Use of Taguchi approach for synthesis of calcite particles from calcium carbide slag for CO2 fixation by accelerated mineral carbonation

This study is focused on the conversion of harmful materials (calcium carbide slag [CCS] and flue gas) into CaCO₃ particles through an accelerated mineral carbonation process. The influences of reaction temperature, amount of Na-oleate, solid-to-liquid ratio, and stirring speed on the properties of CaCO₃ particles were determined using XRF, XRD, SEM, FTIR, TG, and contact angle measurements. Experiments were designed based on an orthogonal array L9 (3⁴) of the Taguchi approach. The gas mixture of CO₂/N₂ (16.3% of CO₂ cons.) gas was used to represent the flue gas for each experiment. The formation of CaCO₃ particles from CCS depending on time was monitored via SEM. Experiments showed that the presence of Na-oleate in the slurry played a curial role in the carbonation process, and the conversion ratio of CO₂ into a solid carbonate phase was higher than that in the experiments conducted without Na-oleate. The crystallite size of CaCO₃ particles varied between 11.55 and 38.11 nm depending on the production conditions. Each obtained CaCO₃ particles were identified as calcite (cubic-like rhombohedral), which is in high demand in many industrial applications.     

碳酸钙晶须合成过程中可溶性磷酸盐的作用机理研究

以可溶性磷酸盐为控制剂,一步碳化法制备了文石相碳酸钙晶须。借助于XRD和FTIR,分析了可溶性磷酸盐在碳酸钙晶须合成过程中的作用机理。研究结果表明：通入CO₂进行碳化反应前,可溶性磷酸盐与Ca(OH)₂反应生成了热力学上最稳定的磷酸钙化合物——羟基磷灰石;在通入CO₂初期,[CO₃^2-(OH)]进入到羟基磷灰石的晶格,部分替代[PO₄^3-],生成碳酸羟基磷灰石,然后以此为结晶中心诱导文石相的异相成核,Ca²⁺、CO₃^2-不断叠加,进而生长为碳酸钙晶须。     

Crystal growth of CaCO3 induced by monomethylitaconate grafted polymethylsiloxane

We report the preparation of a new monomethylitaconate grafted polymethylsiloxane (CO₂H-PMS) copolymer and its effect as template for crystal growth of CaCO₃. The in vitro crystallization of CaCO₃ was carried out using the gas diffusion method at different pH values at room temperature for 24 h. The CO₂H-PMS was prepared using polydimethylsiloxane-co-methylhydrogensiloxane (PDMS-co-PHMS), obtained through cationic ring opening polymerization, from cyclic monomers and monomethyltaconate (MMI) via hydrosilylation reactions with platinum complex as catalyst. FTIR results are in an agreement with the proposed template structure and confirmed that the hydrosilylation was complete. Experimental results from pH values and SEM analysis showed that the carboxylate groups of CO₂H-PMS alter the nucleation, growth and morphology of CaCO₃ crystals. SEM revealed single-truncated (ca. 5 μm) modified at pH 7-9, aggregated-modified (ca. 20 μm) at pH 10-11, and donut-shaped crystals at pH 12. These morphologies reflect the electrostatic interaction of carboxylic moieties with Ca²⁺ modulated by CO₂H-PMS adsorbed onto the CaCO₃ particles. EDS confirmed the presence of Si atoms on the crystals surface. XRD analysis showed the existence of only two polymorphs: calcite and vaterite revealing a selective control of CaCO₃ polymorphisms. In summary, the use of grafted polymethylsiloxane template offer a good alternative for polymer controlled crystallization and a convenient approach for understanding the biomineralization process useful for the design of novel materials.     

A parametric study of CO2/N2 gas separation membrane processes for post-combustion capture

Capture of CO₂ from flue gases produced by the combustion of fossil fuels and biomass in air is referred to as post-combustion capture. Chemisorbent processes are considered to be the most feasible method and are already at an advanced stage of development, but gas separation membranes are attracting more and more attention as a possible alternative. This paper describes a detailed parametric study of mass and energy balances for a simulated single membrane process. Typical operating conditions (CO₂ concentration in the flue gas, pressure and temperature, etc.) together with the influence of the membrane quality (permeability, selectivity) and membrane area on membrane performance (CO₂ separation degree and CO₂ purity) are simulated over a wide range of parameters.     

