Unsteady CFD analysis of kraft recovery boiler fly-ash trajectories, sticking efficiencies and deposition rates with a mechanistic particle rebound-stick model

• A CFD model for ash deposition prediction is presented.
• Differently-sized fine ash particles showed different deposition trends.
• Ash deposition CFD model-specific guidelines regarding accuracy were followed.
• The deposition trends showed spatial variations in different deposition locations.
• An analysis of the propensity of thermophoresis is provided.

This work presents a CFD model of a transversally-periodic bundle of four in-line tubes of a kraft recovery boiler bank for ash deposition calculations. The flue gas was laden with discrete solid ash particles. Particle contact and sticking–rebound mechanics were used. The ash deposition parameters (arrival rate, sticking efficiency, deposit rates) were computed locally, studying the dependency on the particle diameter. Emphasis was put regarding grid resolution, unsteady flow solving, and the differences observed among different tubes and locations.Thermophoresis was responsible for 94.1% of the total deposition rates for submicron (0.7 μmμm) particles, becoming markedly less significant for coarser particles. These small particles showed a much higher sticking efficiency (above 95%) than other studied particles. The particles with the largest diameter studied (18.7 μmμm) had much higher arrival rates to the cold surfaces since they did not follow the flue gas flow paths (inertial impaction). Due to their low sticking efficiency (about 15%), they formed more irregular deposition distributions. Particles with an intermediate diameter (3.62 μmμm) showed the smallest arrival rates among the three particle sizes considered.