NH3 oxidation catalyzed by partially sulphated limestone—modelling and experimental work

The oxidation of NH₃ over three different types of partially sulphated limestone is investigated experimentally and theoretically. The experiments were made in a laboratory fixed bed reactor at atmospheric pressure at 850°C to simulate conditions of fluidized bed combustion. The catalytic activity of calcined limestone decreases strongly when it is sulphated. The decrease in the rate of NH₃ oxidation is mainly caused by the conversion of CaO into less active CaSO₄ and by the formation of an impervious layer of CaSO₄ around the CaO core.A model based on the grain model has been developed to describe the decrease of activity for NH₃ oxidation with increasing conversion of CaO to CaSO₄. The changes in structure and in composition of the limestone particle are incorporated in the model. The simulation results are in good agreement with the experimental data.Besides the decrease in activity, the selectivity for the formation of NO decreases during sulphation of the limestone. In experiments, it was shown that the decrease in selectivity is independent of the NO concentration and therefore is not the result of NO reduction by NH₃ as proposed in the literature. Further, it was shown that while the formation of NO decreases with increasing CaO conversion the formation of N₂ is almost not influenced. From these observations and from simulation results it was concluded that the NH₃ oxidation to N₂ over CaSO₄ is the major cause of the decrease in selectivity. To our knowledge, this is the first validated explanation for the decrease in selectivity for NO during sulphation of calcined limestone.     

Phase characterization and mechanical and flame‐retarding properties of nano‐CaSO4/polypropylene and nano‐Ca3(PO4)2/polypropylene composites

An in situ deposition approach was used for the synthesis of nano‐CaSO₄ and nano‐Ca₃(PO₄)₂. The nanosize particles were confirmed with an X‐ray diffraction technique. Composites of polypropylene (PP) with 0.1–0.5 wt % nano‐ or commercial CaSO₄ or nano‐Ca₃(PO₄)₂ were prepared. The transition from the α phase to the β phase was observed for 0.1–0.3 wt % nano‐CaSO₄/PP and nano‐Ca₃(PO₄)₂/PP composites. This was confirmed by Fourier transform infrared. A differential scanning calorimetry analysis was carried out to determine the thermal behavior of the nanocomposites with increasing amounts of the nano‐CaSO₄ and nano‐Ca₃(PO₄)₂ fillers. Increases in the tensile strength and Young's modulus were observed up to certain loading and were followed by a decrease in the tensile strength. A continuous decrease in the elongation at break (%) was also observed for commercial CaSO₄ and larger nano‐Ca₃(PO₄)₂. A decrease in the mechanical properties after a certain loading might have been due to the agglomeration and phase transition of PP in the composites. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 670–680, 2007     

The Accelerating Effect of CaSO4 Within CMAS (CaO–MgO–Al2O3–SiO2) and Its Effect on the Infiltration Behavior in EB‐PVD 7YSZ

Infiltration and deposition of CaSO₄ in thermal barrier coatings (TBC) in addition to the CMAS deposits was found in many occasions on real aviation engines. The source and role of CaSO₄ on the degradation of TBC is not well understood. CaSO₄ containing CMAS was synthesized and a systematic study of its role on the CMAS infiltration behavior in EB‐PVD 7YSZ is presented in this work. Its influence on the melting and crystallization behavior of CMAS was studied with the help of differential scanning calorimetry. The decomposition of CaSO₄ into CaO and SO₃ was observed at 1050°C in laboratory air under the presence of CMAS using mass spectroscopy and in situ high‐temperature XRD. The same amount of CaO is brought into the CMAS system by means of adding CaCO₃, which will eventually decompose into CaO and CO₂ at 700°C. CMAS infiltration tests were carried out at different temperatures with and without CaSO₄/CaCO₃ and the results demonstrate that the sulfur has no direct effect on the aggressiveness of the anhydrite containing CMAS with regard to its infiltration behavior in EB‐PVD 7YSZ at high temperatures. The extra amount of calcia added to CMAS that is introduced by the evaporating species is responsible for enhanced infiltration of the deposits into the TBC.     

