Synergistic alkali diagnostics: PF-TDLAS and surface ionization detection in a high-temperature reactor

Artikel i vetenskaplig tidskrift, 2026

Alkali plays a critical role in most systems using biogenic fuels, yet quantification is difficult and often method-dependent, due to partitioning between particles and gas, chemical conversion, and strong interactions with surrounding surfaces. Building on the limited prior work combining online alkali diagnostics, we demonstrate a combined diagnostic approach using photofragmentation tunable diode laser absorption spectroscopy (PF-TDLAS; in situ, path-averaged, gaseous potassium chloride (KCl) and potassium hydroxide (KOH)) and surface ionization detection (SID; point-sampled total potassium (K), gas and particles) in a well-controlled heated reactor with potassium aerosol seeding. Reactor temperature, atmosphere (air, N2, CO2), and the amount and chemical form of injected potassium (chloride, hydroxide, carbonate or sulfate) were varied, and the data were interpreted with computational fluid dynamics (CFD) and thermodynamic equilibrium calculations. Together, the diagnostics resolve potassium partitioning and speciation and quantify what reaches an extraction point: at low temperature they indicate particulate K (high SID, negligible PF-TDLAS signals); at higher temperatures, they capture the particle-to-gas transition. In this work, the transition occurred around 700 °C rather than ∼550 °C predicted by equilibrium, since the injected jet remained colder locally (CFD). At 900 °C, both diagnostics respond consistently in terms of absolute K concentrations and as a function of alkali seeding, while revealing speciation differences (KCl(g) for KCl seeding; KOH(g) plus minor KCl(g) for KOH seeding). Atmosphere and seed chemistry further modulate the signals: CO2 reduces delivered K-loading and promotes carbonate formation during KOH seeding, suppressing KOH(g) in PF-TDLAS, while maintaining elevated total-K in the SID. This combined framework shows that alkali measurement is intrinsically multi-factor and that PF-TDLAS and SID should be applied synergistically, not interchangeably. Novelty and significance statement Building on the only earlier combined optical/SID alkali study known to us, we present a coordinated, quantitative PF-TDLAS/SID deployment in a more well-controlled high-temperature reactor, substantially expanding the scope by varying temperature, atmosphere, and the amount of injected potassium, and by adding KOH, K2CO3, and K2SO4 seeding. PF-TDLAS provides in situ, path-averaged gas-phase KCl(g) and KOH(g), while SID measures extractive total potassium (gas and particles). The novelty lies in integrating these complementary measurements with CFD-resolved flow and temperature fields and thermodynamic equilibrium calculations to develop a mechanistic interpretation framework for alkali quantification under non-uniform reactor conditions. This framework rationalizes how phase-partitioning, transport/surface losses, spatial non-uniformity, sampling bias, and chemistry-dependent transformations jointly govern the PF-TDLAS and SID signals. The result is a transferable framework for robust alkali quantification with repeatable sub-ppm sensitivity, improving cross-comparability for combustion-relevant systems where alkali partitioning and losses govern fouling, corrosion, emissions, and catalytic effects.