Producer gas leaves a gasifier at 300-400 °C carrying tar, ash dust and water vapour, and it must be cooled to below 40 °C and passed through a cyclone, a filter and a water trap in that order before it reaches an engine. Skipping any stage ruins an engine slowly or quickly: tar glues valves and rings, dust wears the cylinders, and water makes the engine misfire.
Part of Biomass Gasification
Gas cleaning removes the tar, particles and water that a gasifier’s gas carries, in a train of cyclone, cooler, filter and condensate trap.
Three Contaminants, Three Jobs
The parent says even a good downdraft gasifier produces gas that will damage an engine if used directly. Each contaminant needs a different tool.
| Contaminant | What it does | Tool | Article |
|---|---|---|---|
| Large dust and char | Wears cylinders and rings | Cyclone (removes particles above 10 microns) | Tar Removal (Cyclone & Cooling) |
| Tar vapour | Condenses and glues valves and rings | Cooling, with a drain | same |
| Fine particles | Wear and clog | Fabric, sawdust or sand filter | Particulate Filtration |
| Water | Misfires, dilutes the gas | Cooling and a condensation trap | Moisture Separation |
The Order Matters
- Cyclone first, while the gas is hot and the dust is dry. Dust that meets condensing tar becomes sludge that blocks the filter.
- Cooler next, to take the gas from 300-400 °C to below 40 °C so the tar and water condense and can drain.
- Filter after cooling, so the fine particles are caught on a surface that is not sticky with hot tar.
- Water trap last, with a drain at the lowest point.
The gas must also be able to flow. Every stage resists the flow, and the engine pulls the gas through by suction. As a rough rule, each 1 kPa of suction used in the train costs the engine about 1% of its intake charge (1 kPa against about 101 kPa of atmosphere), so a clogged filter starves the engine of gas and power.
What It Costs When Cleaning Is Poor
The second carrier’s history records that engine life was reduced by about 30-50% by abrasive particles in imperfectly filtered gas, and that tar fires in the gasifier or filter were the most common breakdown. Both are results of cleaning that was either insufficient or not maintained.
Method: Commission The Train
- Build and leak-test every stage before any engine is attached.
- At the first outdoor run, flare the gas before the cleaning train and again after it and compare: a sooty flame upstream and a clean one downstream shows the train working.
- Never send the first minutes of start-up gas to the engine. It is cool, rich in carbon monoxide and the most contaminated of the run.
- Open the drains and clean each stage on the maintenance schedule in the parent.
Common Mistakes
- Filtering before cooling. The tar soaks and blocks the filter.
- No drain at the low point. Condensate collects and floods the line.
- Treating the train as one box. A single large packed drum does the job of none of the stages well.
Safety
Every vessel in the train holds carbon monoxide, and the condensate is toxic
The gas is about 18-22% carbon monoxide by volume. Do not open a cooler, cyclone or filter housing indoors or while the unit is hot and gas-filled; clean them outdoors when the system has cooled and been ventilated. The condensate and tarred filter media contain phenols and other compounds that burn skin and should not reach water; tarred media is also a fire load. See Carbon Monoxide Risks.
What’s Next
- Learn next: Tar Removal (Cyclone & Cooling) — the first two stages
- Combine with: Engine Connection — where the clean gas goes