Insight into Metastable Lifetime of α‐Calcium Sulfate Hemihydrate in CaCl2 Solution

Alpha calcium sulfate hemihydrate (α‐HH), an important kind of cementitious material, is a metastable phase in CaSO₄–H₂O system. In CaCl₂ solution, thermodynamic metastable window for α‐HH preparation from calcium sulfate dihydrate is constructed as a function of CaCl₂ concentration (0–4.0m) and temperature (60°C–130°C), and then the metastability of α‐HH is characterized by the metastable lifetime (MLT). As the concentration of CaCl₂ is raised from 1.5 to 4.0m and temperature from 80°C to 110°C, MLT of α‐HH decreases gradually from 2920 to 60 min. The reducing effect of CaCl₂ on MLT arises from its contribution to lowering the total interfacial Gibbs free energy required for the subsequent phase transition of α‐HH to anhydrite, the thermodynamic stable phase. This work establishes a method to evaluate the phase metastability and provides a potential approach to optimize the reaction time in metastable material synthesis according to the MLT value for purity control.     

Synthesis of charged linear and crosslinked maleic diester polymers with electron‐beam irradiation

Series of maleic mono‐ and diester monomers have been prepared by esterification of maleic anhydride with poly(ethylene glycol) having different molecular weights, and with n‐dodecyl alcohol. These monomers were copolymerized with 2‐acrylamido‐2‐methylpropane sulfonic acid (AMPS) using different dose rates of electron‐beam irradiation ranging from 40 to 150 kGy. The synthesized copolymers were characterized by IR and ¹H NMR analysis. Their aggregation behaviour and viscometric properties in aqueous solutions were investigated. The crosslinked copolymers were prepared in aqueous acidic solutions at pH 1 or in the presence of 1% of N,N‐methylene bisacrylamide (MBA) as crosslinking agent. The final equilibrium water content and swelling capacities for the prepared hydrogels were determined in aqueous solutions at pH = 1, 6.8 and 12 at 298 K. Swelling equilibria for the prepared hydrogels were carried out in aqueous solutions of NaCl, KCl, CaCl₂, Na₂SO₄, K₂SO₄ and CaSO₄ at concentrations ranging from 1 × 10^?6 to 2 M at 298 K. © 2003 Society of Chemical Industry     

应用CaSO4载氧体燃烧技术的CaO再生过程

引言近年来CaO作为CO2吸收剂在近零排放煤直接制氢、生物质气化、吸收增强型天然气重整制氢、焦炉煤气重整制氢、燃煤电站CO2捕集等过程的应用受到国内外的持续关注和大量研究。在这些过程中CO2以CaCO3的形式固化下来,为循环利用CaO吸收剂并收集CO2,需要将产物CaCO3煅烧分解,这一过程称为CaO再生过程。CaO再生是强吸热过程,为该过程提供热量的一     

Synthesis and characterization of polyacrylamide‐calcium carbonate and polyacrylamide‐calcium sulfate nanocomposites

Polyacrylamide‐calcium carbonate (PAM/CaCO₃) and polyacrylamide‐calcium sulfate (PAM/CaSO₄) nanocomposites were prepared via solution‐mixing technique. The resulting PAM‐based nanocomposites with various CaCO₃ and CaSO₄ nanoparticles contents (0–4% w/w) were investigated. Nanoparticles of CaCO₃ and CaSO₄ were synthesized by in situ deposition technique. In this technique, the surface modification of nanoparticles was performed by nonionic polymeric surfactant. The particle size of nanoparticles was recognized by X‐ray diffraction and scanning electron microscope (SEM) analysis which confirms that the particle has diameter of 25–33 nm. As prepared, nanocomposites films (thickness, 40‐μm) were characterized by Fourier transform infrared (FT‐IR), SEM, and energy‐dispersive X‐ray spectroscopy (EDS). FT‐IR shows the chemical structure of nanocomposites where as SEM analysis suggested that the nanofillers dispersed well in polymer matrix and EDS shows the elemental composition of the nanocomposite samples. Thermal properties of the nanocomposites were studied by using differential scanning calorimetric analysis. The PAM/CaCO₃ and PAM/CaSO₄ nanocomposites showed a higher glass transition temperature and a better thermal stability compared to the pure PAM. The glass transition temperature (T_g) of nanocomposites increases with increase in content of nanoparticles. It may be owing to the interaction between inorganic and organic components. POLYM. COMPOS.,, 2012. © 2012 Society of Plastics Engineers     

