Cooling producer gas condenses its water, but gas cooled only to 40 °C is still saturated, carrying about 63 g of water vapour per normal cubic metre of dry gas, which is 7.3% of its volume, whereas at 25 °C it holds about 26 g. The liquid water that does condense must be drained at a low point before the engine, because droplets cause misfires, and the colder the gas the more power the engine keeps.
Part of Gas Cleaning & Filtering
Moisture separation removes condensed water from cooled producer gas with a drain at the lowest point of the pipework before the engine.
How Much Water There Is
The parent says to include a condensation trap after the cooler and before the engine, because even small amounts of water cause misfires. The amount depends on the fuel moisture and the gasifier, and I do not have a sourced figure for it. To show the arithmetic, assume an illustrative 0.2 kg of water leaves per kilogram of dry wood (an assumption, not a measurement). With about 2.5 Nm³ of gas per kg of dry wood (Energy Content of Wood Gas):
- Water in the gas: 0.2 ÷ 2.5 = 0.08 kg = 80 g per Nm³ of dry gas.
- Saturated vapour at 40 °C: vapour pressure 7.38 kPa, so the mole ratio of water to dry gas is 7.38 ÷ (101.3 − 7.38) = 0.0786, which is 0.0786 × 18.0 = 1.42 g per 22.4 L, or 63 g/Nm³.
- Saturated at 25 °C: 3.17 kPa gives 3.17 ÷ (101.3 − 3.17) = 0.0323, or 0.58 g per 22.4 L, 26 g/Nm³.
- Condensate if cooled to 40 °C: 80 − 63 = 17 g/Nm³; at 25 °C: 80 − 26 = 54 g/Nm³.
- At 20 Nm³/h that is 0.34 and 1.08 kg/h; at 60 Nm³/h it is 1.0 and 3.2 kg/h.
Cold Gas Is Worth More Power
At 40 °C the steam fraction is 7.38 ÷ 101.3 = 7.3% of the volume; at 25 °C it is 3.17 ÷ 101.3 = 3.1%. Steam displaces fuel gas and air in the cylinder without adding energy, so the parent’s rule of cooling below 40 °C is a minimum, and cooling further recovers a few percent of power.
Where Water Is Drained
- A drain valve at the lowest point of the cooler and each filter housing, kept closed during running.
- A water-sealed U-trap only if its seal is deeper than the suction at that point. The engine pulls on the gas line, and a seal shallower than the suction head in millimetres of water is pulled dry and then admits air; 1 kPa of suction is about 102 mm of water.
- A catch vessel that the drained liquid runs into, since it contains tar and phenols.
Method
- Cool first, drain at the low point, then filter.
- Drain at start-up, after the first 15 minutes of running, and at shutdown.
- Judge wetness by the amount drained: a sudden increase usually means wetter fuel.
- Keep gas lines sloping to the drains so water does not lie in dips.
Common Mistakes
- Cooling to 40 °C and calling the gas dry. It is saturated.
- Dips in the pipe. Water collects and chokes the flow.
- Pouring condensate on the ground near a well or stream. It carries tar and phenols.
Safety
Condensate drains open into carbon monoxide
The gas behind every drain valve is about 18-22% carbon monoxide by volume. Drain outdoors, standing upwind, and do not open drains on a running unit with the blower off or on a unit that is hot. The condensate is toxic and burns skin, and tarry rags are a fire hazard. Never run or drain a gasifier in a shed, garage or enclosed yard. See Carbon Monoxide Risks.
What’s Next
- Learn next: Engine Connection — the gas, now cool and dry enough, meets the engine
- Combine with: Tar Removal (Cyclone & Cooling) — the stage that produces the condensate