Degree

Doctor of Philosophy (PhD)

Department

Mechanical and Industrial Engineering

Document Type

Dissertation

Abstract

In an effort to reduce harmful effects of greenhouse gas emissions, ammonia is being pursued as a fuel for power generation as it is a carbon-free energy source. However, the use of ammonia-air mixtures in premixed swirl combustors poses challenges due to low flame speed and reactivity resulting in combustor stability issues in addition to high Nitrous Oxide (NOx) emissions due to the presence of fuel-bound Nitrogen. The first part of this study attempts to overcome lean blowout limits of methane-ammonia-air mixtures using a novel, multi-point () injection strategy, whereby micron-sized holes on the swirler vanes generate a co-flowing stream of fuel and air, which is then injected into a swirling air cross-flow. The resulting improvement in mixing facilitated by increases in momentum flux ratio and fine-scale turbulence is found to reduce lean blowout (LBO) limits to equivalence ratios between 0.65–0.7 for mixtures containing ammonia as high as 80–90% by volume. The measurements carried out using a model-swirl combustor setup are analyzed further using CH* and OH* chemiluminescence. Chemiluminescence imaging shows the heat release zone to move downstream and broaden with increase in ammonia content, because of decreasing flame speed. NOx emissions measured using an exhaust gas analyzer are seen to peak as equivalence ratio approaches stoichiometric conditions as well as observed to be maximum for 30-60% Ammonia addition by volume. The second part of this study involves use of inlet air preheating and multipoint fuel injection swirler (MFIS) to increase flame stability and delay blow-off. Experiments and reactor network simulations are carried out to evaluate the effectiveness of the two strategies in expanding combustor operability limits. The corresponding influence of the proposed strategies on NOx emissions are studied and underlying reaction pathways leading to NOx production are analyzed. Results of the study indicate that a distributed fuel injection strategy can significantly expand stability limits of a swirl combustor operating on methane-ammonia-air mixtures. Inlet air preheating provides additional expansion of stability limits; however, this is accompanied by increased NOx production, which is undesirable from an emissions standpoint. NOx is found to be significantly lower for rich fuel-air mixtures primarily through the effect of NHi reaction pathways responsible for NO consumption facilitated by the presence of excess NH3. The third phase of the research extends this work to develop a two-stage combustor. The two-stage combustor is operated in a Rich-Quench-Lean mode with a rich primary and lean secondary section and fuel injection only in the primary. Flow visualization studies reveals that increasing the volumetric air flow rate into the secondary causes an increase in jet velocity, which increases penetration depth. The disappearance of the swirling motion of the air likely corresponds to an increase in jet penetration depth ensuring good mixing of the secondary air jets with the air entering from the primary stage of the combustion chamber. Using reactor network model predictions, H2, CO and unburnt NH3 are found to be the main reactants generated by the rich primary stage combustion, and capable of undergoing complete oxidation in the secondary. Chemiluminescence studies in the primary and secondary combustion stages show that CH* is sensitive to changes in the NH3 volume percentage as well as the primary and global equivalence ratios. OH* and NO* are insensitive to changes in the primary and global equivalence ratios as well as ammonia content in the fuel mixture. NO emissions are found to be relatively insensitive to the global equivalence ratio. For constant and , NO emissions decrease going from 60% NH3 to 70% NH3 to 80% NH3 in the mixture. NO emissions were also found to decrease with increasing going from 1.15 to 1.35 except for the case with 80% NH3. For cases with 60% and 70% NH3, a 20-30% and 70% reduction is seen in NO emissions for the two-stage combustor operation as compared to a single stage combustor operating at the same . This shows the viability of the two-stage combustor operating in an RQL mode with a significantly rich primary stage to reduce NO emissions while ensuring combustor stability through the use of the MFIS for improved fuel-air mixing and addition of CH4 to improve flame stability. NO2 emissions are seen to be relatively insignificant for all cases tested in this work.

Date

11-2-2023

Committee Chair

Menon, Shyam

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