Mechanism study on the effects of Na/K hydroxides and chlorides on NO reduction by biomass volatiles reburning during high-alkali coal combustion.

To clarify the influence of alkali metals on NO reduction by biomass volatiles during high-alkali coal combustion, a three-stage plug flow reactor (PFR) model with detailed C-H-N-O-Cl-Na-K mechanisms was established on the Chemkin platform. This study systematically investigated the influence of alkali-metal forms (NaOH, KOH, NaCl, KCl) on the deNOx performance of biomass volatiles and their individual components (H2、CO、CH4) under different air equivalence ratios (λ) and temperatures in the reburning zone. The results show that alkali metals (denoted as A, A = Na and K) consume H/OH radicals via the "AOH → A → AOH" cycle. When λ ≥ 0.9, they slightly promote deNOx by inhibiting the oxidation of nitrogen-containing intermediates (e.g., NH) by OH to form NO, increasing the NO reduction efficiency by about 10%. Conversely, when λ < 0.9, they switch to inhibiting deNOx, with the inhibition intensity varying significantly with fuel types and alkali metal forms. The inhibition follows the order CO > H2 > CH4 due to distinct deNOx pathways, while biomass volatiles exhibit intermediate inhibition between that of CO and CH4, resulting from multi-component synergistic effects. The effect peaks in the range of 900-1000 °C and weakens at higher temperatures. Chlorides (ACl) mainly participate in the inert cycle "ACl → A → ACl", resulting in weaker effects than hydroxides. Consequently, the influence on NO reduction follows NaOH > KOH ≈ NaCl > KCl. This work elucidates the alkali-metal mechanisms in reburning deNOx and supports low-NOx combustion optimization for high-alkali fuels.