Renewable natural gas (RNG) is methane captured from decomposing organic waste, then cleaned and processed until it’s chemically identical to the fossil natural gas that flows through pipelines and powers homes. Once upgraded, RNG typically contains 96 to 98 percent methane, making it a direct substitute for conventional natural gas in heating, electricity generation, and vehicle fuel. The key difference is where it comes from: fossil natural gas formed underground over millions of years, while RNG is produced continuously from waste that would otherwise release methane into the atmosphere.
Where RNG Comes From
RNG starts as biogas, a mix of methane and carbon dioxide that forms whenever organic material breaks down without oxygen. This happens naturally in several settings, and each one represents a potential RNG source.
Landfills are among the largest producers. As household trash decomposes underground, it generates a steady stream of biogas that can be collected rather than vented or flared. Wastewater treatment plants produce biogas as a byproduct of processing sewage. Dairy farms and livestock operations generate it from manure, often using enclosed tanks called anaerobic digesters. Food processing facilities, paper mills, and breweries also produce biogas during waste treatment.
Beyond these biological sources, there’s a second production pathway: thermal gasification. This process converts dry, woody biomass (think forestry residues, crop stalks, and wood waste) into a gas mixture called syngas, which is then chemically converted into methane. Gasification opens up a much broader range of plant-based materials that aren’t wet enough for anaerobic digestion. When biomass is available cheaply, gasification-produced RNG can potentially compete with the market price of fossil natural gas.
How Biogas Becomes Pipeline-Quality Fuel
Raw biogas is far from ready for a gas pipeline. It contains only 45 to 65 percent methane, with the rest being mostly carbon dioxide along with traces of hydrogen sulfide, ammonia, moisture, and other impurities. Upgrading it to pipeline quality requires several cleaning steps.
The biggest task is removing carbon dioxide, which dilutes the gas and lowers its energy content. One common method is chemical absorption: the raw biogas passes through a liquid solution (often containing sodium hydroxide or potassium hydroxide) that chemically binds to the carbon dioxide and pulls it out. Other facilities use membrane filtration or pressure-based separation to achieve the same result. The goal is to push methane concentration above 95 percent.
Hydrogen sulfide, which is corrosive and toxic even in small amounts, also has to go. So does moisture, since water accumulation inside a pipeline causes corrosion and operational problems. Siloxanes, compounds that originate from products like shampoo and detergent breaking down in landfills, are filtered out as well because they can damage equipment. By the end of this process, what remains is nearly pure methane, indistinguishable from fossil natural gas in how it performs.
Pipeline Injection and Infrastructure
One of RNG’s biggest practical advantages is that it slots directly into existing natural gas infrastructure. Once upgraded, it can be injected into the same transmission and distribution pipelines that carry fossil gas to homes, businesses, and power plants. No new appliances, no equipment swaps, no retrofitting.
That said, gas utilities set strict quality standards for anything entering their pipelines. Methane content must generally fall between 81 and 99 percent, though biomethane-specific tariffs often require a minimum of 94 percent. Carbon dioxide is capped at around 2 to 3 percent by volume for most utilities. Total sulfur and hydrogen sulfide levels are limited to trace amounts, typically single-digit parts per million. Oxygen and nitrogen are also tightly controlled. RNG producers must continuously monitor their output to meet these thresholds before injection is allowed.
For operations where pipeline access isn’t practical, RNG can also be used on-site (a dairy farm powering its own equipment, for example) or piped through a dedicated line to a nearby end user.
RNG as a Transportation Fuel
The transportation sector has become one of the fastest-growing markets for RNG, particularly for heavy-duty trucking. RNG is compressed or liquefied and used in the same engines that run on conventional compressed natural gas (CNG) or liquefied natural gas (LNG). In California, which tracks this closely, 97 percent of natural gas used by commercial vehicles in 2023 was RNG.
For long-haul fleets, natural gas engines are now delivering performance comparable to diesel, which has made the switch more appealing. Some dairy farms and waste facilities that produce their own RNG have started partnering directly with trucking companies, creating a closed loop where waste becomes fuel. The existing CNG and LNG refueling infrastructure gives RNG a logistical advantage over electric trucks in many regions, particularly where grid capacity limits the pace of electrification.
Compared to diesel or gasoline, RNG significantly reduces emissions of nitrogen oxides and particulate matter, the pollutants most responsible for smog and respiratory problems near highways and freight corridors.
How RNG Differs From Fossil Natural Gas
Chemically, RNG and fossil natural gas are close relatives, but not identical twins. Fossil natural gas contains meaningful amounts of ethane, propane, butane, and other heavier hydrocarbons. RNG contains zero to very low levels of these compounds. In practice, this difference rarely matters for end users since both fuels burn the same way in furnaces, stoves, and engines.
The more significant difference is carbon intensity. Fossil natural gas releases carbon that was locked underground for millions of years, adding new carbon dioxide to the atmosphere. RNG releases carbon that was already part of the surface carbon cycle, captured from plants and organic waste within recent years. This makes its net carbon footprint substantially lower. Carbon intensity scores for RNG range from about 5 to 50 grams of CO2 equivalent per megajoule of energy, depending on the feedstock. Dairy manure projects tend to score at the low end because they also prevent methane (a potent greenhouse gas) from escaping into the air. Wastewater treatment plant RNG falls around 42.5 grams per megajoule, still well below fossil fuels.
Policy and Incentives
In the United States, RNG qualifies as a renewable fuel under the EPA’s Renewable Fuel Standard (RFS). This program assigns tradeable credits called Renewable Identification Numbers (RINs) to qualifying fuels. RNG from landfills, agricultural digesters, wastewater treatment digesters, and separated municipal waste digesters can earn D-code 3 classification, which designates it as a cellulosic biofuel, the highest-value credit category. RNG from other waste digesters earns D-code 5, classified as an advanced biofuel. These credits create a financial incentive that helps offset the higher production costs of RNG compared to fossil gas.
State-level programs add further support. California’s Low Carbon Fuel Standard, for instance, rewards fuels based on how much they reduce carbon intensity compared to petroleum, making dairy-derived RNG particularly valuable because of its deeply negative carbon scores.
Current Limitations
RNG is not a limitless resource. The total amount of organic waste available constrains how much can be produced, and it currently represents a small fraction of overall natural gas supply. Production costs remain higher than drilling for fossil gas, though policy incentives narrow the gap. Upgrading equipment is capital-intensive, and smaller farms or waste facilities may struggle to justify the investment without financial support.
There’s also an ongoing debate about whether RNG should be viewed as a long-term climate solution or a transitional fuel. It reduces emissions compared to fossil gas and diesel, but it still produces carbon dioxide when burned. Its strongest environmental case comes from projects that capture methane emissions that would have occurred anyway, like open manure lagoons on dairy farms or uncontrolled landfill gas venting, turning a climate liability into a usable fuel.