CO2 Pressure Research Articles

Unidirectional ice-templating for aerogel adsorbents: Excellent pore structure and high CO2 capture performance for direct air capture

The ultralow atmospheric partial pressure of CO2 (∼40 Pa) presents a significant challenge for direct air capture (DAC). In this study, high-performance porous aerogel adsorbents are prepared using the unidirectional freezing–ice-templating method. The dispersion of the mixed solution is effectively improved by particle size modulation, and functionalized materials with abundant gas channels are constructed by the rapid cooling of liquid N2. The adsorbents feature a cross-structure comprising monolayer active components and nanoscale layered carriers. The unique micro/mesoporous composite structure of the adsorbents facilitates gas-phase transport. When the functional group efficiency is increased by 5.8 times, the prepared adsorbents demonstrate excellent CO2 capture capacity (1.17 mmol/g and 148 mol/m3) and a low adsorption half-time (1.28 min). The unique cross-structure of the adsorbents renders them hydrophobic; thus, they show higher adsorption capacity at high humidity than other moisture swing adsorbents. At the molecular scale, quantum chemical calculations show that appropriate coordination between the active components and carriers enhance the water vapor hindering ability of the adsorbents. The developed aerogel adsorbents significantly expand the application scenarios for moisture swing adsorbents and enhance the efficiency of DAC.

Carbonation mechanism and cementation properties of recycled cement paste based on CO2 injection time and pressure

Current applications of recycled cement paste (RCP) carbonation mainly focus on using carbonized particles as filler materials or enhancing the adhesion of hardened cement paste to aggregates. However, there is limited research on the carbon dioxide (CO2) sequestration effectiveness and strength improvement mechanisms of carbonized waste recycled concrete powder as a cementitious material. This study investigates the carbonation mechanism and cementation properties of RCP under various carbonation conditions. The phase composition and microstructural evolution of RCP are systematically analyzed through detailed characterization tests, exploring the impact of carbonation time and CO2 pressure on the mechanical properties of the RCP samples. These findings demonstrate that prolonged carbonation enhances the formation of calcium carbonate (CaCO3) and crystalline silica (SiO2), which in turn improves particle bonding and reduces porosity. Higher CO2 pressure results in increased carbonation products and tighter interparticle binding. Advanced nuclear magnetic resonance (NMR) analyses indicate that carbonation facilitates the transition of Al from six- to four-coordinated states and the polymerization of SiO4 tetrahedra, leading to enhanced structural cohesion. This study demonstrates the potential of carbonation in reshaping the cementitious properties of RCP, highlighting its environmental and economic benefits for recycling construction waste.

Cenozoic Carbon Dioxide: The 66 Ma Solution

The trend in partial pressure of atmospheric CO2, P(CO2), across the 66 MYr of the Cenozoic requires elucidation and explanation. The Null Hypothesis sets sea surface temperature (SST) as the baseline driver for Cenozoic P(CO2). The crystallization and cooling of flood basalt magmas is proposed to have heated the ocean, producing the Paleocene–Eocene Thermal Maximum (PETM). Heat of fusion and heat capacity were used to calculate flood basalt magmatic Joule heating of the ocean. Each 1 million km3 of oceanic flood basaltic magma liberates ~5.4 × 1024 J, able to heat the global ocean by ~0.97 °C. Henry’s Law for CO2 plus seawater (HS) was calculated using δ18O proxy-estimated Cenozoic SSTs. HS closely parallels Cenozoic SST and predicts the gas solute partition across the sea surface. The fractional change of Henry’s Law constants, Hn−HiHn−H0 is proportional to ΔP(CO2)i, and Hn−HiHn−H0×∆P(CO2)+P(CO2)min, where ΔP(CO2) = P(CO2)max − P(CO2)min, closely reconstructs the proxy estimate of Cenozoic P(CO2) and is most consistent with a 35 °C PETM ocean. Disparities are assigned to carbonate drawdown and organic carbon sedimentation. The Null Hypothesis recovers the glacial/interglacial P(CO2) over the VOSTOK 420 ka ice core record, including the rise to the Holocene. The success of the Null Hypothesis implies that P(CO2) has been a molecular spectator of the Cenozoic climate. A generalizing conclusion is that the notion of atmospheric CO2 as the predominant driver of Cenozoic global surface temperature should be set aside.

