Global natural gas consumption increased 1.4 per cent last year and was the fastest-growing fossil fuel, according to the Energy Information Administration (EIA). Despite its growth, natural gas has presented one major challenge for the Oil & Gas Industry – it has a lower density compared to traditional fuels, making it difficult to transport.
The industry’s solution has been to cool natural gas to temperatures of -260°F (-160°C) to increase its density. Liquefied Natural Gas (LNG) has allowed natural gas to be moved across vast distances safely and cost-effectively. However maintaining and storing LNG at a stable temperature presents major challenges.
What is boil-off?
While the tanks on an LNG carrier are designed to stay cool, they cannot provide perfect insulation against warming. Heat slowly affects the tanks, which can cause the LNG inside to evaporate and produces a substance known as boil-off gas (BOG).
Natural gas remains liquefied by staying at a consistent pressure, but when boil-off occurs and it returns to gas, the larger volume of gas will increase the tank pressure. While the tanks are designed to handle the rise over short distances, prolonged pressure increases cannot be managed effectively and require alternative solutions.
Handling the pressure
To relieve the pressure in LNG tanks, BOG can be re-liquefied, used as fuel or burned in a gasification unit.
Reliquefaction occurs when evaporated LNG is cooled and reverted back to its liquid state. However this presents its own problems. Reliquefaction requires a lot of equipment, meaning tanks become very large. The process is more practical for large land based liquefaction plants, where space is not as much of an issue.
At sea, BOG can also be managed through combustion. Excess gas is fed to the ship’s engines which have a suitable fuel pressure to consume it. The LNG gas is used as fuel, with some tankers relying on BOG to perform certain manoeuvres. Another alternative is to burn the unwanted gas in a gasification unit, but this results in wastage of materials and valuable energy.
The challenge of LNG
Even when safely handled, BOG offers other challenges. As BOG develops, the more volatile components in LNG – nitrogen and methane – boil-off first. This changes the composition and quality of the LNG over time in a process known as ageing – displaying how even the smallest change in LNG can have major effects.
It is crucial that LNG is accurately measured and managed during its cooling, storage and transportation.
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