Controlling sulfidity by recycling sulfur in CNCG, TAPPI Journal July 2026

 


Application: Making sulfuric acid from CNCGs creates a new opportunity to manage mill sulfidity. Instead of dumping ESP ash and buying new caustic to replace what was lost, a mill can make its own acid, which means savings in chemical makeup costs.

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Author: Mariana Carvalho, Katja Kuparinen, Satu Lipiäinen, Esa K. Vakkilainen, and Marcelo Hamaguchi
Controlling sulfidity by recycling sulfur in CNCG, TAPPI Jou
ABSTRACT: Pulp and paper mills have faced a major dilemma. On one hand, mills should not dispose of the side streams by mixing them with effluent water. On the other hand, there has been no other way to dispose of excess sulfur than getting rid of recovery boiler electrostatic precipitator (ESP) ash and buying sodium hydroxide to replace lost sodium. A new, yet old, process has been introduced to alleviate the problem. An alternative is the onsite production of sulfuric acid (H2SO4) to significantly reduce, or to even eliminate, the need for recovery boiler fly ash dumping. A sulfuric acid plant produces concentrated commercial quality H2SO4 from concentrated non-condensable gases (CNCGs). Up to 99.9% of the sulfur in CNCGs can be converted into concentrated sulfuric acid with this wet-gas sulfuric acid (WSA) process. There are already references on pulp mills highlighting that the process is straightforward to use. Mills can increase the amount of sulfur in CNCGs by controlling the heavy back liquor storage time and temperature. This opens up a new way to control the mill sulfidity by recycling sulfur. The effect of recycling sulfur is highlighted with reference mills. Due to integrated paper production, the Northern Mill is a large energy consumer, producing mostly softwood-based end products. The Eucalyptus Mill is a large standalone hardwood (eucalyptus) pulp mill where electricity is produced substantially in excess of its own use. As a sample case, lignin production with and without sulfur recycling is presented.
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