High temperature combustion of methane over thermally stable CoO-MgO catalyst for controlling methane emissions from oil/gas-fired furnaces

Abstract

In order to control the concentration of methane in the hot (>800°C) flue gases of oil/gas-fired furnaces, complete combustion of dilute methane (5000 ppm in N₂-air mixture) over thermally stable CoO-MgO (Co/Mg = 0.5 or 1.0) catalyst (calcined at 950°C, 1200°C and 1400°C) at different space velocities (15000–120000h-¹, measured at 0°C and 1 atm pressure) and temperatures (800–1100°C) has been thoroughly investigated. The catalytic combustion was carried out in quartz reactor with a very low dead volume so that the contribution of homogeneous combustion, particularly at higher temperatures, could be kept low. Involvement of lattice oxygen of the catalyst in the methane combustion was confirmed by methane pulse experiments. The catalysts were characterised by XRD, XPS and also for their surface area and reduction by H² at different temperatures, using pulse technique. Surface area and methane combustion activity of the catalyst is decreased markedly with increasing its calcination temperature. However, the catalyst calcined at a temperature as high as 1400°C, showed a good methane combustion activity. The catalyst was found to exist as a complete solid solution of CoO and MgO. Both the activation energy and frequency factor for the combustion were found to increase markedly with increasing the catalyst calcination temperature. At the higher reaction temperatures and/or lower space velocities, the contribution of homogeneous combustion occurring simultaneously in the voids of the catalyst bed was found to be appreciable. By using the catalyst (calcined at 1200°C) in the combustion, a methane conversion close to 100% could be obtained at a contact time of about 15ms at 950°C. Since, furnace flue gases are at high temperatures and contain enough oxygen, the combustion of methane to CO² and water at high conversion can be accomplished just by passing the flue gases over the thermally stable CoO-MgO catalyst at a small contact time, depending upon the temperature of the flue gases.