Stomatal response to VPD in C4 plants with different biochemical sub-pathways.

C4 NAD-malic enzyme (NAD-ME) species occurs in drier regions and exhibit different drought responses compared to C4 NADP-malic enzyme (NADP-ME) species. However, a physiological mechanism explaining the geographical discrepancies remains uncertain. This study examined gas exchange patterns that might explain different distributions observed between two subtypes of C4 photosynthesis. We measured the response of leaf gas exchange to vapour pressure deficit (VPD) and CO2 in plants from six distinct C4 clades having closely related NAD-ME and NADP-ME species using a Li-Cor 6400 gas exchange system. We found that NAD-ME species exhibited greater relative reductions in stomatal conductance with increases in VPD than NADP-ME species but observed no consistent subtype differences in C4 cycle activity as indicated by the initial slope of the A response to intercellular CO2 concentration. Based on these results, we hypothesise the greater response of gs to increasing VPD may enable NAD-ME plants to outperform NADP-ME plants in hot, dry environments where VPD is normally high.

Enhanced CO2 conversion through confinement of cross-linked ionic polymer within the pores of porous carbon materials

It is crucial to employ an integrated catalyst to avoid the complications of the recovery process. This work reports the fabrication of porous carbon@ionic liquid (PC@IL) composites with readily accessible active ion sites, achieved by confining cross-linked ionic liquid (IL) within the channels of porous carbon (PC). The incorporation of porous carbon not only confines the IL within its framework, creating microsites for CO2 adsorption and conversion, but also simplifies catalyst recovery. The results indicate that PC@IL composites exhibit excellent cycloaddition activity towards CO2 in a co-catalyst- and solvent-free environment. Notably, PC@IL(C)-24 demonstrates remarkable catalytic performance across various epoxides under 1 bar of CO2, with yields above 90 % at 90 °C for 12 h, and achieving a remarkable styrene carbonate yield of up to 92.8 % under a CO2 pressure of 1 bar (at 100 °C for 12 h). Control experiments confirm that the confinement effect exerted by N,S co-doped carbon on cross-linked IL plays a pivotal role in enhancing both stability and activity of PC@IL composites, thereby providing novel insights for designing functionalized porous carbon catalysts for CO2 cycloaddition conversion.

Assessment of carbon flux gradients and dominant processes in a subtropical highly urbanized coastal ecosystem

Highly urbanized coastal ecosystems are vital in the global carbon budget. However, there are limited researches on carbon flux gradients in these nearshore areas, considering both natural and anthropogenic influences. Through on-site measurements and field samplings during wet-to-dry season in 2023, this study investigated spatial variations and factors affecting carbon fluxes, focusing on the impacts of salinity and eutrophic status in five geographically connected coastal waters of the Guangdong-Hong Kong-Macau Greater Bay Area (GBA). By estimating carbon exchange at land-sea-air interface, dominant processes in carbon dynamics were identified as well. Results showed that partial pressure of CO2 (pCO2) varied from 391 to 2290 μatm, and sea-air CO2 exchange fluxes (FCO2) ranged from -3.07 to 70.07 mmol m–2 d–1, indicating significant geographical distinctions among five coastal waters of the GBA. The total carbon transport from rivers to these nearshore waters was approximated at 6.44 Tg C yr-1, with the Pearl River (PR) contributing 99.7%, primarily in dissolved forms. Atmospheric CO2 release was calculated at 0.29 Tg C yr-1 for studied five coastal waters, primarily as carbon sources, except for Dapeng Bay (DPB) as a sink. CO2 emissions inversely correlated with salinity, yet positively with eutrophication status, particularly in river-dominated estuaries. Moreover, CO2 flux decreased 23 times as eco-status shift from eutrophic to non-eutrophic. River plumes, terrestrial pollutant inputs, and economic structure were underlying drivers, influencing carbon species concentrations and fluxes. Elevated CO2 concentrations in eutrophic coastal waters were mainly attributed to terrestrial carbon and nutrients inputs, supporting active biological respiration and microbial decomposition. Conversely, carbon dynamics potentially depend on the balance of respiration and photosynthesis in non-eutrophic coastal waters. This study offers high geographic precision and specificity of carbon species, and provides land-sea integration insight to understand carbon dynamic mechanisms, promoting advancements in water quality management and climate mitigation.

