Synthesis Gas. James G. Speight

with higher methane and hydrogen contents as well as with lower carbon dioxide contents. Furthermore, the very nature of the gasification process used to produce the medium heat-content gas has a marked effect upon the ease of subsequent processing. For example, the CO₂-acceptor product is quite amenable to use for methane production because it has (i) the desired H₂/CO ratio just exceeding 3:1, (ii) an initially high methane content, and (iii) relatively low water and carbon dioxide contents. Other gases may require appreciable shift reaction and removal of large quantities of water and carbon dioxide prior to methanation.

Town gas is also a medium Btu that is produced in the coke ovens and has the approximate composition: 55% v/v hydrogen, 27% v/v methane, 6% v/v carbon monoxide, 10% v/v nitrogen, and 2% v/v carbon dioxide. Carbon monoxide can be removed from the gas by catalytic treatment with steam to produce carbon dioxide and hydrogen.

       2.6.1.3 High Btu Gas

      High Btu gas (heat-content gas) is essentially pure methane and often referred to as synthetic natural gas or substitute natural gas (SNG) (Kasem, 1979; c.f. Speight, 1990, 2013). However, to qualify as substitute natural gas, a product must contain at least 95% methane, giving an energy content (heat content) of synthetic natural gas on the order of 980 to 1080 Btu/ft³).

      The commonly accepted approach to the synthesis of high heat-content gas is the catalytic reaction of hydrogen and carbon monoxide:

      To avoid catalyst poisoning, the feed gases for this reaction must be quite pure and, therefore, impurities in the product are rare. The large quantities of water produced are removed by condensation and recirculated as very pure water through the gasification system. The hydrogen is usually present in slight excess to ensure that the toxic carbon monoxide is reacted; this small quantity of hydrogen will lower the heat content to a small degree.

      The carbon monoxide/hydrogen reaction is somewhat inefficient as a means of producing methane because the reaction liberates large quantities of heat. In addition, the methanation catalyst is troublesome and prone to poisoning by sulfur compounds and the decomposition of metals can destroy the catalyst. Hydrogasification may be thus employed to minimize the need for methanation:

      The product of hydrogasification is far from pure methane and additional methanation is required after hydrogen sulfide and other impurities are removed.

      Synthetic natural gas (SNG) is methane obtained from the reaction of carbon monoxide or carbon with hydrogen. Depending on the methane concentration, the heating value can be in the range of high-Btu gases.

       2.6.1.4 Synthesis Gas

      Synthesis gas is a mixture mainly of hydrogen and carbon monoxide which is comparable in its combustion efficiency to natural gas (Speight, 2008 Chapter 7). This reduces the emissions of sulfur, nitrogen oxides, and mercury, resulting in a much cleaner fuel (Nordstrand et al., 2008; Lee et al., 2006; Sondreal et al., 2004, 2006; Yang et al., 2007; Wang et al., 2008). The resulting hydrogen gas can be used for electricity generation or as a transport fuel. The gasification process also facilitates capture of carbon dioxide emissions from the combustion effluent (see discussion of carbon capture and storage below).

Although synthesis gas can be used as a stand-alone fuel, the energy density of synthesis gas is approximately half that of natural gas and is therefore mostly suited for the production of transportation fuels and other chemical products. Synthesis gas is mainly used as an intermediary building block for the final production of various fuels such as synthetic natural gas, methanol, and synthetic fuel (dimethyl ether – synthesized gasoline and diesel fuel) (Chadeesingh, 2011; Speight, 2013).

      The use of synthesis gas offers the opportunity to furnish a broad range of environmentally clean fuels and chemicals and there has been steady growth in the traditional uses of synthesis gas. Almost all hydrogen gas is manufactured from synthesis gas and there has been an increase in the demand for this basic chemical. In fact, the major use of synthesis gas is in the manufacture of hydrogen for a growing number of purposes, especially in crude oil refineries (Speight, 2014a, 2017). Methanol not only remains the second-largest consumer of synthesis gas but has shown remarkable growth as part of the methyl ethers used as octane enhancers in automotive fuels.

      The Fischer-Tropsch synthesis remains the third-largest consumer of synthesis gas, mostly for transportation fuels but also as a growing feedstock source for the manufacture of chemicals, including polymers. The hydroformylation of olefins (the Oxo reaction), a completely chemical use of synthesis gas, is the fourth-largest use of carbon monoxide and hydrogen mixtures. A direct application of synthesis gas as fuel (and eventually also for chemicals) that promises to increase is its use for integrated gasification combined cycle (IGCC) units for the generation of electricity (and also chemicals) from coal, crude oil coke or high-boiling (high-density) resids. Finally, synthesis gas is the principal source of carbon monoxide, which is used in an expanding list of carbonylation reactions, which are of major industrial interest.

      Since the synthesis gas is at high pressure and has a high concentration of carbon dioxide, a physical solvent, can be used to capture carbon dioxide (Speight, 2008, 2013), which is desorbed from the solvent by pressure reduction and the solvent is recycled into the system.

      2.6.2 Liquid Products

      The production of liquid fuels from coal via gasification is often referred to as the indirect liquefaction of coal (Speight, 2013). In these processes, coal is not converted directly into liquid products but involves a two-stage conversion operation in which coal is first converted (by reaction with steam and oxygen) to produce a gaseous mixture that is composed primarily of carbon monoxide and hydrogen (synthesis gas). The gas stream is subsequently purified (to remove sulfur, nitrogen, and any particulate matter) after which it is catalytically converted to a mixture of liquid hydrocarbon products.

      The synthesis of hydrocarbon derivatives from carbon monoxide and hydrogen (synthesis gas) (the Fischer-Tropsch synthesis) is a procedure for the indirect liquefaction of coal and other carbonaceous feedstocks (Starch et al., 1951; Batchelder, 1962; Dry, 1976; Anderson, 1984; Speight, 2011a, 2011b). This process is the only coal liquefaction scheme currently in use on a relatively large commercial scale; South Africa is currently using the Fischer-Tropsch process on a commercial scale in its SASOL complex (Singh, 1981).

Thus, coal is converted to gaseous products at temperatures in excess of 800^oC (1470^oF), and at moderate pressures, to produce synthesis gas:

      The gasification may be attained by means of any one of several processes or even by gasification of coal in place (underground, or in situ, gasification of coal).

      In practice, the Fischer-Tropsch reaction is carried out at temperatures of 200 to 350^oC (390 to 660^oF) and at pressures of 75 to 4000 psi. The hydrogen/carbon monoxide ratio is typically on the order of 2/2:1 or 2/5:1. Since up to three volumes of hydrogen may be required to achieve the next stage of the liquids production, the synthesis gas must then be converted by means of the water-gas shift reaction) to the desired level of hydrogen:

      After this, the gaseous mix is purified and converted to a wide variety of hydrocarbon derivatives:

      These reactions result primarily in low- and medium-boiling aliphatic compounds