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Combustion and Flame · 2026

Resolving NO formation in ammonia–methane diffusion flames

Formation of NO in counterflow diffusion flames of ammonia-methane: Spatially-resolved measurements and kinetic analysis

Using ammonia alongside natural gas offers a route toward lower-carbon industrial fuels, with NO control a key challenge. This study examines NO-formation mechanisms to inform cleaner combustion.

The motivation

Co-firing ammonia with natural gas offers a gradual route toward lower-carbon industrial heating, but fuel-bound nitrogen introduces an NO-emission challenge. Most fundamental studies have examined premixed flames, whereas industrial furnaces commonly use non-premixed combustion. This study addresses the lack of quantitative counterflow diffusion-flame data and examines how mixing and flame structure alter NO formation.

The approach

Ammonia fraction, oxidizer oxygen fraction and nozzle velocity were varied in a counterflow burner. Axial NO profiles were measured using ceramic microprobe sampling coupled with mid-infrared laser absorption spectroscopy, alongside flame-temperature measurements. Detailed chemical-kinetic simulations, sensitivity analysis and reaction-pathway analysis were used to relate the measured trends to radical distributions.

Counterflow burner and optical arrangement for NO measurements. Gas sampled by a ceramic microprobe is analyzed in a heated absorption cell; the right-hand panels show an enlarged view and a representative flame. Original Fig. 2.
Counterflow burner and optical arrangement for NO measurements. Gas sampled by a ceramic microprobe is analyzed in a heated absorption cell; the right-hand panels show an enlarged view and a representative flame. Original Fig. 2.

Key findings

  1. At 28% oxygen and an inlet velocity of 20 cm/s, peak NO rises monotonically with the ammonia fraction. Kinetic analysis identifies the enhanced fuel-nitrogen route through HNO as a key reason for this trend.
  2. In the pure-ammonia flames studied, increasing strain lowers the temperature but raises peak NO. Mechanistic analysis links this behavior to greater spatial overlap of OH and NH₂, in contrast to the trend in pure-methane flames.
  3. Oxygen enrichment increased NO in both pure-fuel flames, but the ammonia flame showed a weaker response than the methane flame, reflecting their different NO-formation mechanisms.
Effects of ammonia fraction on axial NO profiles and peak NO. The oxidizer oxygen mole fraction is 0.28 and both nozzle velocities are 20 cm/s. Symbols denote measurements and lines denote simulations. Original Fig. 6.
Effects of ammonia fraction on axial NO profiles and peak NO. The oxidizer oxygen mole fraction is 0.28 and both nozzle velocities are 20 cm/s. Symbols denote measurements and lines denote simulations. Original Fig. 6.

About this paper

Qing Li, Bowen Sun, Xuan Zhao, Liuhao Ma, Yu Wang

Combustion and Flame · 2026 · 288 · 114970

Publisher record (DOI)