Calcination Rate Research Articles

Sol–gel combustion synthesis of chromium doped yttrium aluminum perovskites

A water-based sol–gel combustion synthesis was optimized in order to achieve pure yttrium aluminium perovskite (YAlO3, YAP). The method involved three main steps: xerogel formation, combustion treatment and calcination of the combusted precursors. TG DTA, FTIR and XRPD, used to investigate the precursor phase evolution, confirmed the strong influence of gel synthesis conditions, combustion temperature and calcination rate on YAP purity. Both stoichiometry and kinetics control during all the preparation steps are crucial in order to avoid the formation of undesired Y3Al5O12 (YAG) and Y4Al2O9 (YAM) stable phases. This method allowed to synthesize, for the first time from an aqueous sol–gel process, single phase samples of YAP. A series of samples with composition: YAl(1−x)Cr(x)O3 where x is 0, 0.035, 0.135, 0.250, 0.500, 0.750 was obtained. A consistent decrease of calcination temperature, with respect to conventional solid state synthesis, was achieved by sol–gel combustion, avoiding polyphase materials even without the use of mineralizers. This is an important result in several applications like the synthesis of ceramic pigments.

Thermal degradation kinetics of calcium‐enriched bio‐oil

AbstractThe thermal decomposition of calcium‐enriched bio‐oil (CEB) was studied in air by thermogravimetric experiments. The characteristics, to a great extent, are quite similar to that of calcium acetate (CA). The decomposition processes can be divided into four stages: the loss of volatile materials, devolatilization and degradation of pyrolytic lignin, decomposition of organic calcium salts and residual carbon, decomposition of CaCO3 to CaO. The activation energy values are lower than that of CA at the corresponding decomposition stages. The CaCO3 from amorphous CEBs exhibits the higher calcination rate than that from CA. The actual mechanisms of the second and the third stages obey the nucleation and growth model (A1), with integral form of F(α) = −ln(1 − α). The high‐porosity of calcined CEBs should be the further evidence of the mechanism. Correspondingly, the mechanism of the fourth stage obeys three‐dimensional (3‐D) phase boundary reaction (R3) mechanism with integral form of F(α) = 1−(1 − α)1/3. © 2008 American Institute of Chemical Engineers AIChE J, 2008

Decomposition and Calcination Characteristics of Calcium-Enriched Bio-oil

The decomposition and calcination characteristics at high temperatures of calcium-enriched bio-oil (CEB) were investigated from 900 to 1100 °C in the study. The CEB were produced by reacting bio-oil with calcium hydroxide, which combined the functions of desulfurization and providing heat during the decomposition and calcination processes. The decomposition of CEB consisted of four steps, partly similar to the case of pure calcium acetate (CA). The rate of the final step, i.e., the calcination rate to derive CaO, was higher in the case of CEB than in the case of CA. The alkali metal migrated from bio-oil might accelerate the calcination rate of CEB-derived CaCO 3 and sintering rate of CaO. The porosities of CEB-derived CaO particles were higher than that from CA, but the Brunauer−Emmett−Teller (BET) surface areas were lower. Dried CEB of pH 10.0 (CEB10) were typical amorphous solids, which should be the optimum bifunctional material with a medium heating value. The decomposition of CEB10 yielded porous CaO particles of moderate surface area and sintering rate for potential SO 2 capture.

Preparation of nano-MgO using ultrasonic method and its characteristics

MgO is an important inorganic material, which can be used in many aspects, such as catalyst, toxic-waste remediation agent, adsorbent, and others. In order to make use of MgO, nano-MgO was prepared by ultrasonic method using Mg (CH3COO)2.2H2O as precursor, NaOH aqueous solution as precipitant in this paper. Effect factors on MgO nano-particle size were investigated. Characteristics of samples were measured by TGA, XRD, TEM, and others techniques. The results showed that the size of nano-MgO about 4 nm could be obtained under the following conditions (ultrasonic time 20 min, ultrasonic power 250 W, titration rate of NaOH 0.25 mL/min, NaOH concentration 0.48 mol/L, calcinations temperature 410 °C, calcination time 1.5 h, heating rate of calcination 5 °C/min). It was a very simple and effective method to prepare nano-MgO.

