HEART OF A CHEMICAL PULP MILL

The recovery boiler plays a central role in the chemical cycle of a modern pulp mill. The boiler produces energy for the mill.

The spent liquor from the digesting plant goes first to the evaporation plant where the dry solids content of the liquor is increased. Then the evaporated liquor comes to the recovery boiler plant. Fly ash from electrostatic precipitators is mixed into the black liquor.  After additional concentration of the black liquor in the evaporation plant  the liquor is burned in the combustion chamber of the boiler.

Feed water is pumped first to the economizers where it is preheated by flue gas. The water then enters the water circulation system of the boiler. During combustion of  the black liquor high pressure steam is generated in the boiler.  The superheated steam flows from boiler to a turbine generator plant.

The hot smelt flow of the regenerated chemicals is drained from the furnace floor to the dissolving tank. The chemicals are dissolved into weak white liquor and returned to a recausticizing plant for further processing.

EXPERIENCE

Manufacturing of steam boilers started in Varkaus, Finland, in 1872. The first boilers were used in vessels.  The first recovery boiler was manufactured in 1952 for Lohja-Kotka Oy, Finland.  The steam pressure was 45 bar and steam temperature 400 C. The dry solids combustion capacity of the boiler was 110 tons per day.  The combustion capacity of modern recovery boilers is about 30 times higher.