Potential of CO2 sequestration by olivine addition in offshore waters: A ship-based deck incubation experiment

Ocean alkalinity enhancement is considered as an effective atmospheric CO2 removal approach, but currently, little is known about the carbon sequestration potential of implementing olivine addition in offshore waters. We investigated the effect of olivine addition on the seawater carbonate system by carrying out a deck incubation experiment in the Northern Yellow Sea; the dissolution rate of olivine was calculated based on the increase in seawater alkalinity (TA), and the CO2 sequestration potential was evaluated. The results showed that the dissolution of olivine increased seawater TA and decreased partial pressure of CO2, resulting in oceanic CO2 uptake from the atmosphere through sea-air exchange; it also increased seawater pH and mitigated ocean acidification to a certain extent. The addition of 1 ‰ olivine had a more significant effect on the seawater carbonate system than 0.5 ‰ olivine addition. The average dissolution rate constant of olivine was 1.44 ± 0.15 μmol m−2 d−1. Assuming that olivine settles completely on the seabed due to gravity, the theoretically maximum amount of CO2 removed by applying 1 tonne of olivine per square meter area in the Northern Yellow Sea is only 2.0 × 10−4 t/m2. Therefore, when olivine addition is implemented in the offshore waters, it is necessary to consider reducing the olivine size, prolonging the settling time of olivine in the water column; and spreading olivine in well-mixed waters to prolong the residence time through repeated resuspension, thus increasing its potential in carbon sequestration.

The efficient and reusable imidazolium-organoboron catalysts for green CO2 insertion reactions in solvent-free under atmospheric and high-pressure conditions

Organoboron compounds that are available in industrial and scientific research were investigated as organocatalysts for the coupling reaction of CO2 into cyclic carbonates as fine chemicals in solvent-free and under sustainable green atmospheric and high-pressure conditions (1 atm and 1.6 MPa, 100 °C, 2 h). This work synthesized a series of imidazolium-organoboron catalysts (IBC1-IBC8) using the eco-friendly and metal-free methodology and characterized by various analysis techniques. These techniques were NMR (1H, 13C, and 11B), UV–Vis, FT-IR, Mass spectrometry, C/H/N analysis, and melting point measurement together with thermal gravimetric analysis (TGA-DTA), respectively. After that, the Lewis acidity of the synthesized imidazolium-organoboron catalysts, which is important for catalytic CO2 conversion studies, was investigated by the traditional Gutmann-Beckett test, and the Lewis acidity order of the target compounds was found at range 54.96–50.36 ppm. In the presence of 0.1 mol% imidazolium-organoboron catalyst (IBC8) and 0.2 mol% DMAP as co-catalyst, 4-chloromethyl-1,3-dioxalan-2-one was obtained as cyclic carbonate from epichlorohydrin in 44.9 % yield and 97.6 % selectivity (1 atm and 100 °C) and in an excellent 97 % yield with 98.9 % selectivity (1.6 MPa and 100 °C) in 2 h without any solvents. After the imidazolium-organoboron catalyst (IBC8) showed efficient catalytic conversion and high selectivity, the effect of different parameters, including the effect of substrate, Lewis acid-base, reaction time, reaction temperature, CO2 pressure, and catalyst amount, was studied for this catalyst. According to the obtained catalytic conversion results, it was also determined that the optimum Cat./ECH ratio for CO2 cycloaddition reactions is 1/1000.

