Recovery boiler char bed dynamics – measurements and modeling

This paper aims at increased understanding of the dynamics of char bed processes and their impact on the operation of the recovery boiler. As part of a comprehensive measurement campaign of a 4450 tds/day Kraft recovery boiler, different operational modes of char bed burning were used. These included (i) stable operation, (ii) char bed growth, and (iii) char bed depletion. Videos from IR furnace cameras are used for monitoring the response of the char bed to changes in recovery boiler operation. Physical transport processes affecting bed shape are visually identified from the video. The impact of char bed dynamics on boiler steam production is shown, and oxygen consumption calorimetry is used to confirm cyclic changes in in-furnace burning rate as the cause. Computational fluid dynamics (CFD) simulations – including char bed shape – are carried out for comparison with the video observations. The simulations show the capability of current steady-state models to predict long term growth or depletion of the char bed, but time-dependent simulations are needed if true char bed dynamics are to be simulated.