CTL or coal-to-liquid and even BTL (biomass-to-liquid) have the same potential. In fact, any carbon or hydrocarbon source that can be gasified to produce so-called synthesis gas or syngas – a mixture of carbon monoxide and hydrogen – can function as an initial feed.

GTL gets the most attention because it is based on an optimal feedstock in the sense that gasification requires very little correction to the hydrogen-to-CO ratio before sending it to the synthesis unit. There, a catalytic reaction produces mainly a synthetic wax, often called syncrude. A lot of side reactions take place as well that generate oxygenates such as alcohols and some olefins. Fixed-bed and fluidized-bed reactors are alternative synthesis process options.

There are two broad classes of the FT synthesis reaction – differentiated by low-temperature and high-temperature synthesis. Low temperatures are optimal for wax production and the longer hydrocarbon chains needed for base stock production. High temperatures are more suited to short hydrocarbon chains and olefin production.

A molecular excess of hydrogen over CO – ideally around 2:1 – is required in the syngas. However, for CTL, extra steps are needed to reach the optimal ratio for gasification because coal can be regarded as only carbon. These extra steps include hydrogen generating reactions such as the water gas shift reaction. They increase the energy intensity of the CTL process relative to GTL, making it costlier to run.

Various catalysts are used in the low-temperature syncrude production stage, although most are based on cobalt, usually on some proprietary oxide support. This is often the stage where the economics can be tweaked to provide a competitive edge.

Economics are key for FT technology because capital expenditures are usually very significant. Therefore, to generate acceptable paybacks, all processes, but especially syncrude production, must be as efficient as possible.

Up-front CAPEX is the main constraint for FT projects in general. Other API Group III manufacturers dont have to fund the extra feedstock production steps, and in a fuels processing refinery they usually benefit from synergies to provide their hydrowax feedstocks.

Once the FT syncrude or synthetic wax has been produced, the downstream processing is pretty well identical to that used in regular wax isomerization to produce Group III. In fact, the output is Group III+ because the high-quality synthetic wax feedstock is almost pure paraffinic and produces very high VI base stocks.

At one time, natural gas-based FT/GTL projects were predicated on stranded gas as a feedstock; that is, natural gas that could not be easily networked but still required moving up the value chain to liquid products on site. That seems to be less of a consideration today with an ever increasing natural gas supply, for example from fracking. But feedstock cost, including alternative values – as well as the duration of that supply – will still be a critical factor in the business case for any new FT base oil project.

Some refer to Group III and GTL base stocks separately, as if GTL stocks were different from Group III, which they are not. In fact, GTL base stocks meet Group III specifications by default.

FT base stocks, regardless of initial feed, are produced by isomerizing a synthetic wax. This means that the molecular types in the finished base stock are predominantly iso-paraffins. As a result, such stocks are at the top end of the VI range.

In addition, they have among the highest aniline points, grade-for-grade, among Group IIIs because they lack some of the significant naphthenic content of regular crude-derived Group III stocks. However, this factor does not seem to limit potential applications for these high-quality stocks any more than it does for polyalphaolefins, with their similar paraffinic levels and aniline points.

Currently, there is only one major player in FT/GTL base stocks, although this may change as downstream economics improve in the future. Certainly, the market for the highest quality Group III will see the greatest percentage increase among base stocks as more low-viscosity fuel economy oils are required for passenger cars and to a certain extent in the heavy-duty market.

An additional bonus from FT base stock projects is the very high carbon numbers of the wax feedstocks – usually higher than crude oil. This provides the potential to produce a Group III brightstock residual grade, even after hydrocracking. This is largely constrained at present by lack of suitable catalytic dewaxing technologies to produce clear, bright products, even though the pour point can be set. Hopefully, this may change with technology developments, and well have another option to address the Group I brightstock replacement issue.