Under early exploration

Atmospheric Methane Removal Approaches

Photocatalytic 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
photocatalytic 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.

Photocatalytic Reactors

Overview

Photocatalytic reactors are a proposed atmospheric methane removal approach that use artificial ultraviolet light to oxidize methane. 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.

For photocatalytic reactors to be a feasible atmospheric methane removal approach there would have to be a >200x improvement in catalytic efficiency, as well as extremely low energy, essentially free, 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.

Feasibility

Learn more about how we evaluate cost plausibility and climate impacts

Reactors using current photocatalysts would be climate detrimental and cost-implausible.

Apparent Quantum Yield (AQY), the ratio of incident photons to oxidized methane molecules, is a key metric for determining cost and climate impacts. Another consideration is the energy requirement for moving 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, current approaches to photocatalysis reactors don’t yet meet the criteria of being climate beneficial or cost-effective. The process could become climate beneficial above the AQY threshold of 0.1%. Before taking into account airflow requirements, cost-plausibility would require an AQY of at least 1.4% and cost-effectiveness at least 7% (forthcoming research). The best AQY measured so far is roughly 0.03% for 2 ppm methane, meaning that a >200x improvement is necessary to achieve cost-effectiveness.

The energy costs from light generation of reactors using current photocatalysts would exceed the economic benefit of the resulting methane removal by two orders of magnitude, even before materials costs and air handling costs are considered.

Scalability

Learn more about how we evaluate scalability

The potential scale photocatalytic reactors could reach is limited primarily by their feasibility considerations, including the energy requirements of air movement and resource limitations of photocatalytic materials. Given the low atmospheric concentration of methane, any flow through system would have to process massive amounts of air to oxidize 10 million metric tons of methane per year. For example, if you add methane oxidizing catalysts to every carbon dioxide direct air capture system that is supposed 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 photocatalytic reactors are made 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. Thus an approach of this sort would be expected to take decades to scale after feasibility and cost-effectiveness were established.

Health & Environmental Considerations

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

In addition to methane, photocatalysts are expected to react with other atmospheric pollutants, including sulfur dioxide (SO₂), carbon monoxide (CO), ozone (O3) and non-methane volatile organic compounds (NMVOC). These compounds also contribute to climate change and the positive and negative climate impacts of their breakdown would need to be accounted for. Increasing oxidation of these species could have additional air quality benefits near ground level. At the same time, photocatalysts are known to generate undesirable end products (nanoparticles, secondary volatile organic compounds, etc.). More research is needed to better understand  the tradeoffs between these potential positive vs. negative impacts. In closed reactor systems, byproducts could be measured in situ and potentially be removed or neutralized selectively.

Learn More

State of Research
Thank you to
Max Kessler (Stanford) and Richard Randall (Stanford)
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

Stay in touch

Sign up to our newsletter and stay updated!

Your submission has been received!
Oops! Something went wrong while submitting the form.