Optical investigation of gas-phase KCl/KOH sulfation in post flame conditions

A counter-flow reactor setup was designed to investigate the gas-phase sulfation and homogeneous nucleation of potassium salts. Gaseous KOH and KCl were introduced into the post-flame zone of a laminar flat flame. The hot flame products mixed in the counter-flow with cold N₂, with or without addition of SO₂. The aerosols formed in the flow were detected through Mie scattering of a 355 nm laser beam. The temperature distribution of the flow was measured by molecular Rayleigh scattering thermometry. From the temperature where nucleation occurred, it was possible to identify the aerosols formed. Depending on the potassium speciation in the inlet and the presence of SO₂, they consisted of K₂SO₄, KCl, or K₂CO₃, respectively. The experiments showed that KOH was sulphated more readily than KCl, resulting in larger quantities of aerosols. The sulfation process in the counter-flow setup was simulated using a chemical kinetic model including a detailed subset for the Cl/S/K chemistry. Similar to the experimental results, much more potassium sulfate was predicted when seeding KOH compared to seeding KCl. For both KOH and KCl, sulfation was predicted to occur primarily through the reactions among atomic K, O₂ and SO₂, forming KHSO₄ and K₂SO₄. The higher propensity for sulfation of KOH compared to KCl was mostly attributed to the lower thermal stability of KOH, facilitating formation of atomic K. According to the model, sulfation also happened through SO₃, especially for KCl (KCl → KSO₃Cl → K₂SO₄).