Preparation, characterization and reactivity of Me-hexaaluminate (Me=Mn, Co, Fe, Ni, Cr) catalysts in the catalytic combustion of NH 3-containing gasified biomasses

Abstract

The preparation, characterization and reactivity of metal-substituted (Mn, Cr, Co, Ni, Fe) hexaaluminate catalysts in the combustion of NH 3-containing gasified biomass-derived fuels is investigated. It is found that the preparation method adopted (carbonates route) is suitable to prepare microcrystalline matrices of high surface areas with high interspersion of the constituents. The calcination of all the microcrystalline matrices leads to the formation of a final monophasic materials with a Ba–β-Al 2O 3 structure and surface areas of 5–15 m 2/g. In these structures Ni is divalent and octahedrally and pseudo-octahedrally coordinated; Fe and Cr are trivalent in octahedral coordination and Co is both divalent tetrahedrally coordinated and trivalent octahedrally coordinated. The prepared substituted hexaaluminate catalysts are active in the catalytic combustion of the biomass-derived fuels, and present different activity towards the various biogas components. In particular the Fe- and Mn-substituted samples show the highest activity in the combustion of methane, which is the least active biogas fuel. Over all the catalyst samples, ammonia contained in the biogas is oxidized to N 2 but large amounts of NO are also formed. However, the NO formation decreases at high temperatures: this phenomenon has been associated with the occurrence of a selective non-catalytic reduction (SNCR) process involving ammonia as reducing agent and occurring in the first part of the reactor in the simultaneous presence of oxygen, NO and ammonia.