Effect of bed material on ammonia dissociation in a bubbling fluidised bed

Ammonia (NH₃) dissociation is a crucial first step during its combustion. This study systematically examines the characteristics of NH₃ dissociation in a lab-scale bubbling fluidised bed (BFB), with a special concern to the effect of the type of bed material. The dissociation experiments are performed with an initial NH₃ concentration of 11.1 % in NH₃/Ar mixture under incipient fluidisation and different temperatures (750–950 °C) conditions in the presence of SiO₂, Al₂O₃, CaO, Fe₂O₃, and a bottom ash from a coal-fired circulating fluidised bed (CFB) boiler as bed materials, respectively. The reactor exit stream is analysed using a gas chromatograph (GC) for its gas composition, from which the NH₃ conversion is calculated. The bed materials before and after the NH₃ dissociation experimentation are characterised using X-ray diffraction (XRD), scanning electron microscopy (SEM), Brunauer–Emmett–Teller (BET) and NH₃ temperature-programmed desorption (NH₃-TPD) for changes in their physicochemical properties. Results showed that while the NH₃ conversion generally increases with increasing temperature, it is strongly influenced by the bed material at a given temperature, where the NH₃ conversion decreases with the bed material following the order of Fe₂O₃ > CFB bottom ash > CaO > Al₂O₃ > SiO₂. It is believed that the metallic iron formed due to NH₃ reduction of Fe₂O₃ and iron oxides in the CFB bottom ash, as confirmed by XRD analysis, catalyses NH₃ dissociation during experimentation. The effect of CaO, Al₂O₃ and SiO₂ as bed materials is found to depend on the surface acidity and pore structure. The CaO bed material incurs an increase in both pore volume and pore size following the NH₃ dissociation experiment.