Synthetic Natural Gas (SNG) is a manufactured fuel. This gas is chemically engineered to mimic the properties of its underground-sourced counterpart, making it compatible with existing energy infrastructure. SNG is produced by converting various carbon-containing feedstocks or by using renewable electricity to create a gaseous fuel. The resulting product is almost chemically pure methane (CH4), which is the primary energy carrier in all forms of natural gas.
The Chemical Composition of SNG
Synthetic Natural Gas is defined by its high concentration of methane (CH4), the simplest hydrocarbon molecule. To be considered pipeline-grade, SNG must achieve a methane purity exceeding 95%, with some specifications demanding even higher levels. This high degree of purity is necessary to ensure the manufactured gas has a consistent and predictable heating value.
This includes maintaining a specific energy content, generally falling within a narrow range of 950 to 1,150 British thermal units per standard cubic foot. Conventional natural gas, in contrast, often contains small amounts of heavier hydrocarbons like ethane and propane, along with inert components such as nitrogen and carbon dioxide.
SNG production processes also meticulously remove trace contaminants like hydrogen sulfide and excess water vapor. These impurities must be minimized because they can be corrosive to pipeline infrastructure and pose safety risks. The standardized, high-purity composition of SNG ensures it can be safely and efficiently delivered and combusted in appliances designed for fossil natural gas without any modification.
Converting Feedstocks into SNG
The production of SNG follows two main chemical pathways, both focused on synthesizing the CH4 molecule. The first pathway involves the thermal or catalytic conversion of solid or liquid carbon-based feedstocks. Materials like coal, biomass, or municipal waste are first subjected to gasification or pyrolysis at high temperatures to produce a mixture known as syngas.
Syngas is primarily composed of hydrogen (H2) and carbon monoxide (CO). This syngas must be cleaned to remove contaminants like sulfur compounds, which can poison the catalysts used in the next step. After cleaning, the syngas undergoes a catalytic process called methanation, which takes place over a nickel-based catalyst.
The core of this conversion is an exothermic reaction where carbon monoxide and hydrogen react to form methane and water: CO + 3H2 \(\to\) CH4 + H2O. Another reaction converts any remaining carbon dioxide in the syngas, known as the Sabatier reaction: CO2 + 4H2 \(\to\) CH4 + 2H2O. Since these reactions release significant heat, the process design must incorporate cooling systems to manage the temperature and protect the catalyst.
The second key pathway is Power-to-Gas (P2G), which links the electricity grid to the gas grid. This method begins with using surplus renewable electricity from sources like solar or wind to power an electrolyzer, splitting water into hydrogen and oxygen. This generated hydrogen is then combined with captured carbon dioxide, often sourced from industrial emissions or biomass processes.
The mixture of H2 and CO2 is fed into a methanation reactor, again utilizing the Sabatier reaction to produce CH4. P2G technology allows for the creation of a synthetic fuel that is potentially carbon-neutral, as the carbon used in the final methane molecule originates from a captured source rather than being newly extracted from the earth.
Utility and Integration into Energy Grids
SNG is a “drop-in” fuel, meaning it can be injected directly into the established network of natural gas transmission and distribution pipelines. The ability to utilize the existing gas grid provides a massive advantage in terms of delivery and storage capacity.
SNG plays a strategic role in energy storage, particularly in managing the intermittent nature of renewable energy sources. The P2G pathway converts electrical energy into chemical energy, which can be stored in the massive volume of the gas grid for long durations. This process is suitable for seasonal energy storage, allowing energy captured during times of peak renewable generation to be saved and used months later during periods of low generation or high demand.
As a synthetic product, SNG is an important option for decarbonizing the energy system. When produced using renewable electricity and captured CO2, the resulting SNG can be a carbon-neutral fuel, offsetting the need for fossil fuels in hard-to-abate sectors like industry and heating.