CO2 Removal Efficiency Research Articles

Investigation of the indoor CO2 removal efficiency and fresh air energy savings of living walls in office spaces

Elevated indoor CO2 levels might have adverse effects on human health. However, conventional physical and chemical methods for indoor CO2 removal face the challenges of high energy consumption, low efficiency at low CO2 concentrations, and high costs. Furthermore, the introduction of outdoor air to lower indoor CO2 concentrations results in significant HVAC energy consumption. Aligning with office hours and the natural light cycle, the utilization of photosynthesis in living walls (LWs) offers an energy-efficient and sustainable solution for the mitigation of high CO2 levels in office spaces. This study experimentally investigates the impacts of the carbon fixation pathways, light flux density, and substrate moisture content on the CO2 removal rate of LWs in two identical rooms. The findings provide practical suggestions for plant species selection, lighting design, and irrigation management in office spaces to enhance the CO2 removal efficiency of LWs. Additionally, the fresh air energy-saving effects of LWs under different scenarios are accurately simulated in EnergyPlus. The results demonstrate that choosing C3 plants over CAM plants in LWs yields higher CO2 removal efficiency. The activation of artificial lighting is recommended when the surface light flux density of LWs drops below the light compensation point (LCP), ensuring favorable conditions for growth. In a 30-m2 office room accommodating two and three occupants, LWs can respectively reduce the demand for fresh air by 13.9 %–38.5 % and decrease fresh air energy consumption by 11.2 %–28.2 %.

Numerical simulations of CO2 absorption by MgO-based sorbent in a gas–solid fluidized bed

Given that carbon dioxide (CO2) constitutes the predominant proportion of greenhouse gas emissions, its capture, utilization and storage have always been a subject of great concern. Gas-solid fluidized bed has become one of the commonly applied equipment for the removal of CO2 from flue gases owing to its advantages such as exceptional mixing effect and substantial interfacial area between the phases. However, there is a lack of attention to the mechanism of CO2 absorption by individually-tracked moving solid sorbents as well as their effects on the decarbonization performance in a gas–solid fluidized bed. In this study, CO2 removal model by solid particles based on MgO adsorbent was established with the implementation of the shrinking-core model and each particle was tracked individually. Results validated the accuracy of the absorption model and revealed that smaller sizes of the solid absorbent, lower inlet gas velocities and larger inlet CO2 mass fractions exhibit relatively high CO2 removal efficiency. In addition, the decarbonization performance of fluidized and packed beds was compared. The packed bed exhibits more uniform gas flow and higher CO2 removal efficiency, whereas the fluidized bed provides larger interphase contact area, which facilitates the heat transfer process. Through the analysis of a series of parameters, the results provide recommendations for improving the CO2 removal efficiency and help to explore the optimal protocol for the design of bed reactors for CO2 removal.

CFD investigation of post-combustion CO2 capture using an industrial spray tower: Regulation effects of spray schemes

The use of spray towers for post-combustion CO2 capture has attracted growing industrial interest. However, its practical application is hampered by a lack of knowledge about CO2 spray absorption on an industrial scale. Herein, we developed a numerical model to explore the potential of the spray tower for CO2 removal at a 330 MW coal-fired power plant. The model was developed in the Euler-Lagrange framework integrated with interphase transfer models. The variations of gas-droplet flow, CO2 removal efficiency, CO2 loading of rich solution, and temperature distribution under different spray schemes were investigated. It was found that gas flow hydrodynamics and CO2 removal performance can be regulated by the spray scheme. Compared to the conventional downward spray scheme, the lower nozzle layer adopted an upward spray scheme can improve the uniformity of the gas flow and results in several advantages: (1) average droplet residence time is extended by 5.2 %; (2) CO2 removal efficiency increases from 79.1 % to 82.9 %. This improvement is attributed to two factors: one is the droplet’s re-distribution, lowering the gas flow resistance in the region below the first spray layer near the inlet; the other is the change in gas-droplet interaction whereby the upward-moving droplets entrain the gas to flow upwards. Moreover, we investigated the effects of the spray schemes under varying flue gas loads. Under these conditions, the upward spray mode adopted in the first layer shows the best CO2 removal performance. Further theoretical analysis reveals the mechanisms underlying the regulation effect. The findings in this work will benefit the design and optimization of the industrial-scale spray tower for CO2 removal.

