Renewable Natural Gas 101
What is renewable natural gas?
Renewable natural gas (RNG) is a waste–derived fuel. As waste such as existing food waste, animal manure, wastewater sludge and garbage breaks down, it emits methane, which is a potent, naturally occurring harmful greenhouse gas (GHG). RNG projects aim to capture this methane and redirect it away from the environment, repurposing it as an energy source.
The production and use of RNG is growing rapidly in the United States. In the last five years, the number of production facilities has grown about threefold, with about 115 facilities making the fuel, which can be used interchangeably with natural gas. Minnesota’s first RNG facility became operational just this year.
What are the sources of RNG?
Common sources of renewable natural gas include landfills, animal manure, food scraps and wastewater sludge. Through anaerobic digestion, bacteria breaks down this organic matter and produce methane, carbon dioxide and other gases and solids. The resulting biogas is then processed or “upgraded” to get rid of impurities, so that it is nearly pure methane.
Landfills account for over 90 percent of renewable natural gas production, and are often more economical relative to other waste streams because many already have methane collection systems and tend to produce more gas in a concentrated area.
Other potential sources of RNG like animal manure and food waste, have become growing sources of renewable natural gas, but are not as cost-effective to produce and will likely require policy support or incentive to grow.
RNG is still an emerging technology. Researchers are working to find a cost-effective way to derive RNG from “dry” organic wastes from things like corn plants after harvest, grass clippings, tree trimmings and nut shells. These products have significant potential, but are not significant sources of RNG today.
How is RNG used?
Today, RNG is used most often as a fuel in transportation, especially as a substitute for diesel in heavy-duty vehicles. This trend is likely attributed to certain mandates like the federal Renewable Fuel Standard, which credit the environmental attributes of renewable natural gas production. Minnesota has also enacted a clean fuels standard and the 2021 Natural Gas Innovation Act which both bolster efforts to get this new technology in play across the state.
RNG is directly interchangeable with natural gas and therefore has a wide array of applications. There has been a growing interest in its potential as a heat source in existing buildings or in industrial applications.
What are the concerns about RNG?
Renewable natural gas is mainly composed of methane, which emits carbon dioxide when burned as fuel. The release of this carbon is typically offset by other benefits throughout the production chain. However, any methane leaks along the RNG supply chain prior to combustion risks undermining potential benefits because methane is a greenhouse gas 84-86 times more powerful than carbon dioxide.
Concerns about renewable natural gas often include the risk of expanding or locking in fossil fuel infrastructure that enables more fossil fuel use. This is because RNG uses some of the same infrastructure. There are also concerns that alternative uses for certain organic wastes may be more beneficial than their use in RNG. Researchers advise evaluating RNG on a case by case basis to minimize any risk of projects doing more harm than good.
Can the US produce a meaningful amount of RNG?
The United States generates millions of tons of food scraps, sewage, oils and greases, livestock manure and other waste each year, but these waste streams are finite, as is the amount of renewable natural gas that may be produced from them as the country is already using many of these waste streams. For example, wastewater facilities often burn biogas in a generator to supply heat and power to the facility, and food waste may be diverted to composting or animal feed. Currently, estimates suggest that about 4-7 percent of current natural gas consumption in the US could be produced as RNG.
What role can RNG play in a clean energy future?
On its own, RNG cannot replace enough fossil fuel use to decarbonize any one sector of the economy. For this reason, some of the criticism of the source is in its potential to distract from developing other technologies with larger abilities to do so. Nonetheless, renewable natural gas can be a piece of the puzzle to the extent that it results in a net reduction in methane emissions, and it displaces fossil fuel use in otherwise stubborn sectors.
The ability to use RNG in existing infrastructure could make it a cost-effective option in the near term even as electrification efforts move forward. Industries requiring high-density, combustion-based fuel, such as heavy-duty transportation and industrial heating, may be particularly good candidates for renewable natural gas. These considerations can help decision makers identify opportunities for renewable natural gas to contribute to long-term decarbonization.
Why haven’t we deployed more renewable natural gas?
Renewable natural gas faces many barriers — including cost; feedstock availability; and regulatory, operational and market risks. Installing equipment and connecting pipelines can cost projects millions of dollars up front.
Feedstock availability is affected by the seasonal variability of waste production and competing uses for the products. The ever-changing political environment also impacts emerging technologies as they are subject to defunding of programs or the repeal of legislation with changing parties. There are also operational risks – like inconsistency in the specific composition of waste in feedstocks (from landfills, for example) which can impact the production equipment.
Finally, there is a large market risk with RNG because of fluctuating prices which leads to uncertain revenue from RNG sales. From 2016 to 2019 credit prices under the federal Renewable Fuel Standard fell by two-thirds and natural gas prices fell by one-third.