Mathematical modeling of intradeposit alkali chloride enrichment in biofuel boiler superheaters

Mathematical modeling based on Computational Fluid Dynamics (CFD) is carried out to investigate the effect of temperature gradient induced boiler deposit densification and alkali chloride enrichment in a boiler environment. The model considers the upper furnace of the boiler. For each superheater the calculated parameters include an average local deposit thickness, as well as distributions of heat transfer rate and intradeposit alkali chloride transport rate to the steel surface.

The model is applied in simulations of a pulp mill Kraft recovery boiler. The model demonstrate first steps taken toward more detailed predictions of deposit aging processes in boiler environments. The results show considerable variation in the local conditions and magnitude of the alkali chloride transport both within an individual superheater as well as when comparing superheaters to each other. The results suggest a potential for a considerable aging-effect concerning deposit composition and porosity. The predicted change in composition at the steel surface is estimated to correspond to a decrease in deposit first melting temperature of the order of magnitude 1-10 °C. Field data is needed to understand better the role of the intradeposit alkali chloride transport in boiler environments as well as to validate the model.