Direct Ammonia SOFC-Gas Turbine Hybrid Systems Modeling for Heavy-Duty Transportation Sector

Abstract

Ammonia currently stands as being the second most produced chemical in the world [1]. While it is prominently used in the manufacturing of fertilizers, increase in production capacity through renewable sources such as green hydrogen and its potential as a carbon-neutral energy carrier make it a promising candidate to replace fossil fuels for power generation in the future [2]. Moreover, when compared to liquid hydrogen, ammonia can be stored at more practical temperatures (-253⁰C vs -33⁰C at 1 bar) and has a higher volumetric energy density (8.5 MJ/L vs 12.7 MJ/L) [3]. Though the toxicity of ammonia as a fuel is well known, there are well established safety and handling procedures for storage and transport, given its ubiquitous existence as an essential commodity required for agriculture.From a systems perspective, residual chemical exergy available in the SOFC anode gas exhaust can be utilized through a gas turbine to enable increased efficiency [4]. Direct ammonia SOFCs (DASOFCs) can eliminate the need for external cracking to hydrogen, thereby reducing system complexity. In addition, heat loss and thermal management in the stack periphery may be effectively managed via in-situ endothermic decomposition of ammonia [5]. Integration in a hybrid system is also beneficial to SOFCs due to the associated heat of compression for pressurized operation. The different physicochemical timescales of electrochemical fuel cells and mechanical turbomachinery are advantageous to dynamic operation which is important because of fluctuating power loads in a mobile application.In this work, the potential of an ammonia-based hybrid SOFC/GT system concept is assessed. A multiscale stack simulation using constitutive relations for heat, mass and charge transfer based on high power density SOFCs is integrated with turbomachinery and other auxiliary system components within the gPROMS process simulation framework. State variables for each point can then be defined to estimate the operating boundaries of the system.This presentation discusses the thermodynamics of SOFC operation and system concepts to utilize ammonia in a hybrid system for a kW-scale transportation application. This involves evaluation of system indicators such as thermodynamic efficiency, fuel utilization and specific power which can help determine optimal performance. The efficiency and specific power are dictated by the operating temperature of the fuel cell, preheating requirements and the relative utilization of fuel between the SOFC and gas turbine. The proposed system concept is shown to achieve LHV efficiencies of up to 60% (LHV) within reasonable operating limits. The presentation includes an examination of the prospects of ammonia as a power source for the mobility sector in comparison to contemporary and future renewable fuels, and concludes with practical challenges for deployment.