Ammonia production by microbubbles: A theoretical analysis of achievable energy intensity

Abstract

The present paper studies the energy intensity of ammonia production by a freely oscillating microbubble placed in an infinite domain of liquid. The initial content of the bubble is a mixture of hydrogen and nitrogen. The bubble is expanded isothermically to a maximum radius, then it is “released” and oscillates freely. The input energy is composed of the potential energy of the bubble at the maximum radius, the energy required to produce hydrogen, and the pumping work in case a vacuum is employed. The chemical yield is computed by solving the underlying governing equations: the Keller–Miksis equation for the radial dynamics, the first law of thermodynamics for the internal temperature and the reaction mechanism for the evolution of the concentration of the chemical species. The control parameters during the simulations are the equilibrium bubble size, initial expansion ratio, ambient pressure, the initial concentration ratio of hydrogen and the material properties of the liquid. At the optimal parameter setup, the energy intensity is 90.17GJ/t that is 2.31 times higher than the best available technology, the Haber–Bosch process. In both cases, the hydrogen is generated via water electrolysis.