Direct coupling of pressurized gas receiver to a brayton supercritical CO2 power cycle in solar thermal power plants

Three layouts of Brayton supercritical CO2 power cycles directly coupled to the receiver are proposed for Generation 3 solar power plants: conventional recompression, recompression with partial cooling, and recompression with intercooling. To achieve direct coupling, the solar heat is introduced downstream of the turbine, where CO2 pressure is lower. A higher temperature rise diminishes the receiver's dimensions, thus increasing its energy efficiency. It also lowers the average working temperature since the maximum temperature is fixed at 700 °C, thereby reducing losses. However, optical efficiency decreases as the receiver size diminishes. Both intercooling and partial cooling layouts further increase the cycle's net efficiency, which reduces the receiver's size, following similar trends observed with an increase in temperature rise. Considering all these effects, various factors push in opposite directions, affecting overall efficiency and costs. This competitive interplay results in overall efficiencies ranging from 30.26 % to 31.58 % and Levelized Costs of Electricity (LCOEs) between 162.47 €/MWh and 166.81 €/MWh. In conclusion, similar outcomes in terms of energy and economics are achieved with the three layouts, suggesting the simplest layout (recompression) as the most advisable. If thermal storage is incorporated, partial cooling becomes preferable due to its significant increase in the receiver's temperature rise.

Salt-Reduced Fish Sauce Produced under Pressurized Carbon Dioxide Treatment Using Sardinops melanostictus, Trachurus japonicus, Konosirus punctatus, Odontamblyopus lacepedii, Their Collective Mixture, and Unused Fish Mixture.

Fish sauce is produced at high salt concentrations (>20%) to inhibit the growth of harmful microorganisms. The salt-reduced fish sauce (10% salt) was prepared under pressurized CO2 (pCO2) conditions at 30 °C and 5 MPa for 3 months (FSCO2), from Sardinops melanostictus, Odontamblyopus lacepedii, Trachurus japonicus, Konosirus punctatus, and their collective mixture, as well as unused fish mixture obtained from the Ariake Sea in Japan. FSCO2 exhibited significantly better microbial quality and free amino acid content, lighter color, standardized odor (dashi-like odor), and umami richness qualities compared to fish sauces prepared using the conventional method (FScon) (20% salt), as previously demonstrated, after a fermentation period of 2 months. Bacterial flora analysis implied that the standardization of odor and umami richness may not be the result of specific microbial metabolism. Even when using previously unused fish, it was possible to produce FSCO2 equivalent to that produced by conventional sardines and other fish. These results indicate that the quality of fish sauce can be improved. The flavor of FSCO2 became similar regardless of the type of fish and fermentation period using pCO2 during fermentation, leading to the effective utilization of unutilized fish as a resource for high-quality salt-reduced fish sauce.

Corrosion at Top-of-the-Line in High Pressure and Dense CO2 Environments

This study presents unique data on top-of-the-line corrosion (TLC) occurring in high-pressure environments where CO2 was in the gaseous, liquid, or supercritical state. While CO2 is traditionally in a gaseous phase, this form of degradation is referred to as TLC. In this study, similar phenomena with different mechanisms were observed in liquid CO2 and supercritical states all of which are referred to as TLC due to the location of specimens and ease of comprehension. Experiments were conducted to investigate the effect of CO2 partial pressure (ranging from 20 bar to 100 bar) with temperatures (30°C to 50°C) relating to different water condensation rates (0.001 mL/m2/s to 0.1 mL/m2/s). Uniform and localized TLC rates increased with a higher water condensation rate and surface temperature. As long as CO2 remained gaseous, its partial pressure (pCO2) showed a negligible influence on both uniform and localized TLC rates. At the highest gaseous CO2 content tested, the formation of a protective iron carbonate (FeCO3) layer decreased the TLC rate, with this effect being more pronounced at lower water condensation rates. The risk of localized corrosion for specimens exposed to this environment at high and medium water condensation rates remained an issue. In the dense phase CO2 environment, the difference in temperature between the bulk environment and the specimen’s surface caused a similar phenomenon to water condensation, termed water drop-out, which resulted in corrosion. The rate of water drop-out could not be measured experimentally or estimated theoretically but is a complex function of temperature, pCO2, and CO2 physical state. The interplay between high pCO2 and low pH of the dropped-out water led to elevated uniform and localized corrosion rates. The depth of localized corrosion, at the high and medium water drop-out conditions, reached its maximum at the surface temperature of ca. 45°C. At a lower surface temperature of ca. 25°C and a higher surface temperature of ca. 65°C, the maximum penetration rate was decreased due to slower kinetics of reactions and the formation of a more protective FeCO3 layer, respectively. The results presented in this study highlight the significant difference between corrosion rates, especially in the form of localized damage, in gaseous and dense-phase CO2 environments.

