Steam hydration of sorbents from a dual fluidized bed CO2 looping cycle reactor

Results are presented on steam hydration of spent residues obtained from a 75 kW_th dual fluidized bed combustion (FBC) pilot plant unit operating in a CO₂ looping cycle mode. The samples were collected from the unit under various conditions, which included electrical heating of the reactor, as well as firing with coal, and biomass under oxy-fuel combustion conditions. In addition, different operating times, i.e., number of cycles (25 min–455 min/1–25 cycles) were examined, with the carbonator operating at temperatures of 600–700 °C and the calciner at 850–900 °C. The samples collected came from the calciner, carbonator and cyclone. Steam hydration itself was done under atmospheric pressure in saturated steam at 100 °C for periods of 15, 30 and 60 min. The original limestone sample, as well as the spent samples from the pilot plant and the hydrated samples were examined to determine their hydration and carbonation levels, as well as their unreacted CaO content using TGA and XRD analysis. In addition, samples were characterized for pore distribution (nitrogen adsorption/desorption: BET and BJH), skeleton characterization, with density by He pycnometry and particle surface area morphology (SEM/EDX), as well as changes in sample volume during hydration (sample swelling). The results obtained showed successful hydration (typically only ～10% unreacted CaO) even for hydration periods as short as 15 min, and very favorable sample properties. Their pore surface area, pore volume distribution and swelling during hydration are promising with regard to their use in additional CO₂ capture cycles or SO₂ retention. However, their predisposition to fracture is the main disadvantage observed with these samples. This may result in difficulties in terms of their handling in FBC systems, due to intensified attrition and consequent elutriation from the reactor.     

Sequential SO2/CO2 capture enhanced by steam reactivation of a CaO-based sorbent

The steam hydration reactivation characteristics of three limestone samples after multiple CO₂ looping cycles are presented here. The CO₂ cycles were performed in a tube furnace (TF) and the resulting samples were hydrated by steam in a pressure reactor (PR). The reactivation was performed with spent samples after carbonation and calcination stages. The reactivation tests were done with a saturated steam pressure at 200 °C and also at atmospheric pressure and 100 °C. The characteristics of the reactivation samples were examined using BET and BJH pore characterization (for the original and spent samples, and samples reactivated under different conditions) and also by means of a thermogravimetric analyzer (TGA). The levels of hydration achieved by the reactivated samples were determined as well as the conversions during sulphation and multiple carbonation cycles. It was found that the presence of a CaCO₃ layer strongly hinders sorbent hydration and adversely affects the properties of the reactivated sorbent with regard to its behavior in sulphation and multiple carbonation cycles. Here, hydration of calcined samples under pressure is the most effective method to produce superior sulphur sorbents. However, reactivation of calcined samples under atmospheric conditions also produces sorbents with significantly better properties in comparison to those of the original sorbents. These results show that separate CO₂ capture and SO₂ retention in fluidized bed systems enhanced by steam reactivation is promising even for atmospheric conditions if the material for hydration is taken from the calciner.     

Investigation of the carbonation mechanism of CH and C-S-H in terms of kinetics,microstructure changes and moisture properties

The purpose of this article is to investigate the carbonation mechanism of CH and C-S-H within type-I cement-based materials in terms of kinetics, microstructure changes and water released from hydrates during carbonation. Carbonation tests were performed under accelerated conditions (10% CO₂, 25 °C and 65 ± 5% RH). Carbonation profiles were assessed by destructive and non-destructive methods such as phenolphthalein spray test, thermogravimetric analysis, and mercury intrusion porosimetry (destructive), as well as gamma-ray attenuation (non-destructive). Carbonation penetration was carried out at different ages from 1 to 16 weeks of CO₂ exposure on cement pastes of 0.45 and 0.6 w/c, as well as on mortar specimens (w/c = 0.50 and s/c = 2). Combining experimental results allowed us to improve the understanding of C-S-H and CH carbonation mechanism. The variation of molar volume of C-S-H during carbonation was identified and a quantification of the amount of water released during CH and C-S-H carbonation was performed.     

