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.
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
|Moisture Content (wt%)
(Rv, max %)
||Nitrogen (wt%) sulfur
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 H2, 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
||D (m2/s) 10-9
||sorption capacity at equilibrium pressure (12.78–13.03 bar)
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
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.
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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