Steady-state and transient modelling of a microchannel reactor for coupled ammonia decomposition and oxidation

Abstract

A computational fluid dynamic (CFD) model was developed to describe the transient and steady-state behavior of NH3 decomposition in an experimental autothermal microchannel reactor. The effects of NH3 decomposition and NH3 oxidation flows on the axial temperature profile and product gas composition were investigated. During reactor start-up, the NH3 oxidation reaction was initiated at 300 °C, and attained a steady state value of approximately 650 °C after 6 h. Thus modelling of the reactor start-up regime enabled an accurate interpretation of temperature increases and H2 production in this dynamic period. Appropriate governing equations were used to account for mass, momentum, energy, and species transport. As a result of the different time scales associated with the chemical reaction and the heat transfer respectively, it was necessary to develop a new solution method to solve the model, especially in the transient period. This numerical method and results are described in detail in this paper. The experimental and simulated temperature and H2 production performance are found to be within reasonable accuracy during the transient period, while the steady-state results compare very well. Indications are that CFD can be used in future reactor designs to minimize time loss and accelerate H2 production during start-up.