Experimental studies and thermodynamic modeling of the carbonation of Portland cement,metakaolin and limestone mortars

The carbonation of Portland cement, metakaolin and limestone mortars has been investigated after hydration for 91 days and exposure to 1% (v/v) CO₂ at 20 °C/57% RH for 280 days. The carbonation depths have been measured by phenolphthalein whereas mercury intrusion porosimetry (MIP), TGA and thermodynamic modeling have been used to study pore structure, CO₂ binding capacity and phase assemblages. The Portland cement has the highest resistance to carbonation due to its highest CO₂ binding capacity. The limestone blend has higher CO₂ binding capacity than the metakaolin blends, whereas the better carbonation resistance of the metakaolin blends is related to their finer pore structure and lower total porosity, since the finer pores favor capillary condensation. MIP shows a coarsening of the pore threshold upon carbonation for all mortars. Overall, the CO₂ binding capacity, porosity and capillary condensation are found to be the decisive parameters governing the carbonation rate.     

Kinetic model of CaO/fly ash sorbent for flue gas desulphurization at moderate temperatures

Characteristics of the sulphation reaction between SO₂ and CaO/fly ash sorbent were analyzed based on TGA results to develop a kinetic model for a dry moderate temperature (400–800 °C) FGD process. It was found that SO₂ diffusion within sorbent particles involved three sub-processes: inter-particle diffusion, inter-grain diffusion and diffusion through product layers and the diffusion dominated the whole sulphation reaction process. The activation energy for product layer diffusion E_diff of 49.3 kJ mol⁻¹ being greater than the chemical reaction activation energy E_a of 13.9 kJ mol⁻¹ verified the importance of the diffusion. Predictions using the kinetic model in which k₀ varies with temperature agree well with the experimental data.     

Characteristics of absorption/regeneration of CO2–SO2 binary systems into aqueous AMP + ammonia solutions

To examine the characteristics of absorption and regeneration, the simultaneous removal efficiency of carbon dioxide/sulfur dioxide (CO₂/SO₂), the CO₂ absorption amount, and the CO₂ loading value of an ammonia (NH₃) solution added to 2-amino-2-methyl-1-propanol (AMP) were investigated using the continuous absorption and regeneration process. The performances of this system, such as the removal efficiency of CO₂ and SO₂, absorption amount, and CO₂ loading, were evaluated under various operating conditions. Based on the experimental study, the optimum conditions were a liquid circulation rate of 90 mL/min and gas flow rate of 7.5 L/min. The addition of NH₃ into aqueous AMP solution increased the absorption rate and loading ratio of CO₂ and raised the removal efficiencies of CO₂ and SO₂ to over 90% and over 98%, respectively.     

Combined high hydrostatic pressure and carbon dioxide inactivation of pectin methylesterase,polyphenol oxidase and peroxidase in feijoa puree

A combined treatment of high hydrostatic pressure (HHP) and dense phase carbon dioxide (DPCD) was investigated to inactivate pectin methylesterase (PME), peroxidase (POD) and polyphenol oxidase (PPO) in feijoa (Acca sellowiana) puree. The treatments were HHP (HHP); carbonation and HHP (HHPcarb); carbonation + addition of 8.5 mL CO₂/g puree into the headspace of the package and HHP (HHPcarb + CO₂). The different samples were treated at 300, 450 and 600 MPa, for 5 min.The residual POD and PPO activity decreased in the order HHP > HHPcarb > HHPcarb + CO₂ at all pressures used. Treatments with HHP at 300 MPa increased POD activity to 140%. The residual PME activity of HHPcarb and HHPcarb + CO₂ samples at 600 MPa (45–50%) was significantly (p < 0.05) lower than for HHP treatment (65%).The simultaneous application of HHP and DPCD seems to synergistically enhance the inactivation of the enzymes studied, the CO₂ concentration being a key process factor.     

Enhancing the carbonation of MgO cement porous blocks through improved curing conditions

The use of reactive magnesia (MgO) as the binder in porous blocks demonstrated significant advantages due to its low production temperatures and ability to carbonate, leading to significant strengths. This paper investigates the enhancement of the carbonation process through different curing conditions: water to cement ratio (0.6–0.9), CO₂ concentration (5–20%), curing duration (1–7 days), relative humidity (55–98%), and wet/dry cycling frequency (every 0–3 days), improving the carbonation potential through increased amounts of CO₂ absorbed and enhanced mechanical performance. UCS results were supported with SEM, XRD, and HCl acid digestion analyses. The results show that CO₂ concentrations as low as 5% can produce the required strengths after only 1 day. Drier mixes perform better in shorter curing durations, whereas larger w/c ratios are needed for continuous carbonation. Mixes subjected to 78% RH outperformed all the others, also highlighting the benefits of incorporating wet/dry cycling to induce carbonation.     