Preparation of large area three-dimensionally ordered macroporous thin films by confined infiltration and crystallisation

The formation of large-area three-dimensionally ordered macroporous (3-DOM) thin films of TiO 2 using a novel confined crystallisation method is described. The effects of infiltration method, precursor concentration and calcination rate on 3-DOM film quality were investigated and optimised. The templates were prepared as supported thin films which controls their area, thickness and crystallographic orientation. Confinement of the template for infiltration and crystallisation is the key to the preparation of large area 3-DOM films. This new strategy is proposed as a widely applicable route to the formation of large-area, well-ordered 3-DOM materials.

Synthesis of Fe‐MCM‐41 from silatrane and FeCl3 via sol–gel process and its epoxidation acivity

AbstractAn iron‐containing mesoporous molecular sieve, or Fe‐MCM‐41, was successfully synthesized the via sol–gel technique using silatrane and FeCl3 as the silicon and iron sources, and was characterized using various techniques. Many factors were investigated, namely, reaction temperature and time, calcination rate, and iron amount in the reaction mixture. It was found that the optimum conditions in which to synthesize Fe‐MCM‐41 was to carry out the reaction at 60 °C for 7 h using a 1 °C min−1 calcination rate and a 550 °C calcination temperature. The catalytic activity and selectivity of styrene epoxidation using hydrogen peroxide showed that the selectivity of the styrene oxide reached 65% at a styrene conversion of 22% over the 1%wt catalyst. Copyright © 2008 John Wiley & Sons, Ltd.

Preparation of nano-MgO using ultrasonic method and its characteristics

MgO is an important inorganic material, which can be used in many aspects, such as catalyst, toxic-waste remediation agent, adsorbent, and others. In order to make use of MgO, nano-MgO was prepared by ultrasonic method using Mg (CH3COO)2.2H2O as precursor, NaOH aqueous solution as precipitant in this paper. Effect factors on MgO nano-particle size were investigated. Characteristics of samples were measured by TGA, XRD, TEM, and others techniques. The results showed that the size of nano-MgO about 4 nm could be obtained under the following conditions (ultrasonic time 20 min, ultrasonic power 250 W, titration rate of NaOH 0.25 mL/min, NaOH concentration 0.48 mol/L, calcinations temperature 410 °C, calcination time 1.5 h, heating rate of calcination 5 °C/min). It was a very simple and effective method to prepare nano-MgO.

Controlled Synthesis of One-Dimensional Single-Crystal Co3O4 Nanowires

Uniform Co3O4 nanowires with diameters of ~80 nm and lengths of 10–20 μm have been prepared by a hydrothermal reaction and sequential decomposition of the intermediate product. Transmission electron microscopy characterization and selected-area electron diffraction patterns show that the nanowires elongate along the [110] direction. The amount of CO(NH2)2 employed and the heating rate of calcination have a key effect on the morphology of the product. In addition, analysis of the optical properties indicates a blue shift of the bandgaps and reveals the semiconductive nature of the products.

Impact of temperature ramping rate during calcination on characteristics of nano-ZrO 2 and its catalytic activity for isosynthesis

This paper studied the effect of temperature ramping rate during calcination on characteristics of nanoscale zirconia and its catalytic performance for isosynthesis. The physical properties, i.e. BET surface area, cumulative pore volume, cumulative pore diameter and the phase composition in zirconia, acid-base properties and surface properties such as Zr 3+ quantity, were characterized. Increase in the temperature ramping rate of calcination resulted in a higher composition of the tetragonal phase, but it showed insignificant influence on the other physical properties. Considering the catalytic activity, the acid sites did not affect the activity, but the basic sites depended on the fraction of the tetragonal phase in zirconia which was related to the selectivity to isobutene. The intensity of Zr 3+ on the surface varied with the change in the heating rate of calcination. Both the tetragonal phase composition in zirconia and the quantity of Zr 3+ were the key factors affecting the selectivity to isobutene in hydrocarbons. Moreover, the maximum value of the product selectivity to isobutene on the ZrO 2 (5.0) catalyst was attained at the highest concentration of Zr 3+.

