Microalgae have great, yet relatively untapped potential as a highly productive crop for the production of animal and aquaculture feed, biofuels, and nutraceutical products. Compared to conventional terrestrial crops they have a very fast growth rate and can be produced on non-arable land. During microalgae cultivation, carbon dioxide (CO₂) is supplied as the carbon source for photosynthesising microalgae. There are a number of potential CO₂ supplies including air, flue gas and purified CO₂. In addition, several strategies have been applied to the delivery of CO₂ to microalgae production systems, including directly bubbling CO₂-rich gas, microbubbles, porous membrane spargers and non-porous membrane contactors. This article provides a comparative analysis of the different CO₂ supply and delivery strategies and how they relate to each other.

The emission of large amounts of carbon dioxide is of major concern with regard to increasing the risk of climate change. Carbon capture, utilisation and storage (CCUS) has been proposed as an important pathway for slowing the rate of these emissions. Solvent absorption of CO₂ using amino acid solvents has drawn much attention over the last few years due to advantages including their ionic nature, low evaporation rate, low toxicity, high absorption rate and high biodegradation potential, compared to traditional amine solvents. In this review, recent progress on the absorption kinetics of amino acids is summarised, and the engineering potential of using amino acids as carbon capture solvents is discussed. The reaction orders between amino acids and carbon dioxide are typically between 1 and 2. Glycine exhibits a reaction order of 1, whilst, by comparison, lysine, proline and sarcosine have the largest reaction constants with carbon dioxide which is much larger than that of the benchmark solvent monoethanolamine (MEA). Ionic strength, pH and cations such as sodium and potassium have been shown to be important factors influencing the reactivity of amino acids. Corrosivity and reactivity with impurities such as SO_x and NO_x are not considered to be significant problems for amino acids solvents. The precipitation of CO₂ loaded amino acid salts is thought to be a good pathway for increasing CO₂ loading capacity and cutting desorption energy costs if well-controlled. It is recommended that more detailed research on amino acid degradation and overall process energy costs is conducted. Overall, amino acid solvents are recognised as promising potential solvents for carbon dioxide capture.

Membrane and membrane process have been considered as one of the most promising technologies for mitigating CO₂ emissions from the use of fossil fuels. In this paper, recent advances in polymeric membranes for CO₂ capture are reviewed in terms of material design and membrane formation. The selected polymeric materials are grouped based on their gas transport mechanisms, i.e., solution-diffusion and facilitated transport. The discussion of solution-diffusion membranes encompasses the recent efforts to shift the upper bound barrier, including the enhanced CO₂ solubility in several rubbery polymers and novel methods to construct shape-persisting macromolecules with unprecedented sieving ability. The carrier-bearing facilitated transport membranes are categorized based on the specific CO₂-carrier chemistry. Finally, opportunities and challenges in practical applications are also discussed, including post-combustion carbon capture (CO₂/N₂), hydrogen purification (CO₂/H₂), and natural gas sweetening (CO₂/CH₄).

Aqueous ammonia (NH₃) is a promising alternative solvent for the capture of industrial CO₂ emissions, given its high chemical stability and CO₂ removal capacity, and low material costs and regeneration energy. NH₃ also has potential for capturing multiple flue gas components, including NO_x, SO_x and CO₂, and producing value-added chemicals. However, its high volatility and low reactivity towards CO₂ limit its economic viability. Considerable efforts have been made to advance aqueous NH₃-based post-combustion capture technologies in the last few years:in particular, General Electric's chilled NH₃ process, CSIRO's mild-temperature aqueous NH₃ process and SRI International's mixed-salts (NH₃ and potassium carbonate) technology. Here, we review these research activities and other developments in the field, and outline future research needed to further improve aqueous NH₃-based CO₂ capture technologies.

The chemical utilization of CO₂ is a crucial step for the recycling of carbon resource. In recent years, the study on the conversion of CO₂ into a wide variety of C₂₊ important chemicals and fuels has received considerable attention as an emerging technology. Since CO₂ is thermodynamically stable and kinetically inert, the effective activation of CO₂ molecule for the selective transformation to target products still remains a challenge. The welldesigned CO₂ reduction route and efficient catalyst system has imposed the feasibility of CO₂ conversion into C₂₊ chemicals and fuels. In this paper, we have reviewed the recent advances on chemical conversion of CO₂ into C₂₊ chemicals and fuels with wide practical applications, including important alcohols, acetic acid, dimethyl ether, olefins and gasoline. In particular, the synthetic routes for C-C coupling and carbon chain growth, multifunctional catalyst design and reaction mechanisms are exclusively emphasized.

