Purified hydrogen from synthetic biogas by joint methane dry reforming and steam-iron process: Behaviour of metallic oxides and coke formation

Abstract

Mechanical mixtures of nickel oxide and iron oxide doped with small quantities of ceria and alumina (2wt%), have been tested as suitable solids to jointly produce and purify hydrogen from an equimolar mixture of CH4 and CO2 simulating a desulfurized biogas. Since the solid mixture constitutes a reactant within the system, most part of the experiments has been carried out in a thermogravimetric system acting as a differential reactor. Exhaust gases were continuously monitored by mass spectrometry at the exit of the thermobalance. The apparent reaction mechanism consists of a first stage in which CH4 reduces Fe2O3 (hematite) to Fe3O4 (magnetite) and NiO to metallic Ni. This newly formed metallic nickel adopts a catalytic role promoting the methane dry reforming (MDR) reaction between CH4 and CO2. Fe3O4 reduces to metallic iron (Fe0) at the expense of H2 and CO (products of MDR), that transform in the oxidized species H2O and CO2 respectively. This reaction displaces dry reforming towards products what avoids carbon formation at some extent. Carbon deposition has been detected at temperatures lower than 900°C. These deposits were partially removed along isothermal experiments, probably by CO2 (Boudouard), and/or newly formed H2 and CO (MDR). EDS analysis demonstrated that carbon is mostly formed on the surface of nickel particles, while Raman spectroscopy shows that this carbon was highly graphitized. Metallic iron can be reoxidized with steam in a subsequent stage releasing high purity hydrogen.