Ammonia Cracking As Auxiliary System for Gas Turbine: Preliminary Studies

Abstract In the competitive and rapidly evolving market toward the decarbonization of the energy production sector, the gas turbine’s ability to handle different carbon-free fuels is more and more required, as additional fuel flexibility compared to the standard natural gas combustion. Such design requirement for the development of gas turbine combustion systems drives the designers to also evaluate the introduction of pre-conditioning systems for the fuel, as in the case of ammonia combustion. Combustion studies and experiences demonstrate a beneficial effect if ammonia, used as fuel, is doped with a more reactive molecule like hydrogen or other low-carbon species; in this way, flame stabilization and a broader combustor control capability can be guaranteed. Ammonia cracking is a very well-known process that could be used to create a fuel mixture of NH3, H2, and N2 adequately converted to satisfy the minimum hydrogen requirement for combustion stability and NOX minimization. Current commercial cracking reactors operate at elevated temperatures (higher than 700 °C) and low pressures (less than 5 bar) with catalysts made typically of Ru and Pd metals, while recent development trends are oriented to adopt catalysts with an active phase more affordable respect to Ru and Pd, which are expensive and with limited availability. Furthermore, research is also focusing on the reduction of the working temperature below 500 °C and the increase of operating pressure above 30 bars approximately; in this way, it is possible to conceive an ammonia cracking system able to generate an appropriate fuel for a dedicated gas-turbine engine connected to the process. Being the cracking reaction endothermic, gas turbine exhaust heat can be conveniently used to increase the whole process efficiency: the lower the reactor required temperature, the higher the benefit in the thermodynamic achievable efficiency. An additional important key element for the gas-turbine application would be the flexibility of the cracking system to regulate the fuel composition. Gas-turbine operability requirements could involve a change of the fuel characteristics to control combustion or emissions; this could be transferred to the cracking system by controlling one of its degrees of freedom like heat supply or internal bypass. The present work describes the preliminary studies done on Nuovo Pignone Tecnologie (Baker Hughes company) gas turbines family, aimed to understand benefits that could be achieved in terms of thermodynamic cycle efficiency, showing energetic and exergetic analyses for some case studies. The identification of the best cracking reactor that could manage the outlet gas composition, according to the specific requirements of gas-turbine operability, will be described as well.