Page 58 - Handbook of Natural Gas Transmission and Processing Principles and Practices

GTL products have been demonstrated to have potential in improving air quality in cities over
refinery transportation fuels. However, on a full cycle analysis GTL fuels do not significantly
outperform refinery fuels because operation of GTL plants would incur substantial emissions. The
problem is primarily with the low energy efficiency of syngas generation and the low carbon
efficiencies of the conversion processes such as FT (O'Rear and Goede, 2007). Technology
breakthroughs are required to improve capital cost and environmental benefits.
The Fischer-Tropsch GTL (FT-GTL) method is an application of the basic Fischer-Tropsch process,
where synthesis gas (or syngas) is reacted in the presence of an iron or cobalt catalyst. End products
are determined by the length of the hydrocarbon chain which, in turn, is determined by catalyst
selectivity and reaction conditions. FT-GTL is essentially a three stage process (syngas generation,
F-T transformation, and product upgrade). Possible end products include kerosene, naphtha,
methanol, dimethyl ether, alcohols, waxes, synthetic diesel, and gasoline, with water or carbon
dioxide produced as a by-product. FT-GTL has reached a key milestone as an industry in that
world-scale capacity plants are in the process of being planned, constructed, and commissioned.
However, FT-GTL has a lot left to prove in terms of technical, economic, and environmental
efficiency before it can be considered as a rival to LNG on a large scale (Fleisch et al., 2003; Al-
Saadoon, 2005; and Apanel, 2005). Modifications, innovations, and patents are being developed for
this complex, energy-intensive process. Most recent technical advances are focused on lowering
capital expenditures, the economies of scale, and improving operating and energy efficiencies for
large-scale FT-GTL plants. Efforts on improving the safe use of oxygen in syngas reforming plants
have also received attention. Much research is still ongoing with respect to other GTL processes, but
none has yet reached large-scale commercialization although further developments, particularly in
methanol to gasoline plants, should be expected in this high-price gasoline market (Wood and
Mokhatab, 2008 and Wood et al., 2008).
GTL technology development has reached a stage where its marinization, enabling smaller
footprints and more compact plants than other technologies, may be considered for FPSO
application. Floating GTL will provide new opportunities for companies to produce, transport, and
market gas reserves that would otherwise remain stranded. Also, floating GTL plants could be
moved from location to location, enabling access to small fields that would otherwise not justify
building a dedicated GTL facility. The GTL FPSO concept, which is still in the development phase,
could greatly improve potential project economics (Loenhout et al., 2006).

1.10.5. Gas-to-Solid
Gas can be transported as a solid, with the solid being gas hydrate (Børrehaug and Gudmundsson,
1996). Natural gas hydrates (NGHs), which are essentially natural gas in a “frozen” state, form
when water and natural gas combine at low temperatures and high pressures. Gas hydrates are
clathrates where the guest gas molecules are occluded in a lattice of host water molecules. They are
most commonly encountered in the industry as a production problem in pipelines to be avoided as
part of flow assurance.
Gas-to-solids (GTS) involve three stages: production, transportation, and regasification.
Conceptually, hydrate slurry production is simply mixing chilled water and gas. In practice,
processed gas is fed to a hydrate production plant, where a series of reactors convert it into a
hydrate slurry. Each reactor further concentrates the hydrate slurry. It is then stored and eventually
offloaded onto a transport vessel (insulated to near adiabatic conditions). At the receiving terminal,
the hydrate is dissociated and the gas can be used as desired. The water can be used at the
destination if there is water shortage, or returned as ballast to the hydrate generator and, since it is
saturated with gas, will not take more gas into solution.
Gas storage in hydrate form becomes especially efficient at relatively low pressures where
substantially more gas per unit volume is contained in the hydrate than in the free-state or in the
compressed state when the pressure has dropped. When compared to the transportation of natural
gas by pipeline or as liquefied natural gas, the hydrate concept has lower capital and operating
costs for the movement of quantities of natural gas over adverse conditions. NGHs are chemically
stable at about −20°C compared with −162°C for LNG, reducing the costs of transportation and
3
storage. One cubic meter of NGH contains approximately 160 m of natural gas, while one cubic
3
meter of LNG contains 600 m of natural gas, thus limiting the quantity of gas that can be

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