Solution-Based Approaches to Fabrication of YBa2Cu3O7−δ (YBCO): Precursors of Tri-Fluoroacetate (TFA) and Nanoparticle Colloids

Detailed investigation of superconducting films of YBa2Cu3O7-δ (YBCO) prepared from solution-based precursors have been performed. Two precursors have been compared in this study: the presently used trifluoroacetate (TFA) solution and a recently developed colloidal suspension containing nanoparticles of mixed oxide. Detailed analyses of the evolution of microstructure and chemistry of the films have been performed, and process parameters have been correlated with final superconducting properties. Both films need two heating steps: a low temperature calcination and a higher temperature crystallization step. For TFA films, it was seen that the heating rate during calcination needs to be carefully optimized and is expected to be slow. For the alternate process using a nanoparticle precursor, a significantly faster calcination rate is possible. In the TFA process, the Ba ion remains as fluoride and the Y remains as oxyfluoride after calcination. This implies that, during the final crystallization stage to form YBCO, fluorine-containing gases will evolve, resulting in residual porosity. On the other hand, the film from the nanoparticle process is almost fully oxidized after calcination. Therefore, no gases evolve at the final firing (crystallization) stage, and the film has much lower porosity. The superconducting properties of both types of films are adequate, but the nanoparticle films appear to have persistently higher Jc values. Moreover, they show improved flux pinning in higher magnetic fields, probably due to nanoscale precipitates of a Cu-rich phase. In addition, the nanocolloid films seem to show additionally enhanced flux pinning when doped with minute amounts of second phase precipitates. It therefore appears that, whereas the TFA process is already quite successful, the newly developed nanoparticle process has significant scope for additional improvement. It can be scaled-up with ease, and can be easily adapted to incorporate nanoscale flux pinning defects for in-field performance.

The effect of CaO sintering on cyclic CO2 capture in energy systems

AbstractThe importance of calcium‐based sorbents, especially natural limestones, for CO2 removal necessitates an investigation into the sorbent decay mechanism. This study starts from pore size distributions for samples from tests under various calcination/carbonation cycling conditions. A sintering model is formulated to describe the cyclic behavior of sorbents during cyclic calcination and carbonation. It explains the similar reversibility shown by sorbents under different test conditions. A balance between shorter cumulative sintering time and higher calcination rates appears to be responsible for the similar degrees of sintering and sorbent reversibility. © 2007 American Institute of Chemical Engineers AIChE J, 2007

Preparation and pervaporation performance of high-quality silicalite-1 membranes

High-performance silicalite-1 membranes were successfully synthesized on novel porous silica tubes by two-step in-situ hydrothermal synthesis. The flux and separation factor towards ethanol/water mixture at 60°C were 0.56 kg/(m2·h) and 84, respectively. The as-synthesized silicalite-1 membranes were characterized by scanning electron microscopy (SEM). The influence of different synthesis conditions on the separation performance of the silicalite-1 membranes was investigated. It was found that the average flux of silicalite-1 membranes was improved by about 26% after filling the silica tubes with mixed solution containing glycerol and water. After calcinating at 400°C for 5 h repeatedly, membrane synthesized on silica tube still showed high pervaporation performance towards ethanol/water mixture even at a calcination rate of 4°C/min, which suggested that silica support was more suitable for preparing high-performance silicalite-1 membranes.

