Lundberg pioneered the technology of indirect heating of black liquor at firing solids in 1982. There are several benefits to indirect black liquor heating:
- Liquor temperature can be maintained more closely than a direct unit, giving better control of combustion and reduction of carry over.
- There is no dilution of the liquor with steam condensate. This results in an increase in firing solids or evaporator/concentrator.
- Indirect liquor heating allows recovery of the clean steam condensate.
- The absence of direct steam injection eliminates the risk of diluting the black liquor below the safe firing concentration.
Lundberg heating systems utilize liquor circulation to maintain velocity within the heater at varying boiler operating rates. Location of the heater downstream of the boiler nozzle feed pumps and the use of a recirculation pump to offset the pressure drop through the heater means that the customer doesn’t need to modify or change their nozzle feed pumps. Pressure drop in the heater is minimized by the use of flow enhancing REX inserts inside the tubes which promote turbulence, allowing the total amount of recirculated flow to be reduced.
Lundberg’s recirculated system will operate more than a month between cleanings and in many locations they operate until the recovery boiler is taken down for a water wash. The direct contact heater can be used as a backup when cleaning is required. Cleaning of the heater 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.