On optimisation of N2 and CO2-selective hybrid membrane process systems for post-combustion CO2 capture from coal-fired power plants

Membrane Separation Technology in Direct Air Capture

Direct air capture (DAC) is an emerging negative CO2 emission technology that aims to introduce a feasible method for CO2 capture from the atmosphere. Unlike carbon capture from point sources, which deals with flue gas at high CO2 concentrations, carbon capture directly from the atmosphere has proved difficult due to the low CO2 concentration in ambient air. Current DAC technologies mainly consider sorbent-based systems; however, membrane technology can be considered a promising DAC approach since it provides several advantages, e.g., lower energy and operational costs, less environmental footprint, and more potential for small-scale ubiquitous installations. Several recent advancements in validating the feasibility of highly permeable gas separation membrane fabrication and system design show that membrane-based direct air capture (m-DAC) could be a complementary approach to sorbent-based DAC, e.g., as part of a hybrid system design that incorporates other DAC technologies (e.g., solvent or sorbent-based DAC). In this article, the ongoing research and DAC application attempts via membrane separation have been reviewed. The reported membrane materials that could potentially be used for m-DAC are summarized. In addition, the future direction of m-DAC development is discussed, which could provide perspective and encourage new researchers' further work in the field of m-DAC.

Membrane-Based Technologies for Post-Combustion CO2 Capture from Flue Gases: Recent Progress in Commonly Employed Membrane Materials

Carbon dioxide (CO2), which results from fossil fuel combustion and industrial processes, accounts for a substantial part of the total anthropogenic greenhouse gases (GHGs). As a result, several carbon capture, utilization and storage (CCUS) technologies have been developed during the last decade. Chemical absorption, adsorption, cryogenic separation and membrane separation are the most widely used post-combustion CO2 capture technologies. This study reviews post-combustion CO2 capture technologies and the latest progress in membrane processes for CO2 separation. More specifically, the objective of the present work is to present the state of the art of membrane-based technologies for CO2 capture from flue gases and focuses mainly on recent advancements in commonly employed membrane materials. These materials are utilized for the fabrication and application of novel composite membranes or mixed-matrix membranes (MMMs), which present improved intrinsic and surface characteristics and, thus, can achieve high selectivity and permeability. Recent progress is described regarding the utilization of metal-organic frameworks (MOFs), carbon molecular sieves (CMSs), nanocomposite membranes, ionic liquid (IL)-based membranes and facilitated transport membranes (FTMs), which comprise MMMs. The most significant challenges and future prospects of implementing membrane technologies for CO2 capture are also presented.

Synchronous Design of Membrane Material and Process for Pre-Combustion CO2 Capture: A Superstructure Method Integrating Membrane Type Selection

Membrane separation technology for CO₂ capture in pre-combustion has the advantages of easy operation, minimal land use and no pollution and is considered a reliable alternative to traditional technology. However, previous studies only focused on the H₂-selective membrane (HM) or CO₂-selective membrane (CM), paying little attention to the combination of different membranes. Therefore, it is hopeful to find the optimal process by considering the potential combination of H₂-selective and CO₂-selective membranes. For the CO₂ capture process in pre-combustion, this paper presents an optimization model based on the superstructure method to determine the best membrane process. In the superstructure model, both CO₂-selective and H₂-selective commercial membranes are considered. In addition, the changes in optimal membrane performance and capture cost are studied when the selectivity and permeability of membrane change synchronously based on the Robeson upper bound. The results show that when the CO₂ purity is 96% and the CO₂ recovery rate is 90%, the combination of different membrane types achieves better results. The optimal process is the two-stage membrane process with recycling, using the combination of CM and HM in all situations, which has obvious economic advantages compared with the Selexol process. Under the condition of 96% CO₂ purity and 90% CO₂ recovery, the CO₂ capture cost can be reduced to 11.75$/t CO₂ by optimizing the process structure, operating parameters, and performance of membranes.

Activated carbon fiber derived from wasted coal liquefaction residual for CO2 capture

Wasted coal liquefaction residual was used to synthesize activated carbon fibers (ACFs) for CO₂ capture, and the properties of the developed ACFs were optimized by adjusting the activation conditions, including the reaction temperature and soaking time. The yield, element distribution, pore structure, composition, functional group, morphology, and adsorption capacity of the as-synthesized ACFs were characterized by various apparatuses. In addition, static and dynamic adsorption experiments were conducted to investigate the adsorption capacity of CO₂ in flue gas. The results revealed that the synthesized ACFs are mainly composed of carbon, accounting for more than 90% of the total elements. The specific surface area, pore volume, and pore width distribution of the prepared ACFs were optimized by changing the activation conditions, and ACFs with a specific surface area higher than 1400 m²/g were successfully developed by activation at 950 for 3 h. The amount of micropores occupied more than 90% of the total pore volume. The pore width distribution dominated by micropores is beneficial for CO2 adsorption since the diameter of CO₂ is 0.33 nm. From FTIR and XPS analysis, it is found that the main structure of ACFs is a carbon skeleton composed of polycyclic aromatic hydrocarbons with a small number of oxygen-containing functional groups. The adsorption isotherm of ACFs for CO₂ conforms to the Langmuir model, indicating that the adsorption process of CO₂ by ACFs can be attributed to monolayer adsorption. Both the specific surface area and oxygen-containing functional groups have crucial effects on the adsorption capacity of CO₂. The dynamic adsorption experiment determined that ACFs-920-3 had the highest adsorption capacity for CO₂ in flue gas, and adsorption equilibrium was achieved after 7 min of adsorption. The adsorption process of CO₂ in flue gas by the as-synthesized ACFs fits well with the pseudosecond kinetic model. The CO₂ adsorption capacity of the obtained ACFs remained unchanged after 10 cycles of adsorption. A high-value-added route for synthesizing ACFs for CO₂ capture using CLR as a raw material was developed.

Techno-Economic Optimization of Multistage Membrane Processes with Innovative Hollow Fiber Modules for the Production of High-Purity CO2 and CH4 from Different Sources

Within the current climate emergency framework and in order to avoid the most severe consequences of global warming, membrane separation processes have become critical for the implementation of carbon capture, storage, and utilization technologies. Mixtures of CO₂ and CH₄ are relevant energy resources, and the design of innovative membranes specifically designed to improve their separation is a hot topic. This work investigated the potential of modified polydimethylsiloxane and ionic liquid–chitosan composite membranes for separation of CO₂ and CH₄ mixtures from different sources, such as biogas upgrading, natural gas sweetening, or CO₂ enhanced oil recovery. The techno-economic optimization of multistage processes at a real industrial scale was carried out, paying special attention to the identification of the optimal configuration of the hollow fiber modules and the selection of the best membrane scheme. The results demonstrated that a high initial content of CH₄ in the feed stream (like in the case of natural gas sweetening) might imply a great challenge for the separation performance, where only membranes with exceptional selectivity might achieve the requirements in a two-stage process. The effective lifetime of the membranes is a key parameter for the successful implementation of innovative membranes in order to avoid severe economic penalties due to excessively frequent membrane replacement. The scale of the process had a great influence on the economic competitiveness of the process, but large-scale installations can operate under competitive conditions with total costs below 0.050 US$ per m³ STP of treated feed gas.