Improving the Co-removal Efficiency of Dyes from Water by a Novel Four-Component Chitosan Flexible Film Containing Graphene Oxide

Improving the Co-removal Efficiency of Dyes from Water by a Novel Four-Component Chitosan Flexible Film Containing Graphene Oxide

CO2 capture by modified clinoptilolite and its regeneration performance

This study focuses on CO2 capture by pressure swing adsorption (PSA), with modified clinoptilolite as the adsorbent. Natural clinoptilolite is modified by roasting, by acid pickling, by a combination of acid pickling and roasting, and by ion exchange. Modification by acid pickling–roasting and by ion exchange are found to give the highest CO2 adsorption capacities, of 730 mL/g and 876.7 mL/g, respectively. It is found that regeneration of clinoptilolite by a combination of vacuum desorption and heating enables recovery of as much as 89% of its previous CO2 adsorption capacity. To examine the CO2 adsorption capacity of clinoptilolite when applied to mixed gas, a simulated coking exhaust containing 12% CO2 and 4% O2 is used, and it is found that ion exchange modified clinoptilolite achieves a CO2 removal efficiency of 92.5%. A BET test reveals that acid pickling–roasting and Na+ modification enhance the porosity of clinoptilolite, thereby improving its adsorption capacity. This work demonstrates the feasibility of applying modified clinoptilolite as an effective adsorbent for CO2 capture, providing a promising tool for dealing with greenhouse gases.

Post-combustion CO2 capturing by KOH solution: An experimental and statistical optimization modeling study

A pilot-scale bubble column contactor has been utilized for carbon dioxide chemisorption from simulated flue gas in the temperature range 30°C to 50°C. The influence of the most important operating parameters has been investigated. A total of 25 experiments were designed using the response surface methodology (RSM) and were then carried out in the bubble contact column. The results revealed that the liquid volume in the column, the alkaline concentration, and the temperature had a positive effect, while the gas hold-up had a negative effect on the CO2 removal efficiency. A statistical model has been developed using the RSM D-optimal experimental design method. To achieve the highest CO2 chemisorption, several operating conditions have been optimized. The model predicted that the maximum percentage of carbon dioxide removal would be 86.64%, and under the same operating conditions the experimental removal efficiency was 87.12%.

A Comprehensive Parametric Study on CO2 Removal from Natural Gas by Hollow Fiber Membrane Contactor: A Computational Fluid Dynamics Approach

AbstractThis study examined essential factors in the use of hollow fiber membranes that affect CO2 removal efficiency. In the simulation, a finite element model for a membrane with ten fibers was used. Each fiber is 175 mm long with inner radius of 0.75 mm and outer radius of 1.5 mm. Liquid and gas flow rates were set at 100–800 mL min−1, and adsorbent concentration was adjusted in the range of 200–1500 mol m−3 for monoethanolamine, piperazine (PZ), and ethylenediamine absorbents. Increasing the liquid flow rate, gas flow rate, and absorbent concentration leads to an increase, decrease, and increase in efficiency, respectively. Thus, using PZ as absorbent with a concentration of 1080 mol m−3 liquid and gas flow rates of 400 and 180 mL min−1, respectively, showed a CO2 removal efficiency of > 95 % for a membrane effective length of 0.3 m.

Performance and mechanisms of alkaline solid waste in CO2 mineralization and utilization

CO2 mineral sequestration using alkaline solid waste (ASW) is a promising strategy for synergistically reducing CO2 emissions and reusing industrial waste. However, improvement the carbonation degree still remains challenges due to the sluggish leaching rate of Ca/Mg ion at low pH. To the issues, this study proposed an amine-mediated CO2 absorption and mineralization process with six common ASWs, as well an ecological utilization route of CO2-ASW productions. Experimental results indicated that calcium carbide slag (CS) had greater CO2 mineralization capacity (86.2 g-CO2/kg-CS) than other ASWs, while stirring rate and particle size played a more important role during CO2 capture. Amine-mediated CO2 capture was verified to be more excellent with steel slag (SS) as mineral medium. When the MEA concentration was increased to 2 mol/L, the extraction efficiency of Ca2+ was increased by 35 %, leaded to the CO2 removal efficiency significantly promoted from 49 % to 92 %. The characterization of structural morphology referred spherical aragonite or needle-bar calcite was dominant for the porous mineralization products (30.6 m2/g). High germination index of pea seed (112.1 % at a dose of 10 g/L) inferred the negligible toxicological effects of tiny MEA residue over SS mineralization products, after centrifugally washing treatment. Pea seeds cultivated with mineralized products after centrifugal washing can achieve a growth rate of about 4 mm/d. Overall, this work provides a feasible route to apply the porous CO2-ASWs production into water conservation in arid and sandy land.

