Abstract
Ammonia is increasingly recognized as a promising carbon-free fuel and an efficient carrier of green hydrogen, particularly in hard-to-abate sectors where decarbonization remains a challenge. Its use in gas turbine systems can contribute significantly to reducing greenhouse gas emissions and mitigating the environmental impact of conventional fossil fuels in the power generation sector. However, its application is hindered by several combustion and practical challenges. From a combustion perspective, ammonia exhibits a low laminar flame speed and high ignition energy requirements, leading to poor flame stability. Its narrow flammability limits and slow chemical kinetics further restrict stable and efficient operation, while the long ignition delays complicate ignition and lean-burn strategies. In addition, ammonia’s inherent nitrogen content leads to excessive NOx formation, and incomplete oxidation often results in unburned fuel slip (ammonia slip). This study started with assessing the global performance of a 3.2 kWe micro gas turbine (mGT) fueled by ammonia through a comprehensive simulation framework: the algorithm includes detailed thermodynamic modeling of the turbomachinery, combustor, heat exchangers, and auxiliary components, with a one-dimensional combustion model based on Cantera, employed to assess flame development, temperature profiles, and pollutant formation. Initial results demonstrate that with appropriate fuel injection strategies, ammonia can deliver comparable power output to methane fuel, with a marginally lower electric efficiency (by 5.16%). Due to the inherent high NOx emissions associated with ammonia combustion, a rich-lean non-premixed strategy in a three-stage combustor is examined, achieving a 6.6% reduction in NOx emissions and lowering the pollutant concentration to 557 ppm at 15% O2. Building upon this preliminary study, a new combustor geometry featuring staged combustion and a primary equivalence ratio of 1.07 is evaluated in detail using CFD simulations based on specific and validated combustion chemistry reactions for ammonia. The optimized configuration reduces NOx emissions to one-third of the baseline level while maintaining low unburned NH3 and H2 levels and enhancing overall combustion efficiency and heat transfer performance. Additionally, the effect of steam injection is analyzed within a STIG cycle framework. The injection of 1.5 g/s of steam, produced by recovering the residual discharge heat of the mGT enhances power output, lowers fuel compressor demand, reduces NOx emissions by 36%, and significantly decreases unburned NH3 (86%) and H2 (28%). Finally, the impact of increasing the Combustor Inlet Temperature (CIT) from 500 K to 1050 K is assessed. The elevated CIT leads to improved combustion efficiency (by 5%), higher outlet temperature, and further reductions in unburned species, albeit with a 50% increase in NOx emissions. In addition, a numerical comparative study of premixed and non-premixed ammonia burners highlights the trade-offs between the two modes: premixed operation enhances flame stability and efficiency but increases NOx, whereas non-premixed operation suppresses NOx through in-flame reburning but risks NH3 slip. Future work will therefore prioritize experimental validation in a lab-scale mGT test bench, material compatibility studies, and the development of integrated after-treatment solutions, ultimately targeting scalable, low-emission ammonia-based power generation systems.