Non-isothermal Thermogravimetric Research Articles

Oxidation kinetics of magnesium particles determined by isothermal and non-isothermal methods of thermogravimetric analysis

Knowledge of the oxidation kinetics of magnesium (Mg) particles is important for the development of advanced propulsion and power systems based on combustion of metals. However, reports on the high-temperature oxidation of Mg powders in oxygen are contradictory. In the present work, the oxidation of spherical and non-spherical (flakes) Mg particles in O2 flow was investigated using isothermal and non-isothermal thermogravimetry. Three fractions of the spherical powder were tested, with average sizes about 30, 60, and 300 μm. The oxidation of all powders was complete at temperatures lower than the melting point of Mg, 650 °C. The Friedman analysis of non-isothermal and isothermal thermogravimetric (TG) data has shown that the activation energy exhibits only small fluctuations during a large part of the oxidation process, which indicates that the process can be modeled as a single-step reaction. Model-based analysis has shown that the Avrami-Erofeev model of simultaneous nucleation and growth provides the best fit with the experimental isothermal and non-isothermal TG curves. Scanning electron microscopy of the oxidized spherical particles has shown submicron grains, which is consistent with the Avrami-Erofeev model. The Mampel-Delmon analysis of the isothermal TG data has shown that for the 30 and 60 μm particles, the nucleation is relatively slow, leading to the sigmoidal TG curve, described by the three-dimensional Avrami-Erofeev equation. An increase in the particle size decreases the dimension of this equation, and for the 300 μm particles and flakes, the entire process can be considered as a growth of a non-protective oxide layer. The apparent activation energy of Mg oxidation is likely in the range of 200 – 230 kJ/mol, independently of the particle size and shape.

Analysis of Iron Oxide Reduction Kinetics in the Nanometric Scale Using Hydrogen.

Iron nanopowder could be used as a sintering aid to water-atomised steel powder to improve the sintered density of metallurgical (PM) compacts. For the sintering process to be efficient, the inevitable surface oxide on the nanopowder must be reduced at least in part to facilitate its sintering aid effect. While appreciable research has been conducted in the domain of oxide reduction of the normal ferrous powder, the same cannot be said about the nanometric counterpart. The reaction kinetics for the reduction of surface oxide of iron nanopowder in hydrogen was therefore investigated using nonisothermal thermogravimetric (TG) measurements. The activation energy values were determined from the TG data using both isoconversional Kissinger–Akahira–Sunose (KAS) method and the Kissinger approach. The values obtained were well within the range of reported data. The reaction kinetics of Fe2O3 as a reference material was also depicted and the reduction of this oxide proceeds in two sequential stages. The first stage corresponds to the reduction of Fe2O3 to Fe3O4, while the second stage corresponds to a complete reduction of oxide to metallic Fe. The activation energy variation over the reduction process was observed and a model was proposed to understand the reduction of surface iron oxide of iron nanopowder.

Thermal Behavior of Sweet Potato Starch by Non-Isothermal Thermogravimetric Analysis.

In this study, X-ray diffraction (XRD), thermogravimetric analysis (TGA), and differential scanning calorimetry (DSC) methods were used to study the structure, the thermal degradation kinetics, and the thermogram of sweet potato starch, respectively. The thermal decomposition kinetics of sweet potato starch was examined within different heating rates in a nitrogen atmosphere. Different models of kinetic analysis were used to calculate the activation energies using the thermogravimetric data of the thermal degradation process. The activation energies got from Kissinger, Flynn–Wall–Ozawa, and Šatava–Šesták models were 173.85, 174.87, and 174.34 kJ·mol−1, respectively. Thermogravimetry–Fourier transform infrared spectroscopy (TG-FTIR) analysis showed that the main pyrolysis products included water, carbon dioxide, and methane.

