# Special Issue on Carbon Capture and Utilization
## 2 Contributions to This Special Issue  
Methane (CH\({}_{4}\)) and carbon dioxide (CO\({}_{2}\)) often coexist in nature, forming several gas mixtures (e.g., natural gas, syngas, and landfill gas). The separation of CO\({}_{2}\) is essential in enhancing the purity and energetic value of these gas streams. In this context, [PERSON] and [PERSON] [1] evaluated the adsorption of CO\({}_{2}\) and CH\({}_{4}\) with a mixture of activated-carbon (AC) nanoparticles with multiwall carbon nanotubes (MWCNTs) with several ratios. It was expected that the combination of different features of the selected materials could improve the gas-adsorption and -separation processes. The single adsorption capacity (adsorption/desorption isotherms of pure CO\({}_{2}\) and CH\({}_{4}\)) and binary adsorption equilibria and selectivity were assessed. The adsorption capacities for both components were measured at different temperatures, and the dual-site Langmuir (DSL) model was applied to describe experimental data. The adsorption capacity of MWCNTs was found to be significantly lower than that of AC. Future research should be performed on different forms of carbon-carbon nanostructures. The study of interactions between them will enable researchers to track the associated mechanisms and enhance the potential gas-adsorption applications. CO\({}_{2}\) capture can be performed following three technological concepts: post-combustion capture systems, pre-combustion capture systems, and oxy-fuel capture systems. For the latter, [PERSON] et al. [2] investigated the effect of flow rates of oxygen and recirculated flue gas (which modifies the combustion atmosphere) on the heating performance and pollutant emissions (oxy-combustion characteristics) of Australian coal in a 0.3 MW furnace. For low oxygen flow rate, the flue gas contained residual oxygen and NO concentrations. On theother hand, the increased recirculated flue gas flow rate decreases the temperature of the radiative section and increases this parameter in the convective one. A high CO\({}_{2}\) concentration (up to 94.8%) can be achieved with a recirculated flue gas in a range of 3-5% (\(\
icefrac{{v}}{{v}}\)), reducing the energy requirements (and CO\({}_{2}\) emissions) by carbon capture and storage. Compared with air combustion, a reduction of up to 74% of NO emissions was achieved. Besides these advantages, the investment in oxygen plant and flue gas recirculation system (CAPEX) and the energy consumption of these systems (OPEX) limits the application of oxy-fuel combustion to the conventional coal power plant. For post-combustion capture systems, [PERSON] et al. [3] assessed the CO\({}_{2}\) capture and conversion of low volumetric flow rates into carbonates using Sr and Ba alkaline solutions with a simple system to implement in small industries. The study of the reaction showed that a 20-min reaction time is required to maximize the carbonate production. CO\({}_{2}\) conversion of 40% was achieved for the best reaction conditions with Sr(OH)\({}_{2}\). The formed solids were characterized by XRD, SEM/EDS, FTIR, and TGA. The synthesized carbonates have commercial value in several sectors, being an alternative and sustainable solution for recycling CO\({}_{2}\) and reducing its emission to the atmosphere. CO\({}_{2}\) capture from large emission sources cannot be enough to fulfil the established climate targets. About 50% of CO\({}_{2}\) emissions are from diffuse sources (e.g., automobiles, shipping, and aircraft), and mitigation actions have been delayed due to economic interests (high fossil fuel dependence among world power economies). Therefore, considering the long residence time of CO\({}_{2}\) in the atmosphere, atmospheric CO\({}_{2}\) capture may be required. [PERSON] and [PERSON] [4] evaluated the performance of a new radial-flow reactor for CO\({}_{2}\) adsorption from ambient air. The adsorption is achieved with 2 kg of supported amine sorbent, and the reactor can operate in two modes (batch or semi-continuous).