Numerical Study on Nitric Oxide Production of Moderate or Intense Low-Oxygen Dilution Combustion Using Ammonia and City Gas

Abstract

Ammonia (NH3) is attracting attention as a carbon-free fuel that does not emit carbon dioxide, but there is concern that a large amount of nitrogen oxides is emitted during its combustion. The mixtures of NH3 and city gas are used as fuels, and the effectiveness of Moderate or Intense Low-oxygen Dilution (MILD) combustion to reduce nitric oxide (NO) emissions is investigated through zero-dimensional and three-dimensional numerical simulations for various NH3 concentrations of the fuels at the lower heating value basis, ENH3. The zero-dimensional numerical simulations show that NO emissions reach their maximum value for ENH3 = 20–40%, and that dilution of the fuel and oxidizer by exhaust gas recirculation reduces NO emissions. On the other hand, the three-dimensional numerical simulations of an actual furnace show that MILD combustion is valid for reducing NO emissions for ENH3 = 0%, 20% and 100%. For burner design, the nozzle diameters used for the oxidizer, D, and the distance between the fuel and the oxidizer nozzles, L, are important parameters. For ENH3 = 20%, a 0.39-fold reduction in D and a 2.5-fold increase in L make the NO emissions 56% and 74%, respectively.