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<p>NH<sub>3</sub> has emerged as a promising candidate for low-carbon gas turbines, with NO<sub><i>x</i></sub> emission issues being mitigated by air-staged combustion. However, the role of fuel/air mixing quality (represented by unmixedness) in NO<sub><i>x</i></sub> formation in NH<sub>3</sub> systems remains poorly explored. In this study, the characteristics of NO<sub><i>x</i></sub> formation under the effects of unmixedness have been numerically investigated using an NH<sub>3</sub>/CH<sub>4</sub> fired air-staged model combustor consisting of perfectly stirred reactors (PSRs) and plug flow reactors (PFRs), employing the 84-species, 703-reaction Tian mechanism under H/J heavy duty gas turbine conditions. It was found that a primary-stage equivalence ratio of 1.2–1.5 corresponds to a low NO<sub><i>x</i></sub> formation region under perfectly mixed fuel and air conditions. In this region, a relatively low NO<sub><i>x</i></sub> formation is achieved when the unmixedness is less than 0.12 and NO<sub><i>x</i></sub> formation exhibits low sensitivity to fuel/air unmixedness. Based on these findings and the fact that the air-staged combustion loses its advantage in reducing NO<sub><i>x</i></sub> emissions when the unmixedness exceeds 0.12 across all equivalence ratios, recommended mixing quality thresholds for different equivalence ratios are proposed to guide combustor design and operation optimization. A parametric study of chemical reaction pathways at different unmixedness levels in the two stages demonstrates that NO<sub><i>x</i></sub> is mainly formed in the main combustion zone of the secondary stage via the HNO pathway, which results in NO<sub><i>x</i></sub> formation rising to thousand ppm when unmixedness exceeds 0.3, although NO<sub><i>x</i></sub> reduction through NHi and N<sub>2</sub>O pathways partially offsets contributions from the HNO and thermal NO<sub><i>x</i></sub> pathways. To leverage the NO<sub><i>x</i></sub> reduction potential of the NHi and N<sub>2</sub>O pathways, the residence time in both stages should be carefully adjusted to help suppress NO<sub><i>x</i></sub> to as low as 48 ppm. The results of this study are important for engineering applications, providing guidance for the design of NH<sub>3</sub> fired combustors aimed at significantly reducing NO<sub><i>x</i></sub> formation.</p>

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

  • 期刊论文

  • 发表日期：

    2025年6月15日

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  • Shan Li,
  • Long Zhang,
  • Xiaopeng Li,
  • Pengfei Fu,
  • Hua Zhou

摘要

NH3 has emerged as a promising candidate for low-carbon gas turbines, with NOx emission issues being mitigated by air-staged combustion. However, the role of fuel/air mixing quality (represented by unmixedness) in NOx formation in NH3 systems remains poorly explored. In this study, the characteristics of NOx formation under the effects of unmixedness have been numerically investigated using an NH3/CH4 fired air-staged model combustor consisting of perfectly stirred reactors (PSRs) and plug flow reactors (PFRs), employing the 84-species, 703-reaction Tian mechanism under H/J heavy duty gas turbine conditions. It was found that a primary-stage equivalence ratio of 1.2–1.5 corresponds to a low NOx formation region under perfectly mixed fuel and air conditions. In this region, a relatively low NOx formation is achieved when the unmixedness is less than 0.12 and NOx formation exhibits low sensitivity to fuel/air unmixedness. Based on these findings and the fact that the air-staged combustion loses its advantage in reducing NOx emissions when the unmixedness exceeds 0.12 across all equivalence ratios, recommended mixing quality thresholds for different equivalence ratios are proposed to guide combustor design and operation optimization. A parametric study of chemical reaction pathways at different unmixedness levels in the two stages demonstrates that NOx is mainly formed in the main combustion zone of the secondary stage via the HNO pathway, which results in NOx formation rising to thousand ppm when unmixedness exceeds 0.3, although NOx reduction through NHi and N2O pathways partially offsets contributions from the HNO and thermal NOx pathways. To leverage the NOx reduction potential of the NHi and N2O pathways, the residence time in both stages should be carefully adjusted to help suppress NOx to as low as 48 ppm. The results of this study are important for engineering applications, providing guidance for the design of NH3 fired combustors aimed at significantly reducing NOx formation.