Advances in Absorption Processes for Nitrogen Oxides and Sulfur Oxides: Updated Mechanism for NO2-Induced Sulfite Oxidation

Licentiate thesis, 2026

Nitrogen oxides (NO_x) and sulfur oxides (SO_x) are hazardous air pollutants that are released primarily from combustion processes associated with power generation, industrial activities, and transportation. The best available technique (BAT) for handling these emissions from power generation and industrial processes involves two separate steps, applying selective catalytic reduction (SCR) for NO_x and limestone-based scrubbing for SO_x removal. While effective, this approach entails high investment and operational costs and constrains process flexibility due to the narrow temperature window of the catalyst. Absorption-based co-removal technologies are a promising alternative to remove both pollutants simultaneously under ambient conditions. However, a key challenge limiting commercial viability is the high consumption of sodium sulfite (Na₂SO₃), which is required to maintain satisfactory NO₂ absorption. The mechanism underlying the rapid depletion of sulfite in the presence of NO₂ and O₂ remains poorly characterized, with oxidation rates that vary significantly across studies and substantially exceed the reported kinetics.
This thesis addresses the knowledge gap related to the aqueous-phase chemistry governing the simultaneous absorption of NO_x and SO_x through kinetic and process modeling. In Paper I, five reaction sets are proposed to describe the rapid sulfite oxidation and the radical scavenging effect of thiosulfate observed in laboratory-scale experiments. The proposed mechanism shows good agreement with measurement data in terms of the liquid and gas-phase concentrations. In Paper II, the proposed mechanism is integrated into a process simulation, to evaluate process behavior under industrially relevant conditions. A sensitivity analysis identifies the inlet concentrations of NO₂ and sulfite as the key parameters governing process performance, with moderate impacts from mass transfer parameters. The findings contribute a new mechanistic understanding of sulfite oxidation kinetics and provide a basis for improved process design and control in absorption-based co-removal systems.