A high efficiency gas phase photoreactor for eradication of methane from low-concentration sources

Abstract

Despite the urgent need, very few methods are able to efficiently remove methane from waste air with low cost and energy per unit volume, especially at the low concentrations found in emissions from e.g. wastewater treatment, livestock production, biogas production and mine ventilation. We present the first results of a novel method based on using chlorine atoms in the gas phase, thereby achieving high efficiency. A laboratory prototype of the methane eradication photochemical system (MEPS) technology achieves 58% removal efficiency with a flow capacity of 30 l min−1; a reactor volume of 90 l; UV power input at 368 nm of 110 W; chlorine concentration of 99 ppm; and a methane concentration of 55 ppm; under these conditions the apparent quantum yield (AQY) ranged from 0.48% to 0.56% and the volumetric energy consumption ranged from 36 to 244 kJ m−3. The maximum achieved AQY with this system was 0.83%. A series of steps that can be taken to further improve performance are described. These metrics show that MEPS has the potential to be a viable method for eliminating low-concentration methane from waste air.

Exploring the bounds of methane catalysis in the context of atmospheric methane removal

“…Another research focus lies in heterogeneous catalysis aimed at generating reactant species for subsequent gas-phase reactions with methane, such as chlorine [14]. As an example the Methane Eradication Photochemical System relies on gas-phase radicals for methane degradation [15] and is not subject to surface rate limitations. While those approaches exhibit promise and are poised for further development, our study considers solely heterogeneous catalysis for direct surface conversion of methane.…”

“…Figure 6 shows that the estimated methane removal energy intensity (left y-axis) achieves a minimum when the convective E conv and material capital E cap are balanced (red circle) at ∼0.15 GJ tonne −1 CO 2 e (0.38 GJ tonne −1 CO 2 e GWP100). The equivalence E conv and E cap occurs for a duct diameter of d = 0.17 m, corresponding to individual volumetric flow rate, V i = 0.35 m 3 s −1 (equation (15), SI) which balances the increased convective energy due to pressure drop at small diameters with increased mass transport to the catalyzed walls. Critically, at this optimum E conv and E cap have energy intensities comparable to electrocatalytic intensities E Elec ∼0.15 GJ tonne −1 CO 2 e (0.38 GJ tonne −1 CO 2 e GWP100).…”