The objective of this study was to investigate whether the release of sodium, potassium and chlorine from black-liquor particles burning in flight could be limited by chemical equilibrium. A novel approach was developed and tested. Estimates of possible thermodynamic limits were made using a simplified model of the lower-furnace flow and temperature fields. The results of these calculations gave support to the idea that the release of sodium is thermodynamically constrained and that this is the main factor limiting sodium release in the furnace. For chlorine, the results indicated that the extent of release would not be limited by chemical equilibrium and that the release of chlorine would have a significant dependence on how large a share of the total black-liquor combustion occurs in flight. This would result in a considerable scatter in the relative extents of chlorine release for different boilers and different operating modes. For potassium, only preliminary conclusions could be drawn because of uncertainty attached to the thermodynamic data. It is likely that the behaviour of potassium is analogous to that of sodium. For all three elements, the conclusions are in accordance with field measurements and observations. The new methods may be applied in conjunction with more detailed modelling of the furnace combustion process.
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