Modelling lung diffusion-perfusion limitation in mechanically ventilated SARS-CoV-2 patients.

This is the first study to describe the daytime evolution of respiratory parameters in mechanically ventilated COVID-19 patients. The data base refers to patients hospitalised in the intensive care unit (ICU) at Arequipa Hospital (Peru, 2335m) in 2021. In both survivors (S) and non-survivors (NS) patients, a remarkable decrease in respiratory compliance was observed, revealing a proportional decrease in inflatable alveolar units. The S and NS patients were all hyperventilated and their SatO2 was maintained at >90%. However, while S remained normocapnic, NS developed progressive hypercapnia. We compared the efficiency of O2 uptake and CO2 removal in the air blood barrier relying on a model allowing to partition between diffusion and perfusion limitations to gas exchange. The decrease in O2 uptake was interpreted as diffusion limitation, while the impairment in CO2 removal was modelled by progressive perfusion limitation. The latter correlated with the increase in positive end-expiratory pressure (PEEP) and plateau pressure (Pplat), leading to capillary compression, increased blood velocity, and considerable shortening of the air-blood contact time.

The Dependence of CO2 Removal Efficiency on its Injection Speed into Water

Scientific research in past decades has come to the consensus that climate change is making drastic alterations to the world we are living in today. Green-house gas emission, such as carbon dioxide (CO2) emission into Earth’s atmosphere through burning fossil fuel, is the root cause of this existential threat to humankind. Research and technological innovations are necessary to alleviate the climate crisis. This has prompted us to conduct research to find viable ways of destroying CO2, such as by breaking the bonds of the CO2 molecules through high-speed collisions. We hypothesized that injecting CO2 into liquid water could destroy the molecular bonds. Further, we investigated how fast the injection speed needs to be for effective CO2 destruction. We used a quantum molecular dynamics (QMD) method to simulate CO2 collisions with liquid water at a variety of speeds. We collected atomic and molecular positions during and after these collisions through numerical simulations. Analysis of these data suggested that more than 80% of CO2 can be destroyed if the injection speed is greater than 20 km/s, while lower injection speed is less effective in breaking up CO2. Our numerical modeling also showed an unexpected result, in that the CO2 destruction process can also produce hydrogen and CO molecules, which are clean and renewable energy sources. These findings may provide a viable path to mitigate CO2 emissions into the atmosphere, contributing to solving the urgent climate crisis.

A TECHNICAL AND ECONOMIC COMPARISON OF CO2 REMOVAL TECHNOLOGIES IN AMMONIA PRODUCTION PLANTS

The utilization of natural gas as a primary feedstock in ammonia production has resulted in significant carbon dioxide (CO2) emissions, raising environmental concerns. Consequently, several technologies have been developed to capture and remove carbon dioxide. In this study, a comparison was conducted between two CO2 removal technologies (Amine and Benfield) in an Ammonia production plant. The evaluation was performed using rate-based modelling techniques in Aspen Plus software. A base case simulation was built and validated using figures obtained from various literature studies. Optimization and sensitivity analyses were conducted to determine optimal operating conditions. The optimized parameters were used to perform an economic analysis of both technologies based on the energy consumption, solvent regeneration efficiency, and CO2 removal efficiency. The results obtained from the comparison showed that the amine technology was more economical than the Benfield technology in terms of cost and plant efficiency. The economic analysis revealed that the amine technology had a lower Net Present value, and a shorter payback period compared to the Benfield technology. Overall, this study provides insight into the economic and operational considerations of CO2 removal technologies in ammonia production plants and recommends the use of amine technology as the preferred technology for CO2 removal in ammonia production plants.