Carbonation passivation layer of scandium loaded BSCF perovskite

This work investigates the improved CO₂ stability of the partial substitution of scandium (Sc) in the B-site of perovskite ceramic materials thus forming a compound also containing barium (Ba), strontium (Sr), cobalt (Co) and iron (Fe) as Ba_0.5Sr_0.5(Co_0.8Fe_0.2)_1−xSc_xO_3-δ. The concentration of Sc in the B-site was varied between 0 and 40 mol%. It was found that perovskite cubic structure was maintained by the substitution with Sc. Further, the cubic cell lattice parameter increased as a function of Sc. However, XRD parameters showed that not all Sc was fully incorporated into the crystal structure of the perovskite, as Sc₂O₃ phase was also detected. Exposure of as-prepared powders to CO₂ gases resulted in the carbonation reactions occurring at temperatures about 100 °C below those observed for unstable pure BSCF (i.e. no Sc) perovskite. This was attributed to the catalytic effect of Sc, which promoted the carbonation reaction of BSCF-Sc. Subsequently, discs were prepared and exposed to CO₂ gases at 800 °C. The analysis of the discs’ surface showed that the perovskite cubic structure retained 96% of its original values upon CO₂ exposure for the samples loaded with 40 mol% Sc, which then reduced to 32% to samples containing 5 mol% Sc. These results proved to be counterintuitive, as Sc promoted the carbonation reaction at lower temperatures for powders, though enhanced the stability of disc exposed to CO₂ atmosphere. Further analyses of the discs by scanning electron microscopy revealed images containing a rich carbon layer, as a passivation carbonation layer, which protected the remainder of the bulk perovskite structure in the disc.     

Mechanical performance and microstructure of the calcium carbonate binders produced by carbonating steel slag paste under CO2 curing

Calcium carbonate binders were prepared via carbonating the paste specimens cast with steel slag alone or the steel slag blends incorporating 20% of Portland cement (PC) under CO₂ curing (0.1 MPa gas pressure) for up to 14 d. The carbonate products, mechanical strengths, and microstructures were quantitatively investigated. Results showed that, after accelerated carbonation, the compressive strengths of both steel slag pastes and slag-PC pastes were increased remarkably, being 44.1 and 72.0 MPa respectively after 14 d of CO₂ curing. The longer carbonation duration, the greater quantity of calcium carbonates formed and hence the higher compressive strength gained. The mechanical strength augments were mainly attributed to the formation of calcium carbonate, which caused microstructure densification associated with reducing pore size and pore volume in the carbonated pastes. In addition, the aggregated calcium carbonates exhibited good micromechanical properties with a mean nanoindentation modulus of 38.9 GPa and a mean hardness of 1.79 GPa.     

CO2 sequestration with magnesium silicates—Exergetic performance assessment

A staged process for CO₂ sequestration by mineralisation, using magnesium silicates, studied at Åbo Akademi (ÅA) involves the production of magnesium hydroxide from suitable rock (requiring heat at ∼350–450 °C) using recoverable ammonium salts and its subsequent carbonation (generating heat at ∼500 °C). In addition, the process gives substantial amounts of solid by-products making the integration of mineral carbonation with other industries an opportunity to both reduce CO₂ emissions and substitute raw material inputs.Aspen Plus^® v7.2 software is used to optimize the ÅA process towards minimal energy use and to study the impact of the flue gases' CO₂ concentration (conventional coal firing (CCF) vs. oxyfuel combustion (OXY) flue gases) in the carbonation step instead of pre-separated and compressed CO₂ (providing an additional benefit compared to “conventional” CO₂ capture and storage (CCS) options). Also the influence of using either ammonium sulphate (AS) or bisulphate (ABS) as the fluxing salts on the process's exergetics is evaluated. It was concluded that ABS lowers the energy requirements in the Mg extraction by 40% but its regeneration (fundamental feature for the route's success) by: (i) thermal decomposition of AS appears to be unviable, (ii) addition of H₂SO₄ saturates the process with sulphur. The simulation results showed that the extraction with ABS and carbonation with OXY flue gases requires less energy input.     

