Carrier-phase direct numerical simulation and flamelet modeling of alkali metal emissions from pulverized biomass flames

Combustion of biomass, as a CO2-neutral and renewable fuel, is an appealing option in transition to sustainable energy sources. However, biomass combustion is prone to generate harmful alkali metals and corresponding sulfates. In this study, we analyze the potassium-containing species emissions in the early stages of turbulent pulverized biomass combustion (PBC) in an oxy-fuel atmosphere. To this end, a 3D carrier-phase direct numerical simulation of pulverized walnut shells combustion is carried out. A realistic fuel composition including heavy tars, obtained by the state-of-the-art CRECK-S-B model, KOH, KCl, and SO2 is considered and detailed homogeneous kinetics for both hydrocarbons and K-Cl-S containing species is employed. The DNS results show that in the considered conditions, the flame is predominantly non-premixed in the vicinity of the stoichiometric surface, while in richer regions, in the center of the turbulent jet, a mix of premixed and non-premixed combustion modes is prevalent. Potassium sulfate (K2SO4) forms in a high amount in a fuel-rich and strained region in the center of the jet, mainly through the direct reaction of K-radicals with SO2. The chemical time scale analysis reveals the slow processes of KOH consumption and K2SO4 production in this region. Closer to the flame surface, K2SO4, KCl, and KOH are consumed which lead to the production of K and KO2. The predictions from a flamelet/progress variable approach are compared to the DNS data. Overall, the predictions of the flamelet model based on steady non-premixed flamelets are satisfactory. Specifically, temperature, light hydrocarbon species, and Cl-containing species (HCl and KCl) are well predicted across the jet flame. A major tar species (C8H8O3), K, KOH, and K2SO4 predictions show discrepancies with the DNS data in the center of the jet, which can be related to the slow kinetics of these species and a mixed combustion mode in this region.