A. H. Lundberg Associates, Inc. pioneered the technology of indirect black liquor heating in 1982.
The benefits of indirect black liquor
heating are several fold. First is that the liquor temperature can be maintained more closely than a direct unit, giving better control
of combustion and reduction of carry over. Second is that there is no dilution of the liquor with steam condensate. This results in an
increase in firing solids or evaporator/concentrator capacity and allows recovery of the clean steam condensate. Finally, the absence of
direct contact minimizes the risk of diluting the black liquor below the safe firing concentration.
The Lundberg Associates heating system utilizes liquor circulation to maintain velocity within the heater at varying boiler operating rates.
Further, the location of the heater downstream of the boiler nozzle feed pumps and the use of the recirculation pump does not necessitate any
change to the nozzle feed pumps. The additional pressure drop of the heater is taken up by the recirculation pump. This pressure drop is
minimized by the use of flow enhancers inside the tubes which promote turbulence, allowing the total amount of recircultated flow to be reduced.
The black liquor heater is a shell and tube unit with the liquor within the tubes. The unit may be installed in either a slightly inclined
horizontal position or a vertical position. The orientation and piping arrangement must facilitate the removal and drainage of wash condensate
or weak liquor from the heater.
The Lundberg Associates recirculated system will operate more than a month between forced cleaning and in many locations they operate until the
recovery boiler is taken down for cleaning. The direct contact heater is usually used as a back up when cleaning is required. When forced
cleaning of the heater is needed, it is done using weak black liquor, process condensate or mill water. When cleaning is complete, the heater
can be brought back on line without interruption of boiler operation.
