An experimental investigation into the characteristics of ammonia dissociation in a Bubbling Fluidised Bed

Ammonia (NH₃) dissociation is of an initial and very important step during its combustion process. This study therefore systematically investigates the effect of reaction temperature (650–950 °C), fluidisation number (0.83–2.50) and static bed depth (0–25 mm) on NH₃ dissociation in a laboratory-scale Bubbling Fluidised Bed (BFB). By analysing the concentrations of H₂ and N₂ in the effluent using gas chromatography, results show that the conversion ratio of NH₃ in BFB was merely 0.26%–0.29% at 650 °C but increased significantly to 18.1%–25.3% at 950 °C. This highlights the importance of temperature in promoting NH₃ dissociation. However, as fluidisation number increased from 1.0 to 2.5, attributing to a decrease in gas – solid contact time, the conversion ratio of NH₃ decreased correspondingly from the highest 20.7%–5.7%. Moreover, as static bed depth increased from 15 mm to 25 mm, the conversion ratio of NH₃ increased slightly from 21.5% to 26.8%, both of which were found to be higher than that without bed particles. Clearly, NH₃ dissociation is enhanced by the bed material depth but is also dependent on the gas – solid contact time. In addition, by measuring the temperature distribution in the BFB reactor, a temperature drop of ca. 20 °C near the distributor was confirmed, showing a strong effect of endothermicity during NH₃ dissociation. These would provide an improved comprehension on the mechanisms of NH₃ dissociation and offer theoretical guidance for optimising the combustion processes in BFB burning NH₃.