Imitating the respiratory activity of the brain stem by using artificial neural networks: exploratory study on an animal model of lactic acidosis and proof of concept.

Artificial neural networks (ANNs) are versatile tools capable of learning without prior knowledge. This study aims to evaluate whether ANN can calculate minute volume during spontaneous breathing after being trained using data from an animal model of metabolic acidosis. Data was collected from ten anesthetized, spontaneously breathing pigs divided randomly into two groups, one without dead space and the other with dead space at the beginning of the experiment. Each group underwent two equal sequences of pH lowering with pre-defined targets by continuous infusion of lactic acid. The inputs to ANNs were pH, ΔPaCO2 (variation of the arterial partial pressure of CO2), PaO2, and blood temperature which were sampled from the animal model. The output was the delta minute volume (ΔVM), (the change of minute volume as compared to the minute volume the animal had at the beginning of the experiment). The ANN performance was analyzed using mean squared error (MSE), linear regression, and the Bland-Altman (B-A) method. The animal experiment provided the necessary data to train the ANN. The best architecture of ANN had 17 intermediate neurons; the best performance of the finally trained ANN had a linear regression with R2 of 0.99, an MSE of 0.001 [L/min], a B-A analysis with bias ± standard deviation of 0.006 ± 0.039 [L/min]. ANNs can accurately estimate ΔVM using the same information that arrives at the respiratory centers. This performance makes them a promising component for the future development of closed-loop artificial ventilators.

Coating based on chitosan/vancomycin nanoparticles: Patterns of formation in a water-carbon dioxide biphase system and in vivo stability

The patterns of formation of chitosan nanoparticles doped with vancomycin and coatings based on them in carbonate solutions have been investigated for the first time in this study. Using a technique of radioactive indicators, it was found that at a CO2 pressure of 30 MPa, the yield of the nanoparticles was ∼85 %, and a maximum antibiotic encapsulation efficiency of ∼30 % was achieved. By spectrophotometric and high-resolution microscopy, it was found that the coating of stabilized xenopericardial tissue of bioprosthetic heart valve, based on chitosan nanoparticles doped with vancomycin with a zeta potential |ζ| ∼20 mV completely covers collagen fibers by depositing about 60 nm nanoparticles onto them under direct deposition from carbonic acid at a pressure of 30 MPa CO2. The coating preserves the mechanical strength characteristics of collagen tissue and completely suppresses the growth of S. aureus pathogenic biofilm. This is consistent with the observed increase in antibiotic release of 15 % when the medium was acidified. Histological study demonstrated that the structure of pericardial tissues was not significantly altered by the deposition nanoparticles from carbonic acid. It was found that the rate of biodegradation of polymers and vancomycin in the coating differs by half (16 weeks for the rat model). A significantly lower degradation rate of antibiotics (∼50 % of vancomycin total remaining mass and ∼25 % of chitosan) was associated with its reliable encapsulation into nanoparticles.

Assessing the impact of simulated ocean acidification on the photodegradation of selected microplastics

This study investigated the impact of ocean acidification on the photodegradation of three microplastics (MPs): polypropylene (PP), expanded polystyrene (EPS), and ethylene-vinyl acetate (EVA), under accelerated UV radiation at three pH levels (i.e., 8.1, 7.8, and 7.5), simulating marine conditions. The acidification system simulated current and projected future environmental conditions. As expected, an increase in partial pressure of CO2, total inorganic carbon, bicarbonate ion, and CO2 resulted in more acidic pH levels, with the reverse being true for the carbonate ion. Structural changes of MPs were evaluated, revealing that all weathered samples underwent higher degradation rate compared to the virgin samples. The oxidation state and crystallinity of PP and EVA MPs were higher in samples exposed to the lowest pH, whereas no significant increase in the degradation rate of EPS samples was observed. Saltwater acidification in this study contributed to enhance the photo-oxidation of MPs depending on their polymeric composition.

