Numerical investigation and modeling of NO_x formation in pulverized biomass flames under air and oxyfuel conditions

In this study, NO_x formation pathways in biomass combustion in air and oxy-atmospheres are investigated by direct numerical simulations. Solid biomass fuels contain fuel-bound nitrogen, which contributes to NO_x formation and complicates NO_x predictions. The NO_x formation pathways and modeling of NO_x formation in biomass combustion are not fully understood, necessitating further investigation through detailed kinetic models. Reactive biomass simulations are performed in a drop tube configuration under laminar conditions, considering the detailed NO_x chemistry for both the solid fuel and the gas phase. To this end, the detailed CRECK-S kinetic scheme is employed for the solid phase. In addition, due to the lack of biomass-specific gas-phase kinetic models in the literature, particularly in terms of the released biomass volatiles and their impact on NO_x formation pathways, a special gas-phase kinetic model, including the NO_x formation pathways for biomass combustion, is designed and utilized in the simulations. NO_x formation in solid fuel flames can be affected by several parameters, such as solid fuel type and composition, particle injection rate, and ambient conditions. The current study assesses the sensitivity of NO_x formation to these parameters. In particular, a detailed pathway analysis is performed to identify the contributions of fuel-related and thermal pathways on the total NO_x formation. Finally, the released volatile composition effect on NO_x formation is evaluated using the fixed volatile composition assumption, which is required in flamelet-based reduced-order modeling of solid fuel combustion using simplified solid kinetic models, in comparison with the dynamically released volatiles predicted from detailed solid fuel kinetics. An improved approach for the fixed volatile composition formulation is proposed. Novelty and significance statement In this work, NO_x formation pathways were numerically investigated during solid pulverized biomass combustion under different operating conditions using detailed chemical kinetic models for both solid and gas phases. Using the detailed numerical framework, the impact of fixed volatile composition on NO_x formation, which is the required assumption for reduced-order flamelet tabulated chemistry models, was evaluated. The novelty and significance of this work can be summarized in two points. First, the new chemical kinetic model containing the biomass-relevant chemistry based on the released volatile species from biomass enabled the detailed pathway analysis under different operating conditions. Second, the drawbacks of the FVC assumption in predicting NO_x were discovered, and a novel formulation was introduced for correctly capturing the NO_x pollutants. This is of critical importance for the enhancement of the reduced-order models in predicting pollutant emissions during solid pulverized fuel combustion.