Emissions and stability of partially cracked ammonia swirling flames

Davies, Jordan 2025. Emissions and stability of partially cracked ammonia swirling flames. PhD Thesis, Cardiff University. Item availability restricted.

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Abstract

Renewably produced ammonia is a potential carbon-free fuel as it features well-established global infrastructure for safe distribution, is easy to store as it liquefies at modest pressures and contains significant hydrogen content. However, effective combustion of ammonia faces challenges stemming from its low reactivity and narrow flame stability limits. This problem can be alleviated by partially cracking the molecule, resulting in a fuel blend of ammonia, hydrogen and nitrogen with much improved reactivity. Therefore, the last remaining challenge relates to mitigating its propensity for producing high NOx emissions. This study aims to reduce NOx emissions from partially cracked ammonia flames and understand how to maintain flame stability as thermal power is scaled up towards levels relevant to industrial systems. To achieve this aim, a laboratory-scale atmospheric swirl burner is employed to assess NOx reduction methodologies such as fuel stratification, humidification and heat loss management. It is also used to evaluate a range of swirlers with varying swirl number and nozzle designs to identify desirable characteristics to mitigate against flame blowoff risk at elevated gas velocities. Experimental gas analysis and chemiluminescence imaging is used in parallel with chemical reactor network modelling to understand the reactions responsible for NOx reduction. The key findings include that retaining the N2 produced during the NH3 cracking process in the fuel blend reduces NOx emissions compared to equivalent NH3/H2 flames. Fuel stratified swirl flames exhibited sensitivity to the diffusive equivalence ratio, with slightly lean central H2 flames producing the lowest combined emissions. Compared to premixed, this case resulted in a 90% NO reduction and 33% NH3 increase at ΦG = 1.05. Humidification lowered NO emissions at fuel lean conditions by reducing HNO production by 29%, contributing to a 40% drop in peak NO, and at fuel rich conditions by supplying OH radicals for extra NH2 formation, increasing NO consumption. It also reduced H2 emissions at fuel rich conditions by up to 41%, likely due to lower post-flame temperatures reducing thermal cracking of NH3 in the exhaust. Controlling temperatures by increasing heat loss to the burner face reduced NO emissions at Φ = 1.1 by 67% alongside an 11% increase in unburned NH3 by shifting NH2 reactivity closer to its temperature dependent peak, increasing NO consumption. Shortening the nozzle increased blowoff resistance at 100 kW by widening the flame brush, a desirable characteristic for scaling these flames to higher thermal power applications. These findings advance the understanding of NOx reduction methodologies in partially cracked ammonia swirl flames and their effects on combustion efficiency, while also highlighting findings for scaling them to higher thermal powers.

Item Type:	Thesis (PhD)
Date Type:	Completion
Status:	Unpublished
Schools:	Schools > Engineering
Uncontrolled Keywords:	1. Ammonia 2. Hydrogen 3. Combustion 4. Humidification 5. Stratification 6. Scale-up
Date of First Compliant Deposit:	16 December 2025
Last Modified:	17 Dec 2025 10:53
URI:	https://orca.cardiff.ac.uk/id/eprint/183189

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