Multi-field and multi-scale dynamic numerical modeling of supercritical CO2 and fly ash mineralization

Fly ash-CO2 mineralization technology can effectively control gas–solid waste emissions and improve the climate environment. However, rationally describing the evolution mechanism of the fly ash-supercritical CO2 mineralization reaction and accurately predicting the efficiency of the mineralization reaction are of great significance for improving and evaluating the supercritical CO2 mineralization sequestration capacity. Fly ash micro- and nanopore multi-scale effects are the key to influencing the supercritical CO2 seepage-mineralization reaction mechanism. None of the current theoretical models consider the pore multi-scale effects, thus the validity and accuracy of predicting and describing the evolutionary mechanism of supercritical CO2 mineralization rates and efficiencies are questionable. To address this scientific issue, a multi-scale continuous pore structure theoretical model was established based on the structural characteristics of fly ash micro- and nanopores. Additionally, a new model of supercritical CO2 multi-field and multi-scale dynamic mineralization theory was developed by considering parameters such as temperature, pressure, and humidity. The accuracy of the model was verified through the mineralization experiments. The new model effectively describes the dynamic evolution mechanism of supercritical CO2 mineralized fly ash. The effects of parameters such as temperature, pressure, water vapor diffusion coefficient, supercritical CO2 permeability, and pore multi-scale effect on the efficiency and mineralization rate were investigated through numerical simulation. The results showed the following: The mineralization efficiency exhibited a parabolic increase with temperature, with 72℃ being the turning point for the growth rate of mineralization efficiency, and 82℃ being the optimal effective temperature to promote the mineralization efficiency of supercritical CO2. A 3.5-fold increase in supercritical CO2 pressure changes the mineralization efficiency by a maximum of only 0.0493 %. Increasing the water vapor diffusion coefficient by 5 orders of magnitude changes the mineralization efficiency by only 8.3 %. The pore attenuation coefficient increases by 3 orders of magnitude, and the supercritical CO2 mineralization efficiency changes by a factor of 2.27. A model that does not account for pore multi-scale effects overestimates the fly ash- supercritical CO2 mineralization efficiency by a factor of 2.47 at a permeability of 3 × 10-10 m2. Therefore, neglecting the pore continuum multi-scale effect can lead to a serious overestimation of the mineralization efficiency of supercritical CO2.

Long-term variations in ecosystem water use efficiency in the Tibetan Plateau: Vegetation types, attribution methods and main drivers

Ecosystem water use efficiency (WUE) is a key indicator for understanding the response of carbon–water processes to environmental changes. However, as affected by uncertainty of WUE estimation and attribution, the individual contributions of environmental drivers to WUE changes and the underlying mechanisms remain unclear on the Tibetan Plateau (TP). Here, a theory-based analytical WUE model was modified by introducing an optimal stomatal behavior model, and used to estimate monthly WUE and their drivers during 1982–2018 for main vegetation types on the Tibetan Plateau (TP). Rationality of three mainstream analytical attribution methods—analytical-based partial derivative method (APDM), regression-based partial derivative method (NPDM), and climate elasticity method (CEM)—was examined. Furtherly, the contributions of various environmental drivers to WUE variation trend were estimated for various vegetation types. Results indicate that: (1) The modified model performed well in estimating WUE against with observed WUE at five eddy-covariance (EC) flux stations, which decreased the uncertainty of WUE estimation. WUE increased significantly in the TP with a slope of 0.49 × 10-2 g C kg−1 H2O yr−1 during 1982–2018 with forests increased most rapidly; (2) Only APDM captured both the variation and magnitude of WUE trends, and both NPDM and CEM failed to reproduce the magnitude of WUE trends, though CEM was better than NPDM benefited from its nonlinear structure. Through comparing these methods, we found that the nonlinear direct effects of these forcing variables on WUE change could be higher than their indirect effects caused by their interactions. (3) There are some differences in the main drivers of WUE trends for various vegetation types. Increase in WUE was mainly driven by the increase in leaf area index (LAI) for both steppes and meadows, while decreased air water vapor pressure deficit (VPD) and elevated CO2 showed higher positive effects for broad-leaf forests. The distinct response and extremely scarce EC observations of forests suggest that more attention should be paid to forests in the TP. This study could help in selection of attribution methods and understanding the mechanisms of carbon–water coupling of alpine ecosystems under a changing climate.