Kinetics and Structural Characterization of Calcium-Based Sorbents Calcined under Subatmospheric Conditions for the High-Temperature CO2Capture Process

The influence of several process parameters on the calcination of a naturally occurring limestone and a precipitated mesoporous calcium carbonate (CaCO3) sorbent structure to calcium oxide (CaO) is detailed in this study. CaCO3 calcination is an integral part of a multicyclic carbonation−calcination reaction (CCR) process that separates carbon dioxide (CO2) from high-temperature gas mixtures into a pure CO2 stream. Maintenance of high sorbent reactivity over repeated CCR cycles reduces the capital and operating cost of the CCR process. A lower calcination temperature, required to maintain high sorbent reactivity, reduces the calcination rate of CaCO3. This study investigates various process conditions such as subatmospheric calcination, sorbent dispersion in a rotary calciner, use of a high thermal conductivity sweep gas, etc. in enhancing the calcination rate at lower calcination temperatures. The high-temperature gas−solid carbonation studies of the resulting CaO sorbent prove the necessity to maintain a high porosity structure during limestone calcination.

The effect of sintering on sulphur capture by limestone and dolomite

AbstractIn-bed sulphur capture is an attractive method for reducing SO2 emissions from coal-fired IGCC power plants. Sulphur (H2S) capture efficiency is dependent upon four competing rates: (i) calcination of sorbent to the oxide with concomitant surface area growth, (ii) sintering of the oxide with an associated decrease in surface area, (iii) sulphiding of the sorbent/calcined sorbent and (iv) sintering of the sulphide. The factors which influence these competing rates have been investigated. Two limestones and a dolomite were calcined in a drop-tube reactor under a combustion atmosphere to obtain samples of varying degrees of calcination and sintering. The results were modelled to yield apparent calcination and sintering rate constants. The partially calcined materials were sulphided in a fixed-bed reactor under a simulated gasification atmosphere. Conversions to the sulphide of up to 30% were observed, alongside rapid losses in surface area. The sintered sulphide forms an impermeable structure over th...

The influence of the calcination rate on silicalite-1 membranes

Silicalite-1 films with a thickness of 500 nm on asymmetric α-alumina micro filtration filters were calcined at 500 °C with heating and cooling rates varying between 0.2 °C/min and 5.0 °C/min. The membranes were characterized with single gas permeation, porosimetry, and xylene isomer separation experiments. It was found that the quality of the prepared membranes was independent of the heating/cooling rate according to the single gas permeation and porosimetry characterization. Xylene isomer separation data was found to vary between the samples, but none of the variations could be attributed to the heating/cooling rate during calcination since the variations did not follow a trend but occurred randomly. It is thus concluded that the calcination rate does not influence the quality of these membranes.

Simultaneous Calcination and Sulfidation of Calcium-Based Sorbents

This work analyzes the simultaneous calcination and sulfidation of a dolomite and a limestone during the H2S retention under calcining conditions. The experiments have been carried out in a thermogravimetric analyzer at 0.1 MPa and in a pressurized differential reactor at 1 MPa using different CO2 partial pressures. The H2S retention process depended on the relative rate of the reactions of calcination, sulfidation of calcined sorbents, and direct sulfidation. A model considering the simultaneous calcination−sulfidation of the sorbents was developed to predict the experimental H2S retention data at different temperatures and pressures. For reaction temperatures far away from the calcination temperature, the sulfidation took place on the full calcined sorbent. Lower temperatures produced a lower calcination rate and a delay in the sulfidation process. The strong decrease in the calcination rate observed at temperatures near the calcination equilibrium produced different behaviors depending on the type of Ca-based sorbent. In the dolomite, the sulfidation took place on the CaCO3 when the calcination rate was lower than the direct sulfidation rate, even working at calcining conditions. In the limestone, the simultaneous presence of CO2 and H2S in some cases produced an inhibition of the calcination and sulfidation reactions. The model used in this work allowed definition of the application limits (temperature, pressure, and gas concentration) of the different particle reaction models (direct sulfidation, sulfidation of calcined sorbents, or simultaneous calcination−sulfidation) to be used in the design of desulfurization reactors.

Characterization of the enhancement effect of Na2CO3 on the sulfur capture capacity of limestones.

