Heat transfer in the external heat exchanger of oxy-fuel fluidized bed boilers

The external heat exchanger (EHE) plays an essential role in the adequate control of the heat balance in large-scale oxy-fuel circulating fluidized bed (CFB) boilers. However, the EHE of an oxy-fuel CFB presents two particularities compared to the known air-combustion case: the composition of the fluidizing gas and the higher flow of recycled solids. Both these issues are addressed in this paper: Firstly, heat transfer coefficients are measured in a 90 kW bubbling fluidized bed, operated at different temperatures, with air and with a wide range of O2/CO2 mixtures. Thereafter, experiments are also carried out in a cold scaled-down EHE in order to assess the influence of an increment in the solids flow across the EHE on heat transfer. on the results of these experiments, an expression for the heat transfer coefficient is proposed which accounts for gas composition and solids flow rate. Finally, a model for the EHE is integrated into a large oxy-fuel CFB model in order to quantify the influence of oxy-fuel operation mode on the heat transfer surfaces required in the EHE.