Combustion Characteristics and Emissions of 99% Cracked 2 Ammonia Blends in a Gas Turbine Representative Swirl

This study presents the first systematic investigation of pressure-dependent combustion characteristics and emissions of highly cracked ammonia (99% cracking efficiency) in a gas turbine representative swirl burner. The research addresses a critical knowledge gap in understanding how operational pressure affects the combustion behaviour of cracked ammonia fuels, which are emerging as promising carbon-free alternatives for power generation.
Experiments were conducted using a High Pressure Optical Combustor (HPOC) facility across a pressure range of 1.1 to 6 bar absolute, with air preheated to 500 K and a constant thermal power output of 22.7 kW maintained under lean conditions (equivalence ratio approximately 0.545). The fuel composition consisted of 17.5% hydrogen, 1.0% residual ammonia, and 81.5% nitrogen by volume, representing a realistic cracked ammonia blend for gas turbine applications.
The investigation employed advanced optical diagnostics to capture NH₂* chemiluminescence, which serves as an indicator of radical formation and combustion intensity. Results demonstrated that NH₂* intensity increased monotonically with pressure from 1.1 to 6 bar, indicating enhanced radical production at higher collision frequencies. Concurrently, NOx emissions measurements revealed a distinctive pressure-dependent trend: emissions rose sharply from 90 ppmv at 1.1 bar to 189 ppmv at 4 bar before stabilising at higher pressures, forming a characteristic plateau that differs fundamentally from pure hydrogen combustion behaviour.
To elucidate the underlying chemical mechanisms, a Chemical Reactor Network (CRN) model was developed and validated against experimental data using the Stagni et al. kinetic scheme. The model successfully captured the NOx plateau phenomenon and revealed that the stabilisation results from a balance between thermal NOx formation pathways and ammonia-mediated reduction reactions. Rate of production analysis identified pressure-dependent shifts in dominant reaction pathways, with thermal NOx mechanisms (N + O₂ → NO + O) contributing 35-42% across all pressures, while destruction pathways increased from 6% at 1.1 bar to 27% at 6 bar, explaining the emissions plateau.
The findings demonstrate that pressure is a critical control parameter for managing NOx emissions in cracked ammonia combustion systems. The research provides essential data for the design and optimisation of low-emission gas turbine combustors capable of operating on ammonia-derived fuels, supporting the transition to carbon-free power generation while maintaining operational flexibility and emissions compliance. This work contributes fundamental knowledge to the emerging field of ammonia energy and offers practical insights for industrial implementation of cracked ammonia combustion technology