Carter Holt Harvey Ltd.

Environmental regulations require operation of recovery unit operations within emission standardsThis paper describes recent discoveries on achieving TRS (total reduced S) compliance during black liquor processing and explores the often overlooked link with pulping operations.Specifically, TRS compliance projects for recovery boiler mix tanks, black liquor oxidation (BLOX) and crude tall oil (CTO) cooking are described.Black liquor temperature has been shown to influence TRS emissions during recovery processes.Elevated temperature increased organic sulfur emissions from black liquor mix tanks prior to the recovery boiler firing rail.Liquor temperature also requires careful consideration when operating black liquor oxidation stages with low sulfide, to prevent steam stripping of organic sulfur vapors.Anecdotal evidence also suggests pulping process conditions influence TRS emissions from black liquor, through altered black liquor compositionTraditional theory has suggested that the black liquor content of soap is strongly correlated with TRS emissions from tall oil processing.This may be the case when hydrogen sulfide is the primary TRS gas generated.However, experience has shown high turpentine levels in soap will cause high organic sulfur emissions from tall oil vent scrubbers.

This study developed a laboratory method producing smelt and green liquor containing up to ten times normal levels of inorganic process contaminants.Green liquor dregs composition, settling behavior and filtration behavior were determinedDregs settling rates for laboratory green liquors were compared to that of a base case green liquor (similar to current processes).Elevated levels of magnesium or calcium in green liquor enhanced dregs settling.Liquors containing elevated iron levels produced the greatest neg. effect on dregs settling rate.Dregs filtration properties were determined through calculation of bed permeability K, calculated from Darcy's Law.Dregs permeability showed significant reductions from base case behavior when silicon and iron contents were elevated.Validation of the laboratory simulation was completed by comparison with mill smelt and green liquor samples.Results showed the laboratory simulation produced green liquor similar to that produced in the mill process.