Numerical Simulation of Effective Heat Recapture Ammonia Pyrolysis System for Hydrogen Energy

Abstract

This paper proposes a solution to address the challenges of high storage and transport costs associated with using hydrogen (H2) as an energy source. It suggests utilizing ammonia (NH3) as a hydrogen carrier to produce H2 onsite for hydrogen gas turbines. NH3 offers higher volumetric hydrogen density compared to liquid H2, potentially reducing shipping costs by 40%. The process involves NH3 pyrolysis, which utilizes the heat waste from exhaust gas generated by gas turbines to produce H2 and nitrogen (N2). Numerical simulations were conducted to design and understand the behaviour of the heat recapture NH3 decomposition system. The design considerations included the concept of the number of transfer units and heat exchanger efficiency, achieving a heat recapture system efficiency of up to 91%. The simulation of NH3 decomposition was performed using ANSYS, a commercial simulation software, considering wall surface reactions, turbulent flow, and chemical reaction. Parameters such as activation energy and pre-exponential factor were provided by a study utilizing a nickel wire for NH3 decomposition experiments. The conversion of NH3 reached up to 94% via a nickel-based catalyst within a temperature range of 823 K to 923 K which is the exhaust gas temperature range. Various factors were considered to compare the efficiency of the system, including the mass flow of NH3, operating gauge pressure, mass flow of exhaust gas, among others. Result showed that pressure would not affect the conversion of NH3 at temperatures above 800 K, thus a lower amount of energy is required for a compression purpose in this approach. The conversion is maintained at 94% to 97% when lower activation energy is applied via a ruthenium-based catalyst. Overall, this study showed the feasibility of utilizing convective heat transfer from exhaust gas in hydrogen production by NH3 pyrolysis, and this will further enhance the development of NH3 as the potential H2 carrier for onsite production in hydrogen power generation.