Microwave assisted vacuum regeneration for CO2 capture from wet flue gas

Small scale processing of flue gas with the goal of enriching the stream in CO₂ for sequestration or use is an interesting application area for adsorption technology. For example, boiler flue gas which may contain up to 10 % (v/v) CO₂ in air can be readily enriched to a stream containing >70 % CO₂ which may be ideal for use within a process such as acidification, precipitation, stripping, etc. The challenge in these applications is producing high purity CO₂ without excessive energy use and handling high concentrations of water vapor without the added complication of a pre-drying stage. In this study we have examined the use of microwave assisted vacuum as a way of rapidly directing thermal energy to the adsorbent surface to liberate water and CO₂. Preliminary “proof-of-concept” pump down experiments were conducted on a small transparent adsorption column of 13X zeolite pre-saturated with a 12 % CO₂ in N₂ gas mixture. Both wet and dry gas tests were conducted. The addition of microwave radiation improved the rapid desorption of CO₂ and water and improved the integrated CO₂ purity in the blowdown stream from 60 to 80 %. In the case of dry CO₂ mixtures, the enhancement is due to microwave heating of the 13X zeolite facilitated by the high cation density in the faujasite structure. In the case of water and CO₂ desorption, the temperature rise of the adsorbent upon microwave heating was much lower than that predicted by simple heating suggesting that the microwave radiation is absorbed primarily by the adsorbed water. A simplified energy analysis suggests that brief exposure of an adsorbent to microwave radiation will raise the required vacuum level for regeneration of high humidity flue gas streams and may lead to an overall lower energy penalty. The selective ability of microwave radiation to target different species provides scope for optimized, compact, flue gas treatment systems.     

Simultaneous Capture of CO2 Boosting Its Electroreduction in the Micropores of a Metal–organic Framework

Integration of CO₂ capture capability from simulated flue gas and electrochemical CO₂ reduction reaction (eCO₂RR) active sites into a catalyst is a promising cost-effective strategy for carbon neutrality, but is of great difficulty. Herein, combining the mixed gas breakthrough experiments and eCO₂RR tests, we showed that an Ag₁₂ cluster-based metal–organic framework ( 1-NH₂ , aka Ag₁₂bpy-NH₂ ), simultaneously possessing CO₂ capture sites as “CO₂ relays” and eCO₂RR active sites, can not only utilize its micropores to efficiently capture CO₂ from simulated flue gas (CO₂ : N₂=15 : 85, at 298 K), but also catalyze eCO₂RR of the adsorbed CO₂ into CO with an ultra-high CO₂ conversion of 60 %. More importantly, its eCO₂RR performance (a Faradaic efficiency (CO) of 96 % with a commercial current density of 120 mA cm⁻² at a very low cell voltage of −2.3 V for 300 hours and the full-cell energy conversion efficiency of 56 %) under simulated flue gas atmosphere is close to that under 100 % CO₂ atmosphere, and higher than those of all reported catalysts at higher potentials under 100 % CO₂ atmosphere. This work bridges the gap between CO₂ enrichment/capture and eCO₂RR.     

Capture of CO2 from flue gas streams with zeolite 13X by vacuum-pressure swing adsorption

