Gas to liquids (GTL) is a refinery process to convert natural gas or other gaseous hydrocarbons into longer-chain hydrocarbons, such as gasoline or diesel fuel. Methane-rich gases are converted into liquid synthetic 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 its critical 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 its energy 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 mostly carbon dioxide, carbon monoxide, hydrogen gas (and sometimes water and nitrogen). [4] The ratio of carbon monoxide to hydrogen in the raw synthesis gas mixture can be adjusted e.g. using the water gas shift reaction. Removing impurities, particularly nitrogen, carbon dioxide and water, from the raw synthesis gas mixture yields pure synthesis 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 from methane (natural gas) in a series of three reactions:
The methanol thus formed may be converted to gasoline by the Mobil process and methanol-to-olefins.
In the early 1970s, Mobil developed an alternative procedure in which natural gas is converted to syngas, and then methanol. The methanol reacts in the presence of a zeolite catalyst to form various compounds. In the first step methanol is partially dehydrated to give dimethyl ether:
The mixture of dimethyl ether and methanol is then further dehydrated over a zeolite catalyst such as ZSM-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 from coal-derived methanol in China by JAMG. A more modern implementation of MTG is the Topsøe improved gasoline synthesis (TiGAS). [5]
Methanol can be converted to olefins using zeolite and SAPO-based heterogeneous 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 four fixed bed reactors in series in which a syngas 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 of methanotrophs, microorganisms that metabolize methane as their only source of carbon and energy. Aerobic methanotrophs harbor enzymes that oxygenate methane to methanol. The relevant enzymes are methane 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 called methyl-coenzyme M reductases. These organisms effect reverse methanogenesis. 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 microbes Moorella thermoacetica and Yarrowia 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 valuable fuel oils, which can be sold as is or blended only with diesel fuel. The World Bank estimates that over 150 billion cubic metres (5.3×10 12 cu ft) of natural gas are flared 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 as crude oil resources are depleted.
Royal Dutch Shell produces a diesel from natural gas in a factory in Bintulu, Malaysia. Another Shell GTL facility is the Pearl GTL plant in Qatar, the world's largest GTL facility. [14] [15] Sasol has recently built the Oryx GTL facility in Ras Laffan Industrial City, Qatar and together with Uzbekneftegaz and Petronas builds the Uzbekistan GTL plant. [16] [17] [18] Chevron Corporation, in a joint venture with the Nigerian National Petroleum Corporation is commissioning the Escravos GTL in Nigeria, which uses Sasol technology. PetroSA, South Africa's national oil company, owns and operates a 22,000 barrels/day (capacity) GTL plant in Mossel 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 by INFRA 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 novel FPSO for offshore conversion of gas to liquids such as methanol, diesel, petrol, synthetic crude, and naphtha. [29]
GTL using natural gas is more economical when there is wide gap between the prevailing natural gas price and crude oil price on a Barrel of oil equivalent (BOE) basis. A coefficient of 0.1724 results in full oil 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 to LNG 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]
Methanol is an organic chemical compound and the simplest aliphatic alcohol, with the chemical formula CH3OH. It is a light, volatile, colorless and flammable liquid with a distinctive alcoholic odor similar to that of ethanol, but is more acutely toxic than the latter. Methanol acquired the name wood alcohol because it was once produced chiefly by the destructive distillation of wood. Today, methanol is mainly produced industrially by hydrogenation of carbon monoxide.
Syngas, or synthesis gas, is a mixture of hydrogen and carbon monoxide, in various ratios. The gas often contains some carbon dioxide and methane. It is principally used for producing ammonia or methanol. Syngas is combustible and can be used as a fuel. Historically, it has been used as a replacement for gasoline, when gasoline supply has been limited; for example, wood gas was used to power cars in Europe during WWII.
Gasification is a process that converts biomass- or fossil fuel-based carbonaceous materials into gases, including as the largest fractions: nitrogen (N2), carbon monoxide (CO), hydrogen (H2), and carbon dioxide (CO2). This is achieved by reacting the feedstock material at high temperatures (typically >700 °C), without combustion, via controlling the amount of oxygen and/or steam present in the reaction. The resulting gas mixture is called syngas (from synthesis gas) or producer gas and is itself a fuel due to the flammability of the H2 and CO of which the gas is largely composed. Power can be derived from the subsequent combustion of the resultant gas, and is considered to be a source of renewable energy if the gasified compounds were obtained from biomass feedstock.
Propylene, also known as propene, is an unsaturated organic compound with the chemical formula CH3CH=CH2. It has one double bond, and is the second simplest member of the alkene class of hydrocarbons. It is a colorless gas with a faint petroleum-like odor.
The Fischer–Tropsch process (FT) is a collection of chemical reactions that converts a mixture of carbon monoxide and hydrogen, known as syngas, into liquid hydrocarbons. These reactions occur in the presence of metal catalysts, typically at temperatures of 150–300 °C (302–572 °F) and pressures of one to several tens of atmospheres. The Fischer–Tropsch process is an important reaction in both coal liquefaction and gas to liquids technology for producing liquid hydrocarbons.