Resistance to sulfidation of the catalytic reduction of NOx over Ag/Al2O3 by controlling the loadings of Ag and Ag+

SO₂ strongly decreased the catalytic activities of low loading Ag/Al₂O₃ below 500 °C in selective catalytic reduction (SCR) of NO_x by propene with or without the assistance of non-thermal plasma (NTP), which was mainly attributed to the competition between SO₂ and NO. By controlling the loadings of Ag and Ag⁺ over alumina, the resistance of SO₂ was remarkably enhanced between 400 °C and 500 °C in thermal SCR. In the NTP-assisted SCR, most of the NO_x conversions were also apparently recovered from 250 °C to 500 °C.     

Preparation of CaCO3 using mega-crystalline calcite in electrical furnace and batch type microwave kiln

Mega-crystalline calcite (m-CC) breaks apart easily during calcination, and cannot be easily converted to CaO due to its characteristic that requires massive heat consumption. To solve this problem, the calcination characteristics were compared using electrical furnace (EF) and batch type microwave kiln (BM). After hydrating the manufactured CaO, Ca(OH)₂ was produced, and through the carbonation process, CaCO₃ was synthesized.The results of the XRD pattern of CaO that was formed through calcinations indicated that decarbonation reaction occurred as 98.2 wt.% by EF for 240 min, and 97.8 wt.% by BM for 30 min at the same temperature of 950 °C. Hydration results revealed that CaO by EF was high-reactive whereas CaO by BM was medium-reactive. CaCO₃ was synthesized through the carbonation process. At 25 °C, in both cases, colloidal-shaped CaCO₃ was found, and the more spindle-shaped CaCO₃ by cubic-shaped self assembly was synthesized at higher temperatures. However, in case of EF, Ca(OH)₂ existed in products.     

An artificial photosynthesis system based on CeO2 as light harvester and N-doped graphene Cu(II) complex as artificial metalloenzyme for CO2 reduction to methanol fuel

An artificial photosynthesis catalyst composed of CeO₂, N-doped graphene and copper ions (CeO₂–NG–Cu^2 +) was fabricated. The light-harvesting CeO₂–NG was characterized by X-ray diffraction, transmission electron microscopy, Raman spectroscopy, and X-ray photoelectron spectroscopy. The photocatalytic reduction of CO₂ was conducted in an aqueous solution of Na₂SO₃. Results indicated that the reduction rate of CO₂ to methanol approached 507.3 μmol · g⁻¹ cat. · h^− 1 for CeO₂–NG–Cu^2 + artificial photosynthesis system in 80 min, whereas the reduction rate was only 5.8 μmol · g⁻¹ cat. · h^− 1 for bare CeO₂–NG without metalloenzyme. Therefore, artificial metalloenzyme played a vital role in reducing CO₂ to methanol fuel.     

Role of ceria in the improvement of SO2 resistance of LaxCe1 − xFeO3 catalysts for catalytic reduction of NO with CO

Perovskite-type catalysts with LaFeO₃ and substituted La_xCe_1 − xFeO₃ compositions were prepared by sol–gel method. These catalysts were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), CO temperature-programmed reduction (CO-TPR), and SO₂ temperature-programmed desorption (SO₂-TPD). Catalytic reaction for NO reduction with CO in the presence of SO₂ has been investigated in this study. LaFeO₃ exhibited an excellent catalytic activity without SO₂, but decreased sharply when SO₂ gas was added to the CO + NO reaction system. In order to inhibit the effect of SO₂, substitution of Ce in the structure of LaFeO₃ perovskite has been investigated. It was found that La_0.6Ce_0.4FeO₃ showed the maximum SO₂ resistance among a series of La_xCe_1 − xFeO3 composite oxides.     