Investigation of microgravity vortex phase separator for spacecraft liquid amine CO2 removal system

Spacecraft cabin atmosphere revitalization, more specifically CO2 removal, is a key technology to pursue long-duration, crewed space missions, such as the ones to the Moon and Mars. The ISS currently uses the Carbon Dioxide Removal Assembly (CDRA) as the primary system that employs solid sorbent (zeolite) to remove CO2 from cabin air. However, CDRA cannot meet high reliability and low maintenance requirements. Liquid sorbents may be used as an alternative to solid sorbents and are estimated to attain 65 % less power, weight, and volume than solid based CO2 scrubbers. Liquid amines (liquid sorbent) are currently being researched by NASA for CO2 capture, however their implementation for space applications depends on an effective gas-liquid separation method under microgravity conditions. The Vortex Phase Separator (VPS) offers a new approach for a liquid amine CO2 removal system, and initial investigation of the prototype VPS system demonstrated up to 90.3 % CO2 removal from a humid CO2 stream at 1.47 l/min flow rate. Tests were conducted to determine the CO2 removal efficiency considering several operating parameters, including the liquid amine flow rate, initial fill (charge) level, and temperature; CO2 flow rate; extended-time operation; and CO2-amine pre-mixing length. Results provided key insights on design, operation, and performance aspects of a subscale system, and demonstrated the feasibility of the microgravity VPS for liquid amine CO2 removal system as an alternative, novel spacecraft air revitalization approach.

Biomass performance and stability of 5-year outdoor microalgal cultivation for CO2 removal from cement flue gas

The study evaluated removal of industrial CO2 from cement flue gas using algal cultivation. Local polycultures were grown in an up-scaled outdoor photobioreactor over 5 years in northern Europe. Algal biomass was harvested 2–3 times per week and the closed panel system was re-filled with seawater amended with nutrients. Flue gas was fed to the photobioreactor circulatory system in one direction or re-circulated. Removal efficiency of CO2 averaged 9 % in non-recirculation and 17 % in re-circulation modes and reached 20–60 % under best cultivation conditions. Recovery of carbon into algal biomass reached up to 10 g m2d−1 in non-recirculation mode. Biomass performance was explained by circulation mode and shift of polyculture traits. Stability of biomass quality was shown over seasons, with higher relative content of protein in autumn. Toxic elements in biomass were below legal thresholds for upcycling. The study shows feasibility of algal solutions for conversion of waste, applied in temperate climate.

Direct air capture of CO2 using green amino acid salts

Direct air capture (DAC) of CO2 using liquid sorbent technology is gaining attention as a promising approach in tackling the looming climate crisis. Despite technical advancements, critical aspects such as contactor selection, energy efficiency, sustainability, and environmental compatibility still pose uncertainties. In this study, various green amino acid salts performances in a DAC system were explored using non-porous hollow fiber membrane contactors (HFMCs). Two DAC absorption and desorption apparatuses were developed. For the DAC-absorption unit, the thermodynamic and kinetic behavior of five types of aqueous amino acid salt solutions were evaluated in long-term operations. High absorption stability for most of the solutions in different solvent loadings (up to 80% CO2 solvent loaded) were observed and potassium glycinate (GlyK) was selected as the most suitable candidate for DAC. To enhance the CO2 separation efficiency, parametric analysis on air and solvent flow rates, solvent temperature and concentration were conducted using GlyK. Vacuum low-temperature desorption experiments were carried out with GlyK to evaluate the CO2 removal efficiency over a range of solvent temperatures and concentrations, CO2 loadings, vacuum pressures, and vacuum/sweep gas mode. The results successfully quantified the effect of each operational parameter under various conditions on CO2 removal in a DAC system. Finally, to investigate the impact of membrane characteristics on DAC absorption–desorption performance, a developed and validated model was used. Taken all together, hybrid technology of membrane modules and green amino acid salts is shown to be a viable pathway towards a sustainable DAC process.

