Thermocatalytic reactors use heat to activate a catalyst that oxidizes 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.
Thermocatalytic reactors are promising for breaking down methane at its emission sources where its concentration is elevated. For thermocatalytic reactors to be a feasible atmospheric methane removal approach there would have to be a dramatic decrease in operating temperature (from 300°C today to under 30°C) 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.
Thermocatalytic reactors operating at atmospheric methane concentrations are currently climate detrimental and cost-implausible. Significant reductions in operating temperature are necessary to change this.
Using thermocatalytic reactors to remove atmospheric methane would require energy to heat the catalyst and to transport large volumes of air. Both are currently energy-intensive, and therefore costly and carbon-intensive. Either energy costs of air handling and heat generation would have to come down by orders of magnitude, or passive sources of heat and airflow would have to be incorporated into the reactor design.
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 must be considered for active methods. Energy costs of active air flow methods could be lowered by co-locating the system with existing air movement systems, such as cooling towers, heat exchangers, or direct air capture facilities.
Assuming best-case 2030 energy cost and carbon footprint projections are met, current thermocatalytic reactors are not climate beneficial or cost-effective. Before taking into account airflow requirements, the maximum operating temperature thresholds are ~160°C for climate beneficial, ~40°C for cost-plausible, and ~30°C for cost-effective. The current lowest operating temperature demonstrated for atmospheric methane removal is 300°C.
The potential scale thermocatalytic 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 thermocatalytic 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.
Preliminary research suggests that apart from oxidizing methane or other gases, thermocatalysis has minimal negative side effects.
Thermocatalytic reactors already exist for medium concentration methane emitted from point sources. Regenerative Thermal Oxidizers, which use precious metals, transition metal oxides, or non-catalytic ceramics, are currently commercially operated above 2,000 ppm CH4. While they require operating temperatures of ~1,000°C, these systems can still be climate beneficial and cost-plausible because of the high concentration of methane they address. At highly enriched methane sources, excess heat generated by the oxidation of methane can also be extracted for electricity generation.
Increasing the surface area of the catalyst in contact with methane allows these systems to operate more efficiently. Zeolites, high-surface area, Earth-abundant, porous clay materials, have been tested in lab scale reactors using well controlled artificial air mixtures from pure gases. The most studied zeolites are copper and iron based. One copper based zeolite has been shown to oxidize atmospheric methane at 300 °C. Oxidation efficiency improves with increasing temperature and reaction rates improve with increasing methane concentration. Laboratory research is underway to improve the performance of these zeolites at lower temperatures and to structure them to be able to tolerate high airflows.
Performance could potentially be further improved by adding co-catalysts or dopants, increasing reactivity via zeolite geometry improvements, and otherwise altering the preparation process and composition of the material to produce higher reaction rates, access lower concentrations, and lower the operating temperature.
Key questions that need to be answered regarding thermocatalytic reactors include: