Renewable natural gas (RNG), also called biomethane, is a type of clean fuel made from biogas. This biogas is cleaned to be like natural gas, with at least 90% methane. By removing carbon dioxide and other unwanted materials from biogas, the methane level becomes high enough to move through existing gas pipelines. RNG can be used in the same devices that use natural gas, such as vehicles powered by natural gas engines.

The most common way to collect biogas for RNG is through a process called anaerobic digestion. This process happens in special facilities that break down materials like animal waste, household food scraps, or wastewater. Biogas is also created when organic waste in landfills breaks down over time.

RNG can also be made by combining carbon dioxide and hydrogen using methods like biomethanation, the Sabatier process, or electrochemical cells similar to fuel cells. These methods can use carbon dioxide from capture systems or syngas made from burning wood or other plant-based materials. These production methods are still being tested and currently make up a small part of global RNG production.

Production

Most RNG is made from biogas created through a process called anaerobic digestion, which breaks down organic waste without oxygen. Biogas is made as a byproduct of landfills and wastewater treatment. In landfills, organic waste that is buried breaks down through anaerobic digestion, producing biogas that can be collected using wells inside the landfill. Wastewater treatment plants often use tanks to break down sludge from settling tanks through anaerobic digestion. These plants may use biogas for heat or burn extra biogas. In some areas, landfills are required to capture biogas to reduce methane escaping into the air. The captured gas is either burned, used to make electricity, or upgraded into RNG.

Anaerobic digesters can also be built specifically to produce RNG. These digesters usually process waste from farms and cities. Farm waste includes manure, spoiled food, and materials used for animal bedding. City waste includes organic materials separated from trash, fats, oils, and greases, and industrial byproducts. In some places, other materials like energy crops (such as corn or barley) or cover crops (grown to improve soil quality) are used. In France, rules limit the use of energy crops for biogas, which has led to more use of cover crops. Depending on the availability of waste, the material is processed in a facility alone or mixed with other materials.

Commercial anaerobic digesters earn money by selling RNG, selling digestate (a nutrient-rich material from digestion), and charging fees for waste disposal. Costs are reduced by making the facility large and placing it near transportation routes, like highways or ports, for the chosen waste source. Choosing a location close to gas pipelines also helps lower connection costs. Gas storage is often needed to manage low gas demand during summer, depending on the region.

RNG can also be made by converting carbon dioxide or syngas (a gas made through thermal gasification) into methane. These methods are not widely used yet and are still being developed.

Raw biogas is mostly methane and carbon dioxide, usually in a 60:40 ratio. It may also contain small amounts of oxygen and nitrogen from the air and is often full of water. All biogas sources have hydrogen sulfide, and biogas from landfills or wastewater treatment may also contain siloxanes and other chemicals.

Biogas upgrading is the process of removing impurities, water, and non-methane gases to make a product similar to natural gas. Upgrading has two steps: first, removing impurities and water, and then separating the gas. Impurities and water are often removed using materials like activated carbon, activated alumina, or iron oxide. Impurities can also be removed through biological methods using bacteria, chemical methods using ammonia, or regenerative methods like temperature swing adsorption.

After impurities and water are removed, biogas is separated to increase methane concentration. Methods for gas separation include:

• Membrane separation, which uses hollow fibers to separate gases based on their size.
• Pressure swing adsorption, which uses materials like zeolites or carbon sieves to trap gases. During high pressure, one gas is trapped, and the product gas exits. The material is then cleaned using a vacuum.
• Chemical washing, which uses ammonia or water to dissolve carbon dioxide. The chemical is then reused through heating or flashing.

Compatibility with natural gas infrastructure

Renewable natural gas (RNG) is often used similarly to natural gas. Many areas have basic quality rules that determine if RNG is suitable for local natural gas systems. The main difference between RNG and natural gas is that RNG contains only methane as a hydrocarbon, unlike natural gas, which may include other hydrocarbons like propane. This means RNG has a lower heating value and a lower Wobbe index compared to natural gas.

Commercial development

As of 2023, more than 300 renewable natural gas (RNG) facilities are currently working in North America. Over 70% of the gas used in these facilities comes from municipal solid waste and landfills, according to the U.S. trade group RNG Coalition.

Most RNG projects in North America began as ways to use gas from landfills. The first commercial RNG facility started at the Fresh Kills landfill near New York City in 1982. Until 2021, landfill gas projects were the main source of RNG in the United States. After 2021, agricultural projects became the most common type of RNG production. As of 2023, there are 102 landfill RNG projects in the United States.

During wastewater treatment, sludge is settled and can be broken down in a machine called an anaerobic digester. These digesters are widely used because they create biogas, which can be used to produce heat for wastewater treatment. Many wastewater treatment plants create more biogas than they need. This extra gas is either burned to remove methane, used to create electricity and heat, or upgraded to RNG.

