Renewable natural gas (RNG), also known as biomethane, is a renewable fuel made from biogas that has been upgraded to a quality similar to fossil natural gas and has a methane concentration of 90% or greater. By removing carbon dioxide and other impurities from biogas, the concentration of methane is high enough that it becomes possible to distribute RNG via existing gas pipeline infrastructure. RNG can be used in existing appliances, including vehicles with natural gas burning engines (natural gas vehicles). The most common way of collecting biogas with which to produce biomethane is through the process of anaerobic digestion. Anaerobic digestion facilities are either purpose built such as facilities that digest manure, household organic waste, or wastewater treatment plants. Biogas is also byproduct of the decomposition of organic materials in landfills. RNG can also be produced through the methanation of carbon dioxide/monoxide and hydrogen using either biomethanation, the Sabatier process or through electrochemical cells similar to fuel cells. These approaches can be used to methanate carbon dioxide from carbon capture facilities or synthetic gas (syngas) produced from the gasification of wood or other lignocellulosic materials. These approaches to producing RNG are still being developed and account for a small fraction of global production.
Production
Generation of biogas and syngas Most RNG is made from biogas produced via anaerobic digestion of organic waste. Biogas is produced as a byproduct of landfilling and wastewater treatment. At landfills, buried organic waste undergoes anaerobic digestion producing biogas that can be collected through a series of wells installed in the landfill. Wastewater treatment facilities often use anaerobic digestion tanks to degrade sludge from settling tanks. Wastewater treatment plants with anaerobic digesters often use biogas for process heat and flare excess biogas. Landfills in some jurisdictions are required to capture biogas to minimize methane leakage into the atmosphere, the captured gas is either flared, combusted to produce electricity, or upgraded into RNG. Anaerobic digesters can also be purpose-built facilities to produce RNG. Anaerobic digesters typically handle waste biomass from agricultural and municipal sources. Agricultural wastes include manure, spoiled products, and animal bedding. Municipal wastes include source separated organic wastes, fats, oils, and greases (FOG), and industrial byproducts. In some regions, non-waste feedstocks are also used. These feedstocks can be energy crops, such as maize and barley, or cover crops that are grown outside of the growing season to improve soil quality. In France, regulations limit the amount of energy crops that can be used for producing biogas, which has encouraged the use of cover crops. Depending on the availability of biomass feedstocks, the feedstock is processed in a dedicated facility or co-processes with other types of feedstock. Commercial anaerobic digesters typically make money through a combination of selling RNG, selling digestate, and by charging tipping fees for waste disposal. Costs are minimized by maximizing production scale and by locating an anaerobic digestion plant next to transport links (e.g. a port or highway) for the chosen source of biomass. Selecting a site close the gas transmission or distribution grid is also important to reduce interconnection costs. Gas storage is often required to deal with periods of low gas demand during the summer, depending on the region. RNG can also be produced via methanization of either carbon dioxide or syngas produced via thermal gasification. Both approaches are still largely pre-commercial and do not represent a major source of RNG production globally.
Upgrading to RNG Raw biogas is a mixture of primarily methane and carbon dioxide at a ratio of approximately 60:40. Biogas can also contain small amounts of oxygen and nitrogen pulled in from air and is typically saturated with water. All biogas sources contain hydrogen sulphide and biogas produced from waste water treatment or landfills will often contain siloxanes and other volatile organic components. Biogas upgrading refers to the removal of trace contaminants, water, and non-methane gases from biogas to produce a product that is interchangeable with natural gas. Biogas upgrading has two general steps - removal of contaminants and water, followed by gas separation. Contaminants and water are typically removed with sacrificial media beds containing activated carbon, activated alumina, and iron oxide. Contaminants can also be removed through biological scrubbing using sulphur oxidizing bacteria, chemical scrubbing with ammonia, or using regenerative process such as temperature swing adsorption. After contaminants and water have been removed, biogas undergoes gas separation to concentrate methane. There are multiple methods of gas separation used in biogas upgrading:
Membrane separation uses hollow fibres to separate gases using molecular size differences. Pressure swing adsorption separates gases based on their affinity for particular media such as zeolites or carbon molecular sieves. During the high pressure phase, one gas is trapped in the media and the product gas exits the vessel. The media is then regenerated with a vacuum to remove the other gas. Chemical washing uses either ammonia or water to dissolve carbon dioxide. The chemical is then regenerate through flashing or heating.
Compatibility with natural gas infrastructure RNG is generally considered to be interchangeable with natural gas. Many regions have minimum specifications for RNG quality that is deemed acceptable for local natural gas infrastructure. The major difference between RNG and natural gas is that RNG does not contain any hydrocarbons (such as propane) other than methane. This results in RNG having a lower heating value and Wobbe index than natural gas.
Commercial development
Overall As of 2023, more than 300 RNG facilities are currently operational in North America, with more than 70% of supplies drawn from the municipal solid waste and landfill sectors, according to the U.S. trade group RNG Coalition.
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