Gas to Liquids (GTL) is the process that converts natural gas to a diesel like fuel (FT diesel). The process is scalable and can be applied to to both large and small deposits of natural gas.
The future for the technology lies mainly with natural gas - in particular, low-cost remote natural gas or gas associated with oil production that is now flared, but does not justify the cost and scale of liquefied natural gas (LNG) facilities or a pipeline. In these cases, GTL offers an economical way to convert the gas to liquid fuels or a refinery feedstock that can be readily transported.
The processes produces diesel like fuel with an energy density comparable to conventional diesel, a higher cetane number permitting superior performance, low aromatic content and a very low sulfur content. Conventional diesel fuel has increasingly unacceptable particulate matter in its emissions composed of unburnt carbon and aromatics, and compounds of sulfur. The low sulfur content of Fischer-Tropsch (FT) diesel, leads to significant reductions in particulate matter and the low aromatic content reduces the toxicity of the particulate matter.
The GTL process is composed of three steps:
- Reforming of natural gas to produce a synthesis gas (syngas) that has a hydrogen:carbon monoxide ratio of approximately 2:1. The syngas step converts the natural gas to hydrogen and carbon monoxide by partial oxidation, steam reforming or a combination of the two processes.
- The resulting syngas is fed to the Fischer-Tropsch reactor and converted to mostly straight-chain, waxy paraffins in the presence of a catalyst. The catalyst is either iron or cobalt based and the reaction is highly exothermic. The temperature, pressure and catalyst determine whether a light or heavy syncrude is produced. For example at 330C with an iron catalyst mostly gasoline and olefins are produced whereas at 180 to 250C with a cobalt catalyst mostly diesel and waxes are produced.
- The high molecular weight liquid products can be hydro-cracked in a simple low-pressure process to produce naphtha, kerosene and diesel that are virtually free of sulfur and aromatics; These derivative fuels are therefore potentially more valuable, notably in the US, Europe and Japan with high environmental standards.
Until the last few of years diesel from GTL wasn't competitive at oil prices below $35/bbl. Today it is claimed that it can compete with $20 oil. Forbes 5/24/04
Presently there are only three GTL facilities have operated to produce synthetic petroleum liquids at more than a demonstration level: the Mossgas Plant (South Africa), with output capacity of 23 000 barrels per day, Shell Binemerging Bintulu (Malaysia) at 20 000 barrels per day and the subsidized methanol to gasoline project in New Zealand.(Emerging Technologies to Develop Stranded Gas)
Nigeria flares around 600 billion cubic feet of gas each year, a figure that’s equivalent to about 3 percent of total U.S. consumption. So the country has a big incentive to make use of that resource. In April 2005 ChevronTexaco Corp. (San Ramon, CA; www.chevrontexaco.com) awarded a $1.7 billion engineering, procurement and construction contract for its 34,000-bbl/d Escravos gas-to-liquids (EGTL) project in Nigeria. The contract went to a consortium composed of JGC Corp. (Yokohama, Japan), KBR (Houston) and Snamprogetti SpA (Milan). The Nigeria project will benefit from the infrastructure already in place for nearby oil and gas production and export facilities, although it is unclear whether, or to what extent, subsidies or other considerations helped to lower the estimated costs.
In 2004 ExxonMobil, Royal Dutch/Shell, ChevronTexaco and South Africa-based Sasol proposed an assortment of giant gas-to-liquids projects in Qatar, involving more than $20 billion in investments. In April 2005 the government of Qatar announced that all but the Sasol projects would be put on hold in favor of LNG projects.
A joint project between Chevron and Sasol Ltd is under construction in Qatar with a 34,000 barrel per day plant, Oryx I, using the Sasol Slurry Phase Distillate process. As of April 2005 the facility was 80 percent complete with the first production expected early next year. The Oryx facility will convert about a third of a billion cubic feet of lean gas into 34,000 barrels of liquids each day.
Each company in the GTL business has its own proprietary FT technology. Most processes use a slurry-phase reactor with a cobalt-based catalyst. Exceptions are Shell and BP, whose processes use a fixed-bed reactor and Rentech, Inc., which uses an iron-based catalyst. Most companies use autothermal reforming (ATR) rather than steam reforming because it is less expensive in larger plants. Steam reforming capital cost is almost linear with capacity because the amount of tubing required is proportional to plant capacity.
The following description of the GTL technology was excerpted and edited from an article on the website of Chemlink Consultants of Australia. The article contains much additional information for those that are interested.
Sasol is a synfuel technology supplier established to provide petroleum products in coal-rich but oil-poor South Africa. The firm has built a series of Fischer-Tropsch coal-to-oil plants, and is one of the world's most experienced synthetic fuels organizations and is now marketing its natural-gas-to-oil technology. It has developed the world's largest synthetic fuel project, the Mossgas complex at Mossel Bay in South Africa that was commissioned in 1993 and produces a small volume of 25 000 barrels per day. Sasol has commercialized four reactor types with the slurry phase distillate process being the most recent. Its products are more olefinic than those from the fixed bed reactors and are hydrogenated to straight chain paraffins. Its Slurry Phase Distillate converts natural gas into liquid fuels, primarily a superior-quality diesel. The resultant diesel is suitable as a premium blending component for standard diesel grades from conventional crude oil refineries. Blended with lower grade diesels it assists to comply with the increasingly stringent specifications being set for transport fuels in North America and Europe.
The other technology uses the Sasol Advanced Synthol (SAS) reactor to produce mainly light olefins and gasoline fractions. Sasol has developed high performance cobalt-based and iron based catalysts for these processes.
