Dynamical modeling of the gas phase in fluidized bed combustion - accounting for fluctuations

Paper in proceeding, 2009

A model for gas phase mixing in fluidized bed boiler furnaces is presented. The model takes its basis in a description of the dynamics of the dense bottom bed which strongly govern the gas mixing up through the furnace. Thus, a time-resolved approach is used to link the modeling to the physics of the underlying processes determining the gas mixing. As output, the model gives the fluctuating flux of gas species, in contrast to the classical modeling approach which is limited to time-averaged gas fluxes. Such a dynamical approach allows assumption of the volatile combustion system as transport-controlled which avoids complete consumption of either oxygen or combustible gases in each modeled cell. Thus, the time-resolved analysis employed enables application of a realistic criterion for the mixing such as that reactants can coincide in both space and time in order to react. While fitting of kinetics is strongly dependent on the system and operational conditions, the present model integrates key system variables such as the bottom bed height and the characteristic pressure-drop constant over the primary air distributor, allowing application of transport-controlled (i.e. infinitely fast kinetics) volatile combustion. The model of the bottom bed divides the gas flow into two phases, a throughflow and an emulsion gas, and calculates their respective fluctuations in velocity and composition. Having these, in-furnace gas probe measurements can be simulated and compared with in-situ gas suction probe measurements, i.e. the probe signal is modeled. Such an approach is crucial in fluidized bed-boilers furnaces since in-situ gas probe measurements in regions with high fluctuations in gas velocity lead to results which are biased towards reducing conditions. Model results, including simulation of the gas suction probe, are analyzed and compared with experimental data from the Chalmers 12 MWth CFB boiler and a good agreement is obtained.