Abstract: Off gases from black liquor heat treatment, evaporators, wood pulp digesters, etc. are treated to produce primarily hydrogen sulfide and sulfur free fuel gases such as methane, carbon monoxide, hydrogen, and ethylene. Then the hydrogen sulfide is used to produce sodium polysulfide in white liquor for cooking wood chips to produce pulp by the kraft process. Polysulfide may be formed in three different ways: by supplying air to a fuel cell into which white liquor and the hydrogen sulfide containing gas have been introduced; by bringing clarified white liquor and hydrogen sulfide containing gas into contact with a wet-proofed activated carbon catalyst and oxygen containing gas; or by bringing unclarified white liquor into contact with the gas and oxygen containing gas, with lime mud acting as a catalyst, and then clarifying the polysulfide-rich white liquor produced to remove the lime mud. The white liquor produced contains about 0.5-8% on wood sodium polysulfide.