Under early exploration

Atmospheric Methane Removal Approaches

Radical Reactors

Rising temperatures are increasing the risk of natural systems releasing methane, which would drive further warming. Existing efforts towards reducing anthropogenic greenhouse gas emissions and removing atmospheric carbon dioxide are crucial, but may be insufficient to maximally decrease the chance of, and then possible impact of, these risks. Atmospheric methane removal approaches are being researched to determine how to remove methane from the atmosphere faster than natural systems alone, in order to help lower peak temperatures and counteract some of the impacts of large-scale natural systems methane releases.

Atmospheric methane removal, should any approaches prove highly scalable, effective, and safe, could help address some of the current 0.5°C—and rising—of methane-driven warming. All proposed atmospheric methane removal approaches are at a very early stage today: some ideas have been proposed, some are being researched in laboratories, but none are yet ready for deployment. Spark believes that accelerating research to develop and assess which, if any, of these approaches might be possible and desirable is an important additional risk mitigation strategy.

A number of atmospheric methane removal approach ideas have been raised—including
radical reactors
, which is currently
Under early exploration
, with major breakthrough innovations required to change this
.
This approach, based on early analysis, will likely require multiple breakthroughs in order to feasibly address atmospheric methane levels. It may hold the most promise if it also delivers separate benefits (e.g. for climate or pollution), as part of systems deployed for other primary reasons, or to address low-concentration methane sources.

Radical Reactors

Overview

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.

Feasibility

Learn more about how we evaluate cost plausibility and climate impacts

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.

Scalability

Learn more about how we evaluate scalability

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.

Health & Environmental Considerations

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.

Learn More

State of Research
Thank you to
Matthew Johnson (University of Copenhagen)
for their contributions to, and review of, this content.
This is live and evolving content, we are always open to well-referenced updates and suggestions, which can be shared here.

Explore Other Potential Approaches

Approaches Overview

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