Skip to main content

Lund University Publications

LUND UNIVERSITY LIBRARIES

Ignition, combustion modes and NO/N2O emissions in ammonia/n-heptane combustion under RCCI engine conditions

Zhou, Yuchen LU ; Xu, Shijie ; Xu, Leilei and Bai, Xue-Song LU (2025) In Combustion and Flame 280(2025).
Abstract
Ammonia has been considered a promising carbon-free fuel for marine engines. However, its low flame speed and high nitrogen oxides (NOx) and nitrous oxide (N2O) emissions present significant challenges. To address these issues, novel combustion concepts, such as ammonia/diesel dual-fuel Reactivity-Controlled Compression Ignition (RCCI) engines, have been proposed. This paper presents a detailed investigation of ammonia/n-heptane combustion under RCCI engine conditions using direct numerical simulation (DNS) to gain insights into ignition, combustion modes, and emission formation mechanisms. A temporally evolving jet configuration is considered in the DNS, with the computational domain comprising two regions: a fuel-lean premixed... (More)
Ammonia has been considered a promising carbon-free fuel for marine engines. However, its low flame speed and high nitrogen oxides (NOx) and nitrous oxide (N2O) emissions present significant challenges. To address these issues, novel combustion concepts, such as ammonia/diesel dual-fuel Reactivity-Controlled Compression Ignition (RCCI) engines, have been proposed. This paper presents a detailed investigation of ammonia/n-heptane combustion under RCCI engine conditions using direct numerical simulation (DNS) to gain insights into ignition, combustion modes, and emission formation mechanisms. A temporally evolving jet configuration is considered in the DNS, with the computational domain comprising two regions: a fuel-lean premixed ammonia/air mixture and a fuel-rich n-heptane jet/ammonia/air mixing region. The pressure and temperature in these regions are representative of typical marine engine operating conditions. The DNS results reveal multiple reaction layers, including the fuel-lean premixed flame (LPF), fuel-rich premixed flame (RPF), diffusion flame (DF), and rich ammonia oxidation layer (RAOL). The LPF propagates into the ambient ammonia/air mixture, significantly influencing combustion efficiency and NO formation, while the RPF propagates into the fuel-rich n-heptane/ammonia/air mixture due to low-temperature ignition. The DF oxidizes combustion intermediates and NO, while the RAOL facilitates ammonia oxidation, forming intermediate species such as hydrogen (H2), amino radicals (NH2), and nitrene radicals (NH), which eventually participate in the reactions in the DF and RPF. The back-supported propagation of the LPF is influenced by n-heptane mixing, heat, and radical transfer from the DF, and jet-induced vortices and turbulence. Increasing n-heptane jet speed enhances this effect, improving ammonia combustion efficiency. NO primarily forms in the LPF and is consumed in the DF, while N2O is generated in the LPF (continuously) and RPF (during the ignition stage), while being consumed in the RAOL. Higher n-heptane jet velocity accelerates NO consumption but increases N2O formation due to enhanced mixing and ammonia entrainment. Understanding these mechanisms provides valuable insights into optimizing RCCI combustion for reduced emissions and improved efficiency in ammonia-fueled marine engines. (Less)
Please use this url to cite or link to this publication:
author
; ; and
organization
publishing date
type
Contribution to journal
publication status
published
subject
in
Combustion and Flame
volume
280
issue
2025
article number
114352
pages
15 pages
publisher
Elsevier
external identifiers
  • scopus:105012191188
ISSN
0010-2180
DOI
10.1016/j.combustflame.2025.114352
language
English
LU publication?
yes
id
f0353e9d-67ab-4c17-b072-3d328b9a5bb1
date added to LUP
2025-09-12 20:42:38
date last changed
2025-09-30 12:17:07
@article{f0353e9d-67ab-4c17-b072-3d328b9a5bb1,
  abstract     = {{Ammonia has been considered a promising carbon-free fuel for marine engines. However, its low flame speed and high nitrogen oxides (NOx) and nitrous oxide (N2O) emissions present significant challenges. To address these issues, novel combustion concepts, such as ammonia/diesel dual-fuel Reactivity-Controlled Compression Ignition (RCCI) engines, have been proposed. This paper presents a detailed investigation of ammonia/n-heptane combustion under RCCI engine conditions using direct numerical simulation (DNS) to gain insights into ignition, combustion modes, and emission formation mechanisms. A temporally evolving jet configuration is considered in the DNS, with the computational domain comprising two regions: a fuel-lean premixed ammonia/air mixture and a fuel-rich n-heptane jet/ammonia/air mixing region. The pressure and temperature in these regions are representative of typical marine engine operating conditions. The DNS results reveal multiple reaction layers, including the fuel-lean premixed flame (LPF), fuel-rich premixed flame (RPF), diffusion flame (DF), and rich ammonia oxidation layer (RAOL). The LPF propagates into the ambient ammonia/air mixture, significantly influencing combustion efficiency and NO formation, while the RPF propagates into the fuel-rich n-heptane/ammonia/air mixture due to low-temperature ignition. The DF oxidizes combustion intermediates and NO, while the RAOL facilitates ammonia oxidation, forming intermediate species such as hydrogen (H2), amino radicals (NH2), and nitrene radicals (NH), which eventually participate in the reactions in the DF and RPF. The back-supported propagation of the LPF is influenced by n-heptane mixing, heat, and radical transfer from the DF, and jet-induced vortices and turbulence. Increasing n-heptane jet speed enhances this effect, improving ammonia combustion efficiency. NO primarily forms in the LPF and is consumed in the DF, while N2O is generated in the LPF (continuously) and RPF (during the ignition stage), while being consumed in the RAOL. Higher n-heptane jet velocity accelerates NO consumption but increases N2O formation due to enhanced mixing and ammonia entrainment. Understanding these mechanisms provides valuable insights into optimizing RCCI combustion for reduced emissions and improved efficiency in ammonia-fueled marine engines.}},
  author       = {{Zhou, Yuchen and Xu, Shijie and Xu, Leilei and Bai, Xue-Song}},
  issn         = {{0010-2180}},
  language     = {{eng}},
  number       = {{2025}},
  publisher    = {{Elsevier}},
  series       = {{Combustion and Flame}},
  title        = {{Ignition, combustion modes and NO/N2O emissions in ammonia/n-heptane combustion under RCCI engine conditions}},
  url          = {{https://lup.lub.lu.se/search/files/227590659/1-s2.0-S001021802500389X.pdf}},
  doi          = {{10.1016/j.combustflame.2025.114352}},
  volume       = {{280}},
  year         = {{2025}},
}