Nano-scale research for geosequestration wins silver at Pittsburgh

08 October 2009

A research team including CRL Energy’s Research Manager Dr Tony Clemens has again won international accolades, this time a silver award for an “Outstanding Contribution” at the September 2009 Pittsburgh Coal Conference. The award is for a scientific paper produced as a result of their recent research at the US Argonne National Laboratory into the processes that occur when carbon dioxide is injected into various coal types. The work is to understand what happens at the atomic level to the pore spaces in coal if unminable coal seams are used as part of geosequestration.

Geosequestration (the capture and storage of CO₂ in geological structures such as deep aquifers, unminable coal seams, and oil and gas fields) is an increasingly likely option with the pressing need to stabilise CO₂ levels in air, combined with the continued widespread use of fossil fuels predicted to continue at least through to 2050 as the world transitions toward renewable-based energy.

Dr Clemens has been working with Drs Randall Winans and Sönke Seifert of the US Argonne National Laboratory, using in situ Small Angle X-Ray Scattering (SAXS) from Argonne’s Advanced Photon Source (APS) high energy synchrotron. Dr Clemens directly observes changes in coal structure when it is injected with pressurised CO₂. In the past three years, tests have been carried out on a suite of New Zealand coals and US coal samples from the Argonne Data Bank, this year Dr Clemens has focussed on a suite of 10 New Zealand sub-bituminous coals.

“Until the Hadron Collider comes online, Argonne is one of the three most powerful synchrotrons on the planet. The beam intensity provides the level of resolution and observational power that couldn’t be achieved using any other method. We’ve been measuring the low angle elastic scattering of X-rays from samples with non-homogeneities in the nanometre to micron range; this provides the pore size distribution, shape, and the surface morphology over a broad size range. The sample doesn’t need to be crystalline and we’ve injected CO₂ over a wide range of pressures, which gives us an insight into the changes in coal structure as pressurised CO₂ is injected,” says Dr Clemens.

“To understand better what we’re doing, you have to understand the structure of coal. We think there are some types of unminable coal seams that could be ideal CO₂ storage sites. This is because coal itself is actually a heterogeneous organic structure which is riddled with variable pore spaces ranging in size from Angstroms to microns. In our first experiments we put samples of New Zealand and US coals in the beamline and injected CO₂ at a series of pressures.

“We went from ambient to 200 psi, held for a few minutes, then to 400 psi, held for a few minutes, then 600, then 800, then stepped back to 600, 400, and 200. The result was the scattering intensities decreased monotonically with increasing pressure and the scattering variations occurred during pressure changes and the depressurisation data matched that of the pressurisation steps. This strongly suggests we are looking at adsorption of CO₂ on walls of pores or voids and that this is a completely reversible process.

“With out next set of experiments we increased the pressure straight through to 800 psi and held it overnight (15 to 18 hours), we then released the pressure back to ambient. This resulted in decreased scattering intensity which cannot be due to changes in pressure, it is more likely we are seeing the dissolution of CO₂ into the coal and swelling. This process was only partially reversible, with a partial increase in scattering intensity on depressurisation.

“To get more information from this set of experiments, we plotted a function called the Porod Invariant – it is a measure of the variation in volume fraction of the voids and the solid coal matrix on swelling. The Porod invariant plots obtained for high rank coals (bituminous) are completely consistent with an initial pore or void filling process followed by dissolution and diffusion of the CO₂ through the sample. As the CO₂ front proceeds it does so as if it were proceeding through a glassy polymer type material, swelling the material (coal) as it passes via a largely irreversible relaxation process (Class II diffusion).

“This implies that for lower rank coals - which are less like a glassy polymer structure, this effect may not be seen. We tested this with a suite of 10 New Zealand sub-bituminous coals. It happened that three of the sub-bituminous samples (the highest ranked of the ten) behaved like the coals studied previously; that is with void or pore filling followed by Class II diffusion. However, for the other seven samples we saw only the void or pore filling. The dividing line between the two outcomes is very thin.

“The technique is now being applied to other coals from around the world, and there are plans to try using SAXS to observe simulated injection of CO2 into aquifers and oil and gas fields and to compare the results of SAXS direct observation with the models currently used to predict CO₂ behaviour in geological structures.”

As well as the Pittsburgh Coal Conference, Dr Clemens also presented a paper on the research entitled The use of small angle x-ray scattering (SAXS) for direct observation of the injection of CO₂ into coal samples at the New Zealand National Energy Research Institute (NERI) conference in April this year.

Dr Clemens’s research was initiated by support from the NZ/US bilateral climate change research partnership between the NZ Ministry of Foreign Affairs and the US State Department and more latterly by the Foundation for Research Science and Technology and the Ministry for the Environment. The Argonne APS-based research is supported by the US Department of Energy, Basic Energy Science (BES) Program.

The sample holder in the beamline, this time a six sample holder allows Dr Clemens to do six samples in one run.