Synthesis and characterization of pure phase zeolite 4A from coal fly ash

This study was aimed at the synthesis of pure phase zeolite 4A from coal fly ash sourced from Nishat power plant in the Punjab province of Pakistan. Analysis of fly ash (FA) demonstrated that it can be used to produce low silicate type zeolites specifically zeolite 4A due to its Si/Al molar ratio of ∼2. The current investigation emphasized on the addition of induction time (before and after crystallization) to the fly ash extract of Si-Al-Na species that was extracted from fly ash. The induction time helped in catalysing primary and secondary gel formation during the synthesis leading to controlled formation of single phase with high crystallinity and small crystallite size. The physiochemical investigations of synthetic zeolite 4A by using various analytical techniques made the proposed process justified on comparison with commercial zeolite 4A. The X-Ray Diffraction (XRD) analysis for synthesize zeolite 4A confirmed only one crystallographic phase and small crystallite size with 82% crystallinity compared to reference zeolite with 75% crystallinity. Fourier-Transform Infrared spectroscopy (FTIR) spectra revealed the structural aspects of synthesis and reference zeolite by indicating the presence of identical structural units with same height and width of peaks. Morphological analysis by Scanning Electron Microscope (SEM) showed a well-defined cubic shape of synthesis zeolitic crystals. Surface area analyses by Brunauer–Emmett–Teller (BET) showed that synthesized zeolite possessed 122 m2/g surface area that is 3 times higher than reference zeolite of 36.3 m2/g. The synthesized zeolite was also found to have smaller particle size than reference zeolite. Thermal stability comparison of both zeolites was studied by non-isothermal Thermo Gravimetric Analysis-Differential Thermal Analysis (TGA-DTA) thermograms from 25 to 800 °C temperature and both the samples were found to be stable at 800 °C.

Thermal Behavior of Hydrated Iron Sulfate in Various Atmospheres

Iron sulfate, in particular FeSO4·7H2O, is derived from titanium dioxide production and the steel pickling process. Regarding TiO2 manufacturing, the amount of the resultant FeSO4·7H2O can be as high as 6 tons per ton of produced TiO2, leading to a huge amount of ferrous sulfate heptahydrate, which is considered an environmental and economic concern for the titanium dioxide industry in European countries. The present paper focuses on the thermal treatment of ferrous sulfate (heptahydrate and monohydrate) samples under different conditions. Nonisothermal thermogravimetric (TG) analysis was used to study the behavior of iron sulfate samples at temperatures of up to 1000 °C in Cl2 + O2, O2, and N2 atmospheres. Results showed that the dehydration of iron sulfate heptahydrate in nitrogen started at room temperature and resulted in iron sulfate tetrahydrate (FeSO4·4H2O). The ferrous sulfate monohydrate (FeSO4·H2O) was formed at temperatures close to 150 °C, while the anhydrous ferrous sulfate (FeSO4) was obtained when the samples were heated in nitrogen at over 225 °C. The kinetic features of FeSO4 decomposition into Fe2O3 were revealed under isothermal conditions at temperatures ranging from 500 to 575 °C. The decomposition of iron sulfate was characterized by an apparent activation energy of around 250 kJ/mol, indicating a significant temperature effect on the decomposition process. The obtained powder iron oxide could be directed to the agglomeration unit of iron and the steelmaking process.

Catalytic fast pyrolysis of wood plastic composite over microporous zeolites

The catalytic pyrolysis of commercial wood plastic composite (WPC) over microporous catalysts was investigated by thermogravimetric (TG) analysis and tandem micro reactor-gas chromatography/mass spectrometry (TMR-GC/MS). The non-isothermal TG analysis results showed that the decomposition temperatures of polyethylene and polypropylene in the WPC were shifted to lower temperatures by the use of catalysts, from 484 °C to 440 °C over HY(5.1), 457 °C over HBeta(25), and 469 °C over HZSM-5. TMR-GC/MS indicated that the formation efficiency of aromatic hydrocarbons during the catalytic pyrolysis of WPC was affected not only by the catalyst properties, but also by the contact mode between the WPC and catalyst. The in-situ catalytic pyrolysis of WPC showed a higher aromatic formation efficiency than the ex-situ reaction because of the sufficient likelihood of a reaction between the reactant molecules and catalysts during the in-situ reaction. Among the catalysts, HBeta produced the largest amount of aromatics followed by HZSM-5 and HY. Diffusion hindering of the reactant molecules to the small pores of HZSM-5 and the large amount of coke inside the large pores of HY were the main reasons for the lower aromatic formation efficiency over HZSM-5 and HY than over HBeta. Sequential catalytic TMR-GC/MS showed that the increased diffusion hindering effect caused by coke accumulation is the main limitation on the use of HZSM-5. Mesoporous HZSM-5 may be a potential way of overcoming the limitation of the catalysts used in this study.

