Author ORCID Identifier

https://orcid.org/0000-0001-5518-5055

Semester

Fall

Date of Graduation

2025

Document Type

Dissertation

Degree Type

PhD

College

Statler College of Engineering and Mineral Resources

Department

Mechanical and Aerospace Engineering

Committee Chair

Cosmin Dumitrescu

Committee Member

V’yacheslav Akkerman

Committee Member

Hailin Li

Committee Member

John Hu

Committee Member

Omid Askari

Abstract

Ammonia (NH3) is regarded as a promising fuel for future carbon-free power and propulsion systems due to its high hydrogen content, ease of storage, and existing infrastructure. Despite these advantages, NH3 exhibits inherently poor combustion characteristics, including low laminar burning velocity, high ignition energy, narrow flammability limits, and sluggish chemical kinetics. These limitations were solved in prior research by blending NH3 with hydrogen (H2) or hydrocarbons, which improved reactivity but introduced trade-offs such as increased complexity, safety concerns, or carbon emissions. Consequently, there remains a significant lack of fundamental and applied research on neat NH3 combustion, particularly under engine-relevant conditions of elevated pressure, high temperature, and dilution.

The work presented in this dissertation bridged that gap through two complementary investigations. The first focused on evaluating fundamental combustion parameters of NH3/air mixtures. Laminar burning velocities were measured at pressures up to 10 atm, an initial temperature of 298 K, and equivalence ratios between 0.7 and 1.3. Results showed strong buoyancy effects at elevated pressures, particularly near flammability limits where the flame adopted a bean-like morphology. The maximum burning velocity occurred at φ = 1.1, decreasing nonlinearly with pressure. Numerical simulations using Cantera reproduced these trends, and a sensitivity analysis identified NH2- and NO-related reactions as the dominant contributors to flame propagation. The second part investigated neat NH3 operation in a repurposed heavy-duty diesel engine converted to spark ignition. Experiments at speeds from 800 rpm to 1400 rpm, and equivalence ratios from 0.8 to 1.05 demonstrated that NH3 achieved stable and efficient combustion without significant hardware or control modifications. Engine performance matched that of methane (CH4) under equivalent fuel-energy conditions, particularly in lean-burn operation. Despite this, high levels of unburned NH3, NOx and N2O emissions highlighted the need for effective aftertreatment. NO formation increased under leaner mixtures due to additional reaction pathways, but overall NOx levels remained lower than those of CH4 combustion. A kinetic analysis confirmed that unburned NH3 contributes to NO reduction during post-combustion.

Overall, the findings demonstrate that neat NH3 can sustain efficient, stable combustion in conventional engine platforms, paving the way for carbon-free power generation with minimal hardware adaptation and manageable emission control requirements.

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