Substitute Natural Gas (SNG)
Substitute Natural Gas (SNG) is nothing more than methane, which is the main component of natural gas. Methane usually accounts for more than 90% of natural gas.
SNG is often referred to as “Synthetic Natural Gas”, in which the association of “Synthetic” and “Natural” may seem incongruous. "Synthetic Methane" could be a more appropriate name.
SNG can be produced from any hydrocarbons such as coal and biomass.
Three processes are commonly mentioned in literature: steam-oxygen gasification, catalytic steam gasification and hydrogasification. As steam-oxygen gasification is the only process being operated and studied in projects, it is the most developed below.
1) Steam-oxygen gasification
Coal is first gasified with steam and oxygen to produce a synthetic gas or “syngas”, composed of carbon monoxide (CO), hydrogen (H2) and to a lesser extent carbon dioxide (CO2) and methane (CH4) as well as impurities.
The [H2]/[CO] ratio in the syngas needs to be increased to /, which is achieved in a “water-gas shift” reaction, where CO and water are converted to CO2 and H2. Syngas is then cleaned to eliminate dust, tar and acid gases.
The last step is methanation, where the syngas is purified and converted to methane and water in a fixed-bed catalytic reactor.
2) Catalytic steam gasification
This process is under development.
Gasification and methanation occur in the same reactor in the presence of a catalyst.
Advantages are that there is no requirement for pure oxygen (no air separation unit), a thermal compensation occurs between exothermic (methanation) and endothermic (gasification) reactions and lower temperatures are required. Disadvantages are lack of experience, separation of catalyst from solids and the loss of catalyst reactivity.
This process is in the research stage.
Coal is gasified with H2, meaning that a source of H2 is necessary. Gasification can play this role.
Advantages and disadvantages are similar to catalytic steam gasification.
Use of biomass
SNG can be produced from any hydrocarbon by steam-oxygen gasification, including biomass. It is then called “bio-SNG”.
The major advantage is the environmental footprint, as biomass is carbon-neutral, and can generate negative carbon emissions if emitted CO2 is sequestrated.
Disadvantages are the chemical nature of biomass, with lower calorific value, higher moisture content and tar formation, and seasonal supply variations.
Combining coal and biomass presents major advantages in terms of reaction temperature, regular supply and environmental footprint.
Operations and projects
Today, SNG is produced from coal by Dakota Gasification Company in Beulah, North Dakota. The plant uses 6 million tons of lignite coal annually and produces more than 54 billion cubic feet of SNG.
Several plants are in project or in construction in China, South Korea and the USA.