The independent effects of hydrostatic pressure and hypercapnic breathing during water immersion on ventilatory sensitivity and cerebrovascular reactivity.

Head-out water immersion (HOWI) induces ventilatory and hemodynamic changes, which may be a result of hydrostatic pressure, augmented arterial CO2 tension, or a combination of both. We hypothesized that the hydrostatic pressure and elevated CO2 tension that occur during HOWI will contribute to an augmented ventilatory sensitivity to CO2 and an attenuated cerebrovascular reactivity to CO2 during water immersion. Twelve subjects [age: 24 ± 3 yr, body mass index (BMI): 25 ± 3 kg/m2] completed HOWI, waist water immersion with CO2 (WWI + CO2), and WWI, where a rebreathing test was conducted at baseline, 10, 30, and 60 min, and postimmersion. End-tidal pressure of carbon dioxide ([Formula: see text]), minute ventilation, expired gases, blood pressure, heart rate, and middle cerebral artery blood velocity were recorded continuously. [Formula: see text] increased throughout all visits (P ≤ 0.011), was similar during HOWI and WWI + CO2 (P ≥ 0.264), and was greater during WWI + CO2 versus WWI at 10, 30, and 60 min (P < 0.001). When HOWI vs. WWI + CO2 were compared, the change in ventilatory sensitivity to CO2 was different at 10 (0.59 ± 0.34 vs. 0.06 ± 0.23 L/min/mmHg; P < 0.001), 30 (0.58 ± 0.46 vs. 0.15 ± 0.25 L/min/mmHg; P < 0.001), and 60 min (0.63 ± 0.45 vs. 0.16 ± 0.34 L/min/mmHg; P < 0.001), whereas there were no differences between conditions for cerebrovascular reactivity to CO2 (P ≥ 0.163). When WWI + CO2 versus WWI were compared, ventilatory sensitivity to CO2 was not different between conditions (P ≥ 0.642), whereas the change in cerebrovascular reactivity to CO2 was different at 30 min (-0.56 ± 0.38 vs. -0.30 ± 0.25 cm/s/mmHg; P = 0.010). These data indicate that during HOWI, ventilatory sensitivity to CO2 increases due to the hydrostatic pressure, whereas cerebrovascular reactivity to CO2 decreases due to the combined effects of immersion.NEW & NOTEWORTHY Although not fully elucidated, the ventilatory and hemodynamic alterations during water immersion appear to be a result of the combined effects of immersion (i.e., elevated [Formula: see text], central hypervolemia, increased cerebral perfusion, increased work of breathing, etc.). Our findings demonstrate that an augmented ventilatory sensitivity to CO2 during immersion may be due to the hydrostatic pressure across the chest wall, whereas an attenuated cerebrovascular reactivity to CO2 may be due to the combined effects of immersion.