The company claims a single module or the Sasol Slurry Phase Distillate plant, that converts 100 MMscfd (110 terajoules per day of gas) of natural gas into 10 000 barrels a day of liquid transport fuels, that can be built at a capital cost of about US$250 million. This cost equates to a cost per daily barrel of capacity of about US$25,000 including utilities, off-site facilities and infrastructure units. "Gas to Oil: A Gusher for the Millennium," Business Week (May 19, 1997).
In June 1999, Chevron and Sasol agreed to an alliance to create ventures using Sasol's GTL technology. The two companies have conducted a feasibility study to build a GTL plant in Nigeria. Sasol reportedly also has been in discussions with Norway's Statoil, but no definitive announcements have been made.
Shell has carried out R&D since the late 1940s on the conversion of natural gas, leading to the development of the Shell Middle Distillate Synthesis (SMDS) route, a modified F-T process. But unlike other F-T synthesis routes aimed at gasoline as the principal product, SMDS focuses on maximising yields of middle distillates, notably kerosene and gas oil.
Shell has built a 12 000 bbl/day plant in 1993 in Bintulu, Malaysia. The process consists of three steps: the production of syngas with a H2:CO ratio of 2:1; syngas conversion to high molecular weight hydrocarbons via F-T using a high performance catalyst; and hydrocracking and hydroisomerisation to maximise the middle distillate yield. The products are highly paraffinic and free of nitrogen and sulfur.
Shell is investing US$6 billion in gas to liquids technologies over 10 years with four plants. It announced in October 2000, agreement with the Egyptian government for a 75 000 bbl per day (3.8 million tpa) facility and a similar plant for Trinidad & Tobago.
In April 2001, it announced interest for plants in Australia, Argentina and Malaysia at 75 000 bbls/day costing US$1.6 billion.
Exxon has developed a commercial F-T system from natural gas feedstock. Exxon claims its slurry design reactor and proprietary catalyst systems result in high productivity and selectivity along with significant economy of scale benefits. Exxon employs a three-step process: fluid bed synthesis gas generation by catalytic partial oxidation; slurry phase F-T synthesis; and fixed bed product upgrade by hydroisomerisation. The process can be adjusted to produce a range of products. More recently, Exxon has developed a new chemical method based on the Fischer-Tropsch process, to synthesis diesel fuel from natural gas. Exxon claims better catalysts and improved oxygen-extraction technologies have reduced the capital cost of the process, and is actively marketing the process internationally.
The Syntroleum Corporation of the USA is marketing an alternative natural-gas-to-diesel technology based on the F-T process. It is claimed to be competitive as it has a lower capital cost due to the redesign of the reactor; using an air-based autothermal reforming process instead of oxygen for synthesis gas preparation to eliminate the significant capital expense of an air separation plant; and high yields using their catalyst. It claims to be able to produce synthetic crude at around $20 per bbl. The syncrude can be further subjected to hydro-cracking and fractionation to produce a diesel/naphtha/kerosene range at the user’s discretion.
The company indicates its process has a capital cost of around $13 000 per daily barrel of diesel for a 20 000 to 25 000 barrel per day facility and an operating cost of between $3.50 to $5.70 per barrel. The thermal efficiency of the Syntroleum process is reported to be about 60 percent, implying a requirement for about 90 million cubic feet (85 terajoules) per day of dry gas for a $300 to $350 million, 25 000 barrel per day capacity facility. These figures therefore suggests a unit cost of less than $20 per barrel ($3.20 per gigajoule) of diesel fuel. The company claims the required economic scale would be smaller if based on LNG.
Syntroleum Corporation now also licenses its proprietary process for converting natural gas into other synthetic crude oils and transportation fuels. In February 2000, Syntroleum Corporation announced its intention to construct a 10 000 barrel per day natural gas-to-liquids plant for the state of Western Australia to become the first location in the world to acquire full access to Syntroleum technology. The project plans to produce synthetic specialty hydrocarbons (polyalphaolefins lubricating oils), naphtha, normal paraffins and drilling fluids. It is estimated to cost US$500 million generating sales of around US$200 million per year at constant prices.
The process is designed for application in plant sizes ranging from 2 000 barrels per day to more than 100 000 barrels per day. Current licensees include ARCO, Enron, Kerr-McGee, Marathon, Texaco, Repsol-YPF and Australia. The company has advised that it is "working on development plans" for gas-to-liquids specialty chemicals plant and is working with DaimlerChrysler to develop super-clean synthetic transportation fuels.
The small scale of the proposed plant is because the autothermal partial oxidation with air and a once-through reactor design has not yet been proven. The smaller scale also avoids the marketing risk of placing large volumes of specialty chemicals and waxes in the marketplace dominated by large suppliers such as Sasol and Shell.
Rentech (Denver, CO USA; www.rentechinc.com), has been developing an F-T process using a molten wax slurry reactor and precipitated iron catalyst to convert gases and solid carbon-bearing material into straight chain hydrocarbon liquids. In their process, long straight chain hydrocarbons are drawn off as a liquid heavy wax while the shorter chain hydrocarbons are withdrawn as overhead vapors and condensed to soft wax, diesel fuel and naphtha.
Another smaller Nigerian project is being planned by Syntroleum Corp. of the United States and several partners. They are planning an offshore GTL project for the Aje field, 15 miles off Nigeria, and the group is planning to drill a well later this year in 3,100 feet of water. The Aje 3 well follows an exploration well in 1996 and an appraisal well in 1997.