How liquified natural gas works

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Date:30 August 2017 Tags:, , ,

Freezing natural gas and shipping it around the world, explained.

By David Grossman

The US Department of Energy just confirmed gas is a coal-killer in the world of energy. A report released this month by the DOE says the “biggest contributor to coal and nuclear plant retirements has been the advantaged economics of natural gas-fired generation. Responsible for 34 per cent of US electricity generation in 2016, natural gas is the single largest energy source in the country. A big reason for natural gas’s global domination is the spread of liquified natural gas, known as LNG.

A closer look at natural gas

Natural gas is the cleanest fossil fuel, which makes it an imperfect improvement over coal. It’s colourless and has no smell, and is 70-90 per cent methane, with ethane, propane, butane, and pentane making up the rest. Once extracted, the gas goes through a processing station. Here it is stripped of impurities like water, carbon dioxide, and sulphur.

Once purified and transported, the natural gas is burned to produce pressurised gas. That gas spins the turbines which power generators. Magnets spin inside the generator, causing electrons to flow. A heat recovery system captures exhaust heat from the turbine and sends it through the same system. This helps recycle some of the lost energy.

Storing natural gas

One of the problems with natural gas has historically been one of basic physics: Gas is the most expansive of the three traditional types of matter. This is the phase in which atoms spread out the most. It can make natural gas expensive and difficult to store in large quantities. The answer is to change the gas into liquid, something that takes up roughly one-six-hundredth of the space that gas does. In 1886, Polish chemist Karol Olszewski liquefied methane for the first time, and by 1940 a pilot program was opened in Cleveland, Ohio, as the first LNG power plant in the world.

That plant offered up a warning for generations to come about LNG safety. In 1944, during World War II, Ohio city suffered a violent disaster of its own when the plant exploded due to a leak. Over 130 people were killed, most of them workers at the plant, over $10 million in property was destroyed. The flames from the explosion erupted 2,500 feet into the air.

Today, LNG storage and transport is much safer. The impurities are still filtered out as the gas is extracted. The natural gas passes through a water-based solvent that absorbs carbon dioxide and hydrogen sulphide. Anything that would impede the purified natural gas—water, propane, butane, mercury—is also filtered out. Left with mostly pure methane and some ethane, these gases are sent to heat exchangers.

Transforming gas into liquid

Industrial heat exchangers exist to transfer heat from one medium to another. While they differ in technique, they all directly or indirectly expose a warmer medium to a cooler medium. Fans, condensers, belts, coolants, and tubes are used to get the job done. A coolant, for example, can be chilled with giant refrigerators so it absorbs the heat from the natural gas. These coolants can cool the gas to -162 degrees C. This results in a phase change to liquid and that crucial volume shrinkage.

The clear, colourless gas becomes a clear, colourless liquid, stored safely until it is ready for transport, often by ship. When the destination is reached, the liquid is dehydrated and heated up through a series of vaporisers to bring it back above -162 C.

LNG technology is perfect for places that have a lot of natural gas but are somewhat isolated from major population centres. The tiny nation of Qatar, for example, plans to raise its total output of LNG from 77 million tons a year to 100 million tons by 2024. Considering that 258 million tons of LNG were distributed last year, such a move would help Qatar consolidate the market.

Countries like Australia and the USA are going to attempt to increase their LNG exports by the 2020s also. So competition to replicate and improve this technology will only increase. It may not be a perfect zero-emission solution, but LNG can help us in the interim as we transition to a world of clean renewable energy.

From: PM USA

Image and video credit: Shell