Generation-reduction-burnout strategy empowering ultra-low pollutant emissions in biomass chain grate boilers: CFD modeling and industrial experimental validation

Biomass chain grate boilers, which utilize biomass as fuel, encounter significant challenges in controlling nitrogen oxide (NOx) emissions without compromising combustion efficiency. This study proposes a novel low-NOx combustion strategy for biomass chain grate boilers, referred to as the "Generation-Reduction-Burnout" (GRB) strategy. The approach involves establishing a deep pyrolysis-gasification zone (primary air α = 0.7, achieving a low O2 concentration environment within the pyrolysis-gasification zone) to maximize the generation of reducing gases. These gases are further enhanced by injecting recycled flue gas as secondary air to strengthen the reducing atmosphere. Finally, tertiary air is used to achieve directional CO oxidation. Mechanisms are quantified through computational fluid dynamics (CFD) modeling and industrial-scale experiments, which validate the following findings: Within the deep pyrolysis-gasification zone, minimal fuel nitrogen is oxidized to NO, while the recycled flue gas alleviates local overheating in the furnace. The synergy of these measures results in a reduction of NOx emissions to 50 mg/m3 (at 9 % O2). The introduction of tertiary air achieves directed CO oxidation, reducing CO emissions to 35 mg/m3. Furthermore, by verifying the combustible content in the furnace slag, it is demonstrated that this approach effectively ensures combustion efficiency. This strategy provides a dual control of NOx and CO emissions, offering a new perspective for achieving ultra-low emissions in biomass chain grate boilers.