LNG Boil-Off Gas (BOG) Re-liquefaction — StirLNG by Stirling Cryogenics
StirLNG is Stirling Cryogenics’ system for re-liquefying boil-off gas (BOG) from LNG storage tanks. A Stirling cryogenerator liquefies the gas and returns it to the tank — or subcools liquid drawn from the tank and sprays it back — so BOG is not vented and tank pressure stays controlled. The process gas never touches the helium inside the machine, so there is no contamination. The StirLNG-4 also comes in a Maritime version for LNG bunker vessels and LNG-fuelled ships. Two Theta Technology works with Stirling Cryogenics on LNG projects in Thailand, from site survey to per-project quotation.
Specifications
StirLNG-4 Maritime Subcooler specifications
| Pressure (barg) | LNG in (K) | ΔT (K) | LNG out (K) | Input power (kW) | Cooling power (kW) | Capacity (kg/hr) | Capacity (Nm³/hr) |
|---|---|---|---|---|---|---|---|
| 0.0 | 111 | 4.9 | 106.1 | 37.1 | 6.1 | 42.5 | 59.3 |
| 0.2 | 114 | 5.1 | 108.9 | 36.4 | 6.3 | 44.0 | 61.7 |
| 0.5 | 117 | 5.3 | 111.7 | 35.8 | 6.5 | 46.2 | 64.4 |
| 1.3 | 123 | 5.7 | 117.3 | 34.6 | 6.9 | 50.4 | 70.1 |
| 2.5 | 129 | 6.2 | 122.8 | 33.5 | 7.3 | 55.0 | 76.6 |
| 4.0 | 135 | 6.7 | 128.3 | 32.3 | 7.7 | 60.5 | 84.3 |
LNG flow 50 L/min · the Maritime+ version delivers more (up to 69.7 kg/hr at 4.0 barg) · cooling water 4,000 L/hr at 15 °C (4,800 L/hr with 20% glycol) · unit size 1.75 × 0.75 × 1.22 m
Installation options
Two ways to handle BOG
Choose according to your tank and process. The process gas never touches the helium inside the machine, so there is no contamination, and there is no pressure difference between gas inlet and liquid outlet.
Re-liquefy BOG back into the tank
Boil-off gas is drawn from the tank, liquefied in the StirLNG cold head and returned to the storage tank.
Subcool LNG and spray it back
Liquid is drawn from the bottom of the tank, subcooled and sprayed back into the top — lowering overall system pressure so no boil-off gas forms.
StirLNG-4
Four main applications of StirLNG
StirLNG-4 is Stirling’s SPC-4 cryogenerator adapted for small-scale LNG duty. Several units can run in parallel for more capacity and redundancy.
- Micro-scale LNG production from a purified gas source such as pipeline gas or biogas
- Re-liquefaction of BOG in storage systems — LNG fuel stations and storage tanks
- Re-liquefaction of BOG on board vessels
- A Maritime version for LNG bunker vessels and LNG-fuelled ships
LNG production
Small-scale LNG and BioLNG
The same technology liquefies biomethane from biogas plants into BioLNG at −155 °C and a pressure as low as 0.5 bar. Volume drops 600-fold, making it far easier to transport than BioCNG. Stirling’s systems are containerized modules from 1 to 20 tonnes per day.
If you have a biogas plant or gas source you want to convert to LNG, contact Two Theta and we will assess feasibility together with the manufacturer.