A small-scale biomethane liquefier turns pipeline-quality biomethane into liquid biomethane (LBM) at around -162°C, so it can be trucked to customers who are not on a gas grid. A proposal for a 60 tonne-per-day plant in South-East Asia used a nitrogen recycle refrigeration cycle, with an amine unit and dryer ahead of it to remove CO₂ and water that would freeze in the cold box. Roughly 1.9 MW of electricity is needed for the liquefaction itself.
The challenge: biomethane is not yet liquefaction-ready
The feed was biomethane at a minimum of 15 bar g, up to 40°C, with the following composition:
| Component | Share | Why it matters |
|---|---|---|
| Methane | About 92.5% | The product |
| CO₂ | About 6% | Freezes at cryogenic temperatures, so it must be removed to a few ppm |
| Nitrogen | About 1% | Stays in the product |
| Ethane | About 0.3% | Minor |
The feed flow was about 3,500 Nm³/h. Of that, roughly 3,120 Nm³/h was liquefied (about 2,500 kg/h, or 60 tonnes a day), about 250 Nm³/h was the CO₂ removed, and about 130 Nm³/h was used to regenerate the dryer. The regeneration gas cannot be vented, so it has to be burned for energy, for example in a cogenerator, or recovered with a booster compressor.
The approach: purify, then liquefy
1. Feed purification
- A mercury removal barrier, based on adsorption.
- An amine unit with an absorber and a stripper. Gas washed counter-current with solvent loses its CO₂, and the solvent is regenerated with heat. Solvent consumption is small, in the order of 500 kg a year.
- A chiller and condensate separator to cool the gas.
- A two-bed temperature swing adsorption (TSA) dryer using activated alumina, with one bed in service while the other regenerates.
2. Liquefaction
The cold box holds the cryogenic heat exchanger and turbine inside a vertical carbon steel casing filled with perlite insulation. A closed loop of nitrogen is compressed in a recycle compressor, part of it passes through turbines that drive booster compressors, and the nitrogen is cooled and expanded to provide the cold. The biomethane is cooled against the returning nitrogen. This is a reverse Brayton cycle, one of several options (cascade, mixed refrigerant, Linde-Hampson and C3MR cycles) considered in the study.
3. Utilities and nitrogen
- Electric power: about 1,875 kW for the whole liquefaction process, of which the nitrogen compressor and turbine take about 1,787 kW.
- Cooling water: about 360 m³/h in a closed circuit, at 30°C supply and 40°C return.
- Instrument air: about 200 Nm³/h at 7 bar g.
- Liquid nitrogen: about 375 kg/h continuous for seal gas and purging, from an on-site tank. Vaporising it in the liquefier recovers cold and reduces electricity use.
- Steam: about 1.2 tonnes an hour at 4 bar g for the amine unit.
Safety valve and drain gas is collected in a cold flare system, kept purged with a small flow of nitrogen to avoid flame formation. LBM is held in cryogenic tanks, with boil-off recovered, and a pump transfers it to trucks.
Why it works: what is guaranteed
Guarantees apply only at 100% load: production, electrical consumption (with a 3% tolerance) and nitrogen consumption (5%). Part-load figures are expected, not guaranteed. The proposal quoted plant availability of 97%, conditional on utilities and recommended maintenance. Turndown ran from about 30 to 50 tonnes a day by changing the refrigerant charge, with the plant silenced to about 85 dB(A) at 1 m for each machine. Design life was 25 years.
The proposal’s exclusions give a realistic picture of the project beyond the package: civil works, buildings, power supply and switchgear, cooling system, steam and soft-water generation, nitrogen supply, fire and gas detection, firefighting, storage tanks, feed gas metering and chromatography, local permits and all maintenance after start-up. The delivery programme was roughly two years from contract.
For UK context on biomethane, see the government’s biomethane guidance.
What to consider for your own project
- Treat purification as half the plant. CO₂ and water must go to a few ppm.
- Plan what to do with regeneration gas and cold flare vents.
- Budget for utilities. Power, cooling water, nitrogen and steam add up.
- Decide on storage and truck loading. Storage was an option.
- Read the guarantee basis: full load only.
FAQ
Why remove CO₂ before liquefying?
CO₂ freezes at the temperatures needed and would block the cold box.
Why use nitrogen as the refrigerant?
A nitrogen loop is simple and non-flammable, and the reverse Brayton cycle suits small plants.
How much power does liquefaction need?
About 1,875 kW for 60 tonnes a day, excluding utilities such as cooling water.
Can the plant run below full output?
Yes, down to roughly half, but the performance figures at part load are not guaranteed.
Considering biomethane liquefaction?
Tell us your gas flow and composition and we will discuss options.