Key end products from gasification include hydrogen (and synthetic natural gas as a closely related product), electric power, fuels (mainly diesel fuel and gasoline), and fertilizer (which hinges on the large quantities of ammonia produced from gasification). In the context of liquid fuels, methanol can be added as an end product; in some locations (China in particular) methanol is a heavily-used fuel blending stock and feedstock for methyl tert-butyl ether (MTBE) manufacture and commands a large market. At the same time, methanol and ammonia are important not only as fuel/fuel additive/feedstock for gasoline production and fertilizer feedstock, respectively, but they are important as key building blocks for further chemical synthesis; many such chemicals are high-value products and gasification provides the option of using relatively inexpensive carbonaceous solid feedstocks to produce them.
Historically, producing chemicals via gasification of coal has occurred since the 1950s and continues to be embedded in the world chemicals market. Figure 1 shows a diagram depicting an estimate of global flows of fossil fuel feedstocks to chemicals production in 2013. Coal figures prominently in the flows to ammonia and methanol, which in turn feed into a wide range of plastics and chemicals; most of the coal use involves gasification to syngas on these conversion routes. Summary statistics of the Gasification & Syngas Technologies Council’s Worldwide Syngas Database from 2016 clearly illustrate the predominance of operating, under-construction and planned gasification capacity for chemicals production over fuels and electricity end uses, as shown in Figure 2. It should be noted that a sizeable fraction of the chemicals production as described in the Worldwide Syngas Database included ammonia and fertilizers, which are discussed in more detail separately.
Because the slate of chemical products that can be made via gasification can in general use any of the fossil fuel feedstocks shown on Figure 1, the chemical industry tends to use whatever feedstocks are most cost-effective. Therefore, interest in using coal tends to increase with higher oil and natural gas prices, and/or lack of availability of oil and natural gas. Judging by the Worldwide Syngas Database and the preponderance of coal to chemicals plants which are located in China, it is readily apparent that the abundance of coal combined with lack of natural gas resources in China is strong inducement for the coal to chemicals industry pursued there (see additional discussion below about the market for chemicals production from coal). In the United States, there is the well-known example of Eastman Chemical Company’s coal gasification-based chemicals plant at its Kingsport, Tennessee site, in operation since 1983. Similarly, Sasol has built and operated coal-to-chemicals facilities in South Africa.
For more detail on possibilities for chemicals production from coal/petcoke, refer to Figure 3 showing potential products that can be synthesized from coal gasification-derived syngas. Shown in dark green in the figure, primary chemicals that are produced directly from the syngas include methanol (MeOH), hydrogen(H2) and carbon monoxide (CO), which are the chemical building blocks from which a whole spectrum of derivative chemicals can be manufactured, including olefins, acetic acid, formaldehyde, ammonia, urea and others. As a relevant example, the Eastman Chemical plant uses two primary products of CO and MeOH as intermediate chemicals to make acetic acid via a carbonylation reaction; acetic acid can react with additional MeOH to produce methyl acetate, which after further carbonylation forms the final product of acetic anhydride, a raw material to convert cellulose into cellulose acetate, a component for photographic film and coating materials, fibers and other plastic products. Figure 3 also shows methanol being converted to formaldehyde (about 40% of methanol produced follows this route); formaldehyde is widely used in manufacture of plastics, plywood, paints, explosives, and textiles.
Globally, much of ammonia production goes towards synthesis of fertilizers such as urea and ammonium nitrate, as discussed elsewhere, but Figure 3 shows that ammonia also serves as an intermediate chemical for acrylonitrile, used in manufacture of plastics and synthetic rubber.