Dynamic characteristics of high-pressure and high-density carbon dioxide leakage process

The leakage of high-pressure and high-density carbon dioxide (CO2) during storage and transportation can result in the rapid release of a substantial amount of CO2, presenting significant risks to Carbon Capture and Storage (CCS) technology. Investigating the dynamic characteristics of high-pressure and high-density CO2 leakage in storage tank, as well as understanding the variations in CO2 pressure, temperature, density, and phase during the leakage process, is crucial for the design of tank and pipeline leakage prevention. Existing research mainly focuses on leakage at the top or side of storage tank, there is a lack of attention to leakage from the bottom of storage tank. To simulate the bottom leakage processes, a depressurized experimental system with a 0.065 m3 CO2 storage tank volume was constructed. The range of CO2 parameter conditions for filling density is 811 ∼ 998 kg/m3, with initial pressure ranging from 5.72 ∼ 16.50 MPa. According to the change in the slope of the pressure drop in the storage tank, the leakage can be divided into three processes. The closer to the top of the storage tank, the liquid–gas phase transition of CO2 occurs earlier. The pressure shows two inflection points (point I and point II) representing the upper and lower limits of CO2 saturation pressure, respectively. The lower filling density results in a higher saturation pressure, while the higher initial pressure also leads to a higher saturation pressure. The current work can help to better understand pressure, temperature, density, and phase state changes of CO2 during leakage.

Thermodynamic and CO2 sorption investigations on improved Li3BO3-based sorbents by NaOH addition

The development of new solid sorbents with high carbon capture capacity and good reversible sorption properties is considered a key factor for mitigating CO2 emissions. Tri-lithium borate (Li3BO3) has shown to be a high-capacity CO2 sorbent adequate for an intermediate temperature range (500–650 °C). However, the performance of this sorbent has been little studied in particular at low CO2 concentrations, requiring the introduction of changes in Li3BO3 to make it competitive. Here, highly efficient NaOH-modified Li3BO3 sorbents were synthesized by a simple two-step mechano-thermal method at low temperature using different amounts of NaOH. For the first time, the influence of increasing amounts of an additive on the standard enthalpy change of Li3BO3 as well as on the relation between CO2 pressure and temperature equilibrium was determined. A progressive rise in the standard enthalpy change and a decrease in the equilibrium CO2 pressure at a fixed temperature were observed with the increase in the amount of additive. Moreover, the NaOH addition to Li3BO3 improves its CO2 capture capacity, CO2 absorption rate and stability during carbonation/regeneration cycles. The CO2 capture capacity ranges between 50 wt% and 35 wt% at pure CO2 and 15 % CO2, respectively, for low amounts of NaOH added. Further studies showed that CO2 capture/desorption efficiency is kept high after 20 cycles at 525 °C and 15 % CO2. The improved performance of NaOH-modified Li3BO3 sorbents and the changes in the thermodynamic properties were associated with two factors: the partial substitution of boron by carbon in the Li3BO3 structure, and the formation of Li2CO3-Na2CO3 molten carbonates. Therefore, these NaOH-modified Li3BO3 materials represent attractive CO2 sorbents for moderate temperatures, applicable for post-combustion industrial processes.

Hydrogen reduction of iron ore pellets: A surface study using ambient pressure X-ray photoelectron spectroscopy

Using Ambient Pressure X-ray Photoelectron Spectroscopy (APXPS), this study investigates the reduction behavior of iron oxides in direct reduction (DRI) and blast furnace (BF) pellets. H2, CO, and a H2–CO mixture are used as reducing agents at 650 °C. The investigation aimed to elucidate variations in the rate of reduction over time and under different conditions. Additionally, contour plots are generated to visualize the X-ray photoelectron peak intensity variations as a function of time. Furthermore, phase stability diagrams based on Fe–O–C and Fe–O–H systems are employed to enhance the understanding of reduction behavior. Results revealed that an increased gas flow rate significantly accelerated the reduction rate due to enhanced gas diffusion, while elevated pressure facilitated the reduction of wüstite to metallic iron. Notably, the DRI pellet achieves around 90% metallization degree reduction with hydrogen, but the introduction of carbon monoxide into the reducing gas prevented the reduction of the DRI pellet. In the case of BF pellet reduction, approximately 20% metallization degree is observed using H2–CO (50:50), yet subsequent reoxidation of the reduced iron to wüstite and magnetite occurred. Further investigation identified a significant increase in the partial pressure of H2O and CO2, particularly within the surface porosities, as the underlying cause of this reoxidation phenomenon.