CFD-based modeling of kraft char beds - part 1: char bed burning model

Understanding the physical and chemical phenomena governing char bed burning is important for stable and efficient operation of the recovery boiler. Stand-alone char bed models have been developed and evaluated to increase knowledge of the complex char bed conversion processes. Available computational fluid dynamics (CFD)-based recovery furnace models previously have been applied to evaluate lower and upper furnace processes rather than to investigate the behavior of the char bed. In this first of two papers, a stand-alone char bed model for predicting the chemical processes of char bed burning was modified and implemented into a CFD-based furnace model. 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 sulfate reduction. The model was tested by simulating a 3150 tons dry solids/day recovery boiler. Simulations for two different primary air distributions and using two sulfate reduction degrees in droplets arriving to the bed were performed to evaluate the char bed model. The effects on bed conversion processes were clearly revealed by the new char bed model. The model gives a reasonable description of the chemical processes occurring on the char bed in steady state. In the second part of this work, the model is applied to study the effects of droplet size and bed shape.