Combustion of an ammonia bubble with an oxygen bubble in a fluidized bed

The use of ammonia as a carbon-free fuel can significantly reduce carbon dioxide emissions, and fluidized beds are expected to achieve efficient combustion of ammonia. A preferable approach is to introduce NH3 and O2 into the bed separately to form bubbles and induce combustion through their interaction. However, the mechanisms of mass transfer and chemical reactions during bubble diffusion combustion in fluidized beds are still unclear. This study employs an experimental approach and CFD-DEM numerical simulation to investigate the combustion of an NH3 bubble with an O2 bubble in a two-dimensional fluidized bed. It aims to reveal the mechanisms of diffusion combustion between ammonia and oxygen bubbles in fluidized beds, focusing on the impact of mass transfer on combustion reactions. The research results indicate that the rising process of the NH3 and O2 bubbles could be classified into several distinct stages: injection of first bubble, bubble approach, bubble coalescence and bubble rupture. During the bubble approaching stage, two mass transfer processes occur: O2 is transferred from the lower bubble to the upper bubble, while NH3 enters the bubble cloud from the upper bubble and subsequently permeates into the lower bubble through the emulsion phase. When the bubble interval time is 0.3 s, mass transfer coefficient of the former reaches 0.85 m/s, while the maximum value of the latter is about 0.32 m/s. The former exhibits a more pronounced mass transfer, resulting in a huge influx of O2 into the upper bubble and subsequently leading to the combustion of NH3, and mass transfer of O2 is mainly achieved by convection rather than diffusion. As the bubble interval time decreases from 0.4 s to 0.2 s, the fraction of ammonia oxidized in the bubble of the dense phase increases from 7 % to 31 %.