a total of 0.7-1.0 m³ over about 5 hours. Gas made meanwhile: 0.0625 × 5 = 0.31 m³. The store must therefore hold about 0.7 − 0.31 = 0.4 m³ up to 1.0 − 0.31 = 0.7 m³. Round up for the days when the digester is slow. A holder of 0.4-0.7 m³ covers it; make it bigger if there are morning and evening peaks as well.
The Options
- Dome of a fixed-dome plant: stores gas under the roof; see Gas Holder Design.
- Floating drum: stores gas under the drum at constant pressure (Floating-Drum).
- Separate bag or tube: inner tubes and heavy plastic bags in a frame, weighted to give pressure.
Pressure is the second job: a burner wants 5-15 cm of water column (Pressure Regulation). The third is piping (Piping & Valves).
The First Gas Is Not Fuel
When a new digester starts, the space above the slurry holds air. The first gas leaving it is a mixture of air, carbon dioxide and a little methane. A mixture of methane in air burns explosively between 5% and 15% methane; the first gas passes through that range on its way to a richer mix. Vent the first gas outdoors, high and away from any flame, for several days, until the digester has been fed and producing steadily; the parent gives no figure for how much to vent, and a flame is not a safe way to find out whether the holder still contains air. Do not store it for use. The same applies after any repair that lets air into the holder, and a water seal in the gas line stops a flame from travelling back into the holder (Pressure Regulation).
What You Need
- A holder sized as above, in the open and away from any fire
- A regulator and relief (Pressure Regulation)
- A vent pipe to release the first gas outdoors, high and away from people, windows and flames
Method
Step 1: Size the holder from the evening draw.
Step 2: Connect to the regulator and pipe, leak-test with soapy water.
Step 3: Vent the first gas outdoors for the first days.
Step 4: Connect appliances only when production is established and the line has a flashback water seal.
Common Mistakes
- Sizing the holder to the daily total. It need only cover the draw peak.
- Burning the first gas indoors.
- Allowing the holder to empty completely so that air is drawn in.
Gas and fire do not mix
Methane burns and explodes at 5-15% in air, so a leak from a holder in a closed space can reach explosive strength; the raw gas also carries lethal hydrogen sulfide. Keep the holder outdoors, fence it, never use a flame to look for leaks, and never go into a pit or a closed space where gas piping runs. See Anaerobic Digestion Science.
What’s Next
- Learn next: Gas Holder Design
- Combine with: Using Biogas