Membrane separation technology has popularized rapidly and attracts much interest in gas industry as a promising sort of newly chemical separation unit operation. In this paper, recent advances on membrane processes for CO₂ separation are reviewed. The researches indicate that the optimization of operating process designs could improve the separation performance, reduce the energy consumption and decrease the cost of membrane separation systems. With the improvement of membrane materials recently, membrane processes are beginning to be competitive enough for CO₂ separation, especially for postcombustion CO₂ capture, biogas upgrading and natural gas carbon dioxide removal, compared with the traditional separation methods. We summarize the needs and most promising research directions for membrane processes for CO₂ separation in current and future membrane applications. As the time goes by, novel membrane materials developed according to the requirement proposed by process optimization with increased selectivity and/or permeance will accelerate the industrialization of membrane process in the near future. Based on the data collected in a pilot scale test, more effort could be made on the optimization of membrane separation processes. This work would open up a new horizon for CO₂ separation/Capture on Carbon Capture Utilization and Storage (CCUS).

Under the Paris agreement, China has committed to reducing CO₂ emissions by 60%-65% per unit of GDP by 2030. Since CO₂ emissions from coal-fired power plants currently account for over 30% of the total carbon emissions in China, it will be necessary to mitigate at least some of these emissions to achieve this goal. Studies by the International Energy Agency (IEA) indicate CCS technology has the potential to contribute 14% of global emission reductions, followed by 40% of higher energy efficiency and 35% of renewable energy, which is considered as the most promising technology to significantly reduce carbon emissions for current coal-fired power plants. Moreover, the announcement of a Chinese national carbon trading market in late 2017 signals an opportunity for the commercial deployment of CO₂ capture technologies.
Currently, the only commercially demonstrated technology for post-combustion CO₂ capture technology from power plants is solvent-based absorption. While commercially viable, the costs of deploying this technology are high. This has motivated efforts to develop more affordable alternatives, including advanced solvents, membranes, and sorbent capture systems. Of these approaches, advanced solvents have received the most attention in terms of research and demonstration. In contrast, sorbent capture technology has less attention, despite its potential for much lower energy consumption due to the absence of water in the sorbent. This paper reviews recent progress in the development of sorbent materials modified by amine functionalities with an emphasis on material characterization methods and the effects of operating conditions on performance. The main problems and challenges that need to be overcome to improve the competitiveness of sorbent-based capture technologies are discussed.

The increase in energy demand caused by industrialization leads to abundant CO₂ emissions into atmosphere and induces abrupt rise in earth temperature. It is vital to acquire relatively simple and cost-effective technologies to separate CO₂ from the flue gas and reduce its environmental impact. Solid adsorption is now considered an economic and least interfering way to capture CO₂, in that it can accomplish the goal of small energy penalty and few modifications to power plants. In this regard, we attempt to review the CO₂ adsorption performances of several types of solid adsorbents, including zeolites, clays, activated carbons, alkali metal oxides and carbonates, silica materials, metal-organic frameworks, covalent organic frameworks, and polymerized high internal phase emulsions. These solid adsorbents have been assessed in their CO₂ adsorption capacities along with other important parameters including adsorption kinetics, effect of water, recycling stability and regenerability. In particular, the superior properties of adsorbents enhanced by impregnating or grafting amine groups have been discussed for developing applicable candidates for industrial CO₂ capture.

Phase change solvents are attractive energy-efficient absorbents for carbon dioxide (CO₂) capture due to CO₂-rich phase formation. Potassium prolinate + water + ethanol (ProK/W/Eth) solution has shown good capture characteristics as a promising one in our previous work. In this work, absorption rate of CO₂, solubility of N₂O, and heat of absorption for ProK/W/Eth solution were investigated using a stirred cell reactor and a CPA201 reaction calorimeter and these results were also compared with the aqueous ProK and 30 mass% MEA solutions. Using ethanol as a solvent can substantially increase the CO₂ physical solubility and the absorption rate of CO₂ in ProK/W/Eth solutions is far higher than that in aqueous 30 mass% MEA solutions especially at a low CO₂ loading range. Solid precipitation, obtained from the liquid-to-solid phase change absorption, was analyzed by ¹³C NMR and DSC-TGA. The enthalpy change for ProK/W/Eth solutions at various CO₂ loading was also discussed.

