Synthetic Natural Gas: Coal Gasification, Methanation, and Pipeline-Grade SNG Specifications

Synthetic natural gas, abbreviated SNG and sometimes called substitute natural gas, is a manufactured methane-rich gas produced by converting a solid or heavy hydrocarbon feedstock, most commonly coal but also petroleum coke, heavy oil residue, or biomass, into a gas that is interchangeable with conventional pipeline natural gas. The purpose of SNG is to turn an abundant but hard-to-transport or lower-value carbon source into clean, high-heating-value methane that can move through existing natural gas pipelines, feed power plants, or supply industrial users without any modification to downstream equipment. The dominant route is coal gasification followed by methanation, a two-step thermochemical process. In the first step, coal is reacted with oxygen and steam at high temperature and pressure in a gasifier, producing synthesis gas, or syngas, a mixture rich in carbon monoxide and hydrogen along with carbon dioxide, methane, and impurities. Gasifier designs include fixed-bed, fluidized-bed, and entrained-flow types, and the choice depends heavily on coal rank, ash content, and moisture. The raw syngas is then cooled and cleaned to remove particulates, sulfur compounds, and other contaminants, and its hydrogen-to-carbon-monoxide ratio is adjusted through the water-gas shift reaction. In the second step, the conditioned syngas passes over a nickel catalyst in a series of methanation reactors, where carbon monoxide and hydrogen react exothermically to form methane and water. The methanated product is dried and compressed to meet pipeline specifications, typically a heating value near 37 to 39 megajoules per cubic metre, roughly 1,000 British thermal units per standard cubic foot, with tight limits on carbon dioxide, water, and sulfur. Heating heavy hydrocarbons or refining residues can also yield SNG through similar reforming and methanation chemistry. In the Western Canadian Sedimentary Basin (WCSB) context, SNG has historically been of interest because Alberta holds vast subbituminous coal reserves and a mature natural gas pipeline network, and because coal gasification pairs naturally with carbon capture, since the carbon dioxide stream from gasification and shift is concentrated and comparatively easy to capture and sequester. That linkage between SNG and carbon capture and storage is central to any modern economic case for the technology, given both the capital intensity of gasification plants and the carbon pricing regime under which any WCSB facility would operate. SNG is measured and reported in the same units as conventional gas, in e3m3 and Bcf, and is contractually indistinguishable from wellhead gas once it meets tariff specification and enters the grid.

Gasifier Selection and Syngas Cleanup

The gasifier is the heart of an SNG plant, and its design is dictated by the feedstock. Fixed-bed gasifiers suit lump coal and produce syngas with more methane and tars; fluidized-bed units handle reactive low-rank coals and biomass well; entrained-flow gasifiers run pulverized coal at very high temperature and produce a clean, tar-free syngas but demand more oxygen. After gasification, the raw syngas must be cooled and scrubbed to remove particulates, then treated with an acid-gas removal system to strip hydrogen sulfide and carbon dioxide before methanation, because sulfur poisons the nickel methanation catalyst. For a hypothetical Alberta plant fed subbituminous Ardley coal, an oxygen-blown fluidized-bed gasifier would balance the coal's high moisture against acceptable oxygen consumption.

Methanation Chemistry and Heat Management

Methanation is strongly exothermic, releasing large amounts of heat as carbon monoxide and hydrogen combine into methane. Managing that heat is the central engineering challenge, because excessive temperature both damages the nickel catalyst and shifts equilibrium away from methane. Commercial plants use a series of adiabatic methanation reactors with intercooling, recycling part of the product gas to dilute the feed and limit temperature rise, and recover the reaction heat as high-pressure steam that drives the plant's own compressors and oxygen plant. The final methanation stages run cooler to push conversion toward completion, yielding a gas that after drying and carbon dioxide polishing meets the roughly 1,000 Btu/scf pipeline heating value target.

Related Terms

Synthetic natural gas connects to natural gas, the conventional product it substitutes for and must match in specification, and to carbon capture and storage, since the concentrated carbon dioxide from gasification is what makes SNG's emissions manageable. It relates to heating value, the pipeline-tariff property SNG must hit to be interchangeable, and to enhanced oil recovery, because captured gasification carbon dioxide has historically been sold to WCSB and Williston Basin floods.

WCSB Field Scenario: Captured CO2 to a Saskatchewan Flood

A Saskatchewan enhanced oil recovery operator running a miscible carbon dioxide flood in a Weyburn-area Midale carbonate reservoir sourced a large share of its injection gas from captured SNG-plant carbon dioxide delivered by pipeline from a lignite gasification facility across the border. The concentrated carbon dioxide stream, a byproduct of turning coal into pipeline methane, was compressed to injection pressure and pumped into the reservoir to swell the remaining oil and reduce its viscosity, boosting recovery beyond what water flooding alone achieved.

The arrangement illustrates why SNG and carbon capture are economically linked in the WCSB region. The gasification plant gained a revenue stream and an emissions solution by selling its carbon dioxide, while the oilfield secured a reliable, competitively priced injectant. Weyburn-area floods have added tens of millions of incremental barrels over their lives on the strength of that supply.