Subhabrata Banerjee

Catalysts for combustion of methane and lower alkanes

The important greenhouse effect of methane (more than an order of magnitude greater than CO2) makes it essential to eliminate/control the methane emission from natural gas engines/power plants and petroleum industries. Catalytic combustion of methane is favored over homogeneous combustion, because the former greatly facilitates the oxidative destruction of methane. Moreover, use of catalysts for methane combustion in gas-turbines affords lower working temperatures (as compared to gas-fired turbines) and thermodynamically limits NOx (which is an extremely harmful environmental pollutant) emission. A large amount of work has been undertaken to develop catalysts both for controlling methane emission as well as for generating power in high temperature natural gas-turbines. This review will address the different issues related to the variety of catalysts which have been employed for methane/lower alkane combustion. Although all the related important aspects of the combustion catalysts will be addressed, greater emphasis will be placed on recent work in this field.

Low temperature complete combustion of dilute propane over Mn-doped ZrO2 (cubic) catalysts

Combustion of dilute propane (0.9 mol%) over Mn-doped ZrO2 catalysts prepared using different precipitating agents (viz. TMAOH, TEAOH, TPAOH, TBAOH and NH4OH), having different Mn/Zr ratios (0.05—0.67) and calcined at different temperatures (500—800°C), has been thoroughly investigated at different temperatures (300—500°C) and space velocities (25,000–100,000 cm3 g−1 h−1) for controlling propane emissions from LPG-fuelled vehicles. Mn-doped ZrO2 catalyst shows high propane combustion activity, particularly when its ZrO2 is in the cubic form, when its Mn/Zr ratio is close to 0.2 and when it is prepared using TMAOH as a precipitating agent and calcined at 500—600°C. Pulse reaction of propane in the absence of free-O2 over Mn-doped ZrO2 (cubic) and Mn-impregnated ZrO2 (monoclinic) catalysts has also been investigated for studying the relative reactivity and mobility of the lattice oxygen of the two catalysts. Both reactivity and mobility of the lattice oxygen of Mn-doped ZrO2 are found to be much higher than that of Mnimpregnated ZrO2. Propane combustion over Mn-doped ZrO2 catalyst involves a redox mechanism

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

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.