Tightly bound water in smectites

Abstract Smectites are able to retain molecular tightly bound water (TBW) at temperatures above 100 °C, even after prolonged drying. The presence of TBW affects the stable isotope ratios, the dehydroxylation behavior of smectites and smectite-rich samples and also has implications in measuring various properties of clay-rich rocks. Five reference smectites, in Mg-, Ca-, Na-, and Cs-exchanged forms were subjected to different drying protocols followed by the determination of TBW contents using precise thermogravimetric (TG) analysis. Activation energies (Ea) of the removal of different water fractions at temperatures up to 1000 °C were determined in non-isothermal TG experiments using model-independent methods. Additionally, 4A and 13X zeolites were examined in both cases as apparent OH-free references. After drying at 110 °C, all smectites still contained up to 3 water molecules per interlayer cation. The TBW contents in smectites were found to be primarily dependent on the isothermal drying temperature. For a given temperature, TBW contents decreased with respect to the type of interlayer cation in the following order: Mg > Ca > Na > Cs. The influence of the time of drying and the smectite layer charge were found to be negligible. The Ea of dehydration below 100 °C, as determined by the Friedman method, was quite constant within the 45–60 kJ/mol range. The Ea of TBW removal increased along with the degree of reaction from 90 to 180 kJ/mol, while the Ea of dehydroxylation was found in the 159–249 kJ/mol range, highly depending on the sample’s octahedral sheet structure and the interlayer cation. The Mg2+ cation can hold H2O molecules even beyond 550 °C, making it available during dehydroxylation or—for geologic-scale reactions—pass H2O to metamorphic conditions. High similarities between the TBW contents and the Ea of dehydration for smectites and cationic (low Si/Al-) zeolites lead to the conclusion that TBW in smectites is remarkably similar to zeolitic water in terms of cation bonding and diffusion characteristics. The optimal drying protocol for smectites is to substitute interlayer cations with cations of a low-hydration enthalpy, such as Cs, and to dry a sample at 300 °C, provided that the sample is Fe-poor. Fe-rich smectites should be dried at 200 °C to avoid dehydroxylation that occurs below 300 °C.

Reaction kinetics and phase transformation during hydrogen reduction of spherical Fe2O3 nanopowder agglomerates

The reaction kinetics of spherical Fe2O3 nanopowder agglomerates were investigated as a preliminary study to fabricate structure optimized spherical Fe nanopowder agglomerates for processing PM parts. The apparent activation energy Ea for kinetic analysis was determined from nonisothermal thermogravimetric (TG) measurement using isoconversional Kissinger-Akahira-Sunose (KAS) method, and the result is discussed in terms of the phase transformation by XRD analysis and microstructural change by BET analysis. The calculated activation energy was in a reasonable range compared to the reported data for the hydrogen reduction of Fe2O3, but it showed three distinct slopes according to the conversion degree of the reaction. The phase analysis revealed that a two-step phase transformation occurred during the reduction process in the sequence of Fe2O3, Fe3O4, and Fe without an intermediate phase FeO. During the reduction process of Fe3O4 to Fe, the reduction rate was retarded by sintering of oxide particles leading to an abrupt increase of activation energy. As the volume fraction of the metallic iron increased through the reduction progress, the reaction accelerated remarkably. This is believed to be attributed to the catalytic role of the reduced Fe particles facilitating reduction of the adjacent oxide particles. These results provide the primary data for understanding the microstructural development of spherical Fe nanopowder agglomerates fabricated by the hydrogen reduction process.

