Technical Bulletin No. 0098: The Effect of a Wet Bottom Precipitator Upon Recovery Furnace TRS Emission (AQTB)

The selection of dry bottom precipitators for kraft recovery furnaces has been a common choice during the past decade, particularly on furnaces not using contact evaporators. This choice has reflected concern for any practice which would bring black liquor in contact with flue gas, potentially increasing TRS emission. The material in the attached technical bulletin describes studies on a wet bottom precipitator, fed with un-oxidized liquor, to determine the contribution of this black liquor to TRS in the stack gas. The data were generated in part by NCASI and measurements made by a consultant on the same furnace were made available by a member company for inclusion in this bulletin. Problems were encountered in measuring TRS in the high particulate concentration gas in front of the precipitator, namely, loss of TRS in the sample handling system as a result of reaction with particulate. The bulk of the data collected, despite significant efforts to fabricate a sample handling and conditioning system that was capable of maintaining sample integrity for samples taken in front of the precipitator, is that following the precipitator. The TRS concentrations in the flue gas following the precipitator 'were sufficiently low, with liquor sulfidity as high as 28%, that it can be concluded that contributions of TRS by the liquor in the wet bottom precipitator were negligible. This is consistent with observations made on recovery furnaces with contact evaporators fed well oxidized liquor where the average TRS concentrations are 5 ppm or less. The findings of this study provide the basis for selection of alternate precipitator designs with equal protection of the environment.