Thermal–mass transfer and flue gas characteristics of a steam boiler fueled with hydrogen carrier-blended natural gas

Steam boilers are widely used in industrial processes, where combustion performance and pollutant emissions strongly influence energy efficiency and decarbonization. Natural gas blended with hydrogen or ammonia has recently been considered a promising low-carbon combustion strategy. However, the high flame speed and elevated flame temperature of hydrogen may alter flame structures and promote nitrogen oxides (NOx) formation. To evaluate the effects of hydrogen- and ammonia-blended natural gas on heat transfer behavior and emission characteristics in steam boilers, computational fluid dynamics (CFD) simulations are conducted. Temperature distribution, flow characteristics, and exhaust gas composition are analyzed. Results show that, in a single-tube boiler, increasing the hydrogen blending ratio from 0% to 30% raises the outlet temperature by 4.16% compared with pure natural gas, reaching a maximum of 1408.3 K, while the carbon dioxide mass fraction decreases significantly. The increase in outlet temperature indicates enhanced convective heat transfer within the boiler. However, the higher combustion temperature increases NOx concentration by approximately 27.1 ppm. In contrast, ammonia blending, characterized by a slower reaction rate and lower flame temperature, suppresses thermal NO formation and promotes NO reduction reactions, resulting in a maximum NOx reduction of about 12 ppm. Nevertheless, the outlet temperature decreases with increasing ammonia fraction, reaching approximately 1234 K at 30% ammonia blending, while the residual oxygen concentration increases to around 8.0%, indicating incomplete combustion. This study establishes a CFD-based comparative framework under identical boiler geometry and constant heating-value conditions to clarify the combustion and emission trade-offs between hydrogen- and ammonia-blended fuels.