Molecular simulation of CO2 capture using hydrotalcite

The excessive CO2 emissions generated by human activities are the main factor resulting in climate change. A likely increase of 2ºC in the global average temperature is predicted to induce sea level rise and extreme weather events. Carbon Capture, Utilisation and Storage technologies will play a pivotal role in reducing these emissions and lessening the negative impacts. Currently, the most mature technology for carbon capture is chemical absorption using solvents such as mono-ethanolamine (MEA). However, this technology has the disadvantages of high energy requirements, the use of corrosive substances and elevated cost. The search for alternative cheaper and reliable capture methods is ongoing. Adsorption-based post-combustion carbon capture (PCC) has shown promising results, requiring less energy and using innocuous materials. Nevertheless, the development of adequate adsorbent materials and efficient process design for large scale implementation is still in the early stages.

In this work, we focus on adsorption-based PCC. After carrying out an extensive literature review in the advancements on adsorption carbon capture from the experimental and molecular simulation perspectives, and a survey of the bench and pilot-scale projects around the world using this technology, we selected hydrotalcites (HTs) as a potential adsorbent for capturing CO2 from gas-fired power plant flue gases. HTs are better suited to work at the desired temperature (200ºC) in contrast with other adsorbent materials such as zeolites and activated carbon. In addition, they exhibit high CO2 selectivity and are widely available. The main challenge for their large-scale implementation is their relatively low adsorption capacity in contrast with chemical solvents. Since their performance is influenced by their composition, synthesis, and operational conditions, an experimental approach is impractical, thus molecular simulations were employed. Molecular simulations enable systematic studies without the need for columns settings and with the appropriate tools, in less time.

To the best knowledge of the author, this is the first work employing the ReaxFF method for studying CO2 capture using HT as adsorbents. This molecular simulation method allows the simulation of the formation of chemical bonds, even for large and complex systems as the HT. This study is the first step towards gaining a better understanding of CO2 capture on HT at molecular level considering HT calcination, chemisorption and physisorption.

First, we developed a Mg-Al-CO_3 HT structure geometry with Density Functional Theory calculations. The results showed that the developed structure lattice parameters agreed with experimental measurements.

Next, we developed a specialised reactive force field (FF) and employed it for simulating the calcination process HT undergo for activation with molecular dynamics (MD) simulations. To the knowledge of the authors, this is the first FF capable of working with this HT structure. The FF generated with the Covariance Matrix Adaptation Evolutionary Strategy (CMA-ES) which had the lowest error function value was employed to carry out the calcination MD simulations. Finally, we carried out CO_2 adsorption studies with Grand Canonical Monte Carlo (GCMC) simulations at 200ºC and 1 bar to reflect PCC settings.

The MD simulations for analysing HT during calcination showed a similar decomposition trend as the one reported in the literature, starting with dehydration, a subsequent dihydroxylation, and finally a decarbonation, resulting in a mixed metallic oxides structure. For validation, we compared the surface area of the calcined simulated HT against experimental data. The simulated calcined HT exhibited a surface area of 247.63 m2/g, which is in the expected range for calcined Mg-Al-CO3 HT surface area reported by experiments.

The GCMC simulations of the adsorption studies showed the HT structure has an adsorption capacity of 34.78 molCO2/kgHT, which is much higher than reported in experimental studies. We attribute the disparity between the experimental and literature values to many factors related to the incipient nature of the generated FF and structure.