Physical and practical constraints on atmospheric methane removal technologies

Abstract 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 ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msub> <mml:mi>x</mml:mi> <mml:mtext>CH4</mml:mtext> </mml:msub> <mml:mo>&lt;</mml:mo> </mml:mrow> </mml:math> 1000 ppm) and ambient ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msub> <mml:mi>x</mml:mi> <mml:mtext>CH4</mml:mtext> </mml:msub> <mml:mo>&lt;</mml:mo> </mml:mrow> </mml:math> 2 ppm) methane from air. With the space velocities and removal efficiencies of current three-dimensional technologies, volumes of 7–350 km 3 are required to remove 1 Tg CH 4 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 km 2 is needed to remove 1 Tg CH 4 yr −1 at ambient concentration. However, research shows that per-collision reaction probabilities are <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mo>&lt;</mml:mo> </mml:mrow> </mml:math> 10 −8 requiring a surface area of 10 10 –10 15 km 2 . Finally, we examine atmospheric oxidation enhancement, where 4.8 Tg yr −1 of Cl <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msup> <mml:mrow/> <mml:mo>⋅</mml:mo> </mml:msup> </mml:mrow> </mml:math> or 8.8 Tg yr −1 of OH <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msup> <mml:mrow/> <mml:mo>⋅</mml:mo> </mml:msup> </mml:mrow> </mml:math> is required to remove 1 Tg CH 4 yr −1 , with precursors such as H 2 O 2 or O 3 . 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.