CFD Based Modelling of Recovery Boiler Char Beds

摘要

Understanding the physical and chemical phenomena governing char bed burning is extremely important for stable and efficient operation of the recovery boiler. In previously published works stand alone char bed models have been developed and evaluated in order to increase knowledge of the complex char bed conversion processes. However, available computational fluid dynamics (CFD) based recovery furnace models have rather been applied to evaluate lower and upper furnace processes than to investigate the behaviour of the char bed. In this work, a stand alone char bed model was modified and implemented into a CFD based furnace model developed at Abo Akademi, in order to analyse the interaction of the char bed and the lower furnace gas atmosphere and flow distribution for given furnace conditions of a smaller recovery boiler firing 450 t DS/day. The char bed model, which is fully coupled with the gas phase and a simplified black liquor droplet model, solves equations describing the mass and energy balance on the surface of the char bed and includes carbon conversion via direct oxidation, gasification reactions and sulphate reduction. Simulations for two different air distributions in the furnace have been performed to evaluate the char bed model. The impact of the two air distributions on the lower furnace gas atmosphere, and thus on bed conversion processes, could be clearly revealed by the new char bed model. The model gives a reasonable description of the processes occurring on the char bed in steady-state and the obtained results show a clear correlation between the conditions in the gas phase above the surface of the char bed and the processes occurring on the bed. Furthermore, the developed char bed model has shown to be numerically very efficient and to have negligible impact on the overall simulation times.