Vacuum swing adsorption (VSA) capture of CO₂ from flue gas streams is a promising technology for greenhouse gas mitigation. In this study we use a detailed, validated numerical model of the CO2VSA process to study the effect of a range of operating and design parameters on the system performance. The adsorbent used is 13X and a feed stream of 12% CO₂ and dry air is used to mimic flue gas. Feed pressures of 1.2 bar are used to minimize flue gas compression. A 9-step cycle with two equalisations and a 12-step cycle including product purge were both used to understand the impact of several cycle changes on performance. The ultimate vacuum level used is one of the most important parameters in dictating CO₂ purity, recovery and power consumption. For vacuum levels of 4 kPa and lower, CO₂ purities of >90% are achievable with a recovery of greater than 70%. Both purity and recovery drop quickly as the vacuum level is raised to 10 kPa. Total power consumption decreases as the vacuum pressure is raised, as expected, but the recovery decreases even quicker leading to a net increase in the specific power. The specific power appears to minimize at a vacuum pressure of approximately 4 kPa for the operating conditions used in our study. In addition to the ultimate vacuum level, vacuum time and feed time are found to impact the results for differing reasons. Longer evacuation times (to the same pressure level) imply lower flow rates and less pressure drop providing improved performance. Longer feed times led to partial breakthrough of the CO₂ front and reduced recovery but improved purity. The starting pressure of evacuation (which is not necessarily equal to the feed pressure) was also found to be important since the gas phase was enriched in CO₂ prior to removal by vacuum leading to improved CO₂ purity. A 12-step cycle including product purge was able to produce high purity CO₂ (>95%) with minimal impact on recovery. Finally, it was found that for 13X, the optimal feed temperature was around 67°C to maximize system purity. This is a consequence of the temperature dependence of the working selectivity and working capacity of 13X. In summary, our numerical model indicates that there is considerable scope for improvement and use of the VSA process for CO₂ capture from flue gas streams.     

Preparation of superhydrophobic nanocalcite crystals using Box–Behnken design

Superhydrophobic nanocalcite crystals were prepared via an adjusted aqueous reaction of CaO, CO₂ gas and sodium oleate. Box–Behnken design was used to optimize the preparation parameters such as CaO concentration, CO₂ gas flow rate and surfactant concentration. The results revealed that the produced CaCO₃ is indexed to the calcite phase. The crystallite size, particle size, morphology, hydrophobicity and surface charge of CaCO₃ are significantly affected by changing the preparation parameters. The addition of sodium oleate helps in reducing the crystallite size from 101 nm to 48 nm, reducing the particle size from 1.5 μm length scalenohedral particles to 40 nm rhombohedral particles and modifying the properties of pure CaCO₃ from highly hydrophilic to superhydrophobic.     

Preparation and budding growth of whiskers in a homogeneous system

Ca₁₀(PO₄)₆(OH)₂ (HAP) and NH₄Al(OH)₂CO₃·H₂O (AACHH) whiskers were prepared by a homogeneous precipitation method based on urea hydrolysis reaction. To clarify the growth process of whiskers in the homogeneous system, XRD and SEM results of the products obtained at different reaction time were investigated in detail. A novel observation about budding growth in preparing both whiskers was described. It was indicated that the growth of whiskers went through three stages, which were oversaturation, nucleation, and budding growth. The growth units of whiskers budded from the surfaces of substrates, which were crystallized flakes for HAP preparation and amorphous spherical nuclei for AACHH preparation. Subsequently, the whiskers grew up accompanying with the disappearing substrates. One-dimensional whiskers with fine morphology and large slenderness ratio were finally obtained. Besides, according to the crystal growth and the interface diffusion theories, the effects of the templates and the budding growth mechanism were discussed.     

Non-3d metal modulated zinc imidazolate frameworks for CO2 cycloaddition in simulated flue gas under ambient condition

Cycloaddition of CO₂ and epoxide into cyclic carbonate is one of the most efficient ways for CO₂ conversion with 100% atom-utilization. Metal–organic frameworks are a kind of potential heterogeneous catalysts, however, high temperature, high pressure, and high-purity CO₂ are still required for the reaction. Here, we report two new Zn(II) imidazolate frameworks incoporating MoO₄^2– or WO₄^2– units, which can catalyse cycloaddition of CO₂ and epichlorohydrin at room temperature and atomospheric pressure, giving 95% yield after 24 h in pure CO₂ and 98% yield after 48 h in simulated flue gas (15% CO₂ + 85% N₂), respectively. For comparison, the analogic Zn(II) imidazolate framework MAF-6 without non-3d metal oxide units showed 71% and 33% yields under the same conditions, respectively. The insightful modulation mechanisms of the MoO₄^2– unit in optimizing the electronic structure of Zn(II) centre, facilitating the rate-determined ring opening process, and minimizing the reaction activation energy, were revealed by X-ray photoelectron spectroscopy, temperature programmed desorption and computational calculations.     

