Numerical analysis of NH3-CH4-air mixing quality effects on NOx formation in an air-staged gas turbine model combustor

Research Background

Amid the global drive toward carbon neutrality, ammonia (NH₃) has emerged as a promising carbon-free fuel for gas turbines, thanks to its high energy density, easy transportation, and storage convenience. However, ammonia’s nitrogen-containing nature leads to excessive NO_x emissions during combustion, posing a critical barrier to its widespread adoption. Air-staged combustion is a proven proactive strategy for NO_x reduction, but the role of fuel/air mixing quality, which is quantified by unmixedness in ammonia-methane (NH₃-CH₄) blended combustion remains insufficiently explored. While previous studies have focused on NO_x control via equivalence ratio adjustment and reaction pathway optimization, a systematic quantitative analysis of how unmixedness impacts NO_x formation in air-staged systems is lacking, creating a gap for engineering guidance.

Research Content

The study employed numerical simulations using a chemical reactor network (CRN) composed of perfectly stirred reactors (PSRs) and plug flow reactors (PFRs) to model an air-staged combustor. Researchers adopted the Tian chemical mechanism (84 species, 703 reactions) to analyze NO_x formation under typical H/J-class heavy-duty gas turbine conditions (23 atm pressure, 1873 K outlet temperature). The fuel blend consisted of NH₃ (40% volume ratio) and CH₄, with key variables including the primary-stage equivalence ratio (1.0–2.0) and fuel/air unmixedness (0–0.4). The study quantified NO_x formation across different mixing quality levels, analyzed dominant reaction pathways (HNO, thermal NOx, NHi, N2O), and explored the impact of residence time allocation (total 20 ms) on emission reduction.

Research Results

1. 1. Optimal Equivalence Ratio Range: Under perfect fuel/air mixing, NO_x formation remained low when the primary-stage equivalence ratio was 1.2–1.5. Within this range, NO_x emissions were insensitive to unmixedness as long as the value was below 0.12.
   2. Unmixedness Threshold: When unmixedness exceeded 0.12, the NO_x reduction advantage of air-staged combustion was lost across all equivalence ratios, highlighting the critical need for precise mixing control.
   3. Dominant Reaction Pathway: Most NO_x (up to thousands of ppm when unmixedness > 0.3) was formed in the secondary-stage main combustion zone via the HNO pathway. While NHi (NH₂+NH) and N₂O pathways partially offset NO_x production from HNO and thermal routes, their mitigation effect was limited.
   4. Residence Time Optimization: By adjusting residence time allocation in the primary and secondary stages, NO_x emissions could be further reduced to as low as 48 ppm (at unmixedness = 0.04 and primary-stage post-combustion residence time = 18 ms).

Research Significance

This study fills a critical gap in understanding the role of fuel/air mixing quality in ammonia-fueled combustion systems, providing actionable engineering guidance for low- NO_x combustor design and operation. The proposed unmixedness thresholds (e.g., <0.04 for NO_x <100 ppm at primary equivalence ratio 1.2) and residence time optimization strategies offer practical solutions to mitigate NO_x emissions. By clarifying the dominant reaction pathways and mixing-sensitive mechanisms, the research advances the technical feasibility of ammonia-methane blends in gas turbines, accelerating the adoption of carbon-free fuels and supporting global carbon neutrality goals. The findings are particularly valuable for the energy industry’s transition from fossil fuels to low-carbon alternatives, enabling efficient and environmentally friendly power generation.