Applications of Gasification
Rising fuel costs and a desire for energy independence have revived interest in another market for coal gasification technology: the production of liquid transportation fuels. Commonly called coal-to-liquids (CTLs), or Fischer-Tropsch (FT) liquids, after the German inventors of the primary chemical conversion process (indirect liquefaction), CTL can help increase fuel supply diversity and energy security.
In CTL, clean synthesis gas (syngas) from gasification of coal is converted to a liquid hydrocarbon or alcohol for use as fuel or otherwise. First, the syngas must be cleaned of sulfur and other impurities before it is reacted into its liquid fuel form. The FT process is one possible CTL path. FT catalysts are used to facilitate the formation of hydrocarbons or alcohols from the carbon monoxide (CO) and hydrogen (H2) in the syngas. The end product of the process can be determined by changing the catalyst, feed composition, and reactor conditions such as internal temperature and pressure. The main products of the FT process are typically straight-chain, saturated hydrocarbons, of the form CnH2n+2 (this class of molecule are called paraffin), from which 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 small amounts by CTL but are generally discouraged by the process designers (see Coal-to-SNG for information on SNG as a primary product). 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. Different catalysts can facilitate the formation of alcohols like methanol, ethanol and propanol that can be used as fuel or fuel additives.
Methanol to Gasoline (MTG) is another alternative path for CTL production. In this route, syngas is reacted to form methanol, from which gasoline is then formed. 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.
SASOL II and III, Secunda, S. Africa. FTL Facility. Photo courtesy: SASOL
While FT technology has been commercialized in South Africa (by SASOL) for decades—the largest CTL plant there produces 124,000 barrels per day of light olefins (alkenes) and gasoline from coal—cost reductions are necessary before a market can successfully emerge in the United States. While gasification can currently meet all environmental standards, carbon capture may need to be more fully implemented to ease public concerns about greenhouse gas emissions. High crude oil prices have made CTL more economical, while high gasoline prices have created growing national interest in CTL. The price of crude oil is the main factor in market penetration of CTL.
Baseline Technical and Economic Report
A 2007 NETL report examined the feasibility of a commercial-sized CTL plant located in the United States. The report found that a plant using Midwestern bituminous coal (specifically Illinois #6, in the analysis) shows promising economic opportunities. The theoretical plant would consume just over 24,500 tons per day (tpd) of coal and produce about 50,000 barrels of product (diesel and naphtha) per day. Under baseline assumptions (current and predicted trends in oil, diesel, naphtha, prices, etc.), financial analysis predicts just under 20% return on investment (ROI), a net present value of over $1.5 billion, and a payback period of five years.