Optimization of hydrogen production by Chemical-Looping auto-thermal Reforming working with Ni-based oxygen-carriers

Abstract

Chemical-Looping auto-thermal Reforming (CLRa) is a new process for hydrogen production from natural gas that uses the same principles as Chemical-Looping Combustion (CLC). The main difference with CLC is that the desired product is syngas (H 2 + CO) instead of CO 2 + H 2O. For that, in the CLRa process the air-to-fuel ratio is kept low to prevent the complete oxidation of the fuel. The major advantage of this technology is that the heat needed for converting CH 4 to syngas is supplied without costly oxygen production and without mixing of air with carbon containing fuel gases. An important aspect to be considered in the design of a CLRa system is the heat balance. In this work, mass and heat balances were done to determine the auto-thermal operating conditions that maximize H 2 production in a CLRa system working with Ni-based oxygen-carriers. It was assumed that the product gas was in thermodynamic equilibrium at the exit of the air- and fuel-reactors and the equilibrium gas compositions were obtained by using the method of minimization of the Gibbs free energy of the system. It was found that to reach auto-thermal conditions the oxygen-to-methane molar ratio should be higher than 1.20, which means that the maximum H 2 yield is about 2.75 mol H 2/mol CH 4. The best option to control the oxygen-to-methane molar ratio is to control the air flow fed to the air-reactor because a lower air excess is needed to reach auto-thermal conditions.