Synthetic liquid transportation fuels, collectively known as xTL fuels, are produced through specialized conversion processes. These production methods, including the Fischer-Tropsch process, produce fuels from carbon-based feedstocks, such as biomass, coal, or natural gas, and can yield many useful fuels, including gasoline, diesel, ethanol, and methanol.
Liquid Fuels from Coal and Natural Gas
Liquid fuels from coal and natural gas are produced primarily through the Fischer-Tropsch process described below. Producing liquid fuels from coal and natural gas can result in greenhouse gas (GHG) emissions, but carbon sequestration can mitigate this disadvantage.
In addition, coal can be converted directly into liquids through liquefaction. The Bergius hydrogenation process is a primary method that involves reacting low-grade coal with hydrogen at a high temperature and pressure to produce liquids that can be refined into synthetic fuels. China, India, and the Philippines are studying and deploying direct liquefaction technologies.
The Fisher-Tropsch process produces liquid transportation fuels by converting syngas—a mixture of carbon monoxide and hydrogen produced from biomass or fossil fuels, such as natural gas and coal—into Fischer-Tropsch (F-T) diesel. F-T diesel can substitute for conventional petroleum diesel to fuel diesel vehicles without modifying the engine or fueling infrastructure.
In 1923, Franz Fischer and Hans Tropsch studied converting coal-derived syngas into useful compounds, such as diesel. The key to F-T synthesis is the catalysts—substances that facilitate chemical reactions without being consumed by the reaction. The process includes three steps that occur in the presence of catalysts:
- Syngas Formation
Old Hydrocarbon + Oxygen → Syngas
- Fischer-Tropsch Reaction
Syngas → New Hydrocarbon + Water
New Hydrocarbon → Fuels, Chemicals, etc.
The F-T process is beneficial because it converts relatively inflexible energy sources—such as coal or biomass indicated as "Old Hydrocarbon" in the first step—into useful transportation fuels. Because petroleum-based fuels are in high demand, F-T diesel is a valuable substitute.
Liquid Fuels from Biomass
Liquid fuels converted from biomass feedstocks are produced primarily through two processes:
- Gasification—heating biomass by partial oxidation to produce synthesis gas (syngas)
Biomass is converted to gas by heating it in the presence of about one third of the oxygen needed for combustion. The resulting syngas is a mixture of carbon monoxide and hydrogen that can be burned to produce electricity or converted into hydrocarbons (such as gasoline and diesel), alcohols, ethers, or chemical products. The petroleum and petrochemical industries have developed commercial processes for converting syngas into fuels and chemicals.
- Pyrolysis—heating biomass in the absence of oxygen to produce liquid oil
Sometimes called bio-oil, pyrolysis oil can be burned like fuel oil or refined into chemicals and fuels. Upgrading pyrolysis oil to high-quality hydrocarbon fuels has been demonstrated at a non-commercial scale.
Both processes use heat and chemical reactions to convert biomass into fuels, chemicals, and power. The products are cleaner and more efficient than the original biomass. These processes can also convert biomass, such as wood and agricultural residues, that is difficult to handle with other biofuel production processes.
xTL fuels can be alternatives to conventional transportation fuels. The benefits of xTL fuels include:
- More Compatibility—xTL fuels are compatible with current diesel- and gasoline-powered vehicles and fuel distribution infrastructure. xTL fuels do not require new or modified pipelines, storage tanks, or retail station pumps.
- Increased Energy Security—Using the United State's vast coal reserves and natural gas to produce transportation fuels would reduce U.S. reliance on imported petroleum and increase energy security. xTL fuels produced domestically can also create jobs in the United States.
- Improved Vehicle Performance—Liquid fuels from natural gas and F-T diesel can provide similar or better vehicle performance than conventional fuels.
- Reduced Exhaust Emissions—With F-T diesel, emission-control catalysts can reduce nitrogen oxide emissions, little or no particulate emissions exist because it has low sulfur and aromatic content, and there are fewer hydrocarbon and carbon monoxide emissions.
- Fewer GHG Emissions—Fuels converted from stranded natural gas reserves, which are otherwise not economical to recover, require no gas flaring and produce fewer GHG emissions. Fuels from biomass can produce fewer GHG emissions because carbon dioxide captured during feedstock growth offsets carbon dioxide emissions from burning fuel.
Research and Development
Gas-to-liquid and coal-to-liquid production processes are in relatively advanced stages of development and in commercial production. Biomass-to-liquid processes are less mature. For all three processes, research and development efforts are improving the efficiency and economics of production as well as quantifying costs and benefits of production and use in vehicles.
Companies involved in xTL fuel production include Sasol, Shell, Syntroleum, and Rentech. Since the late 1990s, most major oil companies have announced plans to investigate producing diesel through gas-to-liquid processes. The U.S. Department of Energy and several national laboratories support xTL fuel production research and development.
Learn more about xTL fuels from the links below. The AFDC and U.S. Department of Energy do not necessarily recommend or endorse these companies (see disclaimer).
- American Fuels Coalition
- Clean Alternative Fuels: Fischer-Tropsch
- Coal: Liquid Fuels (World Coal Association)
- Fire in the Hole: Underground Coal Gasification (Lawrence Livermore National Laboratory)
- Fischer-Tropsch Archive (Emerging Fuels Technology)
- Green Car Congress
- International Energy Agency
- National Energy Technology Laboratory
- Baseline Technical and Economic Assessment of a Commercial Scale Fischer-Tropsch Liquids Facility
- Coal and Power Systems: Gasification
- National Renewable Energy Laboratory
- Biomass Research: Thermochemical Conversion Capabilities
- From Biomass to Biofuels: NREL Leads the Way
- Preliminary Screening - Technical and Economic Assessment of Synthesis Gas to Fuels and Chemicals with Emphasis on the Potential for Biomass-Derived Syngas
- Petroleum Fuels (U.S. Department of Energy Office of Fossil Energy)
- Thermochemical Conversion (U.S. Department of Energy Bioenergy Technologies Office)