Radical reactors – a proposed atmospheric methane removal technology – use artificial ultraviolet light to split molecules through photolysis (the decomposition of molecules with light). This generates radicals that oxidize reactive species including methane. This method mimics the primary natural methane sink of hydroxyl and chlorine radicals generated through photolysis in the atmosphere. 

Laboratory research is underway to improve the efficiency of radical generation in order to apply it to methane destruction at emissions sources, or if it becomes efficient enough, to atmospheric methane removal. 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 >450x increase in catalytic efficiency and extremely low energy airflow. The required airflow to achieve large-scale atmospheric methane removal would be tremendous, far beyond planned carbon dioxide direct air capture capacity in the next decade.

Radical reactor approaches for atmospheric methane removal are currently cost-implausible and climate detrimental. Major innovations in process efficiency would be 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. 

Assuming “best-case” 2030 energy cost and carbon footprint projections, current radical reactors don’t yet meet the criteria for being climate beneficial or cost-effective. To be climate beneficial, the AQY would need to be above 0.1%.  To be cost-effective, AQY would need to be at least 7% (Abernethy et al. 2023). The best measured photolysis AQY for chlorine is currently <0.015% at 2 ppm (forthcoming research), so at least a 450x increase in AQY would be required to be both climate beneficial and cost-effective.

The potential scale radical reactors could reach is limited primarily by feasibility considerations, including the energy requirements and volume of air movement, and any resource constraints on other inputs. Given the low atmospheric concentration of methane, any flow through system would have to process massive amounts of air to oxidize a scale benchmark of 10 million metric tons of methane (830 Mt CO2e using GWP20). For example, if you add methane oxidizing catalysts to every carbon dioxide direct air capture system that is predicted 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 would be removed.

Even if radical reactors are made feasible, it will be extremely challenging and resource intensive to reach meaningful scale because the amount of methane addressed will be directly related to the number and scale of the reactors built. Thus an approach of this sort would be expected to take decades to scale after feasibility and cost-effectiveness were established.

Compared to the feasibility and scalability concerns outlined above, the side effects of radical reactors are relatively minor.

It is unclear whether, in addition to oxidizing methane and other desirable outcomes, radical reactors also produce undesired byproducts such as chlorinated gasses. In closed reactor systems, byproducts could be measured in situ and potentially be removed or neutralized selectively, reducing uncertainty as to what gasses are produced and emitted.