Secondary air induced flow structures and their interplay with the temperature field in fixed bed combustors

Air staging in solid fuel combustion features widely in small scale domestic boilers to large scale moving grate combustors. Whilst the effects of air staging on the combustion characteristics of these systems are generally known, there is very little insight available into the role of secondary air on the flow and temperature field in the freeboard of batch-type fixed bed biomass combustors. Three-dimensional gas phase simulations using the Transition 𝑘kl-𝜔 and Finite Rate/Eddy Dissipation models were validated against freeboard temperatures and emissions (CO2 and O2) measured on the same set-up. Results show that secondary air at Qs/Qt ≥0.25 induces two recirculation zones, upstream and downstream of its injection point with maximum freeboard temperatures generally observed around these recirculation zones, if Qs/Qt drops to 0.12-0.18 only an upstream recirculation zone is observed until it diminishes by Qs/Qt = 0.06. However, there is a trade-off caused by what appears to be a cooling effect if Qs/Qt is increased between 0.25 and 0.71. Modelling results show that in reacting cases, unlike non-reacting modelling on the same geometry, the strength of the secondary air induced upstream recirculation zone appears significantly stronger.