Chapter 13 - Substitute Natural Gas and Fischer-Tropsch Synthesis

Abstract

This chapter presents the direct conversion of syngas to hydrocarbons, either substitute natural gas, or higher hydrocarbons via the Fischer–Tropsch synthesis. Substitute natural gas (SNG), also known as synthetic natural gas, is methane that is formed during gasification. The syngas generally contain carbon dioxide that can be hydrogenated to produce methane. Fischer–Tropsch (FT) synthesis is a polymerization reaction. An alkane chain, absorbed on a catalyst site, reacts with carbon monoxide and hydrogen to add a methyl unit (CH2). The heat of reaction per methyl group, estimated from the gas phase heat of formation for dodecane, is –155.5 kJ/mole. This heat of reaction is about 20% of the heat of combustion of the feed gases. FT diesel fuel, due to its high alkane content, has very high cetane numbers. Blends of FT and conventional diesel are sold as premium products in Europe. Normal alkanes, however, have high melting points compared to branched or cyclic hydrocarbons. An FT diesel produced simply by distilling an FT fluid would have poor low temperature properties. Most FT diesel, however, is not simply an FT fluid distillation product, but a blend of this distillation product and products of hydrocracked FT wax.