Visual and Raman spectroscopic observations of hot compressed water oxidation of guaiacol in fused silica capillary reactors

Using fused silica capillary reactors (FSCRs), we investigated the decomposition of guaiacol during hot compressed water oxidation (HCWO), with H₂O₂ added in stoichiometric ratios from 100 to 300%. Reactions were performed between 180 and 300 °C for durations from 2 to 10 min while the concurrent generation of CO₂ during the oxidation process was followed by Raman spectroscopy and the phase behavior of guaiacol in HCW, with or without H₂O₂, was observed visually under a polarized microscope configured with a heating/cooling stage. We found that complete conversion of guaiacol and 100% yield of CO₂ were achieved with a 150% stoichiometric ratio of oxidizer after 10 min at 200 and 300 °C, respectively. Based on the global reaction kinetics for the complete conversion of guaiacol to CO₂, the reaction is considered to be first order. The activation energy and pre-exponential factor for CO₂ formation are 18.62 kJ mol⁻¹ and 12.81 s⁻¹, respectively.     

Impact of hydrated magnesium carbonate additives on the carbonation of reactive MgO cements

Reactive magnesia (MgO) cements have emerged as a potentially more sustainable and technically superior alternative to Portland cement due to their lower production temperature and ability to sequester significant quantities of CO₂. Porous blocks containing MgO were found to achieve higher strength values than PC blocks. A number of variables are investigated to achieve maximum carbonation and associated high strengths. This paper focuses on the impact of four different hydrated magnesium carbonates (HMCs) as cement replacements of either 20 or 50%. Accelerated carbonation (20 °C, 70–90% RH, 20% CO₂) is compared with natural curing (20 °C, 60–70% RH, ambient CO₂). SEM, TG/DTA, XRD, and HCl acid digestion are utilized to provide a thorough understanding of the performance of MgO-cement porous blocks. The presence of HMCs resulted in the formation of larger size carbonation products with a different morphology than those in the control mix, leading to significantly enhanced carbonation and strength.     

New data on arsenic sorption properties of Zn–Al sulphate layered double hydroxides: Influence of competition with other anions

The adequacy of synthetic Zn–Al-sulphate LDHs to remove arsenic from aqueous systems was tested through sorption experiments, using a series of aqueous solutions with dissolved HAsO₄^2 − together with other anions (Cl⁻, SO₄^2 −, MoO₄^2 −, HCO₃⁻, CO₃^2 −) to assess their competition influence on the As removing process. The competitors were added into the solution both simultaneously and afterwards with respect to HAsO₄^2 − in order to verify the effectiveness and the possible reversibility of the As sorption process. The results showed that only carbonates species, in particular in the fully deprotonated form CO₃^2 −, affect significantly the otherwise high efficacy of the sorption process. In fact, up to ~ 90% of HAsO₄^2 − can be removed from the solution, decreasing to ~ 60% in the presence of CO₃^2 −, whilst up to ~ 30% of HAsO₄^2 − can be desorbed when CO₃^2 − is added afterwards into the solution. Considering the very restricted range of pH where HAsO₄^2 − and CO₃^2 − are simultaneously the predominant species in the solution (~ 10 < pH < ~ 11.5), Zn–Al-sulphate LDHs could be successfully used for the treatment of As contaminated waters with pH ranging from circum-neutral to moderately alkaline.     

A shrinking core model and empirical kinetic approaches in supercritical CO2 extraction of safflower seed oil

Utilization of supercritical CO₂ in safflower seed extraction was performed using a semi-batch extractor. Different extraction parameters, such as 40–60 MPa pressure, 323–347 K temperature, 20–76 min time, and 1–3 mL/min CO₂ flow rate were applied. A two-stage experimental design application was performed in order to maximize the oil yield. First of all, a 3² factorial design was applied to estimate the effect of the main factors and their interactions. The second part of the experimental design was improved and accelerated using the steepest ascent method. Optimum extraction parameters were determined to be 50 MPa pressure, 347 K temperature and 76 min time at a constant CO₂ flow rate (3 mL/min) according to the 2² design. Under these conditions, the oil yield obtained was 39.42%, comparable with Soxhlet extraction (40%) for 8 h. Shrinking core and empirical kinetic models were applied in order to generalize the extraction process. The predicted data was compatible with the experimental data.