(114d) A Solid-Gas Ideal Gibbs Reactor Model for Natural Gas Reforming in the Presence of CaO

An ongoing increase of carbon dioxide concentrations in the atmosphere during the last decades has been well documented. This has lead to concerns regarding the possible consequences of this phenomenon, resulting in tighter industrial regulation of carbon dioxide emissions. Hydrogen has been proposed as a viable and effective energy carrier for small and medium vehicles. Natural gas reforming is the main industrial route for hydrogen production. The process consists of a first step in which a gaseous mixture of natural gas (methane) and water reacts at high temperature (about 1000 K) and pressure (5-20 bar) to generate a mixture of methane, water, carbon dioxide, carbon monoxide and hydrogen. This mixture is then processed, in a second step, through a gas-shift reactor sequence which aims to eliminate the carbon monoxide from the mixture. This process however takes place at high temperatures and emits carbon dioxide into the atmosphere.

In this work, we carry out a reaction equilibrium study of the natural gas reforming process in the presence of calcium oxide. Under the right temperature conditions, calcium oxide can react with carbon dioxide while not reacting with water, thus enhancing the conversion of the reforming and gas-shift reaction processes based on the Le Chatelier principle. To quantify this effect, we minimize Gibbs free energy for the solid-gas system: CH₄, CO₂, CO, H₂O, H₂, CaO, CaCO₃, Ca(OH)₂. For fixed temperature and pressure, the total Gibbs free energy minimization problem is non-linear and convex under a species exclusivity assumption. The results of our study identify equilibrium compositions for all species as a function of temperature and pressure and thus establish the ranges within which this process can by implemented industrially.