Physical and practical constraints on atmospheric methane removal technologies

Despite their apparent utility in mitigating climate change, technologies for removing methane from air are in early stages of development. Here we evaluate the limiting physical constraints, for three types of systems: two- and three-dimensional infrastructure and atmospheric oxidation enhancement, focusing on removing low ( x CH4 < 1000 ppm) and ambient ( x CH4 < 2 ppm) methane from air. With the space velocities and removal efficiencies of current three-dimensional technologies, volumes of 7-350 km3 are required to remove 1 Tg CH4 yr−1. Two-dimensional solutions are limited by the transport rate of methane to a surface. If every molecule of methane that collides with the surface is removed, an area of 1130 km2 is needed to remove 1 Tg CH4 yr−1 at ambient concentration. However, research shows that per-collision reaction probabilities are < 10−8 requiring a surface area of 1010-1015 km2. Finally, we examine atmospheric oxidation enhancement, where 4.8 Tg yr−1 of Cl ⋅ or 8.8 Tg yr−1 of OH ⋅ is required to remove 1 Tg CH4 yr−1, with precursors such as H2O2 or O3. However, limitations arise concerning multiple environmental impacts. We conclude that the physical and practical constraints are considerable, and identify the main barriers that must be addressed.