Gas to liquids (GTL)

The GTL process can also be used to improve sustainability in aviation. Kerosene made with the GTL process can be blended with conventional kerosene up to 50% of a full aircraft tank. Here, too, benefits such as cleaner combustion and  reducing emissions apply. Also, GTL kerosene is more thermally stable, which means that fewer dirty particles end up in the engine, making it more efficient and longer lasting.

Converting gas into liquid products is a complex chemical process. The core of the GTL process consists of three steps, which Shell researchers have been optimising in the Amsterdam lab since the 1970s. The three steps have been aligned to get the highest possible efficiency and the best product quality.

1. Making synthesis gas (syngas, a mixture of hydrogen and carbon monoxide): natural gas is converted into syngas with pure oxygen.
2. Synthesis: The syngas is converted into synthetic hydrocarbons via the so-called Fischer-Tropsch synthesis, which produces a wax-like product. At room temperature, this is a solid substance that can best be compared to candle wax. This product is called paraffin. The Fischer-Tropsch synthesis was developed by German scientists in the 1920s.
3. Hydroprocessing to products: by using a catalyst and hydrogen, the pure paraffin is ‘cut’ via hydrocracking to shorter hydrocarbon chains. This provides a range of liquids with different viscosities. These are further processed into transport fuels, lubricants and chemical raw materials.

The first GTL test plant that could produce 3 barrels of GTL per day (one barrel is approximately 160 litres) started up at the Shell lab in Amsterdam in 1982. About ten years later, in 1993, Shell started its first commercial GTL plant in Bintulu in Malaysia, the current capacity of which is 14,700 barrels per day. In 2011, the giant Pearl GTL plant was started in Qatar. This plant takes up a 1.5 km by 1.5 km site and is the world’s largest plant for converting natural gas into synthetic products. After the natural gas has been cleaned, the installation produces 140,000 barrels of synthetic GTL products every day.

GTL and the energy transition: green hydrogen and carbon dioxide (CO₂) instead of natural gas

The researchers at ETCA are now using their years of experience in the field of the GTL process to investigate how this process can play a role in the energy transition. Instead of natural gas, the GTL process can use green hydrogen together with carbon dioxide (CO₂) as feedstock. In this case, only the first step of the conventional GTL process changes: synthesis gas is not made with natural gas and oxygen, but with hydrogen and CO₂. This synthesis gas then follows the same steps 2 and 3 as the conventional GTL process.

When renewable power is the only energy source, we call the process power-to-liquids (PTL). A so-called electrolyser uses the renewable power and water to produce green hydrogen. In addition to hydrogen, carbon is also required: this can come in the form of CO₂. Industrial installations can be a source of CO₂, such as a steel or cement factory or a refinery. Reuse prevents the CO₂ to be emitted. CO₂ can also come from biogas, for example, when processing residues from agriculture or water purification into green natural gas. The so-called Direct Air Capture technology can also capture the CO₂ straight from the atmosphere.

In the “hybrid” GTL-PTL process, natural gas as feedstock is not entirely, but partially replaced by green hydrogen and CO₂. This hybrid form primarily uses renewable power during the day in the case of solar energy or during periods with high wind in the case of wind energy. And it can switch to natural gas when no renewables are available. This means that the GTL-PTL hybrid process requires less expensive energy storage in the form of batteries or hydrogen storage than the PTL process, which reduces costs. Due to the flexibility of the green element in the GTL-PTL hybrid process and the lower costs, this concept is closer to commercial feasibility than the PTL process.