Indirect Liquefaction Processes

Indirect coal liquefaction requires first gasifying the coal into a syngas. Therefore, while direct coal liquefaction (DCL) is a single-step process, indirect coal liquefaction (ICL) consists of two major steps: (a) gasification to produce a synthesis gas (syngas); and (b) conversion of the carbon monoxide (CO) and hydrogen (H₂) in the sygnas to a range of hydrocarbon (fuels/products such as synthetic natural gas, methanol and chemicals) via Fischer-Tropsch (FT) synthesis.

Direct coal liquefaction requires an external source of hydrogen, which may have to be provided by gasifying additional coal feed and/or the heavy residue produced from the DCL reactor. Many argue that indirect liquefaction with the current state-of-the-art technologies is more competitive than direct liquefaction. ICL has been demonstrated commercially by Sasol since the 1950s, and the ICL process is more amenable to carbon dioxide (CO₂) capture.

Typical FT Process
Indirect coal liquefaction involves first gasifying the coal to produce a syngas, which mainly consists of CO and H₂.  After cleaning and conditioning to remove contaminants such as sulfur, the CO and H₂ can then react catalytically to form hydrocarbon liquids via Fischer-Tropsch synthesis. Figure 1 shows a simplified block flow diagram of an ICL process.  The gasification island consists of all the supporting process technologies of coal handling & feed preparation, heat recovery, syngas cleanup and conditioning, water-gas-shift, sulfur recovery, etc. The clean syngas leaving the gasification island is sent onto the FT synthesis island, where the clean shifted syngas is converted into primary products of wax, hydrocarbon condensate, tail gas, and reaction water. The wax is sent on to an upgrading unit for hydrocracking, in the presence of hydrogen, where it is cracked into smaller molecular weight hydrocarbon liquids. A hydrogen recovery unit is used to extract the required quantity of hydrogen from the tail gas as shown, or alternatively from the feed syngas stream. The reaction product, along with that from the upgrading section, is fractionated into the final products of diesel, naptha, and other light ends, depending on the desired product mix. The ICL facility is supported by several utility plants, including the power train.

Commercially available FT catalysts are normally either cobalt or iron based.  Only iron-based FT catalysts are currently used commercially for converting coal derived syngas into FT liquids.

Figure 1: Simplified ICL Process Scheme

The huge Sasol facility in South Africa uses a proven process for indirect liquefaction. The process is described in Figure 2 below.

	CLICK ON GRAPHIC TO ENLARGE
	Figure 2: Sasol Indirect Liquefaction Process (Sasol)

Typical Methanol to Liquids Process
Indirect liquefaction using a methanol to gasoline (MTG) process for syngas upgrading has been demonstrated in several locations and is being proposed for new facilities. Instead of the traditional FT technology, this process follows a methanol synthesis unit with a methanol to gasoline synthesis process. Figure 3 shows the simplified MTG process. This technology is being developed and marketed by ExxonMobil for coal to liquids applications. ExxonMobil and Uhde demonstrated the process in New Zealand and have plants proposed for Wyoming and West Virginia.

Figure 3: Methanol-to-Gasoline process
(Prenflo)

Return to Applications of Gasification