Flame Inhibition by Potassium-Containing Compounds

A kinetic model of inhibition by the potassium-containing compound potassium bicarbonate is suggested. The model is based on the previous work concerning kinetic studies of suppression of secondary flashes, inhibition by alkali metals and the emission of sulfates and chlorides during biomass combustion. The kinetic model includes reactions with the following gas-phase potassium-containing species: K, KO, KO₂, KO₃, KH, KOH, K₂O, K₂O₂, (KOH)₂, K₂CO₃, KHCO₃ and KCO₃. Flame equilibrium calculations demonstrate that the main potassiumcontaining species in the combustion products are K and KOH. The main inhibition reactions, which comprise the radical termination inhibition cycle are KOH+H=K+H₂O and K+OH+M=KOH+M with the overall termination effect: H+OH=H₂O. Numerically predicted burning velocities for stoichiometric methane/air flames with added KHCO₃ demonstrate reasonable agreement with available experimental data. A strong saturation effect is observed for potassium compounds: approximately 0.1% volume fraction of KHCO₃ is required to decrease burning velocity by a factor of 2, however an additional 0.6% volume fraction is required to reach a burning velocity of 5 cm/s. Analysis of the calculation results indicates that addition of the potassium compound quickly reduces the radical super-equilibrium down to equilibrium levels, so that further addition of the potassium compound has little effect on the flame radicals.