Abstract
Nitric oxide (NO x ) emissions from a partial premixed bluff body (PPBB) hydrogen burner under varying operational conditions have been investigated numerically. The PPBB burner employs a conical bluff body to stabilize the flame, and its design allows for the dilution of the fuel-air mixture by internally recirculated flue gas. The degree of premixing can be adjusted via primary and secondary fuel ports that are controlled independently. Steady-state computational fluid dynamic (CFD) simulations were conducted for 12 different combinations of secondary fuel fractions and thermal loads to investigate the complex flow structure in the burner and the source of NO x formation. All simulations were validated against experimental data and underpredicted NO x emissions by 7% on average. A detailed analysis of the combustion characteristics was conducted and showed that primary fuel is burned in a multiregime mode at a wide range of mixture fractions/equivalence ratios, while secondary fuel is burned closer to stoichiometry. Utilizing a secondary fuel fraction of 30% (i.e., the base operational condition) leads to a fuel-rich mixture within the inner recirculation zone compared to a fuel-lean mixture when the burner is operated without secondary fuel. The concave mean curvature of the stoichiometric isosurface between primary and secondary fuel leads furthermore to a local peak of the NO x formation rate when the burner operates at its base conditions. The analysis of the mass flow rates in the inner and outer recirculation zones showed that increasing the thermal load reduces the amount of internally recirculated flue gas. The conducted simulations indicate that these effects contribute to an increase in NO x emissions with increasing thermal load and with increasing secondary fuel fraction. © 2019 American Chemical Society.
