A cyclone separates dust and char from hot producer gas but does little against tar, which is vapour at cyclone temperatures and condenses later as a fine mist, so the tar is removed in the cooler, where the gas is brought from about 350 °C to below 40 °C. For 20 normal cubic metres of gas an hour that is about 2.3 kW of heat, roughly 1 m² of bare pipe in moving air, and the pipe must slope to a drain.
Part of Gas Cleaning & Filtering
The first two stages of gas cleaning: a cyclone that takes out coarse particles, and a cooler that condenses tar and water.
A Correction To The Parent On What A Cyclone Does
The parent says heavy tar droplets are flung to the walls of the cyclone. In a gasifier running as intended, almost all the tar leaves the vessel as vapour, not droplets, and a cyclone sitting in 300 °C gas has nothing liquid to throw. The cyclone’s job is the solid dust and char. Tar that condenses later does so as a mist of very small droplets (recalled: often below a micron) that a cyclone cannot catch. Cyclone first for the dust; the cooler and then the filters for the tar. This is a qualitative correction from general practice; I could not source a figure for droplet size.
Sizing A Cyclone
A Stairmand high-efficiency cyclone, a standard design, has these proportions in terms of the body diameter D: inlet height 0.5D, inlet width 0.2D, gas outlet diameter 0.5D, total height 4D (recalled from chemical-engineering references). Its inlet area is 0.5D × 0.2D = 0.1D², and inlet speeds of roughly 15-20 m/s are normal practice (recalled).
Worked example for a 15 cm body, using the vehicle gas flows of 50-80 Nm³/h (second carrier) at 300 °C:
- Actual volume factor: (273 + 300) ÷ 273 = 2.1.
- Actual flow: 50 × 2.1 = 105 and 80 × 2.1 = 168 m³/h, i.e. 0.0292 and 0.0467 m³/s.
- Inlet area: 0.1 × 0.15² = 0.00225 m².
- Inlet speed: 0.0292 ÷ 0.00225 = 13.0 and 0.0467 ÷ 0.00225 = 20.7 m/s.
So the parent’s 15-20 cm diameter suits the vehicle gas flow. For a 5 kW stationary set at 20 Nm³/h (42 m³/h = 0.0117 m³/s hot) at 15 m/s the inlet area must be 0.0117 ÷ 15 = 0.00078 m², so D = √(0.00078 ÷ 0.1) = 0.088 m: a body of about 9 cm. A cyclone far larger than the flow needs runs slow and catches little; one far smaller wears quickly and costs suction.
The Cooler Duty
- Specific heat of the gas: about 1.35 kJ per Nm³ per K (an estimate from the component gases; assumption).
- Cooling from 350 to 40 °C, a drop of 310 K: 20 × 1.35 × 310 = 8,370 kJ/h = 2.3 kW.
- Mean temperature difference to 20 °C air: (330 − 20) ÷ ln(330 ÷ 20) = 310 ÷ 2.80 = 111 K.
- With an assumed heat transfer coefficient of 20 W/m²K for bare pipe in moving air: area = 2,300 ÷ (20 × 111) = 1.04 m².
At 60 Nm³/h the duty is three times that, about 3 m² of bare pipe, which is why vehicles use a finned radiator (Radiator Setup). Condensing the water also releases latent heat, adding about 10-30% on top depending on how much condenses (Moisture Separation).
Method
- Put the cyclone in the hot line immediately after the gasifier, insulated, with a sealed collection pot.
- Run the cooler pipe with a continuous fall toward a drain, so condensed tar and water leave instead of pooling.
- Provide a way to rod or flush the pipe when cold.
- Drain regularly. Wood tar condensate resembles the tars in Creosote and Wood Tar Production, and treat it with the same care.
Common Mistakes
- Expecting the cyclone to remove tar. It removes dust.
- A cooler laid flat. Tar collects and plugs it.
- A cyclone sized for the wrong flow. Speed is the design variable.
Safety
A hot pipe full of carbon monoxide, and a tar fire hazard
The gas is about 18-22% carbon monoxide by volume. The cyclone pot and cooler drains hold gas as well as condensate; empty them outdoors only, with the unit cold. Tar condensed in pipework burns. Never run, drain or clear any of it indoors, in a shed or in an enclosed yard. See Carbon Monoxide Risks.
What’s Next
- Learn next: Particulate Filtration — the stage after cooling
- Combine with: Moisture Separation — the condensate from the same cooling