Assessment of Oxyfuel Circulating Fluidized Bed Boilers – Modeling and Experiments in a 5 MW Pilot Plant

As in air-firing, oxy-fired combustion using the Circulating Fluidized Bed (CFB) technology may offer advantages in that the CFB technology provides high fuel flexibility, in-furnace reduction of SOx emissions and a relatively smooth distribution of the extracted heat flux. Furthermore, the thermal flywheel induced by the solids flow and the possibility of heat extraction in the solids recirculation system (i.e. outside the furnace) represent a potential to achieve high oxygen concentrations while limiting the temperature level. Allowing such higher oxygen concentrations would imply significantly more compact (and thus less costly) furnaces than those used in air combustion.The present work summarizes the current status of an ongoing project which has the aim to assess the oxyfuel CFB technology by means of pilot testing and modeling. The pilot testing is carried out in a 5 MW oxyfuel CFB pilot plant (representing the largest oxy-fired CFB unit running at present date). The model is an extended version of a model for air-fired CFB combustion developed over the last six years by the authors and validated for utility-scale units.Considering that the model has previously undergone an extensive validation against data from air-fired large-scale boilers, the model is expected to represent a useful tool for the assessment, design and scale-up of oxyfuel boilers once validated with measurements from the 5 MW oxyfuel unit,. An obvious application of the model is in the dimensioning and designing heat extraction surfaces. High oxygen concentrations require more heat extraction in the solids recirculation system, which is allowed by means of increased external circulation of solids. Such increased solids circulation can be established with modifications to the furnace design and a change in operating conditions and/or solids properties