Design and Assessment of Membrane-supported Ammonia Cracking for Hydrogen Refuelling Stations

Abstract

As Canada aims to reduce greenhouse gas emissions, there is a growing shift toward cleaner energy and fuel sources. Hydrogen has emerged as a promising alternative fuel source due to its high gravimetric energy density and ability to power fuel cell electric vehicles without producing direct carbon dioxide emissions. However, there are currently challenges in storing and transporting large amounts of hydrogen. Ammonia is gaining attention as a hydrogen carrier because it can be stored under moderate pressure or refrigeration and leverages existing infrastructure. Once delivered, hydrogen can be extracted from ammonia through on-site decomposition and purification. Although this pathway shows promise, its competitiveness depends on the system's energy requirements, operating costs and emissions. Furthermore, most existing ammonia decomposition and hydrogen refuelling models are proprietary, limiting accessibility for researchers and small-scale developers. This thesis addresses this gap by developing an open-source process model for a palladium membrane supported ammonia decomposition process at hydrogen refuelling stations. The process delivers 500 kg of hydrogen gas at 350 bar per day, and its cost and emissions performance were compared to other hydrogen production pathways. A Python-based model was created using Cantera, a chemical kinetic and thermodynamic library, to simulate the isothermal Pd membrane reactor. In the reactor, ammonia decomposes to nitrogen and hydrogen, while hydrogen is separated using the membrane. This eliminates the need for additional hydrogen purification steps. The base case achieved 99.92% conversion and 95.9% hydrogen recovery. To preheat the ammonia feedstock to the membrane reactor, the unconverted ammonia and unrecovered hydrogen were mixed with some ammonia feedstock and combusted with air. The combustion generates NOx emissions, which were reduced by 85% using a selective catalytic reduction unit, bringing NOx emissions well below provincial limits. While the system has no direct carbon dioxide emissions, indirect emissions from electricity consumption, ammonia feed and transportation for the process were estimated at 4.86 kg CO₂e/kg H₂, with an electricity requirement of 9.77 kWh/kg H₂. An economic analysis shows a capital expenditure of approximately $204,000 and an annual operating cost of $1.6 million for the base case. The levelized cost of hydrogen (LCOH) at 350 bar was estimated at $10.38 kg/H2. A sensitivity analysis was also conducted to evaluate the impact of temperature, pressure and membrane permeance on conversion, hydrogen recovery, NOx emissions, and LCOH. The impact of capital and operating expenditure on LCOH was also analyzed, with the price of ammonia being the main contributor to changes in LCOH. These results from a detailed study of the ammonia to hydrogen pathways contribute to a better understanding of clean hydrogen technologies for transportation applications and also provide key insights for future deployment in clean fuel strategies across Ontario and beyond.