Some features of isothermal multicomponent mass transfer in the convective instability of gas mixture

Isothermal multicomponent diffusion in the H₂ + Ar–N₂ and CH₄ + Ar–N₂ three-component gas mixtures has been experimentally studied at various pressures and certain concentrations of components in binary mixtures. It has been shown that, in systems where diffusion coefficients differ significantly from each other, convective instability occurs with increasing pressure, which significantly intensifies multicomponent mass transfer. The parameters of transition diffusion mixing to convective can be defined in the framework of stability theory. The comparison carried out between experimental and calculated data shows satisfactory agreement between them.     

Sulfation rates of cycled CaO particles in the carbonator of a Ca‐looping cycle for postcombustion CO2 capture

Calcium looping is an energy‐efficient CO₂ capture technology that uses CaO as a regenerable sorbent. One of the advantages of Ca‐looping compared with other postcombustion technologies is the possibility of operating with flue gases that have a high SO₂ content. However, experimental information on sulfation reaction rates of cycled particles in the conditions typical of a carbonator reactor is scarce. This work aims to define a semiempirical sulfation reaction model at particle level suitable for such reaction conditions. The pore blocking mechanism typically observed during the sulfation reaction of fresh calcined limestones is not observed in the case of highly cycled sorbents (N > 20) and the low values of sulfation conversion characteristic of the sorbent in the Ca‐looping system. The random pore model is able to predict reasonably well, the CaO conversion to CaSO₄ taking into account the evolution of the pore structure during the calcination/carbonation cycles. The intrinsic reaction parameters derived for chemical and diffusion controlled regimes are in agreement with those found in the literature for sulfation in other systems. © 2011 American Institute of Chemical EngineersAIChE J, 2012     

Factors influencing the reaction between phlogopite and CaSo4·2H2O plus CaO mixtures

According to earlier studies phlogopite reacts with CaSO₄.2H₂O and a mixture of CaSO₄.2H₂O and CaO exchanging 40–80% of its potassium with calcium. The same reactions occur only poorly in the case of muscovite. The structural difference between phlogopite and muscovite is small so that some unknown factors must effect the exchange reaction. In the present work the factors influencing the reaction between phlogopite and a CaSO₄.2H₂O/CaO mixture have been studied by means of X-ray diffraction and an electron microscope. A rod mill has been employed in the separation of the solid phases produced during the reaction. The results have been calculated and drawn by computer programs created for the study. The effects of gypsum, calcium oxide, iron, aluminium and magnesium have been treated in the exchange reaction. Also the poor reactivity of muscovite in the reaction conditions used has been discussed.     

An analytical model for real gas flow in shale nanopores with non‐circular cross‐section

An analytical model for gas transport in shale media is proposed on the basis of the linear superposition of convective flow and Knudsen diffusion, which is free of tangential momentum accommodation coefficient. The present model takes into the effect of pore shape and real gas, and is successfully validated against experimental data and Lattice–Boltzmann simulation results. Gas flow in noncircular nanopores can be accounted by a dimensionless geometry correction factor. In continuum‐flow regime, pore shape has a relatively minor impact on gas transport capacity; the effect of pore shape on gas transport capacity enhances significantly with increasing rarefaction. Additionally, gas transport capacity is strongly dependent of average pore size and streamline tortuosity. We also show that the present model without using weighted factor can describe the variable contribution of convective flow and Knudsen diffusion to the total flow. As pressure and pore radius decrease, the number of molecule‐wall collisions gradually predominates over the number of intermolecule collisions, and thus Knudsen diffusion contributes more to the total flow. The parameters in the present model can be determined from independent laboratory experiments. We have the confidence that the present model can provide some theoretical support in numerical simulation of shale gas production. © 2016 American Institute of Chemical Engineers AIChE J, 62: 2893–2901, 2016     

