Behavior of Hydrogen Adsorption in Australian Sub-Bituminous Coal

By Laura ThomsonMar 28 2022

Hydrogen—a clean energy source—potentially replaces fossil fuels in a hydrogen economy, therefore, alleviating climate change. One of the main hindrances in the transition to a decarbonized energy society is the lack of storage space for industrial-scale hydrogen (H₂). This article looks at Keshavarz et al.'s study looking at hydrogen diffusion in coal and its implications for hydrogen geo‐storage. The research was published in Journal of Colloid and Interface Science.

 Image Credit: Alexander Limbach/Shutterstock.com

It is important to find new solutions for widespread hydrogen storage. Hydrogen underground storage (UHS) is one option that has been suggested in this context. Here, H₂ is stored in underground geological formations, such as salt caverns or depleted oil and gas reservoirs.

In the recent research, the team studied the behavior of hydrogen adsorption in an Australian sub-bituminous coal sample. In the current literature, however, no data is available for hydrogen adsorption rate or diffusion kinetics. Hence, in this work, the researchers quantified such kinetic parameters in an Australian coal sample. This study offers basic petrophysical data for UHS and thereby helps in the large-scale hydrogen economy implementation.

Methodology

On an Australian anthracite sample, H₂ and CO₂ kinetic sorption tests were conducted. Using a blade grinder, the coal sample was ground to <500 µm. A size fraction of 250–500 µm was employed for the kinetic experiments. The coal was completely analyzed, and final and proximate analyses, helium density, and petrographic analyses findings are presented in Table 1.

Table 1. Essential analysis properties of the tested Australian anthracite coal sample. Source: Keshavarz, et al., 2022

The experimental setup’s schematic view is illustrated in Figure 1.

Figure. 1. Experimental set up: 1. Sample cell; 2. Coal sample; 3. Manual valve; 4. Automatic valves; 5. Pressure transducers; 6. Big reference cell; 7. Small reference cell; 8. Temperature controller; 9. Vacuum line; 10. Vent line; 11. Test gas line; 12. Calibration gas line; 13. Control panel and data acquisition system. Image Credit: Keshavarz, et al., 2017

In Table 2, parameters linked to H_2, and CO₂ diffusion are tabulated.

Table 2. H₂ and CO₂ diffusion parameters and sorption capacities, at equilibrium pressure (approximately 13 bar), for the tested coal at different temperatures. Source: Keshavarz, et al., 2022

Results and Discussion

Evidently, adsorption of H₂ and CO₂ reached equilibrium quicker at a higher temperature, as anticipated (Figure 2). Such quicker equilibration was made to occur by the improved gas molecular kinetic energy at greater temperatures.

Figure. 2. Comparing H₂ and CO₂ adsorption rate parameters: Adsorption rate profile for H₂ and CO₂ as a function of temperature (equilibrium pressures were in the range of 12.78–13.03 bar in all H₂ and CO₂ kinetics tests). Image Credit: Keshavarz, et al., 2022

Moreover, in all diffusion tests (that is, for both H₂ and CO₂), β was nearly constant (β = 0.36 ± 7.5%) (see Table 2 and Figure 3).

Also, 1/t0, and, as a result, D—the diffusion coefficient, increased as temperature increased for both gases (see Figure 4 and Table 2).

Figure. 3. β values for H₂ and CO₂ tests at different temperatures. Image Credit: Keshavarz, et al., 2022

Moreover, it is well-known that gas adsorption (along with that of CO₂ and H₂) decreases with an increase in temperature (see Figure 4).

Figure. 4. H₂ and CO₂ diffusion coefficients at different temperatures. Image Credit: Keshavarz, et al., 2022

Furthermore, increasing the temperature increased the H₂–CO₂ diffusion coefficient ratio, whereas the H₂–CO₂ adsorption ratio stayed the same (see Figure 5).

Figure. 5. Diffusion coefficient ratio (H₂/CO₂) and sorption capacity ratio (CO₂/H₂) at different temperatures. Image Credit: Keshavarz, et al., 2022

It is to be noted that for all tested temperatures, CO₂ adsorption ability was considerably higher than that of H₂ (by five times approximately), whereas adsorption capability decreased slightly for both gases with an increase in temperature (see Figure 6).

Figure. 6. H₂ and CO₂ adsorption capacities at equilibrium pressure (12.78–13.03 bar) as a function of temperature. Image Credit: Keshavarz, et al., 2022

Conclusion

Even though gas diffusion in coal is a well-established phenomenon, there is a severe lack of data for hydrogen diffusion in coal, majorly owing to the novelty of the geologic hydrogen storage concept.

Hence, in this study, depending on already reported gas diffusion measurements in coal, the researchers quantified kinetic adsorption profiles of H₂ for four different temperatures (20 °C, 30 °C, 45 °C, and 60 °C) and equilibrium pressures of ∼13 bar on an Australian anthracite coal sample at isothermal conditions. CO₂ diffusion coefficients were quantified for the same sample at similar equilibrium pressure and temperatures for comparison.

The results reveal that a higher temperature results in a higher rate of H₂ and CO₂ adsorption, but leads to a lower gas adsorption capacity. They also show that H₂ diffusion coefficients are larger than the equivalent CO₂ diffusion coefficients with one order of magnitude. The H₂−CO₂ diffusion coefficient ratio improved from 10 at 20 °C to 22 at 60 °C.

Furthermore, CO₂ adsorption capacities are approximately five times larger than the equivalent H₂ adsorption capacities, at all studied temperatures and equilibrium pressure (∼13 bar). In addition, H₂ kinetic research is underway to find the coal characteristics’ effect on H₂ diffusion in coal.

Journal Reference:

Keshavarz, A., Abid, H., Ali, M., Iglauer, S. (2022) Hydrogen diffusion in coal: Implications for hydrogen geo‐storage. Journal of Colloid and Interface Science. Volume 608, Part 2, P. 1457–1462. Available Online: https://www.sciencedirect.com/science/article/pii/S0021979721017276.

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