Steam reforming or steam methane reforming (SMR) is a method for producing syngas (hydrogen and carbon monoxide) by reaction of hydrocarbons with water. Commonly natural gas is the feedstock. The main purpose of this technology is hydrogen production. The reaction is represented by this equilibrium:
In industrial chemistry, coal gasification is the process of producing syngas—a mixture consisting primarily of carbon monoxide (CO), hydrogen, carbon dioxide, methane, and water vapour —from coal and water, air and/or oxygen.
Catalytic reforming is a chemical process used to convert naphthas from crude oil into liquid products called reformates, which are premium "blending stocks" for high-octane gasoline. The process converts low-octane linear hydrocarbons (paraffins) into branched alkanes (isoparaffins) and cyclic naphthenes, which are then partially dehydrogenated to produce high-octane aromatic hydrocarbons. The dehydrogenation also produces significant amounts of byproduct hydrogen gas, which is fed into other refinery processes such as hydrocracking. A side reaction is hydrogenolysis, which produces light hydrocarbons of lower value, such as methane, ethane, propane and butanes.
The methanol economy is a suggested future economy in which methanol and dimethyl ether replace fossil fuels as a means of energy storage, ground transportation fuel, and raw material for synthetic hydrocarbons and their products. It offers an alternative to the proposed hydrogen economy or ethanol economy, although these concepts are not exclusive. Methanol can be produced from a variety of sources including fossil fuels as well as agricultural products and municipal waste, wood and varied biomass. It can also be made from chemical recycling of carbon dioxide.
Coal liquefaction is a process of converting coal into liquid hydrocarbons: liquid fuels and petrochemicals. This process is often known as "Coal to X" or "Carbon to X", where X can be many different hydrocarbon-based products. However, the most common process chain is "Coal to Liquid Fuels" (CTL).
Synthetic fuel or synfuel is a liquid fuel, or sometimes gaseous fuel, obtained from syngas, a mixture of carbon monoxide and hydrogen, in which the syngas was derived from gasification of solid feedstocks such as coal or biomass or by reforming of natural gas.
Biomass to liquid is a multi-step process of producing synthetic hydrocarbon fuels made from biomass via a thermochemical route.
The Bergius process is a method of production of liquid hydrocarbons for use as synthetic fuel by hydrogenation of high-volatile bituminous coal at high temperature and pressure. It was first developed by Friedrich Bergius in 1913. In 1931 Bergius was awarded the Nobel Prize in Chemistry for his development of high-pressure chemistry.
A methane reformer is a device based on steam reforming, autothermal reforming or partial oxidation and is a type of chemical synthesis which can produce pure hydrogen gas from methane using a catalyst. There are multiple types of reformers in development but the most common in industry are autothermal reforming (ATR) and steam methane reforming (SMR). Most methods work by exposing methane to a catalyst at high temperature and pressure.
Second-generation biofuels, also known as advanced biofuels, are fuels that can be manufactured from various types of non-food biomass. Biomass in this context means plant materials and animal waste used especially as a source of fuel.
The Glossary of fuel cell terms lists the definitions of many terms used within the fuel cell industry. The terms in this fuel cell glossary may be used by fuel cell industry associations, in education material and fuel cell codes and standards to name but a few.
Reactive flash volatilization (RFV) is a chemical process that rapidly converts nonvolatile solids and liquids to volatile compounds by thermal decomposition for integration with catalytic chemistries.
Syngas to gasoline plus (STG+) is a thermochemical process to convert natural gas, other gaseous hydrocarbons or gasified biomass into drop-in fuels, such as gasoline, diesel fuel or jet fuel, and organic solvents.
E-diesel is a synthetic diesel fuel for use in automobiles. Currently, e-diesel is created at two sites: by an Audi research facility Germany in partnership with a company named Sunfire, and in Texas. The fuel is created from carbon dioxide, water, and electricity with a process powered by renewable energy sources to create a liquid energy carrier called blue crude which is then refined to generate e-diesel. E-diesel is considered to be a carbon-neutral fuel as it does not extract new carbon and the energy sources to drive the process are from carbon-neutral sources.
Chemical looping reforming (CLR) and gasification (CLG) are the operations that involve the use of gaseous carbonaceous feedstock and solid carbonaceous feedstock, respectively, in their conversion to syngas in the chemical looping scheme. The typical gaseous carbonaceous feedstocks used are natural gas and reducing tail gas, while the typical solid carbonaceous feedstocks used are coal and biomass. The feedstocks are partially oxidized to generate syngas using metal oxide oxygen carriers as the oxidant. The reduced metal oxide is then oxidized in the regeneration step using air. The syngas is an important intermediate for generation of such diverse products as electricity, chemicals, hydrogen, and liquid fuels.