Microbial H2 consumption at conditions relevant for H2 underground storage
Bundesanstalt für Geowissenschaften und Rohstoffe, Germany
Underground storage of hydrogen could be an alternative way to store large amounts of energy. However, microbial consumption of H2 is still a major uncertainty factor. Since microbial life is widespread in the crust of the earth an underground storage site needs to be seen as a habitat for microorganisms. Microbial activity at the H2 storage site might affect the stored H2 as well as the integrity of the storage site itself.
There is great need for more information about microbial H2 transformation activity at conditions relevant for underground H2 storage i.e. elevated pressure, high temperature and about potential geochemical interactions with surrounding fluid and rock material.
In this study, different fluids from potential subsurface storage sites representing storage in salt caverns or porous rock reservoirs were investigated. While some fluids were inactive, long lasting hydrogen consumption was observed by a porous rock reservoir fluid. Microbial H2 oxidation tolerated high pressure as well as pressure and temperature fluctuations reflecting cycles of H2 storage. In this fluid microbial H2 consumption was shown to be sulfate dependent and led to the formation of sulfide. Furthermore, an increase of sulfate reducing bacteria during microbial H2 consumption was identified by high-throughput sequencing of 16S rDNA. These results indicated the oxidation of H2 by sulfat reducing bacteria to be the presumed process in this porous rock reservoir fluid. Due to the heterogeneity of the investigated fluids, microbial H2 oxidation activity at different H2 underground storage sites cannot be generalized but requires site specific investigations.
Experimental simulations of hydrogen migration through potential storage rocks
Helmholtz-Zentrum Potsdam Deutsches GeoForschungsZentrum GFZ, Germany
The save and effective storage of hydrogen in geological formations is an important part towards the implementation of renewable energy use. Due to fluctuating power supply from wind or solar plants, it is envisaged to use excess energy for electrolytical hydrogen production and the subsequent temporary storage in geological formations, as buffer for energy at “high-demand-low-production-times”.
The preferred geological storage formations are either salt deposits or porous sandstones with a gas-tight caprock. To date, these formations are generally confirmed to be appropriate for natural gas storage. However, the deviant physical properties of hydrogen, in terms of density, viscosity and hence mobility, require a reassessment of migration characteristics in these rocks.
For this purpose, an experimental set-up was designed, constructed and tested to quantify hydrogen migration rates in rocks. It comprises two gas chambers, separated by a through flange, containing the epoxy-embedded sample section with an exposed area of 7 cm². The retentate chamber is filled with a gas mixture of 2 Vol% hydrogen in synthetic air at 0.1 MPa. The permeate chamber contains air and includes an amperometric hydrogen sensor. Since there is no pressure gradient, the driving force for hydrogen movement is solely the concentration gradient between both sides of the rock sample. The hydrogen break-through and transport rates are monitored. In initial tests, core pieces of various length of sandstone and salt at dry and wet conditions are employed. The results approve the functional capability of the set-up and allow for a first-approach characterisation of hydrogen gas transport.
Mechanistic Insights of Mild Hematite Reduction in Hydrogen Storage Sites
Federal Institute for Geosciences and Natural Resources, Germany
Hydrogen is a prospective energy carrier whose storage in extensive volumes is still an unsolved problem. One approach is underground hydrogen storage, in which geological formations such as salt caverns or depleted natural gas and oil reservoirs are used to hold large amounts of gas under pressure. However, in those formations minerals can react with the hydrogen stored and therefore deplete or contaminate the gas recovered.
In our previous project we have shown that various minerals (e.g. pyrite, smectite, hematite) reacted with hydrogen under storage conditions (~120°C, <100 bar). Especially the Fe3+/Fe2+ switch in a reaction in which hematite is reduced to magnetite forming water (3 Fe2O3 + H2 → 2 Fe3O4 + H2O) was found to be active. Mechanistic data of that reduction is abundantly available at high temperatures (>500°C). However, studies at storage conditions (45-120°C) are rare up to this point. Especially the influence of pressure is unclear.
The work presented aims to understand the processes by which hematite is reduced under those conditions. For that purpose, we built a system in which we measure the decreasing hydrogen concentration periodically. Is consists of a heated pressure vessel on whose outlet a 10-port-valve flushes a gas-sample to a mass spectrometer. The resulting H2-peaks in the MS-spectra are quantified using Ne as internal standard. This way we are able to obtain time-resolved data on the consumption of H2 as well as formation of H2O by hematite. Additionally we quantify the hematite to magnetite ratio using XRD after the experiment.