Hydrogen from synthetic biogas via SIP using NiAl2O4 catalyst: Reduction stage

A mechanical mixture of doped iron oxide and non-stoichiometric nickel aluminate (NiO in excess over stoichiometric NiAl2O4) acting as catalyst, was tested for producing purified hydrogen by methane dry reforming reaction (MDR) in combination with steam-iron process (SIP). For such purpose a mixture of CH4 and CO2 was used simulating as sweetened biogas as reducing raw material. Both processes were carried out simultaneously within the same reactor (PFR) seeking the intensification of the whole process (production + purification of hydrogen in the same vessel). SIP is a cyclic redox process consisting of two consecutive steps. In the reduction step haematite converts into metallic iron by reaction with syngas produced from biogas dry reforming, which is catalysed by nickel present in the solid bed. In the subsequent oxidation step, steam oxidizes iron releasing high purity hydrogen while recovering iron oxide (magnetite). Lower reduction temperatures favour coking limiting the performance of the experimental set-up. On the other hand, working at high temperature could cause severe sintering which diminishes the efficiency of the oxidation and therefore hydrogen yield. Besides the effect of temperature, several ratios CH4/CO2 have been checked in order to determine the effect of the composition of biogases from different origins. The higher the methane partial pressure, the shorter is the time needed to reduce the solid mixture, but also higher is the amount of carbon deposited along the bed. Also the effect of several repetitive redox cycles (up to 3) has been part of the study showing that there is a significant loss of reducing capacity of the solid mixture from one cycle to the next. Raman spectroscopy ensures that carbon residue acts as inert during oxidation steps with steam.