Abstract
Keywords: perovskite, methane, combustion, CNG, Pd
Advanced compressed natural gas (CNG) engines entail considerable advantages over conventional gasoline and diesel engines. Natural gas (NG) is a largely available fossil fuel and therefore non-renewable. However, NG has some advantages compared to gasoline and diesel from an environmental perspective. Its emissions are lower. The low flame temperature of lean operated CNG engines helps to limit the formation of NOx. Furthermore, since NG contains only 75 wt% carbon versus 86-88 wt% for gasoline or diesel, it produces less CO2 per unit of energy released. Furthermore, soot particulate can hardly be formed from methane combustion. Other benefits lie in the fact that NG is neither toxic, carcinogenic, nor caustic.
However, unconverted methane in CNG flue gases is much harder to oxidise than gasoline-derived unburned hydrocarbons (UHC). The strong greenhouse effect of methane (more than one order of magnitude higher than that of CO2) forces a higher and higher concern at a legislation level and, as a consequence, the development of new after-treatment technologies to abate these emissions. Catalytic combustion of methane on honeycomb converters similar to those used for the treatment of gasoline engine exhaust gases is the way to go. Commercial catalysts are mostly based on gamma-Al2O3-supported Pd, having a at least three fold higher noble metal loading compared to that of conventional three-way catalysts (up to 300 g/ft3 against 80 g/ft3 ). A research line of ours is aimed at developing nanostructured Pd-perovskite-type-oxide catalysts employing an overall noble metal load significantly smaller than that used in conventional converters, the catalytic performance being the same. Several perovskite-type oxide catalysts (LaMnO3, LaMn0.9Pd0.1O3, LaFeO3, LaFe0.9Pd0.1O3, LaCrO3, LaCr0.9Pd0.1O3) were prepared by SCS, characterized, and tested as catalysts for methane combustion. The comparative analysis of the catalysts activity was carried out with pure perovskites in powder. The best catalyst was found to be LaMn0.9Pd0.1O3 (T50 = 425°C) and therefore it was selected to be deposited on a cordierite monolith in a -Al2O3 supported form (catalyst weight percentage 15%) and tested in a lab-scale test rig under realistic conditions. Half methane conversion (T50) was achieved at 340°C (GHSV = 10 000 h-1), nearly the same T50 value but with a six-fold lower amount of the expensive noble metal than that used in commercial 4wt% Pd/-Al2O3 catalysts.