A comprehensive model of CFB combustion

Paper in proceeding, 2008

This paper presents a new comprehensive model for circulating fluidized bed boilers to be used to support design and scale up as well as a learning tool of the process. The model covers the entire circulating loop and provides a 3-dimensional description of the in-furnace processes in mixing, combustion and heat transfer. The model is unique in that the input data are limited to operational data and that a large number of experimental boiler data has been used to verify the different submodels as well as the overall model. The model is built up by combination of several submodels, each covering main phenomena in the three key fields of CFB boiler modeling: fluiddynamics including gas and solids mixing, char and gas combustion and heat transfer. The modeling of the gas mixing (which governs the gas combustion, assumed to be transport controlled) has a dynamical basis in the modeled fluctuations in velocity and concentration values of the gas phase originating from the dense bottom bed dynamics. This unique feature enables verification of the model by means of time-averaged gas concentration measurements also in the lower region of the furnace. Such measurements sense fluctuations between high-velocity oxidizing gas phase and low velocity oxygen lean phase, yielding results biased towards low oxygen concentrations. The model includes phenomena such as solids backflow effect at the riser exit and corner effects in wall layers and can handle input data on fuel fragmentation. Model main output data consists of spatial distributions of the following parameters: pressure, temperature, heat flux, concentration and flow of main gas species and solid fractions (i.e. inert solids and fuel particles) and particle size distribution of solid fractions. The model gives generally good agreement with experiments from large CFB units for which required input data is available.