Chilling biogas to a dew point of about 20°C and filtering it protects gas engines from condensate and fine particles. In a proposal for a 1,000 Nm³/h landfill gas site in southern Africa, a single skid combined a chiller, shell and tube condenser, cyclonic knockout pot and 3 micron filter to cool the gas from an inlet of up to 45°C to a 20°C dew point, with discharge relative humidity of no more than 80%, before it reaches the generators.
The challenge: wet, contaminated gas
Biogas is usually delivered saturated. When it cools in pipework, condensate forms and often carries contaminants, including ammonia, which can damage a generator very quickly. Biogas may also carry fine particles that wear engine components. Pre-treatment improves gas quality and delivers it at a regulated pressure.
The approach: cool, separate, filter
Design conditions
| Parameter | Value | What it means |
|---|---|---|
| Maximum flow | 1,000 Nm³/h | Sized to the site’s gas yield |
| Methane | 40 to 60% | Typical landfill gas |
| Ambient temperature | 20 to 45°C | The chiller must cope with heat |
| Dew point at discharge | 20°C | Water stays in the gas as vapour in normal pipework |
| Cooling capacity | About 40 kW | Small relative to the gas energy |
| Electrical power | About 12 kW | Mainly the chiller |
| Pressure drop | About 20 mbar | Added load on the blower |
A larger version of the same skid, for three times the flow, was also quoted for another site; the arrangement is identical and only the duty changes.
The equipment, in the order the gas meets it
- A galvanised inlet riser with a removable coarse screen and inspection flange, separating free liquid.
- A 304 stainless steel shell and tube condenser, insulated to improve efficiency and reduce condensate pools in humid weather.
- An electric air-cooled water chiller with a chilled water circuit: pump, buffer storage, control valves and expansion tank.
- A cyclonic knockout pot, with a one-piece polypropylene spiral demister, where condensate drops out and drains.
- A replaceable 3 micron pleated filter on the outlet, with average efficiency above 95% (EN 779 class F9) and an initial resistance of no more than 175 Pa.
The skid is hot-dip galvanised, with stainless steel gas-contact pipework, a PLC-controlled weatherproof (IP56) panel with an HMI, a level-switch trip and start/stop and emergency stop controls. It is used with a separately supplied flare and booster.
Why it works, and what it excludes
Chilling lowers the dew point so moisture drops out in a controlled place instead of in the engine feed line. The pressure drop of about 20 mbar must be allowed for in the booster duty. The supply excludes connecting pipework, electrical supply, foundations, drainage and treatment of the condensate, which is likely to be contaminated. It is not designed for gas with excessive entrained liquid, so a knockout pot or riser must deal with slugs first.
For UK practice on gas treatment ahead of engines, see the Environment Agency’s landfill gas guidance.
What to consider for your own project
- Know the engine maker’s moisture limit and size the dew point to it.
- Allow for pressure drop in the booster.
- Plan condensate disposal. It carries ammonia.
- Allow power for the chiller.
- Put an H₂S scrubber upstream if the gas is high in sulphide; see biological H₂S removal.
FAQ
Why chill biogas?
Lower temperature means lower moisture capacity, so water condenses and is removed before the engine.
What does the filter remove?
Fine particles that cause wear; the 3 micron element is above 95% efficient on average.
Is the condensate harmless?
No. It can carry ammonia and other contaminants and needs proper drainage and treatment.
Can the skid work without a booster?
It is supplied for use with a separate flare and booster set.
Conditioning gas for engines?
Tell us your gas flow and engine requirements and we will size a skid.