It has been known for a long time that certain additives (e.g., NaCl, CaCl2, Na2CO3, Fe2O3) can increase the sulfur dioxide capture-capacity of limestones. In a recent study we demonstrated that very small amounts of Na2CO3 can be very beneficial for producing sorbents of very high sorption capacities. This paper explores what contributes to these significant increases. Mercury porosimetry measurements of calcined limestone samples reveal a change in the pore-size from 0.04-0.2 microm in untreated samples to 2-10 microm in samples treated with Na2CO3--a pore-size more favorable for penetration of sulfur into the particles. The change in pore-size facilitates reaction with lime grains throughout the whole particle without rapid plugging of pores, avoiding premature change from a fast chemical reaction to a slow solid-state diffusion controlled process, as seen for untreated samples. Calcination in a thermogravimetric reactor showed that Na2CO3 increased the rate of calcination of CaCO3 to CaO, an effect which was slightly larger at 825 degrees C than at 900 degrees C. Peak broadening analysis of powder X-ray diffraction data of the raw, calcined, and sulfated samples revealed an unaffected calcite size (approximately 125-170 nm) but a significant increase in the crystallite size for lime (approximately 60-90 nm to approximately 250-300 nm) and less for anhydrite (approximately 125-150 nm to approximately 225-250 nm). The increase in the crystallite and pore-size of the treated limestones is attributed to an increase in ionic mobility in the crystal lattice due to formation of vacancies in the crystals when Ca is partly replaced by Na.

Experimental Investigation of the Decomposition and Calcination of Calcium-Enriched Bio-Oil

The subject of this study is the experimental investigation of the decomposition of calcium-enriched bio-oil (CEB), the product of the reaction of bio-oil with calcium hydroxide, at high temperatures and of the calcination of the CaCO3 material that is obtained when the decomposition is carried out in the presence of CO2. Decomposition and calcination experiments are conducted in a thermogravimetric analysis system, and the pore structures of dried, decomposed, and calcined samples are characterized using nitrogen adsorption−desorption, mercury intrusion−extrusion, and photomicrographic examination. The calcination results for CEB-derived CaCO3 are compared with those for two naturally occurring calcitic solids of high CaCO3 content. The pore structure characterization results show that the decomposition and calcination products of CEB have very high porosities, higher than 80−90%. Because of its high porosity, the calcination product exhibits a much higher calcination rate than particles of limestones an...

Modeling of in-line low-NO x calciners—a parametric study

Simulations with a heterogeneous model of an in-line low-NO x calciner, based on non-isothermal diffusion-reaction models for char combustion and limestone calcination combined with a kinetic model for NO formation and reduction, are reported. The analysis shows that the most important hydrodynamic parameter is the mixing rate of preheated combustion air into the sub-stoichiometric suspension leaving the reducing zone and the most important combustion parameter is the char reactivity. Also, the calcination rate modifies very much the temperature in the calciner, char and limestone conversion and NO emission. Carbon monoxide is a key component for the reduction of NO and reliable data for the kinetics of NO reduction by CO over CaO are very important for the prediction of the NO emission. The internal surface area of char and limestone particles influences the combustion and calcination rates and thereby the char and limestone conversion and the NO emission.

Mathematical modeling of an in-line low-NO x calciner

The reduction of the NO x content in in-line-calciner-type kiln systems can be made by optimization of the primary firing in the rotary kiln and of the secondary firing in the calciner. Because the optimization of calciner offers greater opportunities the mathematical modeling of this reactor is very important. A heterogeneous, dynamic mathematical model for an in-line low-NO x calciner based on non-isothermal diffusion–reaction models for char combustion and limestone calcination has been developed. The importance of the rate at which preheated combustion air was mixed into the main flow was particularly studied. The results of the simulations indicate that the external heat and mass transfer to the char particles is not limiting. Internal diffusion of O 2, CO, NO and CO 2 is important especially in the reducing zone and the first part of the oxidizing zone of the calciner and the internal heat transport limitation is significant for the endothermic limestone calcination. The rate at which preheated combustion air is mixed into the main flow directly influences the coal combustion rate, and thereby through the rate of heat release from combustion, it also influences the calcination rate and the temperature profile. The mixing rate has some influence on the CO concentration profile and an important influence on the overall degree of fuel-N to NO conversion.