Simulation of a Natural Gas Steam Reforming Reactor at Different Scales

The concept of sustainable energy is often associated to the so-called hydrogen economy. However, hydrogen cannot be regarded as an energy source, since it is not present in nature as free H2. Therefore, it must be produced using chemical processes. Among them, natural gas steam reforming (NGSR) is the most widespread and economically feasible process. Natural gas (NG) is a mixture with no well-defined and constant composition. However, methane is the prevailing component (around 85- 90%), but also higher hydrocarbons (i.e. ethane, propane, butane...) can be found. NGSR involves the proper endothermic reaction of reforming which produces syngas (a mixture of H2, CO and CO2). Then, the slight exothermic reaction of water gas shift, further converts CO in CO2 producing more hydrogen. The overall process is highly endothermic, so requires a large amount of heat. Therefore, the reactors are tubes placed in a furnace which provides direct heat to the tubes. Even though this process implements a well-established technology, it still presents some issues, such as carbon formation and deposition. Usually, a high steam to carbon (S/C) ratio allows to reduce carbon formation and its deposition: a value of S/C higher than 2.5 is generally believed to be safe for coke-free operation, nevertheless the problem of carbon formation and deposition is still not solved. The aim of the first part, is the development of an accurate model for these reactors. The mathematical model underlying the chemical and physical system is made of the mass and energy balances. The constitutive equations are then coupled with the kinetic equations for all the reaction involved in the process. The kinetic equations for the NGSR process are retrieved from the literature (i.e. Xu and Froment) but, with a simplified approach, they are adapted to our specific case. The overall set constitutes a partial differential and algebraic equation (PDAE) system which and requires boundary conditions which generally are chosen to be flowrate and composition of the feeding mixture. The resolution of the PDAE system needs the implementation of a numerical method through a finite element method (FEM), implemented through COMSOL Multiphysics®. The major problem which has shown up is the numerical method convergence. However, at the end of the simulation it is possible to obtain plots and maps of the main physical and chemical quantities of interest. Furthermore, an experimental analysis of end-of-life commercial catalyst coming from a full-scale industrial SMR reactor is carried out. This experimental analysis provides interesting results regarding catalyst structure and the eventual carbon deposition. Therefore, a possible qualitative explanation for the carbon formation can be given.