Ti species regulated CuO/CeO2 catalyst for efficient simultaneously NH3-SCR denitration and CO oxidation under oxygen-rich conditions

Synergistic control of multiple pollutants has high benefits during the control of emissions from fuel utilization, and catalysis can contribute to efficient removal of multiple pollutants. In this work, Ti species regulated CuO/CeO2 (Cu/Ce) catalyst was constructed to efficient simultaneous NH3-SCR denitration and CO oxidation under oxygen-rich conditions. It is found that the introduction of Ti species into Cu/Ce catalyst induces the formation of three-dimensional pore structure and promotes the phase transition from crystalline to amorphous, which significantly enhances the specific surface area and the dispersion of CuO and CeO2. The well dispersed CuO and CeO2 species provide a larger number of adsorbed sites for CO and NO and enhance the Cu-Ce interaction, which can contribute to reducing the competitive adsorption between CO and NO and the formation of a richer surface oxygen vacancies. Owing to above improvements, the Ti species regulated Cu/Ce catalyst can achieve more than 90% removal efficiency of NOx and CO at 148–218 °C, and the active temperature windows widened by 36 °C in comparison with Cu/Ce catalyst. Through this work, we expect to shed some light on the synergistic control of CO and NOx under oxygen-rich conditions.

Amine-promoted gypsum carbonation for efficient CO2 capture and selective synthesis of CaCO3 polymorph integrating with amine regeneration by bipolar membrane electrodialysis

CO2 mineralization of waste gypsum not only reduces the carbon emissions of power plants but also improves gypsum utilization by recovering high-value-added CaCO3. However, the extensive consumption of various reagents limited its development. This paper proposed an amine-promoted waste gypsum carbonation process integrating with amine regeneration by bipolar membrane electrodialysis (BMED). The alkaline amine was expected to act as a multifunctional reagent for gypsum carbonation — alkaline reagent, CO2 absorbent, gypsum dissolution promoter, and CaCO3 crystal regulator. Seven typical amines with different amino types and pKa values were selected to investigate the feasibility of this process. Results show that promising CO2 removal efficiency (>95 %) from an artificial exhausting gas, gypsum carbonation efficiency (>93 %), and CO2 sequestration capacity (>340 g-CO2/kg-gypsum) can be achieved due to the additional alkalinity, fast CO2 absorption, high CO2 concentrations, and enhanced CaSO4 dissolution in amine solutions. In addition, the interactions of amine-CO2-gypsum and the CaCO3 growth mechanism in the gas–liquid-solid system were explored. Monoethanolamine (MEA), 3-amino-1-propanol (MPA), piperazine (PZ), and 1,3-Diaminopropane (DAP) could prolong the CaCO3 precipitation due to their strong affinity for Ca2+ and CO32−. In this case, CO2 absorption by the liquid phase dominated the carbonation process in the initial 30 min for these four amines, while CaCO3 precipitation mainly occurred within 30 ∼ 120 min. Amines’ different affinities for Ca2+ and CO32− provided an approach to selective synthesis of CaCO3 polymorph: 100 % calcite by PZ, 100 % vaterite by DAP, a calcite-vaterite mixture by MEA, and a calcite-vaterite-aragonite mixture by MPA. After the carbonation process, amines were recycled from the liquid residue by BMED, which could provide OH− for amines regeneration from protonated amines, recovery of H2SO4, and decrease the solution salinity.

Engineering bifunctional Pd/CeNbTiOx catalyst for the simultaneous removal of NO and CO under oxygen-rich conditions at low temperature

The simultaneous elimination of CO and NO has practical significance for the control of air pollutants in the flue gas operating temperature range of 240–320 °C. CeNbTiOx catalyst was prepared by evaporation-induced self-assembly method (EISA), and the 0.3 % Pd/CeNbTiOx catalyst was obtained by appropriate Pd anchoring at the defects of CeNbTiOx. CO and NO removal efficiency were above 90 % from 240 to 320 °C, which exhibited satisfactory N2 selectivity. The CO and NO removal efficiency of the 0.3 % Pd/CeNbTiOx catalyst exceeded 92 % during stability and water resistance. Oα and Oβ were involved in the oxidation of NO and CO, respectively, promoting the fast SCR and CO oxidation reaction within the ordered mesoporous of the 0.3 % Pd/CeNbTiOx catalyst. In addition, the efficiency of CO and NO elimination had been further improved due to the enhancement of defects after resistance of H2O. Therefore, this work provided a new and efficient way to control CO and NO by the efficient using of oxygen species.

