Understanding Boil-Off Gas (BOG) in the LNG Industry and How BOG Compressors Work

Liquefied Natural Gas (LNG) is natural gas that has been cooled to around -162°C until it turns into a liquid. In liquid form, its volume shrinks to roughly 1/600 of its gaseous volume, which makes LNG far more efficient to store and ship between islands and across borders. However, no matter how good a storage tank's insulation is, heat from the surroundings will still find its way in, little by little.

This incoming heat causes part of the LNG to evaporate back into gas. This vapour is called boil-off gas (BOG). If left unmanaged, BOG keeps raising the pressure inside the tank. This is where BOG compressors play a critical role: keeping pressure at safe levels while turning "lost" gas into something of value.

What Is Boil-Off Gas (BOG)?

Boil-off gas is natural gas, mostly methane, that forms when LNG evaporates due to added heat energy. The phenomenon cannot be eliminated entirely, only minimised and controlled. That is why every LNG facility, from liquefaction plants, receiving terminals and Floating Storage Regasification Units (FSRUs) to LNG carriers, is designed with a complete BOG handling system.

The amount of BOG is usually expressed as a boil-off rate, the percentage of LNG volume that evaporates per day. The figure depends on insulation quality, tank size, and operating conditions.

What Causes BOG to Form?

Heat from the Surroundings and from the Process

• Heat ingress: heat from ambient air and solar radiation passing through the tank walls and insulation.
• Pump energy: in-tank LNG pumps add energy to the liquid they move.
• Loading and unloading: when LNG is transferred from ship to tank or vice versa, vapour displacement occurs, along with additional evaporation in pipelines and loading arms.
• Pressure changes: a drop in tank pressure makes LNG evaporate more easily (flashing).
• Differences in LNG composition: mixing LNG of different densities in a single tank can cause stratification, which in extreme cases can lead to rollover, a sudden release of large volumes of vapour.

Why Must BOG Be Controlled?

Safety. LNG tanks have a design pressure limit. Uncontrolled BOG build-up forces safety valves to release gas to a flare or to the atmosphere.

Economic value. BOG is natural gas, just as valuable as the product itself. Burning it in a flare means wasting energy and money.

Emissions. Methane is a potent greenhouse gas. Increasingly strict emission regulations push operators to make use of BOG rather than discard it.

BOG Handling Options

Once BOG is collected from the tank, there are several ways to use it:

• Fuel gas: used for power generation at the terminal, in boilers, or in the propulsion engines of LNG carriers.
• Recondenser: at regasification terminals, BOG is compressed and then liquefied again by mixing it into a cold LNG stream.
• Reliquefaction: BOG is re-liquefied through a refrigeration cycle and returned to the tank.
• Pipeline send-out: BOG is compressed to pipeline pressure and delivered to users.
• Flare: only as a last resort during emergencies or system upsets.

Almost all of these routes share one thing in common: BOG must first be raised to a higher pressure. That is why the compressor is the heart of any BOG handling system.

How a BOG Compressor Works

In principle, a BOG compressor draws low-pressure vapour from the tank's vapour space and compresses it to the pressure required by the next process. The control system then adjusts compressor capacity to keep tank pressure stable, since the amount of BOG varies throughout the day, especially during loading.

Common Compressor Types

How a Screw Compressor Works for BOG

A screw compressor uses two helical rotors, a male rotor and a female rotor, that rotate while meshing with each other. The process takes place in three stages:

1. Suction: gas enters the cavities between the rotor lobes as they open to the inlet side.
2. Compression: as the rotors keep turning, the gas-filled cavities shrink and the gas pressure rises.
3. Discharge: the pressurised gas exits through the discharge port to the next process.

In oil-injected designs, oil is injected into the compression chamber to cool the gas, seal the clearances between the rotors, and lubricate the bearings. The oil is then separated from the gas through a multi-stage oil separator so that the delivered gas stays clean. Capacity can be adjusted with a slide valve or a variable speed drive (VSD), allowing the compressor to follow fluctuations in BOG volume.

Key Considerations When Selecting a BOG Compressor

• Inlet gas temperature and composition: BOG can be extremely cold and may contain nitrogen. The compressor's materials and design, including any need for a heater or heat exchanger on the suction side, must be suitable.
• Capacity range (turndown): BOG volumes during normal operation and during loading can differ greatly.
• Required discharge pressure: determines the number of compression stages and the type of compressor.
• Hazardous areas: the compressor package must meet hazardous area requirements (for example ATEX or IECEx).
• Reliability and redundancy: many facilities use a standby configuration (for example 2 x 100%) so that BOG handling never stops.

Through its Engineered Project Solutions division, Ingersoll Rand provides screw gas compressor packages engineered to project specifications, with applications that include LNG boil-off gas, vapour recovery, flare gas capture, and fuel gas conditioning.

Conclusion

Boil-off gas is an unavoidable consequence of storing LNG, but it can be controlled. With the right BOG compressor, operators can keep tank pressure safe, reduce flaring and methane emissions, and recover valuable gas for use as fuel or product.

As the official Ingersoll Rand distributor in Indonesia, FMM is ready to help you review BOG handling requirements at your facility and recommend a gas compressor configuration that fits your operating conditions, safety standards, and project efficiency targets.