Anaerobic digesters can also be used on farms. The main material for these digesters is manure, which provides microorganisms and a steady supply of liquid waste. Farmers often add other materials, like spoiled food or animal bedding, to the digesters. Some agricultural projects may use waste from outside the farm. These digesters break down waste, create liquid fertilizer, and help farmers earn extra money.

Municipal anaerobic digesters are facilities that use organic waste collected from green bin programs. These digesters differ from agricultural ones because the waste they process has more solid material than manure. Before processing, the waste is cleaned to remove items like plastic and glass. Cities use these digesters to reduce organic waste sent to landfills and lower methane emissions.

Syngas is a mix of methane, hydrogen, carbon dioxide, and carbon monoxide. Like biogas, syngas can be burned directly or changed into RNG or hydrogen. The process begins with creating syngas from materials like wood through gasification or pyrolysis. The syngas is then cleaned to remove harmful substances like hydrogen sulfide and tar. To improve syngas, the balance of hydrogen and carbon monoxide is adjusted using a chemical reaction called the water gas shift. When the syngas reaches the right balance, it is turned into methane using the Sabatier reaction. Carbon dioxide is then removed using methods similar to those used for upgrading biogas.

Renewable natural gas plants that use wood can be divided into two types. One type, called allothermal, gets heat from an external source, such as a double-chambered fluidized bed gasifier, which has separate areas for burning and gasifying materials. Another type, called autothermal, creates heat inside the gasifier but requires pure oxygen to avoid mixing in nitrogen.

A key benefit of making RNG from wood waste is that it is more efficient than producing liquid fuels using the Fisher-Tropsch method and can be done on a smaller scale than other second-generation biofuel systems. The Energy Research Centre of the Netherlands has studied large-scale RNG production from wood, often using wood imported from other countries.

Growth Outlook

Renewable natural gas can be made and sent through the current gas grid. This makes it a good way to provide renewable heat and gas energy to existing buildings. Renewable natural gas can also be changed into liquefied natural gas (LNG) or compressed natural gas (CNG) for use as fuel in vehicles.

In the United States, different studies have estimated how much renewable natural gas (RNG) could be produced. A 2011 study by the Gas Technology Institute found that RNG from waste materials like agricultural waste could add up to 2.5 quadrillion Btu each year. This amount could meet the natural gas needs of about half of American homes. Another study by the Environmental and Energy Study Institute suggested that RNG could replace up to 10% of all natural gas used in the United States. A third study by the National Association of Clean Water Agencies and the Water Environment Federation found that waste from wastewater treatment could create enough biogas to meet up to 12% of the country’s electricity needs.

A 2019 study by ICF, commissioned by the American Gas Foundation, predicted that between 1.6 and 3.78 trillion cubic feet of RNG could be produced annually for use in pipelines in the United States by 2030.

When combined with power-to-gas, a process that changes carbon dioxide and carbon monoxide from biogas into methane using hydrogen from water, the energy potential of raw biogas is about doubled.

In the United Kingdom, anaerobic digestion is growing as a way to make renewable biogas. Nearly 90 sites now inject biomethane into the national gas grid. Ecotricity plans to provide green gas to UK homes through the national grid. Centrica also plans to inject gas made from sewage into the gas grid.

In Canada, FortisBC, a gas company in British Columbia, adds naturally produced gas to its existing gas system. A company named Divert, which also helps reduce food waste by donating food, will use a $1 billion investment from Enbridge to expand its network of anaerobic digesters for food waste across North America.

In Sweden, Göteborg Energi started the first large-scale plant to produce bio-synthetic natural gas (SNG) from forest waste as part of the GoBiGas project. The plant could make 20 megawatts of bioSNG from about 30 megawatts of biomass, aiming for 65% efficiency. It operated fully from December 2014 and supplied gas to the Swedish grid, meeting quality standards with over 95% methane content. The plant closed permanently in April 2018 due to financial issues. Göteborg Energi had invested 175 million euros in the project, but attempts to sell it to new investors failed.

Environmental concerns

Biogas produces similar harmful substances as natural gas, including carbon monoxide, sulfur dioxide, nitrogen oxide, hydrogen sulfide, and tiny particles. If gas is not burned completely and escapes into the air, it releases methane, a powerful greenhouse gas that stays in the atmosphere for a long time. Unlike fossil natural gas, biogas is often seen as partly or fully carbon neutral because the carbon dioxide in the biomass comes from plants that naturally absorb carbon dioxide from the air, rather than from ancient fossil fuels that release stored carbon dioxide into the atmosphere.

A big challenge is that the amount of biogas that can be made is much smaller than the current supply of fossil gas (also called natural gas). This has caused companies that sell natural gas to oppose efforts to use more electricity instead of gas, which would reduce the need for gas. Because of this, Southern California Gas Company (SoCalGas) secretly helped create a nonprofit group called Californians for Balanced Energy Solutions (C4Bes). This group then worked to support the gas industry and fight efforts to switch to electricity. The Sierra Club discovered that SoCalGas was involved in forming C4Bes (a practice called astroturfing), which led C4Bes to stop its lobbying efforts, though it still promoted the use of gas.