Ash Deposition Modeling and Deposit Properties

1. Ash Deposition Modeling and Deposit Properties Larry Baxter Brigham Young University Provo, UT 84602 17th Annual ACERC Conference Salt Lake City, UT February 20, 2003

2. Focus Is On Three Properties • Thermal Conductivity • Major factor is deposit structure • Varies with sintering • Upper and lower bounds can be set by theory • Precise value difficult to establish without porosity and structural information. • Strength • Parallels thermal conductivity in many ways • Emissivity • Difficult to predict for particulate/porous material • Good theoretical grounding • Good data • Poor parameters (optical constants)

3. q q Upper Limit for Thermal Conductivity • Columnar Structure : Deposit Surface Solid Phase Gas Phase f Deposit Surface Robinson, A. L., S. G. Buckley and L. L. Baxter (2001). Experimental measurements of the thermal conductivity of ash deposits: Part 1. Measurement technique. Energy and Fuels,15: 66-74. Robinson, A. L., S. G. Buckley, N. Yang and L. L. Baxter (2001). Experimental measurements of the thermal conductivity of ash deposits: Part 2. Effects of sintering and deposit microstructure. Energy and Fuels15: 75-84.

4. q q Lower Limit for Thermal Conductivity • Layered structure: Deposit Surface Gas Phase kg = 0.06 W/(m K) f Solid Phase kg = 1.3 W/(m K) Deposit Surface

5. In Situ Thermal Conductivity Schematic

6. Details of Probes

7. Laser Triangulation Diagnostic

8. Measurements of Deposit Thickness Test Section Deposition Probe

9. Limits of Deposit Thermal Conductivity

10. Probe and Deposit Surface Temperature

11. Probe and deposit surface temperatures

12. Probe and deposit temperature profiles

13. Dynamic Conductivity Variations

14. Structural Parameter Variations

15. Other Deposits Behave Differently

16. Strength as a Function of Porosity

17. Radiative Properties are Important • Deposit surface temperature and heat flux depend strongly on thermal conductivity and emissivity • Between the theoretical bounds or thermal conductivity lies a large variation in performance • It is essential that in situ thermal conductivity data are collected

18. Radiative Properties are Important • Theoretically rigorous approaches are being attempted to describe emissivities • Fundamental data (optical constants) are in significant disagreement • New optical constants are being calculated using several approaches

19. Reflectivity Depends on Size and Direction • Small, non-absorbing particles are highly reflective and are typical of recovery boiler deposits • Reflectivity/emissivity depend strongly on direction • Strong local effects are common, especially with small transparent particles, near the angle of the incident beam

20. In Situ Experimental Data are Available

21. Oxygen Isosurfaces

22. Temperature Variations

23. Impaction efficiency: Inertial impaction (clean surface)

24. BL mechanisms Inertial deposition flux [g/m2/h] BL deposition flux [g/m2/h]

25. Vapor deposition Vapor deposition flux [g/m2/h]

26. Initial Deposition Rates Vary

27. Temporal Deposition Variation

28. Entrained particles Small particles capture in recirc. Most large particles impact lower region of SH2

29. Oxygen Contours

30. Deposition rates cont’d

31. Conclusions • Major deposition mechanisms are quantified. • Deposit properties depend in predictable ways on deposit microstructure. • Comprehensive computer codes can accommodate deposition mechanisms. • Predictions suggest major changes in design and operation for some facilities that can optimize operations.