Gas to liquids (GTL) is arefinery process to convertnatural gas or other gaseoushydrocarbons into longer-chain hydrocarbons, such asgasoline ordiesel fuel.Methane-rich gases are converted into liquidsynthetic fuels. Two general strategies exist: (i) direct partial combustion of methane to methanol and (ii)Fischer–Tropsch-like processes that convert carbon monoxide and hydrogen into hydrocarbons. Strategy ii is followed by diverse methods to convert the hydrogen-carbon monoxide mixtures to liquids. Direct partial combustion has been demonstrated in nature but not replicated commercially. Technologies reliant on partial combustion have been commercialized mainly in regions where natural gas is inexpensive.[1][2]
The motivation for GTL is to produce liquid fuels, which are more readily transported than methane. Methane must be cooled below itscritical temperature of -82.3 °C in order to be liquified under pressure. Because of the associated cryogenic apparatus,LNG tankers are used for transport. Methanol is a conveniently handled combustible liquid, but itsenergy density is half of that of gasoline.[3]
A GtL process may be established via the Fischer–Tropsch process which comprises several chemical reactions that convert a mixture of carbon monoxide (CO) and hydrogen (H2) into long chained hydrocarbons. These hydrocarbons are typically liquid or semi-liquid and ideally have the formula (CnH2n+2).
In order to obtain the mixture of CO and H2 required for the Fischer–Tropsch process,methane (main component of natural gas) may be subjected to partial oxidation which yields a raw synthesis gas mixture of mostlycarbon dioxide,carbon monoxide,hydrogen gas (and sometimes water and nitrogen).[4] The ratio ofcarbon monoxide to hydrogen in the raw synthesis gas mixture can be adjusted e.g. using thewater gas shift reaction. Removing impurities, particularly nitrogen,carbon dioxide and water, from the raw synthesis gas mixture yields puresynthesis gas (syngas).
The pure syngas is routed into the Fischer–Tropsch process, where the syngas reacts over an iron or cobalt catalyst to produce synthetic hydrocarbons, including alcohols.
Methanol is made frommethane (natural gas) in a series of three reactions:
The methanol thus formed may be converted to gasoline by theMobil process and methanol-to-olefins.
In the early 1970s,Mobil developed an alternative procedure in which natural gas is converted to syngas, and thenmethanol. The methanol reacts in the presence of azeolite catalyst to form various compounds. In the first step methanol is partiallydehydrated to givedimethyl ether:
The mixture of dimethyl ether and methanol is then further dehydrated over a zeolite catalyst such asZSM-5, and in practice is polymerized and hydrogenated to give a gasoline with hydrocarbons of five or more carbon atoms making up 80% of the fuel by weight. The Mobil MTG process is practiced fromcoal-derived methanol in China byJAMG. A more modern implementation of MTG is the Topsøe improved gasoline synthesis (TiGAS).[5]
Methanol can be converted to olefins using zeolite and SAPO-basedheterogeneous catalysts. Depending on the catalyst pore size, this process can afford either C2 or C3 products, which are important monomers.[6][7]
Methanol to olefins technology is widely used in China in order to produce plastics from coal gasification. It is also discussed as a method to make fossil-free plastics in the future.[8]
A third gas-to-liquids process builds on the MTG technology by converting natural gas-derived syngas into drop-in gasoline and jet fuel via a thermochemical single-loop process.[9]
The STG+ process follows four principal steps in one continuous process loop. This process consists of fourfixed bed reactors in series in which asyngas is converted to synthetic fuels. The steps for producing high-octane synthetic gasoline are as follows:[10]
With methane as the predominant target for GTL, much attention has focused on the three enzymes that process methane. These enzymes support the existence ofmethanotrophs, microorganisms that metabolize methane as their only source of carbon and energy. Aerobic methanotrophs harbor enzymes that oxygenate methane to methanol. The relevant enzymes aremethane monooxygenases, which are found both in soluble and particulate (i.e. membrane-bound) varieties. They catalyze the oxygenation according to the following stoichiometry:
Anaerobic methanotrophs rely on the bioconversion of methane using the enzymes calledmethyl-coenzyme M reductases. These organisms effect reversemethanogenesis. Strenuous efforts have been made to elucidate the mechanisms of these methane-converting enzymes, which would enable their catalysis to be replicated in vitro.[11]
Biodiesel can be made from CO2 using the microbesMoorella thermoacetica andYarrowia lipolytica. This process is known as biological gas-to-liquids.[12]
Using gas-to-liquids processes, refineries can convert some of their gaseous waste products (flare gas) into valuablefuel oils, which can be sold as is or blended only withdiesel fuel. TheWorld Bank estimates that over 150 billion cubic metres (5.3×10^12 cu ft) of natural gas areflared or vented annually, an amount worth approximately $30.6 billion, equivalent to 25% of the United States' gas consumption or 30% of the European Union's annual gas consumption,[13] a resource that could be useful using GTL. Gas-to-liquids processes may also be used for the economic extraction of gas deposits in locations where it is not economical to build a pipeline. This process will be increasingly significant ascrude oil resources aredepleted.
Royal Dutch Shell produces a diesel from natural gas in a factory inBintulu,Malaysia. Another Shell GTL facility is thePearl GTL plant inQatar, the world's largest GTL facility.[14][15]Sasol has recently built theOryx GTL facility inRas Laffan Industrial City, Qatar and together withUzbekneftegaz andPetronas builds theUzbekistan GTL plant.[16][17][18]Chevron Corporation, in a joint venture with theNigerian National Petroleum Corporation is commissioning theEscravos GTL inNigeria, which uses Sasol technology.PetroSA, South Africa's national oil company, owns and operates a 22,000 barrels/day (capacity) GTL plant inMossel Bay, using Sasol GTL technology.[19]
New generation of GTL technology is being pursued for the conversion of unconventional, remote and problem gas into valuable liquid fuels.[20][21] GTL plants based on innovative Fischer–Tropsch catalysts have been built byINFRA Technology. Other mainly U.S. companies include Velocys, ENVIA Energy, Waste Management, NRG Energy, ThyssenKrupp Industrial Solutions, Liberty GTL,Petrobras,[22] Greenway Innovative Energy,[23] Primus Green Energy,[24] Compact GTL,[25] and Petronas.[26] Several of these processes have proven themselves with demonstration flights using their jet fuels.[27][28]
Another proposed solution to stranded gas involves use of novelFPSO for offshore conversion of gas to liquids such asmethanol,diesel,petrol,synthetic crude, andnaphtha.[29]
GTL using natural gas is more economical when there is wide gap between the prevailing natural gas price and crudeoil price on aBarrel of oil equivalent (BOE) basis. A coefficient of 0.1724 results in fulloil parity.[30] GTL is a mechanism to bring down the diesel/gasoline/crude oil international prices at par with the natural gas price in an expanding global natural gas production at cheaper than crude oil price. When natural gas is converted in to GTL, the liquid products are easier to export at cheaper price rather than converting in toLNG and further conversion to liquid products in an importing country.[31][32]
However, GTL fuels are much more expensive to produce than conventional fuels.[33]