Innovative design and operational strategies to improve CO2 mass transfer during photosynthetic biogas upgrading

Several innovative strategies of design and operation, such as biogas recirculation, centrate pH manipulation and liquid nanoparticle addition, were tested to assess their potential to improve CO2 mass transfer during photosynthetic purification of biogas in a microalgae-bacteria pond connected to a biogas scrubbing column. Biogas recirculation in the column was not effective since the biogas and cultivation broth had reached chemical equilibrium under the operational conditions and configuration without biogas recirculation. Feeding the centrate at pH 10 (with and without ammonium desorption) directly to the absorption column substantially improved CO2 removal efficiency (from 58 to 91 %) achieving a biomethane complying with European standards. The supplementation of liquid nanoparticles considerably increased biomass concentration in the pond (from 1.2 to 3.5 g/L), revealing an enhanced photosynthetic activity. However, this promising approach requires additional research to elucidate the best conditions to boost CO2 absorption and guarantee a biomethane fulfilling most international standards.

Achieving net-zero CO2 emissions from indirect co-combustion of biomass and natural gas with carbon capture using a novel amine blend

The application of indirect co-combustion of biomass (in the form of producer gas) and natural gas with CCS technology has been identified as an alternative approach for electricity generation while achieving net zero CO2 emissions. Using the benchmark 5 M MEA (as a basis) and LCA methodology, it was identified that to achieve net-zero CO2 emissions, 14.5 % producer gas (energy basis) is required. Different amine blends were developed and assessed against 5 M MEA. The best performing solvent was 4 M (2:2) AMP: 1-(2HE) PRLD with CO2 absorption and desorption parameter of 4.5 % and 1667 % higher than 5 M MEA at absorption and desorption temperatures of 40 °C and 90 °C, respectively. At the desorption temperature of 110 °C, the lean loading of this optimum solvent and 5 M MEA were 0.02 and 0.24 mol CO2/mol of amine, respectively implying a higher cyclic capacity. The CO2 removal efficiency of the final solvent at 110 °C was 31 % higher than that for 5 M MEA. Finally, Using this optimum solvent as the absorbent for CO2 capture and applying the increased CO2 removal efficiency to the LCA data, it was determined that 8.2 % of producer gas, on energy basis, is sufficient for achieving net-zero CO2 emissions. Thus, using a solvent with a better absorption and desorption parameter allows for a reduction in the volume of biomass required for achieving net-zero CO2 emissions. If the ratio of biomass volume requirement for 5 M MEA is maintained in the scenario where the optimum solvent is used, negative CO2 emissions will be achieved.

Experimental and kinetic study on the cyclic removal of low concentration CO2 by amine adsorbents in confined spaces

Amine-based adsorbents show great promise for removing low-concentration CO2 from environmental control and life support systems (ECLSSs) to ensure crew safety and task execution. In this work, granular silica gel (SG) support was prepared by the extrusion–spheronization (ES) method, and amine-based adsorbent pellets were synthesized by impregnating the SG support with tetraethylenepentamine (TEPA). Dynamic CO2 circulation removal tests were carried out under simulated confined space conditions in a 100 L chamber to evaluate the low concentration CO2 removal performance of the adsorbent pellets. The effect of operating parameters on CO2 removal performance was investigated by orthogonal experiments. CO2 adsorption capacity and removal efficiency were significantly affected by CO2 concentration and reaction temperature, while reaction rate was intimately depended on CO2 concentration and gas flow rate. The maximum CO2 adsorption capacity (0.55 mmol CO2/g), removal efficiency (83.25%) and reaction rate (0.93 mmol CO2/min) were achieved under the optimized conditions of 20 °C, 1.25%CO2 and 2 L/min. A kinetic equation well representing the correlation between reaction rate and CO2 concentration and gas flow rate was established. The TEPA-SG-ES adsorbent pellets exhibited satisfactory working stability, with CO2 removal efficiency and adsorption capacity slightly decreased from 83.25% and 0.55 mmol CO2/g to 75.45% and 0.53 mmol CO2/g in 10 cycles, respectively. The slight decay in CO2 adsorption performance was associated with the oxidative degradation of amine species in oxygenated atmosphere to form amide, nitrite and imine phases, and the change of amine distribution over the pellets in multiple cycles. Overall, the desired TEPA-SG-ES adsorbent pellets could be nice scavenger candidates for circulating purification of CO2 in confined spaces.