Strength development due to long term sulfation and carbonation/sulfation phenomena

CFBC boilers firing high sulfur, low ash fuels can produce deposits consisting of almost pure CaSO₄. Test work has shown that if sorbents or bed materials are sulfated for long times (several days or more), they develop compositions similar to those deposits. Sulfation increases with temperature from 650°;C to 900°C, and strength increases continuously in the temperature range of 650°C to 950°C. When sulfation occurs under conditions where CaCO₃ is stable, the overall strength of the deposit increases but the degree of sulfation diminishes. Finally, this work suggests that pellet tests using crushed and calcined sorbents can give misleading information and should be used with caution to study the phenomena described here.     

Thermodynamic and kinetic studies of the MgCl2‐NH4Cl‐NH3‐H2O system for the production of high purity MgO from calcined low‐grade magnesite

To improve the overall sustainability of MgO‐based refractory production, a novel process to produce high purity MgO from calcined low‐grade magnesite in ammonium chloride solution was developed. The process was designed on the basis of the phase equilibria of the NH₄Cl‐MgCl₂‐NH₃‐H₂O system obtained using the Mixed Solvent Electrolyte model embedded in OLI software. The optimum calcination temperature of low‐grade magnesite was determined to be 650°C in terms of the conversion ratio of magnesium and calcium in the leaching experiments. An apparent activation energy of Mg extraction was 30.98 kJ/mol, which is slightly lower than that of Ca leaching. An empirical kinetic model of magnesium extraction was also developed to describe the effects of NH₄Cl concentration, particle size of calcined magnesite, and solid‐to‐liquid ratio on the extent of extraction of magnesium. At leaching time of 10 min, the leachate with high Mg/Ca molar ratio was obtained. Then, MgO with a purity of 99.09% was produced through the decomposition of intermediate 4MgCO₃·Mg(OH)₂·4H₂O. © 2015 American Institute of Chemical Engineers AIChE J, 61: 1933–1946, 2015     

Removal of DDT in drinking water using nanofiltration process

The removal of DDT[(1,1-bis(4-chlorophenyl)-2,2,2-trichloroethane)] with synthetic waters was carried out on a nanofiltration (NF) pilot unit. The influence of initial DDT concentration, pH, flux and recovery on the removal of DDT was studied. The presence of humic acid and some inorganic (CaCl₂, NaCl, and CaSO₄) matters was also tested in the experiment. The removal percent and that of their adsorption on the membrane have been calculated. The results reveal that DDT was easy to be adsorbed on the membranes and the higher the applied pressure the more rapidly saturation of the membrane was achieved. At the initial concentration of 77 μg/L, the equilibrium for DDT adsorption can be achieved in 30 min. With the initial DDT concentration from 5 to 20 μg/L, the removal percent was from 95 to 85%. On condition that recovery was not changed, higher flux can lead to low rejection of DDT. On the other hand, low recovery can have a high rejection when the fluxes were the same. Humic acid can hinder DDT from passing through the membrane by adsorption and inorganic matter (NaCl, CaCl₂ and CaSO₄) can improve the removal percent by reducing the pore size of the membrane.     

CaO‐containing LaCO3OH nanogears and their luminescence and de‐NOx properties

Large‐scale, uniform, monodisperse LaCO₃OH cherry‐blossom‐like nanogears and/or nanocubes have been synthesized under hydrothermal reaction conditions. Upon the addition of only 5 mol% Ca²⁺ ions into a La nitrate salts solution with pH 8.5, LaCO₃OH crystals with novel cubic or nanogear structures are formed in the hexagonal phase. The hydrothermal reactions were carried out without the addition of a template or catalysts. Both 24 hour and 48 hour hydrothermal reactions yield 100% pure LaCO₃OH with no irregular particles. We examined the photoluminescence properties of the as‐synthesized powders of the pure LaCO₃OH nanogears and found one broad emission band centered at 394 nm after excitation at λ  =  280 nm. The NO reduction activity was also examined over highly dispersed CaO‐containing La₂O₃ obtained after calcination the LaCO₃OH at 800^○C for 2 hours. The CaO‐containing La₂O₃ catalysts showed good stability for NO reduction with CH₄ in the presence of O₂ and H₂O vapor.     

