Volatile fuel costs and concern over the sustainability of fossil fuel resources and sourcing are driving interest in decarbonized or net-zero carbon fuels and chemicals. Traditionally refined liquid transportation fuels such as gasoline, diesel fuel, and aviation fuel have a significant greenhouse gas/carbon footprint that conflicts with goals for extensive decarbonization across the economy of the United States. Routes to synthesis of liquid fuels from solid feedstocks such as wastes, waste coal, and biomass could add substantial diversity in fuel supply capability and increased energy security that accompany these factors. Gasification’s abilities to accept these otherwise difficult to convert feedstocks, co-gasification options, plus the relative ease of carbon capture in gasification process systems, enables the viability of gasification-based production of sustainable, decarbonized liquid fuels.
Although there are a number of different demonstrated process routes for production of liquid fuels from solid feedstocks like coal (e.g. direct coal liquefaction), the most important methods have based on production of 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.
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, diesel, and aviation fuel 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 sections.