Diffusion combustion of NH3 in a single bubble of fluidized bed

Abstract

Fluidized bed is a potential equipment for NH3 combustion due to its efficient gas–solid contact and high temperature particles as heat source for ignition. This paper studies the diffusion combustion of NH3 in a single bubble of fluidized bed, using an experimental approach and CFD-DEM simulation incorporated with a skeleton mechanism of 98 elementary reactions. The results show that the diffusion combustion of NH3 intermittently occurs in the bubble injection period and the oxygen is mainly consumed by H + O2 = OH + O. Slow oxidation, instead of combustion, takes place in the bubble rising period and the oxygen is mainly consumed by NNH + O2 = N2 + HO2. With the rupture of the bubble, unreacted NH3 re-burns, generating flames in the freeboard of the bed. The chemical reaction in the bubble is highly correlated to the mass transfer between the emulsion phase and bubble phase. Increasing the fluidization gas velocity promotes the accumulation of oxygen and enhances the reaction rate of slow oxidation of NH3 in the bubble due to the increasing inter-phase mass transfer. The increase of the bed temperature reduces the ignition delay time, and increases the chemical reaction rate and mass transfer coefficient. The changes in oxygen concentration of fluidizing gas not only changes the chemical reaction rate, but also affects the oxygen consumption path. The fraction of NH3 oxidized in the dense bed can be enhanced by increasing the fluidization gas velocity, bed temperature and oxygen concentration of the fluidizing gas, the latter exerting the greatest influence.