The process models for an equilibrium CO₂ absorber and a rate based CO₂ absorber using potassium carbonate (K₂CO₃) solvents were developed in Aspen Custom Modeller (ACM) to remove CO₂ from a flue gas. The process model utilised the Electrolyte Non-Random Two Liquid (ENRTL) thermodynamic model and various packing correlations. The results from the ACM equilibrium model shows good agreement with an inbuilt Aspen Plus^® model when using the same input conditions. By further introducing a Murphree efficiency which is related to mass transfer and packing hydraulics, the equilibrium model can validate the experimental results from a pilot plant within a deviation of 10%. A more rigorous rate based model included mass and energy flux across the interface and the enhancement effect resulting from chemical reactions. The rate based model was validated using experimental data from pilot plants and was shown to predict the results to within 10%. A parametric sensitivity analysis showed that inlet flue gas flowrate and K₂CO₃ concentration in the lean solvent has significant impact on CO₂ recovery. Although both models can provide reasonable predictions based on pilot plant results, the rate based model is more advanced as it can explain mass and heat transfer, transport phenomena and chemical reactions occurring inside the absorber without introducing an empirical Murphree efficiency.

Membrane gas-solvent contactors are a hybrid technology combining solvent absorption with membrane gas separation, which demonstrates potential for CO₂ capture through the ability of the membrane to rigidly control the mass transfer area. Membrane contactors have been successfully demonstrated for CO₂ absorption, and there is strong research interest in using membrane contactors for the complimentary CO₂ desorption process to regenerate the solvent. However, understanding and modelling the various stages of mass transfer in the desorption process is less well-known, given the existing mass transfer correlations had been developed from absorption experiments. Hence, mass transfer correlations for membrane contactors are reviewed here, and their appropriateness for desorption analysed. This is achieved through simulating CO₂ desorption through a membrane contactor from loaded 30wt% monoethanolamine solvent to enable comparison of the correlations. It was found that the most cited correlations by Yang and Cussler were valid for shell side parallel flow, while that of Kreith and Black was viable for shell side cross flow. A limitation of all of these correlations is that they assume single phase flow on both sides of the membrane; however, the high temperature of CO₂ desorption can lead to partial solvent vaporisation and hence two phases present on one side of the membrane contactor during desorption. A mass transfer correlation is established here for two phase parallel flow on the shell side of a membrane contactor, based on experimental results for three composite and one asymmetric hollow fibre membrane contactors stripping CO₂ from loaded MEA at 105-108℃. This correlation is comparable to that reported in the literature for mass transfer in other two phase systems, but differs from the standard format for membrane contactors in terms of the exponent on the dimensionless Schmidt and Reynolds numbers.

The deactivation mechanism of Co/MgO catalyst for the reforming of methane with carbon dioxide was investigated. The conversion of CH₄ displayed a significant decrease in the initial stage caused by carbon deposition. There were two types of cokes, carbon nanotubes (CNTs) and carbon nano-onions (CNOs). The number of the CNO layers that coated on the surface of Co nanoparticles (NPs) increased rapidly in the initial reforming time, which was responsible for the deactivation of the Co/MgO catalyst. The deposition of CNOs was attributed to the oxidation of Co NPs. Therefore, the deactivation of the Co/MgO catalyst was originated from the first oxidization of the Co NPs into Co₃O₄ by O species (OH intermediate, CO₂, H₂O) during the reforming reaction, which accelerates the formation of coke that blocked the active metal, thus led to catalyst deactivation.

Post-combustion CO₂ capture (PCC) process faces significant challenge of high regeneration energy consumption. Biphasic absorbent is a promising alternative candidate which could significantly reduce the regeneration energy consumption because only the CO₂-concentrated phase should be regenerated. In this work, aqueous solutions of triethylenetetramine (TETA) and N,N-diethylethanolamine (DEEA) are found to be efficient biphasic absorbents of CO₂. The effects of the solvent composition, total amine concentration, and temperature on the absorption behavior, as well as the effect of temperature on the desorption behavior of TETA-DEEA-H₂O system were investigated. An aqueous solution of 1 mol·L^-1 TETA and 4 mol·L^-1 DEEA spontaneously separates into two liquid phases after a certain amount of CO₂ is absorbed and it shows high CO₂ absorption/desorption performance. About 99.4% of the absorbed CO₂ is found in the lower phase, which corresponds to a CO₂ absorption capacity of 3.44 mol·kg^-1. The appropriate absorption and desorption temperatures are found to be 30℃ and 90℃, respectively. The thermal analysis indicates that the heat of absorption of the 1 mol·L^-1 TETA and 4 mol·L^-1 DEEA solution is -84.38 kJ·(mol CO₂)^-1 which is 6.92 kJ·(mol CO₂)^-1 less than that of aqueous MEA. The reaction heat, sensible heat, and the vaporization heat of the TETA-DEEA-H₂O system are lower than that of the aqueous MEA, while its CO₂ capacity is higher. Thus the TETA-DEEA-H₂O system is potentially a better absorbent for the post-combustion CO₂ capture process.

