Multicomponent and multi-dimensional modeling and simulation of adsorption-based carbon dioxide separation

Assessment of Appropriate Geometry for Thermally Efficient CO2 Adsorption Beds

Carbon capture is one of the recently raised technologies to mitigate greenhouse gas emissions. Adsorption was introduced as an energy-efficient carbon capture process, and the literature primarily shows the utilization of circular cross-sectional adsorption beds for this purpose. In this regard, this paper investigates different shapes of adsorbent beds to determine the thermal and adsorption uptake enhancements. Three geometries are considered: circular, square, and triangular cross-sectional beds. Mg-MOF-74 is used as an adsorbent, and numerical simulation is developed using a user-defined function coupled with ANSYS-Fluent. The results show that the triangular cross-sectional bed exhibits better adsorption capacity and thermal management compared to other beds. For example, the triangular cross-sectional bed shows 6 K less than the circular one during the adsorption process. It is recommended that the triangular cross-sectional bed be used for temperature swing adsorption when pumping power is not important. The square bed comes second after the triangular one with a lower pressure drop, suggesting such beds as good candidates for pressure swing adsorption. The square bed could be an excellent choice for compact beds when CO2 uptake and pumping power are both important.

Adsorption of CO2 on In Situ Functionalized Straw Burning Ashes. An Innovative, Circular Economy-Based Concept for Limitation of Industrial-Scale Greenhouse Gas Emission

A new, innovative approach in the search for an effective and cheap carbon dioxide sorbent, in line with the circular economy and sustainable development principles, directs the attention of researchers to various types of waste ashes generated as a result of biomass combustion. In addition to the use of environmentally safe materials that have been experimentally identified, and that, in some way, have adjustable sorption capacity, it is also possible to rationally develop a widely applicable, simple, and inexpensive technology based on large amounts of this type of post-industrial waste, which is also an equally important issue for the natural environment (reducing the need for ash storage and accumulation). Even the lower sorption capacity can be successfully compensated for by their common availability and very low cost. Thus, the CO2 adsorption capability of the ashes from the combustion of straw biomass was experimentally investigated with the use of a high-pressure adsorption stand. The presented original technological concept has been positively verified on a laboratory scale, thus a functionalization-based approach to the combustion of substrate mixtures with nano-structural additives (raw, dried, calcined halloysite, kaolinite), introduced to improve the performance of straw biomass combustion and bottom ash formation in power boilers, clearly increased the CO2 adsorption capacity of the modified ashes. This allows for an advantageous synergy effect in the extra side-production of useful adsorbents in the closed-loop “cascade” scheme of the CE process. The addition of 4 wt.% kaolinite to straw biomass caused an over 2.5-fold increase in the CO2 adsorption capacity in relation to ash from the combustion of pure straw biomass (with a CO2 adsorption capacity of 0.132 mmol/g). In the case of addition of 4 wt.% nano-structured species to the straw combustion process, the best effects (ash adsorption capacity) were obtained in the following order: kaolinite (0.321 mmol/g), raw halloysite (0.310 mmol/g), calcined halloysite (0.298 mmol/g), and dried halloysite (0.288 mmol/g). Increasing the dose (in relation to all four tested substances) of the straw biomass additive from 2 to 4 wt.%, not only increase the adsorption capacity of the obtained ash, thus enriched with nano-structural additives, but also a showed a significant reduction in the differences between the maximum adsorption capacity of each ash is observed. The experimental results were analyzed using five models of adsorption isotherms: Freundlich, Langmuir, Jovanović, Temkin, and Hill. Moreover, selected samples of each ash were subjected to porosimetry tests and identification of the surface morphology (SEM). The obtained results can be used in the design of PSA processes or as permanent CO2 adsorbents, based on the environmentally beneficial option of using ashes from biomass combustion with appropriately selected additives.

Thermodynamic Properties for Carbon Dioxide

We first report three reliable analytical expressions of the entropy, enthalpy and Gibbs free energy of carbon dioxide (CO₂) and perform predictions of these three thermodynamic quantities on the basis of the proposed analytical expressions and in terms of experimental values of five molecular constants for CO₂. The average relative deviations of the calculated values from the National Institute of Standards and Technology database over the temperature range from 300 to 6000 K are merely 0.053, 0.95, and 0.070%, respectively, for the entropy, enthalpy, and Gibbs free energy. The present predictive expressions are away from the utilization of plenty of experimental spectroscopy data and are applicable to treat CO₂ capture and storage processes.

Review of post-combustion carbon dioxide capture technologies using activated carbon

Carbon dioxide (CO₂) is the largest anthropogenic greenhouse gas (GHG) on the planet contributing to the global warming. Currently, there are three capture technologies of trapping CO₂ from the flue gas and they are pre-combustion, post-combustion and oxy-fuel combustion. Among these, the post-combustion is widely popular as it can be retrofitted for a short to medium term without encountering any significant technology risks or changes. Activated carbon is widely used as a universal separation medium with series of advantages compared to the first generation capture processes based on amine-based scrubbing which are inherently energy intensive. The goal of this review is to elucidate the three CO₂ capture technologies with a focus on the use of activated carbon (AC) as an adsorbent for post-combustion anthropogenic CO₂ flue gas capture prior to emission to atmosphere. Furthermore, this coherent review summarizes the recent ongoing research on the preparation of activated carbon from various sources to provide a profound understanding on the current progress to highlight the challenges of the CO₂ mitigation efforts along with the mathematical modeling of CO₂ capture. AC is widely seen as a universal adsorbent due to its unique properties such as high surface area and porous texture. Other applications of AC in the removal of contaminants from flue gas, heavy metal and organic compounds, as a catalyst and catalyst support and in the electronics and electroplating industry are also discussed in this study.