Gas to Liquids describes a chemical process to convert gas into products such as fertilizers, methanol or liquid hydrocarbons, which can be readily transported to any location.
A Gas-to-Liquids plant converts natural gas (mostly methane) into liquid hydrocarbon products, for example diesel, kerosene and waxes. Not to be confused with LNG, which makes gas liquid by cooling it (a physical process), a GtL plant performs a chemical transformation. This produces ultra pure "oil" products, including specialties, that have a very high market value. A scheme of the process is shown in the below image:
Shell developed the Shell Middle Distillate Synthesis (SMDS) technology using natural gas as a feed-stock to produce middle distillates such as naphtha and gas-oil.
Compared to pipeline and LNG transport, GtL has significant logistical advantages.
- First of all there is a large open market for middle distillate products.
- Secondly, the SMDS technology produces fuels with virtually no aromatic and sulphur components, which can be used directly, or as refinery blending components to improve the quality of crude-derived gas-oil. Blends of SMDS gas-oil with conventional gas-oil give significant reductions in regulated emissions (NOx, SOx, HC, CO and particulates).
- Lastly, SMDS gas-oil can also be used as a neat fuel in diesel engines with minor modifications.
At the heart of any GtL plant is the Fischer-Tropsch (FT) synthesis, which uses a catalyst to convert hydrogen (H2) and carbon monoxide (CO) into higher hydrocarbons, mostly normal paraffin. The FT synthesis step is preceded by the manufacturing of synthesis gas, and followed by the work-up of the raw FT product stream. Really, a mini refinery sits at the end of a GtL plant.
What is Syngas
The mixture of CO and H2 that is required as feed-stock for the FT synthesis step is referred to as synthesis gas, or short: syngas.
Types of Syngas
Syngas can be produced from other sources than natural gas:
-biomass (BtL)
-coal (CtL)
-heavy oil residue (LtL) are all possible.
Natural gas is particularly convenient for several reasons:
-Often gas produced to feed a GtL plant yields LPG and gas condensate, valuable by-products, which greatly helps the economics
-Gas is easy to transport over short distance by pipeline (compared e.g. to biomass handling)
-Gasifying methane yields H2 and CO in about the right proportion for FT synthesis: 2 to 1
-Gas is usually relatively clean, especially compared to coal, which limits the need for gas processing
-A gas field provides a convenient accumulation of feed-stock (compared say to biomass, which may have to be gathered from a large area)
Presently, there are only two companies operating commercial GtL plants: Sasol and Shell