Diffusional effects on differences of coal char reactivity between air and oxy-fuel combustion in thermogravimetric experiments

Oxy-fuel combustion can realize large-scale CO2 capture and low NOx emission from coal-fired power plants, while the evaluation of coal reactivity differences between air and oxy-fuel conditions is of importance for retrofitting existing conventional boilers to oxy-fuel ones. Here, two sets of specially designed experiments were contrastively conducted to assess the diffusional limitation effects on differences of coal char reactivity between air (O2/N2) and oxy-fuel (O2/CO2) combustion in non-isothermal thermogravimetric (TG) experiments, which were seldom investigated previously. Experimental results show that the TG/DTG curves of char combustion present distinct differences before and after reducing diffusional limitation. The differences of char combustion measured in non-isothermal TG experiments between O2/CO2 and O2/N2 conditions are shrinking with the reduction of diffusion resistance. Compared with conventional air combustion, change of combustion atmosphere in oxy-fuel condition mainly exhibits influence on diffusion process, while it has no observable effect on char-O2 chemical reaction in TG experiments. Obviously different results between air and oxy-fuel combustion can be obtained when the diffusional limitation in TG tests is significant, while the change of combustion atmosphere has negligible influence on char reactivity under minimized inhibition of heat and mass transfer. Knowledge of the respective influence of intrinsic reaction and diffusion resistance on the measured TG experiments is of considerable significance for the exploration of reactivity and kinetics differences between air and oxy-fuel combustion.

Approximation-based integral versus differential isoconversional approaches to the evaluation of kinetic parameters from thermogravimetry

The relative accuracies of approximation-based integral versus differential isoconversional approaches for ‘actual’ E determination were investigated on experimental dehydration data of roxithromycin monohydrate from thermogravimetric (TG) analysis. The dehydration kinetic parameters and the relationship to the structural characteristics of the monohydrate and anhydrate forms from differential scanning calorimetry (DSC) and single-crystal X-ray diffractometry (SC-XRD) are also reported. Integral methods versus the differential Friedman isoconversional method evaluated E correspondences in both iso- and non-isothermal TG methods. The reliability in E from Friedman approached that of estimates from current most accepted integral isoconversional methods and was even superior to methods (for non-isothermal studies) that employ an approximation to the temperature integral (modified Kissinger–Akahira–Sunose, Senum–Yang fourth degree). Structural characterization (DSC, SC-XRD) and kinetic analysis from generalized kinetic master plots concluded that coordinated water occupied interlinked voids in crystal structure which may have facilitated the multidimensional diffusional loss of water upon heating without disruption of the crystal structure.

Thermal and spectral properties of alkali metal complexes of vitamin K3 analogue phthiocol

Hydroxynaphthoquinones, alkali, and alkali metal salts were used to synthesize several biologically important molecules. As hydroxynaphthoquinones act also as potential ligands, it is possible that alkali metal complexes may be formed as intermediates in such reactions. To ensure this, the reactions of 2-hydroxy-3-methyl-1,4-naphthoquinone (phthiocol, a vitamin K3 analogue) with sodium metal, alkali such as NaOH and KOH, and alkali metal salts such as CH3COONa, Na2CO3 and K2CO3 were studied in the present investigation. The products were characterized by various analytical techniques, namely elemental analysis, FT-IR, 1H NMR, UV–Vis, thermogravimetric (TG) analysis, GC–MS, and DSC studies. 100 % reduction of the ligand was observed in Phth-1 with sodium metal as reducing agent. The solution of CH3COONa, NaOH, KOH, and K2CO3 in methanol facilitates the reduction of phthiocol ligand. The reduction of the ligands may be probably due to disproportionate reaction of the ‘catechol’ form and the fully oxidized naphthoquinone form of the ligand. The radical signatures of the ligands in complexes were detected by EPR studies and also by carbonyl frequencies in FT-IR spectra. Non-isothermal TG studies confirm the presence of adsorbed and coordinated water molecules. Residue formed after complete decomposition was observed to be 1/2 Na2O in Phth-1 to Phth-3, K2O in Phth-4, and 1/2 K2O in Phth-5. Based on GC–MS and TG studies, three decomposition mechanisms are being proposed for all complexes.

Thermogravimetric studies and kinetics of decomposition of ammonium yttrium fluoride

The kinetics of thermal decomposition of ammonium yttrium fluoride intermediates, (NH4)3Y2F9 and NH4Y2F7 were studied by a non-isothermal thermogravimetric (TG) technique at different heating rates. Kinetic parameters were determined from the TG curves by using two model-free methods, i.e. Flynn–Wall–Ozawa (FWO) and Kissinger–Akahira–Sunose (KAS). The activation energy (E) for the decomposition of (NH4)3Y2F9 was dependent on the fraction converted, α; its value was 152 kJ/mol (for α = 0.1–0.3) and 232 kJ/mol (for α = 0.4–0.8) from both models; and the pre-exponential factor (ln(A)) was 43 (for α = 0.1–0.3) and 59 (for α = 0.4–0.8). For the decomposition of NH4Y2F7, the activation energy (E) was independent of the fraction converted, α, and its value was 123 kJ/mol; the pre-exponential factor (ln(A)) was 31. The decomposition of (NH4)3Y2F9 proceeds by one-dimensional diffusion (for α = 0.1–0.3), followed by two-dimensional diffusion (for α = 0.4–0.8). The decomposition of NH4Y2F7 occurs by random nucleation and growth.

