N-containing pollutant formation in pyrrole counterflow diffusion flames

Abstract

Biomass conversion through direct combustion (energy production) or pyrolysis (bio-oil production) are novel concepts to reduce CO₂ emission in the energy sector. However, the nitrogen content in biomass feedstocks may result in elevated N-containing pollutants, e.g., NO_x, NH₃, HCN, and nitriles, yet their formation chemistry remains unclear. In this work, we studied N-containing pollutant formation in counterflow diffusion flames fuelled by pyrrole, a biomass tar surrogate component that accounts for the majority of fuel nitrogen. 27 species, including 8 N-containing species, were identified and measured in three flames with designed boundary conditions to reveal the influence of flame temperature and methane addition. Species mole fraction comparisons showed that methane addition and higher flame temperature promoted C₂–C₆ hydrocarbon formation, but mole fractions of N-containing species did not change much, reflecting less dependence on flame temperature or hydrocarbons in the species pool. An existing kinetic model for pyrrole pyrolysis and combustion was developed by updating the formation reactions of N-containing species based on recent literature studies. Numerical simulations using the kinetic model well reproduced mole fractions of most species except for NO and NO₂. Model analyses illustrated the nitrogen conversion pathways from pyrrole to individual N-containing pollutant species, and indicated the possible reactions for underestimated mole fractions of NO and NO₂. This work contributes to a better understanding of the combustion properties of N-containing fuels and N-containing pollutants in the context of biomass energy clean utilization.