Oxidization of SO2 by Reactive Oxygen Species for Flue Gas Desulfurization and H2SO4 Production

A strong ionization dielectric barrier discharge was used to produce a high concentration of reactive oxygen species that were then injected into a simulated flue gas in a duct to remove SO₂ by oxidation. Sulfuric acid (H₂SO₄) was produced through the following two reactions: (1) O₃ oxidation of SO₂–SO₃, which then reacted with H₂O to produce H₂SO₄; and (2) reaction of O₂ ⁺ with H₂O to produce ·OH radicals, which then rapidly and non-selectively oxidized SO₂–H₂SO₄. When the molar ratio of reactive oxygen species to SO₂ was 4:1, the SO₂ removal efficiency was 94.6%, the energy consumption per cubic meter of flue gas was 13.3 Wh/m³, the concentration of recovered H₂SO₄ was 4.53 g/l, and the H₂SO₄ recovery efficiency was 28.8%. The H₂O volume fraction in the simulated flue gas affected the SO₂ removal efficiency, whereas the O₂ and CO₂ volume fractions did not. These results prove that oxidation by reactive oxygen species is a feasible method for flue gas desulfurization.     

Proton‐Transfer Reaction Dynamics and Energetics in Calcification and Decalcification

CaCO₃‐saturated saline waters at pH values below 8.5 are characterized by two stationary equilibrium states: reversible chemical calcification/decalcification associated with acid dissociation, Ca²⁺+HCO₃^??CaCO₃+H⁺; and reversible static physical precipitation/dissolution, Ca²⁺+CO₃^2??CaCO₃. The former reversible reaction was determined using a strong base and acid titration. The saturation state described by the pH/P_CO2‐independent solubility product, [Ca²⁺][CO₃^2?], may not be observed at pH below 8.5 because [Ca²⁺][CO₃^2?]/([Ca²⁺][HCO₃^?]) ?1. Since proton transfer dynamics controls all reversible acid dissociation reactions in saline waters, the concentrations of calcium ion and dissolved inorganic carbon (DIC) were expressed as a function of dual variables, pH and P_CO2. The negative impact of ocean acidification on marine calcifying organisms was confirmed by applying the experimental culture data of each P_CO2/pH‐dependent coral polyp skeleton weight (Wskel) to the proton transfer idea. The skeleton formation of each coral polyp was performed in microspaces beneath its aboral ectoderm. This resulted in a decalcification of 14 weight %, a normalized CaCO₃ saturation state Λ of 1.3 at P_CO2 ≈400 ppm and pH ≈8.0, and serious decalcification of 45 % and Λ 2.5 at P_CO2 ≈1000 ppm and pH ≈7.8.     

Thermal decomposition of calcium oxalate: beyond appearances

The goal of this study is twofold: to take a fresh look at the decomposition of calcium oxalate and to warn users of thermogravimetric analysis against the hasty interpretation of results obtained. Since the pioneer work of Duval 70 years ago, the scientific community has agreed unanimously as to the decomposition of anhydrous calcium oxalate (CaC₂O₄) into calcium carbonate (CaCO₃) and CO gas, and that of the calcium carbonate into calcium oxide (CaO), and CO₂ gas. We will demonstrate how these reactions, simple in appearance, in fact result from a succession of reactive phenomena involving numerous constituents both solid (CaCO₃, free carbon) and gaseous (CO₂ and CO) produced by intermediary reactions. The mass losses evaluated in the two distinct domains correspond closely to the molar masses of CO and CO₂, respectively. The simple mathematical calculation of that mass loss has simply concealed the existence of other reactions, and, most particularly the Boudouard reaction and that of solid phases between CaCO₃ and C. It just goes to show that appearances can be deceiving.

