Abatement of dilute methane using combined ozone and Pd-ion-exchanged zeolite

Abstract

Dilute methane emissions found in industrial exhausts such as waste water treatment plants and coal mine ventilation systems are problematic for energy recovery, while their quantity imposes significant global warming effects. In this study, the catalytic combustion of methane with combined ozone and Pd-ion-exchanged zeolite was investigated. Without zeolite, gas phase reaction among methane, air and ozone gave 1-11% of methane conversion at 295-435 oC. The presence of inlet ozone (1-4 vol%) to the zeolite 13X system enhanced methane removal efficiency by 24-56%, giving overall performance of 93% in this low temperature range. There existed a range of reaction temperature where methane conversion was most enhanced by ozone in the zeolite system. Without ozone, methane oxidation showed an activation energy of 136 kJ/mol. This was reduced to 116 kJ/mol by using 4 vol% of ozone. The enhanced methane removal was attributed to the decomposition of ozone to form atomic oxygen reacting with adsorbed methane. Higher methane conversion was observed at lower space velocities under various inlet ozone concentrations indicating the importance of mass diffusion of ozone and methane to the active sites in the zeolite. As the inlet ozone and methane concentration increased, the methane removal efficiency was limited by the number of available active sites in the zeolite, which governs the amount of adsorbed methane and atomic oxygen available for subsequent reactions.