Bed Cooling Following an ESP, 1998 International Chemical Recovery Conference Proceedings

Initiating an Emergency Shutdown Procedure (ESP) on a recovery boiler, can leave a large hot bed on the hearth which may contain a considerable amount of molten smelt. Because of the potential for a smelt-water explosion, the bed must be allowed to cool before waterwashing the furnace as part of the repair procedure. Thus there is a need for reliable means to determine when the bed and smelt have cooled sufficiently so that water could safely enter the furnace cavity. This paper examines procedures for measuring temperatures in char beds and for using such temperature data in assessing the thermal state of the cooling bed. The starting point is a review of a study of char bed cooling after ESP’s that was carried out by Arthur D. Little Inc. (ADL) for the AF&PA Recovery Boiler Committee about 20 years ago. Bed cooling from the bottom is at least as important as cooling from the top. Calculations indicate that a minimum of about 24 hours is needed to freeze a typical smelt pool by cooling from the bottom only. Longer times would be needed if the smelt is located away from the bottom or in isolated pockets. Continuing combustion adds significantly to the cooling load and increases the molten smelt inventory. Combustion should be minimized or eliminated, and specific steps to achieve this may be warranted. The main resistance to heat transfer is internal to the bed. Enhancement of heat conduction by interpore radiation is an important factor in heat transfer within the bed and bed porosity and pore size distribution are key factors. Except in isolated locations, temperature gradients within the bed are not excessive. Thermoprobes inserted into the bed on the order of a meter can provide an effective measurement of the average thermal state of the cooling bed.