     

燃煤烟气中共存杂质对膜分离CO2性能影响的实验研究

利用自行搭建的膜分离实验台,考察了共存气态组分以及颗粒物对于聚二甲基硅氧烷/聚砜(PDMS-PSF)复合膜分离CO₂性能的影响.结果表明,共存气态组分中O₂对于膜分离CO₂有抑制作用;由于SO₂浓度显著低于CO₂,在短时间内对膜分离CO₂没影响;水汽可以促进CO₂的分离;燃煤飞灰细颗粒在分离膜表面沉积会导致膜性能的恶化.在此基础上,采用模拟湿法烟气脱硫系统装置,进行了燃煤湿法脱硫净烟气环境下的膜分离CO₂实验;在测试的50 h以内,水汽、SO₂和O₂的共同作用导致膜分离性能在前期有一定的提高,随着运行时间的延长,细颗粒物对膜的影响程度加大,导致PDMS-PSF复合膜的分离性能逐渐恶化,最终导致膜的CO₂/N₂分离因子和CO₂渗透速率分别下降了17.91%和28.21%.     

Adsorption technology for direct recovery of compressed,pure CO<Subscript>2</Subscript> from a flue gas without pre-compression or pre-drying

The effects of the sorption and the regeneration temperatures on the performance of a novel rapid thermal swing chemisorption (RTSC) process (Lee and Sircar in AIChE J. 54:2293–2302, 2008) for removal and recovery of CO₂ from an industrial flue gas without pre-compression, pre-drying, or pre-cooling of the gas were mathematically simulated. The process directly produced a nearly pure, compressed CO₂ by-product stream which will facilitate its subsequent sequestration. Na₂O promoted alumina was used as the CO₂ selective chemisorbent, and the preferred temperatures were found to be, respectively, 150 and 450 °C for the sorption and regeneration steps of the process. The specific cyclic CO₂ production capacity of the process and the pressure of the by-product CO₂ gas were substantially increased over those previously achieved by using the sorption and regeneration temperature of, respectively, 200 and 500 °C (Lee and Sircar in AIChE J. 54:2293–2302, 2008). The net compressed CO₂ recovery from the flue gas (∼92%) did not change. However, substantially different amounts of high and low pressure steam purges were necessary for comparable degree of desorption of CO₂. A first pass estimation of the capital and the operating costs of the RTSC process was carried out for a relatively moderate size application (flue gas clean up and CO₂ recovery from a ∼80 MW coal fired power plant). Both costs were substantially lower than those for a conventional absorption process using MEA as the CO₂ solvent (Desideri and Paolucci in Energy Convers. Manag. 40:1899–1915, 1999).     

Reversible chemisorption of carbon dioxide: simultaneous production of fuel-cell grade H2 and compressed CO2 from synthesis gas

One vision of clean energy for the future is to produce hydrogen from coal in an ultra-clean plant. The conventional route consists of reacting the coal gasification product (after removal of trace impurities) with steam in a water gas shift (WGS) reactor to convert CO to CO₂ and H₂, followed by purification of the effluent gas in a pressure swing adsorption (PSA) unit to produce a high purity hydrogen product. PSA processes can also be designed to produce a CO₂ by-product at ambient pressure. This work proposes a novel concept called “Thermal Swing Sorption Enhanced Reaction (TSSER)” which simultaneously carries out the WGS reaction and the removal of CO₂ from the reaction zone by using a CO₂ chemisorbent in a single unit operation. The concept directly produces a fuel-cell grade H₂ and compressed CO₂ as a by-product gas. Removal of CO₂ from the reaction zone circumvents the equilibrium limitations of the reversible WGS reaction and enhances its forward rate of reaction. Recently measured sorption-desorption characteristics of two novel, reversible CO₂ chemisorbents (K₂CO₃ promoted hydrotalcite and Na₂O promoted alumina) are reviewed and the simulated performance of the proposed TSSER concept using the promoted hydrotalcite as the chemisorbent is reported.     

