Volatile fuel costs and a desire for energy independence have revived interest in another market for coal gasification technology: the production of liquid transportation fuels, chiefly gasoline and diesel fuel. For the United States, routes to synthesis of liquid fuels from coal add substantial diversity in fuel supply capability, a large capacity for fuels production considering the great extent of domestic coal reserves, and increased energy security that accompany these factors.
Although there are a number of different demonstrated process routes for production of gasoline and diesel fuel from coal, not all of which involve coal gasification (e.g. direct coal liquefaction), the most important methods are based on production of synthesis gas (syngas) from gasification of coal, which is converted to liquid hydrocarbons or alcohol for use as fuel or fuel refining feedstock. Because the coal is first gasified, followed by conversion of the syngas to liquid products, these are termed indirect liquefaction methods. Since impurities such as sulfur and mercury are removed from the syngas prior to fuels synthesis, the result is ultra-clean liquid fuels that burn with lower emissions than conventional gasoline and diesel fuel. In fact, South Africa’s Sasol has been producing large amounts of these clean-performing, coal-derived fuels since 1955, with 30% of the entire country’s gasoline and diesel needs produced from indigenous coal. Sasol’s coal to liquid fuels also include jet fuel, meeting stringent approval for utilization in commercial jet aircraft. Environmental considerations, national energy concerns, and global oil markets could play a role in more extensive development of these applications.
Fischer-Tropsch (FT) synthesis is a very important liquefaction technology used since the World War II era. FT catalysts are used to facilitate the formation of hydrocarbons or alcohols from the carbon monoxide (CO) and hydrogen (H2) in syngas. The end products of the process are influenced by choice of catalyst, feed composition, and reactor conditions such as internal temperature and pressure. The FT synthesis step produces a range/mixture of straight-chain, saturated hydrocarbons, of the form CnH2n+2 (termed paraffin hydrocarbons), aromatic hydrocarbons, olefins, and other species. From these, gasoline and diesel can be refined. Fuel gases like methane (SNG) and liquefied petroleum gas (LPG; mostly propane and butane) are usually also formed in FT synthesis but are generally minimized or recycled to yield the maximum amount of high-value liquid products. Waxes (longer-chain paraffin with 20 to 40 carbon molecules that are solid at standard conditions) are also formed, but can be "cracked" to shorter, liquid forms.
As opposed to FT synthesis, syngas may be converted to methanol, which may be further transformed into gasoline via the ExxonMobil Methanol to Gasoline (MTG) process. Developed by Mobil throughout the 70s and early 80s, a first-of-its-kind plant was built in New Zealand in 1985, where it successfully produced gasoline for 10 years. The process has been continuously refined since then to its current state as a viable alternative to conventional gasoline sources. The synthetic gasoline produced by MTG is a very low sulfur, low benzene high quality gasoline, which is a valuable blending component for meeting environmental regulations specific to sulfur and benzene.
These important fuels synthesis routes are described in detail in the following discussion.