Methane is removed from the environment naturally by a chemical reaction called oxidation, which primarily produces carbon dioxide (CO₂) and water (H₂O). The primary methane sink is atmospheric oxidation by the hydroxyl radical (OH), with additional oxidation occurring atmospherically through chlorine, and in soils by methanotrophic bacteria.
Multiple categories of methane removal techniques are currently deployed or being researched for oxidation of concentrated methane streams, as well as atmospheric methane levels (2 ppm), taking inspiration from the current natural methane sinks:
Flaring is the least expensive way to oxidize methane, and is fairly broadly commercialized across the fossil fuel sector. However, it requires a point source with a methane concentration of at least 44,000 ppm, so it’s not applicable to many sources of methane.
Photocatalyst, thermo-catalyst, and electrocatalyst approaches, which lower the temperature and/or minimum concentration of methane required to oxidize methane are under development. Some of these approaches will only ever apply to point-sources, while others may eventually be able to address atmospheric concentrations.
Thermal catalysts, including Regenerative Thermal Oxidizers (RTOs) and Regenerative Catalytic Oxidizers (RCOs), are the most commercially developed, and have been implemented in the fossil fuel industry. Though they are much more expensive than flaring, they can oxidize methane down to 2,500 ppm for RTOs and 1,000 ppm for RCOs. Methane can also be oxidized by metal catalysts embedded in high surface area zeolites or porous polymer networks, which depend on elevated temperatures to function.
Photocatalysts, like titanium dioxide, which use the sun’s energy to catalyze the oxidation of methane, are still in the early research phase of development.
Electrocatalysts are even earlier, mostly theoretical, and use electricity to catalyze oxidation. These catalysts have the potential to function at very low concentrations of methane, as low as 100 ppm.
Methanotrophic soil bacteria consume methane and their effectiveness can be improved by creating a better environment for them to flourish and/or by genetically modifying them to consume methane faster or at a lower concentration.
We can also harness bacteria to oxidize methane, potentially down to about 500 ppm with currently researched methods (Yoon 2009). Biocovers, a commercialized approach to removing methane from the soil above landfills, create an enhanced environment for the growth of methanotrophs and a degraded environment for methanogens, functioning as both area source removal and prevention simultaneously. Biocovers are a subclass of a larger solution: soil amendments. By adding things like biochar, gypsum, or rice straw to soil, especially in inundated areas like rice paddies, more methanotrophs and fewer methanogens can be fostered. Soil amendments are still in early research. Managing water levels in natural environments, such as wetlands, may also be able to reduce their methane emissions.
Another point source solution, which could potentially function for lower concentrations of methane, are bioreactors/biofilters. These are artificial, temperature and moisture controlled environments created to grow genetically engineered methanotrophic bacteria capable of consuming methane at lower concentrations than they would otherwise. These are still in development and have not yet been deployed in pilots. They could be installed to capture and remove methane from wastewater treatment plants, manure lagoons at livestock farms, and landfills.
Sunlight-activated free radicals are the primary natural methane oxidation pathway (“methane sink”). This strategy involves enhancing this sink by increasing the availability of free radicals. This strategy could be implemented as a point-source solution in a contained environment, or potentially as a direct atmospheric solution, though this implementation will require significant additional research into safety and effectiveness.
Methane is currently oxidized in the atmosphere by OH and Cl radicals. In early trials, Cl radicals were generated in an enclosed environment at a point source of methane from a cattle feedlot, oxidizing the methane.
Some methane emissions, including historical emissions and natural emissions, are nearly impossible to address without oxidizing methane at atmospheric concentration: 2 ppm. There are currently no demonstrated catalysts or methanotrophs which can effectively oxidize at that concentration, though some may be developed. Radicals can effectively oxidize at 2 ppm, but deploying them requires releasing either directly generated radicals or precursors—which as iron salt aerosols, which stimulate the generation of Cl radicals—into the atmosphere. Atmospheric Oxidative Enhancement (AOE), as it is called, currently has poorly understood consequences, and needs much more research before any future potential deployment.
Atmospheric methane removal could be a critical tool in our toolkit, particularly as we face increasing risks of methane-emitting natural feedbacks and tipping points, such as the collapse of the permafrost and rising natural wetland emissions. This is an important, and underdeveloped, research area in order to identify which approaches may be efficacious, scaleable, and safe. Much more research is needed before we have those answers.