Formation of calcium sulfate whiskers from CaCO3-bearing desulfurization gypsum

Calcium sulfate whiskers can be used as the reinforcing agents in many composites, such as polymers, ceramics, cements, and papers, etc. This paper investigated the feasibility of preparing calcium sulfate whiskers using desulfurization gypsum as the raw material. The desulfurization gypsum composed mainly of CaSO₄·2H₂O (93.45 wt%) and CaCO₃ (1.76 wt%) were treated with dilute H₂SO₄ at room temperature to convert CaCO₃ to CaSO₄; the latter was then treated at 110?C150 °C to form CaSO₄·0.5H₂O whiskers. The removal of the CaCO₃ impurity from the desulfurization gypsum favored the formation of CaSO₄·0.5H₂O whiskers with high aspect ratios.     

Studies on the thermal decomposition of calcium carbonate in the presence of alkali salts (Na2Co3, K2CO3 and NaCl)

Mixtures of CaCO₃ and varying amounts of Na₂CO₃, K₂CO₃ and NaCl were subjected separately to thermal analysis. DTG, DTA, TG analyses indicate that the presence of alkali salts in CaCO₃ influences its decomposition behaviour. A minimum DTA peak temperature of CaCO₃ decomposition is noticed at low concentrations of alkali salts (K₂CO₃ and Na₂CO₃); an increase in concentration increases the DTA peak temperature. However, in the case of NaCl no appreciable lowering of the DTA peak temperature of CaCO₃ decomposition is observed. Similarly, the minimum temperature at which decomposition completes is found to correspond to the concentration of 1 per cent salt (K₂CO₃ and Na₂CO₃) in CaCO₃.     

Adsorption separation of carbon dioxide, methane and nitrogen on monoethanol amine modified B-zeolite

A new type of composite adsorbents was synthesized by incorporating monoethanol amine (MEA) into β-zeolite. The parent and MEA-functionalized β-zeolites were characterized by X-ray diffraction (XRD), N₂ adsorption, and thermogravimetric analysis (TGA). The adsorption behavior of carbon dioxide (CO₂), methane (CH₄), and nitrogen (N₂) on these adsorbents was investigated at 303 K. The results show that the structure of zeolite was well preserved after MEA modification. In comparison with CH₄ and N₂, CO₂ was preferentially adsorbed on the adsorbents investigated. The introduction of MEA significantly improved the selectivity of both CO₂/CH₄ and CO₂/N₂, the optimal selectivity of CO₂/CH₄ can reach 7.70 on 40 wt% of MEA-functionalized β-zeolite (MEA(40)-β) at 1 atm. It is worth noticing that a very high selectivity of CO₂/N₂ of 25.67 was obtained on MEA(40)-β. Steric effect and chemical adsorbate-adsorbent interaction were responsible for such high adsorption selectivity of CO₂. The present MEA-functionalized β-zeolite adsorbents may be a good candidate for applications in flue gas separation, as well as natural gas and landfill gas purifications.     

Experimental Study on Demercurization Performance of Wet Flue Gas Desulfurization System

The demercurization performance of wet flue gas desulfurization (WFGD) system was investigated by measuring mercury concentrations at the inlet and outlet of WFGD system with a QM201H mercury analyzer. The selected desulfurizer included NH₃·H₂O, NaOH, Na₂CO₃, Ca(OH)₂ and CaCO₃. The influences of adding oxidant and coagulant such as KMnO₄, Fenton reagent, K₂S₂O₈/CuSO₄ and Na₂S into desulfurization solutions were also studied. The results show that elemental mercury is the main component of gaseous mercury in coal‐fired flue gas, and the proportion of oxidized mercury is less than 36%. Oxidized mercury could be removed by WFGD system efficiently, and the removal efficiency could amount to 81.1%–92.6%. However, the concentration of elemental mercury slightly increased at the outlet of WFGD as a result of its insolubility and re‐emission. Therefore, the removal efficiency of gaseous mercury is only 13.3%–18.3%. The mercury removal efficiency of WFGD system increased with increasing the liquid‐gas ratio. In addition, adding KMnO₄, Fenton reagent, K₂S₂O₈/CuSO₄ and Na₂S into desulfurization solutions could increase the mercury removal efficiency obviously. Various additives have different effects, and Na₂S is demonstrated to be the most efficient, in which a mercury removal efficiency of 67.2% can be reached.