Numerical investigation of NOx production from premixed hydrogen/methane fuel blends

Hydrogen (H2) fuel is a promising means for long duration energy storage and dispatchable utilization of intermittent renewable power, which can be combusted without CO2 emissions. Several recent studies have noted that hydrogen blended systems have elevated NOx emissions relative to natural gas, prompting significant concerns about the air quality impacts of a hydrogen economy. However, many of these results are for nonpremixed systems and it is not entirely clear what is being held constant for these comparisons (temperature, power, etc.). Given the strong temperature sensitivity of NOx production, these results cannot be applied more generally to understand NOx emissions tendencies. This study reports calculations of NO emissions of perfectly premixed H2/CH4 blends at constant flame temperatures, presenting results as a function of residence time, over a range of pressure and temperature conditions. Four major findings are presented: (1) At constant temperature, NO emissions, quantified as mass per energy input, via ng/J, actually decrease with the addition of H2 for NO measurements taken at typical gas turbine residence times (10–20 ms). This effect is less pronounced at elevated temperatures/pressures; (2) NO volume concentrations, when measured in a dried sample and corrected to 15% O2, do not increase by the same amount as emissions quantified in ng/J over the same conditions. This demonstrates that one cannot directly compare volume concentration-based NO values when H2 composition is varied; (3) the post-flame NO production rate negligibly increases with addition of H2 at most conditions, especially at high power; (4) for realistic gas turbine combustor residence times, the dominant NO production under atmospheric pressure conditions occurs in the flame itself, while they occur post-flame via thermal-NO at practical high pressure conditions. In other words, atmospheric pressure fuel sensitivity NOx studies will not capture the controlling NO production physics that are present in practical applications, such as gas turbines.