Substitute Natural Gas, which is interchangeable with Natural Gas can be manufactured from other fossil fuels by combining three main reaction stages; the gasification of the feedstock with steam, or a mixture of steam and oxygen, to produce a gas from which Methane can be synthesized from the carbon oxides and hydrogen, and the removal of carbon dioxide.
A number of large plants were built in the United States and Japan in the early 1970s to make SNG from Sight distillate oils to meet predicted shortages in natural gas supplies. The process used was the CRG (Catalytic Rich Gas) process developed by British Gas. In this process, the distillate oil is first hydro-desulfurized and it is then gasified catalytically with steam in an adiabatic reactor to produce a gas containing about 64% methane, 17% hydrogen and 21% carbon dioxide, by volume, on a dry basis. The gas leaving this reactor at 500°C is cooled to 300°C and passed through an adiabatic catalytic reactor, or methanator, in which equilibrium is attained in the following reactions:
in which a low temperature favors methane formation. After cooling the gas and passing it through a second methanator, it contains 77% methane and 22% carbon dioxide. After removal of most of the carbon dioxide, the gas contains more than 99% methane.
The application of this process was restricted by the availability of light distillate oils and, in the 1970s, work started on the development of a number of different processes for the gasification of coal. All of the coal gasification processes that use steam and oxygen produce gas containing carbon oxides and hydrogen and they are potentially capable of making SNG. However, the methane synthesis reaction, Equation (2), is highly exothermic and there are large efficiency and cost advantages in using a process, such as the Lurgi process that makes methane during the gasification of the coal.
The predicted shortages and high costs of natural gas did not materialize and the only coal based SNG plant that now operates is in North Dakota. Commissioned in 1984, the plant produces 3.9 × 106 m3/day of SNG from lignite using 14 Lurgi dry-ash gasifiers. The crude gas leaving the gasifiers contains 12% methane, 16% carbon monoxide, 39% hydrogen and 32% carbon dioxide, by volume on a dry basis. The gas is cooled, tars and oils are separated and it is washed with methanol at -40°c to remove carbon dioxide and sulfur compounds. The purified gases then passes through a series of methanation reactors to bring the gas to equilibrium at progressively lower temperatures to raise the methane content; gas recirculation is used to moderate the temperatures in the first methanators and the final methanator is water cooled. The final gas contains 96% methane.
Davis, H. S., Lacey, J. A., and Thompson, B. H. (1968) Processes for the Manufacture of Natural Gas Substitutes, The Institute of Gas Engineers, CC155.
Elliott, M. A. (1981) The Chemistry of Coal Utilisation, Ch. 24, Wiley Interscience.