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 800oC (1470oF), 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 350oC (390 to 660oF) 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 suitable for gasoline and diesel fuel.
2.6.3 Tar
Another key contribution to efficient gasifier operation is the need for a tar reformer. Tar reforming occurs when water vapor in the incoming synthesis gas is heated to a sufficient temperature to cause steam reforming in the gas conditioning reactor, converting condensable hydrocarbon derivatives (tars) to non-condensable lower molecular weight molecules. The residence time in the conditioning reactor is sufficient to also allow a water gas shift reaction to occur and generate increased amounts of hydrogen in the synthesis gas.
Thus, tar reforming technologies – which can be thermally driven and/or catalytically driven – are utilized to break down or decompose tars and high-boiling hydrocarbon products into hydrogen and carbon monoxide. This reaction increases the hydrogen-to-carbon monoxide (H2/CO) ratio of the synthesis gas and reduces or eliminates tar condensation in downstream process equipment. Thermal tar reformer designs are typically fluid-bed or fixed-bed type. Catalytic tar reformers are filled with heated loose catalyst material or catalyst block material and can be fixed or fluid bed designs.
Typically, the tar reformer is a refractory lined steel vessel equipped with catalyst blocks, which may contain a noble metal or a nickel-enhanced material. Synthesis gas is routed to the top of the vessel and flows down through the catalyst blocks. Oxygen and steam are added to the tar reformer at several locations along the flow path to enhance the synthesis gas composition and achieve optimum performance in the reformer. The tar reformer utilizes a catalyst to decompose tars and high boiling hydrocarbon derivatives into hydrogen and carbon monoxide. Without this decomposition the tars and high boiling hydrocarbon derivatives in the synthesis gas will condense as the synthesis gas is cooled in the downstream process equipment. In addition, the tar reformer increases the hydrogen/carbon monoxide ratio for optimal conversion. The synthesis gas is routed from the tar reformer to downstream heat recovery and gas cleanup unit operations.
References
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