Development of a Scalable Method for Manufacturing High‐Temperature CO2 Capture Sorbents

An engineered process for scalable manufacture of a calcium aluminum carbonate CO₂ sorbent with production amounts of about 1000 g per hour has been developed. The process includes mixing and heating, solid‐liquid separation, drying and extrusion, crushing and conveying, and calcined molding steps. The sorbent preparation involves the coprecipitation of Ca²⁺, Al³⁺, and CO₃^2– under alkaline conditions. By adjusting the Ca:Al molar ratio, a series of Ca‐rich materials could be synthesized for use as CO₂ sorbents at 750 °C. A calcium acetate‐derived sorbent exhibited better cyclic stability than sorbents originating from CaCl₂ and Ca(NO₃)₂. The initial sorption capacity increased with CaO concentration. High stability of more than 90 % was maintained by the Ca:Al sorbents after 40 looping tests.     

Studies on scaling of membranes in desalination by direct contact membrane distillation: CaCO3 and mixed CaCO3/CaSO4 systems

Scaling of membranes by CaCO₃ and CaSO₄-CaCO₃ is of considerable concern in membrane desalination processes. It is particularly relevant for porous crossflow hollow fiber-based membrane distillation (MD) processes which can achieve high water recovery and can encounter heavy precipitation of scaling salts. Therefore an analysis of the scaling potential for CaCO₃ and mixed CaSO₄-CaCO₃ systems is presented first in terms of the saturation index profiles throughout the crossflow hollow fiber membrane module as a function of the location in the module for feed solutions resulting from high water recovery. Scaling experiments during DCMD with tap water, CaCO₃ and mixed CaSO₄/CaCO₃ were conducted over a wide range of values of saturation index (SI) (10<SI_calcite<64, 1.1<SI_Gypsum<1.5) using porous fluorosilicone coated crossflow hollow fiber membrane desalination modules. The effects of flow rates, flow patterns (cross vs. parallel flow) and the nature of the membrane surface on possible scaling scenarios were further investigated for the scaling salt CaSO₄. Experimental results at high saturation indices show that even when the precipitation rate was fast in the CaCO₃ system at elevated temperatures or high concentrations, no significant loss in water vapor permeation was observed suggesting no effect of scaling on membrane flux. However, for a few of the mixed CaSO₄-CaCO₃ systems, the water vapor flux dropped somewhat. Possible explanations have been provided and a method to solve this problem has been illustrated. Fast feed flow rate resulted in a shortened induction period. Crossflow flow pattern and the nature of the hydrophobic porous coating on the membrane surface were proven to be helpful in developing the resistance to scaling. Results of modeling show that concentration polarization effects are far more important than temperature polarization effects.     

Molecular dynamics simulations to compute diffusion coefficients of gases into polydimethylsiloxane and poly{(1,5‐ naphthalene)‐co‐[1,4‐durene‐2,2′‐bis(3,4‐dicarboxyl phenyl)hexafluoropropane diimide]}

