Ammonia Combustion in Fixed-Bed and Fluidised-Bed Reactors: the Concept, Knowledge Base, and Challenges

In considering ammonia (NH3) as a carbon-free fuel for large-scale power generation, this review examines the current state of knowledge of NH3 as a fuel in terms of its thermophysical properties and burning characteristics compared to conventional hydrocarbon fuels. The proceeding analysis portrays the challenges associated with NH3 combustion in traditional systems and suggests fluidised-bed NH3 combustion as a plausible means to provide reliable ignition, stable combustion, and reduced NOx emission. A fixed-bed is considered as a research tool, as well as a special case of fluidised-bed, to study NH3 oxidation and combustion in the presence of solid bed material to provide foundational information key to understanding the more complicated fluidised-bed NH3 combustion. The thermophysical properties and burning characteristics of NH3, along with an examination of the combustion of other fuels in the presence of solid media, enable expectations for NH3 combustion in fixed- and fluidised-beds. A general fluidised-bed NH3 combustion system design, along with suggested operating conditions, is presented to provide an appreciation for a practical large-scale fluidised-bed NH3 fired power generation system. The NH3 combustion chemistry and associated NOx formation and destruction pathways are also discussed to appreciate the impact of operating conditions on combustion performance. Finally, the review identifies key knowledge gaps and technical challenges which warrant further research to advance fluidised-bed NH3 combustion technology for large-scale electric power generation in a carbon constrained future.