Radical reactors use artificial ultraviolet light to split molecules (through photolysis) which generates radicals that oxidize methane. This method mimics the primary natural methane sink of hydroxyl and chlorine radicals generated through photolysis in the atmosphere. Air must be moved through the localized reactor, either through active methods such as fans, or passive methods such as solar updraft chimneys that use convective heat transfer to move air.
Radical reactors are promising for breaking down methane at its emission sources where its concentration is elevated. For radical reactors to be a feasible atmospheric methane removal approach there would have to be a >10x increase in the catalytic efficiency of radical generation and extremely low energy airflow. The required airflow to achieve megaton-scale atmospheric methane removal would be far beyond planned carbon dioxide direct air capture capacity in the upcoming few decades.
Radical reactors operating at atmospheric methane concentrations are currently climate detrimental and cost-implausible. Significant improvements in catalytic efficiency are necessary to change this.
A key metric for determining cost and climate impacts for photolysis is Apparent Quantum Yield (AQY), the ratio of incident photons to oxidized methane molecules. This determines the energy input required to produce artificial light to oxidize methane. Cost and climate impacts are also driven by the energy requirements to move air through the reactor. This could be negligible for passive air flow, but significant for active air flow methods. Where artificial light is required, the energy requirements to generate it add to the cost. Sunlight does not provide a high enough photon flux to be a viable alternative.
Assuming best-case 2030 energy cost and carbon footprint projections are met, current radical reactors are not climate beneficial or cost-effective. Before taking into account airflow requirements, the minimum AQY thresholds are ~0.1% for climate beneficial, ~1.4% for cost-plausible, and ~9% for cost-effective. The best measured photolysis AQY so far (using chlorine gas) is 0.8% at 50 ppm, which corresponds to ~0.03% at 2 ppm. Therefore, a ~300x increase in AQY is required to be cost-effective at atmospheric levels of methane.
The potential scale radical reactors could reach is limited by the energy requirements of air movement and potentially resource limitations of raw materials. Given the low atmospheric concentration of methane, any flow through system would have to process massive amounts of air to oxidize a benchmark annual scale of 10 million metric tons of methane (830 Mt CO2e using GWP20). For example, if you add methane breakdown reactors to every carbon dioxide direct air capture system that is projected to be installed by 2030 (~60 Mt CO₂/yr or around 5 billion cubic feet per minute), only around 0.1 Mt/yr of methane could be removed.
Even if radical reactors become feasible, it will be extremely challenging and resource intensive to reach meaningful scale, as the amount of methane addressed will be directly related to the number and scale of the reactors built. Therefore we estimate an approach of this sort would take decades to scale after feasibility was established.
In a closed reactor system, byproducts could be measured in situ and potentially be selectively removed, reducing the uncertainty as to what gasses would be produced and emitted. This drastically lowers the health and environmental risks compared to open system interventions.
More research is needed to determine whether, in addition to oxidizing methane and other desirable outcomes, radical reactors also produce undesired byproducts such as chlorinated gasses.
Radical-based reactors using photolysis to produce hydroxyl and chlorine radicals have been tested in the lab, usually with well controlled artificial air mixtures from pure gases, and often in batch reactors.
Gas-phase hydroxyl reactors have been used in industry to remove VOC emissions from wastewater, windmill blade production, and paint fuels. Unfortunately, methane has the lowest reactivity of any hydrocarbon with hydroxyl radicals, so it appears to not be feasible to generate sufficient hydroxyl concentrations for atmospheric removal. Chlorine-based reactors may be able to overcome this limitation because chlorine radicals react faster than hydroxyls and higher chlorine concentrations can be maintained. Research is ongoing to determine the methane concentration at which radical reactors could prove feasible for breaking methane down at its sources.
An Apparent Quantum Yield of at least 9% will be required to make radical reactor energy costs approach the social cost of methane. Currently, the state-of-the-art AQY is 0.8% for 2 ppm methane (for chlorine gas photolysis).
Key questions that need to be answered regarding the radical reactor approach include: