Carbon monoxide for FT catalysis is derived from hydrocarbons. In gas to liquids (GTL) technology, the hydrocarbons are low molecular weight materials that often would be discarded or flared. Stranded gas provides relatively cheap gas. For GTL to be commercially viable,  gas must remain relatively cheaper than oil.
Several reactions are required to obtain the gaseous reactants required for FT catalysis. First, reactant gases entering a reactor must be desulfurized. Otherwise, sulfur-containing impurities deactivate ("poison") the catalysts required for FT reactions.
Several reactions are employed to adjust the H:CO ratio. Most important is the water-gas shift reaction, which provides a source of hydrogen at the expense of carbon monoxide:
For FT plants that use methane as the feedstock, another important reaction is dry reforming, which converts the methane into CO and H: