A two-stage cyclone can clean the dusty, sometimes tarry gas a biomass gasifier produces during start-up, before it is burned in a flare. The hard part is not the cyclone itself but the unknowns: particle size, velocity, tar and the pressure available. This post follows a proposal for a cold cyclone at a biomass gasification power plant in England and shows how those unknowns were handled.
The challenge: dirty gas before the plant settles
During start-up, a gasifier does not yet make the clean, steady gas it will in normal running. The gas carries dust and may carry tar, and it has to go to a flare. Without cleaning, the flare burner can be fouled. The client had design data from the gasifier technology supplier, including expected loadings, flue gas analysis, start-up design data and particle size distribution charts from earlier test work.
The approach: two stages and a wide size range
The measured particle sizes suggested a narrow range, but a working unit usually meets a broader one than test data predict. The proposal therefore used a two-stage cyclone, arranged horizontally on a common centre line, with each stage designed for a different size band so that together they cover roughly 2 to 100 microns.
- Material: 304 stainless steel, built for up to 5 bar working pressure (an ASME VIII plated vessel was available if third-party approval was needed).
- Size: about 8 m tall including the dust outlet, and about 2 m in diameter.
- Rectangular inlet duct, 430 mm wide by 860 mm high; 860 mm flanged dust outlet; 1.08 m circular gas exit.
- Mounted on three brackets for installation into a structure by others.
Two risks, and the options for each
Velocity falling outside the design range during start-up. A cyclone only separates well within a velocity band. Two fixes were proposed: an auxiliary air fan to make up lost flow, controlled by a pressure transducer, although extra air would load the flare burner; or a splitting fan on the gas exit, on a speed controller, so the point of cut can be tuned.
Tar. Tars would clog the cyclone through rapid deposit build-up. If it becomes an issue, the option is in-situ, on-line cleaning with a solvent spray. This was listed as an exclusion, to be added if needed.
The pressure-drop trade-off
In follow-up correspondence the client wanted a smaller exit duct. The consequence was clear: with the original dimensions the pressure loss was approximately 20 mbar per stage, matching the roughly 20 mbar the client had said was available. Narrowing the exit to 500 mm while keeping the cyclone design relationships would raise it to roughly 150 mbar per stage. The alternative was to reduce the duct size after the cyclone outlet. The supplier asked for a target pressure loss to re-run the calculations.
The lesson is that cyclone geometry, efficiency and pressure drop are linked. Squeezing one dimension changes the others.
What to consider for your own project
- Get the available pressure drop in writing before sizing.
- Design for a wider size range than the test data suggest.
- Ask about tar early. It changes the cleaning strategy.
- Decide who covers installation, ductwork and support steel.
- Check emissions rules for flares on thermal plants with your regulator.
FAQ
What does a cyclone do?
It spins gas so heavier particles are thrown to the wall and drop out.
Why two stages?
Each stage is designed for a size band, so together they cover a wider range.
Why does exit diameter matter?
A narrower exit raises pressure loss sharply: about 20 mbar per stage at the original size, around 150 mbar at 500 mm.
Cleaning dirty process gas?
Share your gas conditions and dust data and we will advise. Related: flaring difficult vent gases.