Evaluation of an Ammonia Fueled Combine-cycle Gas Turbine Power System on Maritime Usage

Abstract

Ammonia fuel has been widely considered as an attractive solution for reducing the green house gas emissions over recent years. Adapting ammonia as fuel on ships would highly reduce the carbon footprint of international shipping and off-shore transportation. Among the different systems currently under development for carbon-free power production in the near future, the combined-cycle gas turbine system stands out for its relatively high system efficiency and a potential for running on pure ammonia fuel while maintaining a low level of NOx emission. This thesis project puts a sight on a special design of this type of COGAS power system and tries to adapt it for maritime usage onboard future ships. Previous researches have pointed out the key design features of the ammonia COGAS power system being running under a high fuel-air ratio with a cooling method based on EGR technology and cracking the additional ammonia fuel into hydrogen in the gas turbine system, then this created hydrogen concentration could be used for re-heating the exhaust before it is used by the combined steam cycle. However, current understanding of this type of COGAS system is still limited under static analysis and designed working points. This thesis project tries to provide a basic view on the off-design performance and dynamic behaviors of this COGAS system, and examines if this system is still able to maintain a low level NOx emission under such working conditions. This thesis project combines a dynamic model of an ammonia gas turbine and a chemical thermodynamic model for simulating the chemical behavior of the work fluid inside the gas turbine system. It is found that the fuel-air equivalence ratio of the gas turbine needs to be designed at a high value to ensure the flammability of the hydrogen consisting exhaust in the re-heating process. A very low NOx emission is observed in the gas turbine exhaust under an assumption of complete chemical reactions. The final NOx emission of the COGAS system is found to be within the EEDI Tier III limitation under both rated and part-load working conditions. The thermal efficiency of the gas turbine is relatively low due to the high equivalence ratio, while a system efficiency comparable with current oil-fueled COGAS power system is able to be expected for the full system of ammonia COGAS. On the phase of dynamic analysis, this project has concluded that traditional fuel control method for controlling gas turbine power generation is not adaptable to gas turbine systems working at fuel-rich conditions. A non-linear behavior is observed due to this high equivalence ratio. This thesis provides a new controlling method with controlling both the fuel injection ratio and the EGR ratio with an additional feedback controlling system attached to the original feed-forward system of the fuel control. Basic tests shows that such method is able to generate a dynamic output with the correct tendency. This thesis project also observes a high sensitivity of NOx emission with the presence of additional oxygen in the exhaust under a complete chemical reaction. In this thesis project it is found that the considered ammonia COGAS system maintains the advantage of traditional COGAS power systems and a is able to take an advantage in comparing with medium-speed diesel systems under an ammonia economy. Power output of the ammonia gas turbine is able to be controlled with a combination of fuel control and EGR control. A low NOx emission within the limit of EEDI Tier III is observed under both rated and part-loading conditions of the power system, but this is achieved with the assumption of a complete chemical reaction. Whether this assumption is adaptable to maritime scaled ammonia gas turbine system requires further kinetic analysis on the combustion process and further research efforts in the designing of the combustor system.