Hydrogen enrichment of natural gas by catalytic decomposition of methane

An iron ore has been investigated for its ability to catalyse the decomposition of methane contained in natural gas. The purpose of which is lowering the carbon content of natural gas by partially replacing methane with hydrogen. Materials have been characterised using an assortment of techniques e.g. Powder X-ray diffraction (PXRD) and Raman spectroscopy which focus on the bulk material, physical measurements such as thermal gravimetric analysis (TGA) and hydrogen temperature programmed reduction (H2-TPR) and short-range techniques such as Brunauer Emmett Teller (BET) surface analysis and scanning electron microscopy (SEM).

Characterisation of the bulk iron ore using PXRD then Raman spectroscopy indicated that the ore was predominantly comprised of magnetite (Fe3O4) with smaller fractions of haematite (a-Fe2O3). BET surface analysis pointed to the iron ore being a highly non-porous material with a specific surface area (SSA) of 0.06 m2/g. Experiments to optimise the catalytic activity of the iron ore by increasing the surface area using high energy ball milling at 300 rpm for times ranging from 30 to 330 minutes were carried out. This study to determine the effect of ball milling on the degree of methane conversion concluded that optimal grinding of the ore resulted in a five-fold increase in methane conversion, from ~ 1 to 5 %. Milling for times greater than the optimum caused a reduction in methane conversion to 4 %. This reduction in conversion was shown by SEM to tally with an increase in particle size brought about by agglomeration of the iron ore particles. Mössbauer and Raman spectroscopy and H2-TPR data provided evidence for the occurrence of a phase transformation from magnetite to maghemite (-Fe2O3) and haematite (at the surface of the iron ore particles) as the milling time increased. This study also examined the purity of the carbon generated from the catalytic decomposition of methane (CDM) process, since carbon itself is a valuable commodity, for example it is used in steel production. Analysis carried out on the carbon by-product showed the material was of high purity with a carbon content of circa 86 %.

The development of a thermally resistant material to mitigate the sintering of the active species (Fe0) in the iron ore-based catalyst was also investigated. Experiments to explore the formation of catalysts from a composite containing different proportions (10 to 90 wt.%) of the iron ore mixed with graphite were performed. The effect of the various composites on the catalytic performance was also examined. The results showed that as the amount of iron ore in the composite increased from 10 to 90 wt.%, the corresponding methane conversion rose from 8 to 35 %. Furthermore, the SSA of the composite decreased as the amount of added graphite decreased. SEM images showing the microstructure of the composite catalysts indicated sintering/conglomeration of the iron particles in the catalysts formed with low amounts of added graphite (10 and 20 wt.%). The composite catalyst that exhibited the best methane conversion was formed from adding 40 wt.% graphite (60Fe-G). Comparatively, the catalyst productivity of 60Fe-G was up to 9.4 mol(H2)/mol(Fe), approximately seven times higher than the 1.4 mol(H2)/mol(Fe) achieved by a catalyst formed solely from iron ore.

The solid residue, consisting of the used catalyst and deposited carbon generated during the CDM process, was analysed for potential application as a catalyst for CDM at temperatures ranging from 600 to 800 °C. Data from PXRD indicated the presence of iron, wustite (FeO), iron carbide (Fe3C) and carbon in the solid residue, while SEM revealed forms of carbon, including carbon nanotubes. CDM reactions, using the solid residue, at 600, 700, and 800°C showed that methane conversion increased with temperature: 4%, 40%, and 70%, respectively. An evaluation of the impact of activating the material by reducing it showed that despite a much higher methane conversion in the first hour, 92%, compared to 74 % for the non-activated sample, the average conversion over nine hours of testing was similar: 67% for the activated sample compared to 69% for the non-activated sample. This demonstrated that the solid residue from the CDM process could serve as a catalyst without requiring additional chemical treatment.