Combustion and Emission of Additional Fuels in Laboratory Non-premixed Flames

Abstract

Additive strategies are widely used in combustion and post-combustion systems to modify emissions, oxidation behavior, and heat-release characteristics. Their reported benefits, however, are often obtained under limited configurations and may not transfer directly across changes in mixing, temperature field, loading level, oxygen access, particle history, or heterogeneous contact. Rather than treating additive performance as an intrinsic property of the additive alone, this thesis examines how additive-induced responses depend on the reacting configuration in which the additive acts. Four selected case studies are used to evaluate this problem across gas-phase fuel addition, dispersed metal-particle combustion, flame-scale effective metal addition, and catalytic heterogeneous oxidation. The first case study examines NOx formation in ethylene-ethanol dual-fuel counterflow flames using combined experiment and detailed chemical-kinetic analysis. Relative to the ethylene-only reference flame, ethanol addition increases and broadens the high-temperature region, modifies intermediate-species distributions, and increases NO formation under the investigated conditions. Pathway-isolation, sensitivity, and rate-of-production analyses show that the baseline ethylene flame is primarily prompt-NO-influenced, whereas ethanol addition strengthens the thermal-NO contribution by extending the high-temperature region and modifying the radical environment. These results show that the NOx consequence of ethanol addition cannot be interpreted from fuel chemistry alone, but must be understood through the coupled changes in flame structure, temperature field, and pathway balance. The second case study investigates dispersed micro-sized aluminum particles in a non-premixed methane-air flame using high-speed imaging, two-color pyrometry, thin-filament pyrometry, and supporting numerical simulation. Individual particles exhibit substantial heterogeneity in trajectory, radiative temperature history, optical-emission lifetime, fragmentation behavior, molten aluminum ejection, and continued oxidation after breakup. Particle temperatures frequently exceed the surrounding gas temperature, and particle behavior correlates with local temperature gradients, oxygen availability, and transient particle-level processes rather than with bulk flame properties alone. This study provides the central experimental evidence that aluminum-particle combustion in a non-premixed flame is strongly particle-history-dependent. The third case study extends the aluminum investigation to a bounded flame-scale numerical analysis of higher effective aluminum loading in a non-premixed methane flame. Aluminum is represented as an effective gaseous reactive component, so the model is not intended to reproduce discrete-particle shell rupture, molten ejection, or fragmentation. Instead, it focuses on how stronger aluminum participation can alter the reacting field. The results show that increasing effective aluminum loading shifts and broadens the high-temperature region, redistributes methane-flame and aluminum-bearing species, modifies the energy-normalized CO2 response, and affects the modeled NO field under the adopted gas-phase mechanism. This analysis provides a controlled numerical bridge between individual-particle observations and flame-scale aluminum-addition effects. The fourth case study examines ceria-catalyzed carbon oxidation as a heterogeneous catalytic analogue for post-combustion particulate treatment. By comparing bulk soot-ceria powder oxidation measured using TGA/DSC-DTG with local carbon-ceria contact oxidation observed using in-situ ESTEM, the study shows that the apparent catalytic response depends on contact geometry, oxygen delivery, particle morphology, and the physical configuration in which carbon, ceria, and oxygen interact. Bulk measurements reflect ensemble powder behavior and oxygen transport through the packed sample, whereas ESTEM observations resolve local interface-controlled oxidation near evolving carbon-ceria contacts. The same catalytic system therefore produces different apparent oxidation behavior depending on the contact and oxygen-access conditions. Across the four case studies, the dissertation shows that additive-induced changes in emissions, combustion, or oxidation behavior are configuration-dependent responses rather than intrinsic additive properties. The principal contribution is the development of an experimentally and numerically grounded basis for identifying when additive effects remain physically interpretable and engineering-relevant as mixing, temperature, loading, oxygen access, particle history, and heterogeneous contact vary. The dissertation also provides structured experimental and numerical datasets, including dual-fuel flame measurements, individual aluminum-particle trajectory and temperature histories, methane-aluminum numerical flame fields, and ceria-catalyzed soot-oxidation data, that can support future model validation, comparative additive studies, and data-driven or AI-assisted analysis of reacting systems.