Physics-based analytical correlation for NO x formation in partially premixed swirl-stabilized lean H2 combustors

Hydrogen combustion is gaining attention for its potential to enable low-emission energy conversion in gas turbines. Since hydrogen is carbon-free, it produces no carbon-based pollutants and primarily forms water vapor. However, due to its higher adiabatic flame temperature with respect to hydrocarbons, hydrogen combustion is more prone to increased nitrogen oxides (NO x ) formation. Accurately predicting NO x formation remains a major challenge, particularly when scaling from laboratory experiments to industrial applications. While scaling laws are widely used in fluid dynamics, their application to NO x emissions is challenging due to the complex nature of NO x formation. This study develops a semi-empirical, physics-based correlation to estimate NO x emissions in swirl-stabilized, partially premixed lean hydrogen–air burners. The proposed correlation expresses the Emission Index of NO x (EINO x ) as a function of key operating and design parameters, including fuel mass flow rate, pressure, adiabatic flame temperature, equivalence ratio, residence time and swirl number. It builds upon Westenberg’s NO x formation rate equation, incorporating composition-dependent effects via equivalence ratio. The slow formation nature of NO x is accounted for via the combustor mean residence time. Additionally, the influence of swirl on the mixing process is modeled through a swirl-modified effective equivalence ratio, acknowledging that while partial premixing is a design choice, swirl intensity can either enhance or disrupt the degree of premixing before combustion. The model’s parameters were calibrated using experimental data from published literature. By providing a predictive tool for NO x scaling across different operating conditions, this model supports the development and design of experiments and devices for hydrogen–air combustion. • Novel model predicts nitrogen oxide levels in hydrogen swirl flames. • Swirl intensity correction models imperfect fuel–air mixing effects. • Simple formula links emissions to pressure, temperature, and flow. • Extensive hydrogen burner emission data compiled and analyzed. • Model robustly predicts NOx across diverse burner configurations.