have been the main markets for NGVs. Both of these uses have occurred mainly in response to the Clean-Fuel Fleet Program set up by the U.S. Environmental Protection Agency to reduce air pollution.
The GTL process for producing synthetic diesel fuel is similar to the indirect liquefaction of coal. Instead of syngas production via the gasification of coal, however, syngas is produced by the steam reforming of natural gas. The synthesis gas can then be converted to an olefinic distillate, called synthol light oil, and wax using a catalytic modification of the FT process discussed earlier. The olefinic distillate and wax are hydrocracked to produce high-quality diesel, as well as naphtha and other streams that form the basis of specialty products such as synthetic lubricants.
Although it is technically difficult, the naphtha can also be upgraded to gasoline. Naphtha is an ideal feedstock for manufacture of chemical building blocks (for example, ethylene), and GTL diesel provides high-quality automotive fuel or blending stock (Johnson-Matthey, 2006) like coal-to-liquids technology. GTL is an option for producing diesel from “stranded” natural gas, such as that which exists in the Middle East and Russia. However, a couple of GTL plants would produce enough naphtha to swamp the chemical market for this material.
Hypothetically, there are several advantages to converting natural gas into GTL diesel rather than into CNG. All diesel vehicles can run on GTL diesel, which gives gas producers access to new market opportunities. Vehicle driving range for diesel is much higher than for compressed natural gas because of diesel’s higher energy density. Engine efficiency and performance are not compromised by the adjustment to GTL diesel fuel. GTL diesel can be shipped in normal tankers and unloaded at ordinary ports (The Economist, 2006).
Currently, there are several commercial GTL plants. Sasol in Nigeria and Qatar, as well as Shell in Malaysia and Qatar, produce GTL diesel fuel; a number of companies, including World GTL and Conoco Phillips, have plans to build GTL plants in the next several years. Because the economics of GTL plants are very closely tied to the natural gas price, viability depends in large part on inexpensive stranded gas. GTL diesel is viewed mainly as an alternative to liquified natural gas for monetizing large natural gas accumulations such as the one in Qatar. The high cost to produce GTL diesel makes its development in the United States unlikely unless an abundant and inexpensive source of natural gas is found (for example, natural gas hydrates).