Gas-to-liquid fuels (GTL) are fuels that can be produced from natural gas, coal and biomass using a Fischer-Tropsch chemical reaction process. The liquids produced include naphtha, diesel and chemical feedstocks. The resulting GTL diesel can be used neat or blended with today’s diesel fuel and used in existing diesel engines and infrastructure. These fuels provide an opportunity to reduce dependence on petroleum-based fuels and reduce tailpipe emissions.
Natural gas can be used to produce bulk petrochemicals, including methanol and ammonia, but these are relatively small users of the gas reserves with limited markets. Liquid and other petroleum products are cheaper to transport, market and distribute to large markets. These can be moved in existing pipelines or products tankers and even blended with existing crude oil or product streams. Further, no special contractual arrangements are required for their sale with many suitable domestic and foreign markets.
GTL not only adds value but is also capable of producing products that could be sold or blended into refinery stock as superior products with less pollutant for which there is growing demand. Reflecting its origins as a gas, GTL processes produce diesel fuel with an energy density comparable to conventional diesel but with a higher cetane number permitting a superior performance engine design. Low sulfur content leads to significant reductions in particulate matter that is generated during combustion, and the low aromatic content reduces the toxicity of the particulate matter reflecting in a worldwide trend toward the reduction of sulfur and aromatics in fuel.
Technology
Synthesis gas is produced by reacting methane (or carbon) with steam at elevated temperatures to yield a useful mixture of carbon oxides and hydrogen. It can be produced by a variety of processes and feedstocks. It may require the indicated compositional adjustment and treatment before use.
Fischer-Tropsch
The discovery of Fischer-Tropsch chemistry in Germany dates back to the 1920s and its development has been for strategic rather than economic reasons, as in Germany during World War II and South Africa. Mobil developed the “M-gasoline” process to make gasoline from methanol and implemented it in 1985 in a large integrated methanol-to-gasoline plant in New Zealand.
Syngas
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 partial oxidation route provides the desired 2:1 ratio and the preferred routes in isolation of other needs. There are two routes: One uses oxygen and produces a purer syngas without nitrogen, and the other uses air creating a more dilute syngas. However, the oxygen route requires an air separation plant that increases the cost of the investment.
Conversion
Conversion of the syngas-to-liquid hydrocarbon is a chain growth reaction of carbon monoxide and hydrogen on the surface of a heterogeneous 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 330 C, mostly gasoline and olefins are produced whereas at 180 C to 250 C, mostly diesel and waxes are produced.
There are mainly two types of Fischer-Tropsch reactors. The vertical fixed tube type has the catalyst in tubes that are cooled externally by pressurized boiling water. For a large plant, several reactors in parallel may be used presenting energy savings. The other process uses a slurry reactor in which preheated synthesis gas is fed to the bottom of the reactor and distributed into the slurry consisting of liquid wax and catalyst particles. As the gas bubbles upward through the slurry, it is diffused and converted into more wax by the Fischer-Tropsch reaction. The heat generated is removed through the reactor’s cooling coils where steam is generated for use in the process.
For more information on Fischer-Tropsch chemistry, visit www.fischer-tropsch.org. For more information on petroleum geology, visit www.aapg.org. For more information on SPED, visit www.spedweb.com or call (832) 286-7678.
