Ammonia decomposition for hydrogen production: a thermodynamic study

Abstract

The need for COx-free H2 in proton-exchange membrane fuel cells (PEMFC) has driven ammonia (NH3) decomposition to the forefront of H2 production technologies, taking NH3 as a potential and viable hydrogen storage material. Herein, a detailed derivation of thermodynamics governing equations has been applied to analyze the thermodynamics of ammonia decomposition reaction. The study utilizes MATLAB optimization tool ‘fmincon’ to solve the objective function, in a bid to find Gibbs free energy minima. The present study supports that if NH3 decomposition proceeds without molecular hindrance, almost 100% ammonia conversion, with close to 99.85% H2 yield, is achievable at 1 bar pressure and ≥ 700 K (427 ℃) temperature but also noticeable that 98% NH3 conversion is achievable at 600 K (327 ℃). The total free energy of ammonia decomposition system becomes more negative with increasing extent of reaction until equilibrium is reached. As the reaction temperature increases at a pressure of 1 bar, the extent of ammonia decomposition reaction also increases, reaching 0.61, 0.84, 0.91, 0.97 and 0.99 mol at 450, 500, 600, 700, and 773 K, respectively. The conversion of ammonia increases with increasing temperature and a negative effect of pressure was observed as per Le-Chatelier’s principle.