Diffusion coefficients of N₂, O₂, CO₂ and CH₄ at 298 K in polydimethylsiloxane (PDMS) and poly{[(1,5‐naphthalene)‐co‐[1,4‐durene‐2,2′‐bis(3,4‐dicarboxyl phenyl)hexafluoropropane diimide]} (6FDA‐1,5‐NDA) polymers have been estimated using molecular dynamics (MD) simulations. Estimated diffusion coefficients in PDMS decrease systematically with increasing size of the penetrant gas molecules following the experimental observations. For 6FDA‐1,5‐NDA polymer, diffusion coefficients decrease in the same order of magnitude, but differ in their sequential order, due to varying side group interactions of the polymer with the gaseous molecules. Cohesive energy density, solubility parameter and free volume of the polymers were determined using MD simulations. Reliability and accuracy of the simulations have been tested typically with the computed values of the diffusion coefficient of O₂ in PDMS polymer, which compare well with the literature data. X‐ray scattering profiles of 6FDA‐1,5‐NDA have been generated to understand the interrelationship between the morphology and diffusion coefficients. The radial distribution function was evaluated to find the contribution of atoms that are important in understanding the molecular interactions during gas diffusion in polymers. Copyright © 2007 Society of Chemical Industry     

Study on the oxidation of calcium sulfide using TGA and FTIR

CaS, generated from H₂S removal process using calcium-based sorbents during coal gasification, is a hazardous chemical. The unstable CaS can be converted to stable CaSO₄ via oxidation. Using Fourier Transform Infrared Spectroscopy (FTIR) technique, the experiments of CaS oxidation were performed in a thermogravimetric analyzer (TGA) qualitatively and quantitatively. FTIR coupled with TGA, which can provide in-situ data in real time, was applied to illuminate the SO₂ evolution in the product gases during CaS oxidation. There is a maximum concentration of SO₂ about 4 min after the CaS reaction starts. The FTIR analyses for the solid products obtained at ambient pressure show that the molar contents of CaSO₄ and CaO both increase with temperature, and the content of CaS diminishes accordingly. However, an exception is that at 950 °C, the conversion of CaS to CaSO₄ by FTIR analyses is lower than that at 900 °C due to the sintering of the product CaSO₄ and the change of the mechanism at higher temperatures. The higher the pressure (1.0 MPa), the more molar contents of CaSO₄ and CaO in the products can be attained, which is more pronounced at lower temperatures. The experimental results indicate that the FTIR technique can be applied to the oxidation of CaS experiments effectively to obtain the contents of the solid products.     

Reactivity of a CaSO<Subscript>4</Subscript>-oxygen carrier in chemical-looping combustion of methane in a fixed bed reactor

Chemical-looping combustion (CLC) is a promising technology for the combustion of gas or solid fuel with efficient use of energy and inherent separation of CO₂. A reactivity study of CaSO₄ oxygen carrier in CLC of methane was conducted in a laboratory scale fixed bed reactor. The oxygen carrier particles were exposed in six cycles of alternating reduction methane and oxidation air. A majority of CH₄ reacted with CaSO₄ to form CO₂ and H₂O. The oxidation was incomplete, possibly due to the CaSO₄ product layer. The reactivity of CaSO₄ oxygen carrier increased for the initial cycles but slightly decreased after four cycles. The product gas yields of CO₂, CH₄, and CO with cycles were analyzed. Carbon deposition during the reduction period was confirmed with the combustible gas (CO+H₂) in the product gas and slight CO₂ formed during the early stage of oxidation. The mechanism of carbon deposition and effect was also discussed. SO₂ release behavior during reduction and oxidation was investigated, and the possible formation mechanism and mitigation method was discussed. The oxygen carrier conversion after the reduction decreased gradually in the cyclic test while it could not restore its oxygen capacity after the oxidation. The mass-based reaction rates during the reduction and oxidation also demonstrated the variation of reactivity of CaSO₄ oxygen carrier. XRD analysis illustrated the phase change of CaSO₄ oxygen carrier. CaS was the main reduction product, while a slight amount of CaO also formed in the cyclic test. ESEM analysis demonstrated the surface change of particles during the cyclic test. The reacted particles tested in the fixed bed reactor were not uniform in porosity. EDS analysis demonstrated the transfer of oxygen from CaSO₄ to fuel gas while leaving CaS as the dominant reduced product. The results show that CaSO₄ oxygen carrier may be an interesting candidate for oxygen carrier in CLC. This work was presented at the 7 ^th China-Korea Workshop on Clean Energy Technology held at Taiyuan, China, June 26–28, 2008.