Air port flow patterns in kraft recovery boilers

Severe localized corrosion on the coldside of the primary ports of many high pressure kraft recovery boilers is believed to be due to the condensation of molten alkali hydroxides which are produced in the combustion zone. The air flow patterns in the windbox and in the vicinity of the primary air ports of a kraft recovery boiler were therefore numerically simulated to investigate the possible transport of gaseous alkali hydroxides to the port coldside. The flow pattern close to the air port opening is sensitive to the air port geometry but less so to the char bed geometry and fireside boundary conditions. Flue gases from the combustion zone are entrained towards the air ports by the air jets due to the turbulent shear stress at the jetflue gas interface. The close proximity of adjacent air jets and the entrained flue gases combine to create a recirclation zone adjacent to the port openings. The intensity and position of the recirculation zone is strongly affected by the air port geometry. Sharp changes in the opening shape lead to backward flow towards the coldside of the air port. These results indicate that air port design Inay play an important role in the transport of alkali hydroxides from the combustion zone to the coldside of the air ports and hence corrosion of tubes.