A biogas cooking burner is a gas jet that draws in primary air and burns with a blue flame, and because biogas is only about 60% methane it needs a much larger jet than a propane burner: about 2.4 times the diameter at the same heat output, not the one or two drill sizes the parent suggests, and a much smaller air intake, since biogas needs about 5.7 volumes of air per volume against propane’s 23.8. Start from a natural-gas burner and tune by flame colour, with the room ventilated.
Part of Biogas Digesters
A cooking burner mixes biogas with air and burns it under a pot; its jet size and air intake are set by the gas’s heating value and pressure.
The Parent’s Advice And The Arithmetic
The parent says a standard propane or natural gas burner works with slightly larger orifice openings, to drill out by 1-2 sizes, adjust the air for a blue flame, and that 0.3-0.4 m³ cooks a meal for 4-5 people. Check against the gas properties.
Air needed. Methane burns as CH₄ + 2 O₂. Air is 21% oxygen, so 2 ÷ 0.21 = 9.5 volumes of air per volume of methane; biogas at 60% methane needs 0.6 × 9.5 = 5.7. Propane (C₃H₈ + 5 O₂) needs 5 ÷ 0.21 = 23.8. A propane burner’s air intake is therefore set for four times the air, and with biogas it must be closed down sharply or the flame blows out or lifts.
Jet size. For equal heat, the gas volume flow scales with heating value. Propane is about 91 MJ/m³ (recalled), biogas about 21 MJ/m³ at 60% methane, so 91 ÷ 21 = 4.3 times the volume of gas. Jet speed depends on supply pressure and gas density: propane supplied at about 2.8 kPa (the common 11-inch water column, recalled) and density 1.88 kg/m³; biogas at 1.0 kPa (10 cm of water) and 1.14 kg/m³ (0.6 × 0.668 + 0.4 × 1.842, at about 15°C). Speed ∝ √(pressure ÷ density): √((1.0 ÷ 1.14) ÷ (2.8 ÷ 1.88)) = √0.59 = 0.77 as fast. Area must rise by 4.3 ÷ 0.77 = 5.6 times, so the diameter by √5.6 = 2.4 times. A 0.9 mm propane jet becomes about 2.1 mm for biogas. Even at the same supply pressure the area is 3.4 times, 1.8 in diameter.
So the parent’s “1-2 sizes larger” is far short for a propane burner. A natural-gas burner, whose jet and air intake are already closer to biogas, is the better starting point, but still needs enlarging and tuning.
What You Need
- A burner with a replaceable jet and an adjustable air shutter
- Drill bits in small steps and a way to measure the jet
- Pot supports to hold the pot at the right height
- A ventilated kitchen with a chimney or opening
- An accessible shut-off valve (Piping & Valves)
Method
Step 1: Connect to the gas line at 5-15 cm of water column (Pressure Regulation).
Step 2: Enlarge the jet in small steps, testing the flame between.
Step 3: Read the flame. Blue with a short sharp inner cone means a good mix. Yellow or orange tips mean too little air (or too much gas); a flame that lifts off the burner, roars or goes out means too much air or too much pressure; a flame that burns back inside the burner head means too little gas speed.
Step 4: Set the air shutter to the blue flame; keep soot off the pot bottoms as a check.
Step 5: Shut the valve at the appliance after every use.
Common Mistakes
- Drilling a propane jet by a size or two and calling it done.
- Leaving the air shutter wide open.
- A yellow flame accepted as normal. It makes carbon monoxide and soot.
- Cooking in an unventilated room.
Burner hazards
An unlit burner with the valve open fills the room with methane (explosive at 5-15% in air) and hydrogen sulfide. A yellow or sooty flame produces carbon monoxide, which poisons silently. Scrubbed biogas has no smell, so do not rely on the nose to find a leak. Light with the flame first, shut the valve if the flame blows out, ventilate, and never use a flame to find a leak (Gas Hazards (Methane, CO) describes carbon monoxide).
What’s Next
- Learn next: Lighting
- Combine with: Using Biogas