Ammonia combustion in fixed-bed and fluidised-bed reactors: The concept, knowledge base, and challenges

Abstract

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.