A 20 wt% Ni/bentonite catalyst was prepared by a solution combustion synthesis (SCS), which exhibited higher activity for the CO₂ methanation than that of an impregnation method (IPM), and the catalyst prepared by SCS showed a CO₂ conversion of 85% and a CH₄ selectivity of 100% at 300℃, atmospheric pressure, and 3600 ml·(g cat)^-1·h^-1, and the catalyst exhibited stable within a 110-h reaction. The results showed higher metallic Ni dispersion, smaller Ni particle size, larger specific surface area and lower reduction temperature in the Ni/bentonite prepared by SCS than that of IPM. And the Ni/bentonite prepared by the SCS moderated the interaction between NiO and bentonite.

Using CO₂ as gasification agent instead of steam in in-situ coal gasification chemical looping combustion (iG-CLC) power plant can eliminate energy consumption for steam generation, thus obtaining higher system efficiency. In this work, a comparative study of iG-CLC power plant using steam and CO₂ as gasification agent is concentrated on. The effects of steam to carbon ratio (S/C) and CO₂ to carbon ratio (CO₂/C) on the fuel reactor temperature, char conversion, syngas composition and CO₂ capture efficiency are separately investigated. An equilibrium carbon conversion of 88.9% is achieved in steam-based case as S/C ratio increases from 0.7 to 1.1, whereas a maximum conversion of 84.2% is obtained in CO₂-based case with CO₂/C ranging from 0.7 to 1.1. Furthermore the effects of oxygen carrier to fuel ratio (φ) on system performances are investigated. Increasing φ from 1.0 to 1.4 helps to achieve char conversion from 75.9% to 88.9% in steam-based case, by contrast the char conversion can achieve 66.3%-84.2% in CO₂-based case within the same φ range. In terms of iG-CLC power plant, recycling partial CO₂ to the fuel reactor improves the overall performance. Approximately 3.9% of net power efficiency are increased in CO₂-based plant, from steam-based plant. Higher CO₂ capture efficiency and lower CO₂ emission rate are observed in CO₂-gasified iG-CLC power plant, expecting to be 90.63% and 85.18 kg·MW^-1·h^-1, respectively.

Supported ionic liquid (IL) sorbents for CO₂ capture were prepared by impregnating tetramethylammonium glycinate ([N₁₁₁₁] [Gly]) into four types of porous materials in this study. The CO₂ adsorption behavior was investigated in a thermogravimetric analyzer (TGA). Among them, poly(methyl methacrylate) (PMMA)-[N₁₁₁₁] [Gly] exhibits the best CO₂ adsorption properties in terms of adsorption capacity and rate. The CO₂ adsorption capacity reaches up to 2.14 mmol·g^-1 sorbent at 35℃. The fast CO₂ adsorption rate of PMMA-[N₁₁₁₁] [Gly] allows 60 min of adsorption equilibrium time at 35℃ and much shorter time of 4 min is achieved at 75℃. Further, Avrami's fractional-order kinetic model was used and fitted well with the experiment data, which shows good consistency between experimental results and theoretical model. In addition, PMMA-[N₁₁₁₁] [Gly] remained excellent durability in the continuous adsorption-desorption cycling test. Therefore, this stable PMMA-[N₁₁₁₁] [Gly] sorbent has great potential to be used for fast CO₂ adsorption from flue-gas.

The development of defect-free composite membrane (CM) is often challenging due to poor dispersion and distribution of filler particles in the polymer matrix. Despite the attractive physicochemical properties and gas separation performance of carbon nanotube (CNT) based CM, CNT displayed poor dispersion characteristics in most polymer matrix domain. Instead of incorporating CNT, a viable alternative, carbon nanofiber (CNF) which exhibits similar properties as CNT, but improved dispersion quality in the polymer matrix is found. In this work, CNF particles were incorporated in poly(2,6-dimethyl-1,4-phenylene oxide) (PPO_dm) polymer continuous phase for CM development. The optimum gas separation performance of the PPO_dm-CNF CM (11.25 at 197.02 barrer of CO₂ permeability) was obtained at 3 wt% of CNF loading. Compared to pristine PPO_dm membrane, CO₂ permeability and CO₂/CH₄ selectivity of PPO_dm-3 wt% CNF CM were enhanced by 180% and 55%, respectively. At 3 wt% CNF loading, the filler particles were dispersed and distributed more homogenously, in which no obvious CNF agglomeration was observed. In addition, the incorporation of CNF particles also enhanced the mechanical and thermal properties of the resultant CM.