The corrosion problems in the pulp and paper industry are numerous and varied. In a Kraft paper mill the economics of the process are governed by the recovery of spent chemicals from the pulp digester. The recovery boiler plays several roles in the operation of the mill. The spent chemicals from the digester which are dissolved in water along with the herbaceous residues that were extracted from the pulp are the fuel for the boiler and are known as black liquor. When the black liquor is sprayed into the boiler, the water flashes to steam and becomes part of the flue gas. The spent chemicals are chemically reduced as a result of combustion of the organics with deficient air. The reduced chemicals fuse and fall to the bed of the furnace and are drawn off to be recycled into the digester. At higher elevations in the furnace, additional air is introduced to complete the burning of the organic materials. This heat is utilized to produce steam for a conventional steam turbine and generator. The turbogenerators are typically capable of producing from 10 to 50 MW of power for running the plant. Sometimes the surrounding communities or power companies purchase excess power from the mill.