Thermal Analysis and Decomposition Kinetics of Chinese Forest Peat under Nitrogen and Air Atmospheres

The mechanism of forest ground fire is thermal decomposition and smoldering combustion of forest peat or duff. The availability of oxygen is believed to influence the processes. This paper aims to investigate the thermal decomposition of peat under inert and oxidative atmospheres. Experiments were monitored under nitrogen and air atmospheres, using the non-isothermal thermogravimetric (TG) and differential thermal analysis (DTA) methods. The pyrolysis curves of peat showed three main stages, i.e., the stage of moisture evaporation (together with low stability of organic compounds) (315−432 K, with the heating rate of 10 K/min), organic matter pyrolysis (432−805 K), and inorganic compound decomposition (805−1075 K). The stage of organic matter pyrolysis also contained three steps, corresponding to hemicellulose, cellulose, and lignin pyrolysis. Because the temperature of inorganic compound decomposition was higher than the peat-smoldering temperature, this stage of inorganic compound decomposition was not ...

Co-firing of biomass with coals

The chemical composition and reactivity of fir (Abies bornmulleriana) wood under non-isothermal thermogravimetric (TG) conditions were studied. Oxidation of the wood sample at temperatures near 600 °C caused the loss of aliphatics from the structure of the wood and created a char heavily containing C–O functionalities and of highly aromatic character. On-line FTIR recordings of the combustion of wood indicated the oxidation of carbonaceous and hydrogen content of the wood and release of some hydrocarbons due to pyrolysis reactions that occurred during combustion of the wood. TG analysis was used to study combustion of fir wood. Non-isothermal TG data were used to evaluate the kinetics of the combustion of this carbonaceous material. The article reports application of Ozawa–Flynn–Wall model to deal with non-isothermal TG data for the evaluation of the activation energy corresponding to the combustion of the fir wood. The average activation energy related to fir wood combustion was 128.9 kJ/mol, and the average reaction order for the combustion of wood was calculated as 0.30.

Fundamental Study on a Salt Distillation from Mixtures of Rare Earth Precipitates and LiCl-KCl Eutectic Salt

A fundamental study on the distillation rate on LiCl-KCl eutectic salt under different vacuums from 66 to 6600 Pa (0.5 to 50 mm Hg) was performed by using both a nonisothermal and an isothermal thermogravimetric (TG) analysis. Based on the nonisothermal TG data, distillation rate equations as a function of the temperature could be derived. Calculated flux by these model flux equations was in agreement with the distillation rate obtained from isothermal TG analysis. A salt distillation operation with a moderated distillation rate of 10–4 to 10–5 mol·cm–2·s–1 is possible at temperatures of <1300 K and vacuums of 660 to 6600 Pa. An ~99% salt distillation efficiency was obtained after 1 h at a temperature above 1150 K under 6600 Pa. An increase in the vaporizing surface area is relatively effective for removing residual salt in the remaining particles, when compared to that for the vaporizing time. More than 99.95% of total distillation efficiency was obtained for a 1-h distillation operation by increasing the inner surface area from 4.52 to 12.56 cm2 (about 3 times increase).

Thermal Stability, Equilibrium Vapor Pressure and Standard Enthalpy of Vaporization of 1-Butyl-3-methylimidazolium Tetrafluoroborate

The thermal stability, equilibrium vapor pressure, and standard enthalpy of vaporization of 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim][BF4]) ion liquid were investigated using both non-isothermal and isothermal thermogravimetric analysis (TGA) under high purity nitrogen as a carrier gas. The non-isothermal thermogravimetric (TG) curve showed that the onset and peak decomposition temperatures were 697 and 734 K, respectively. However, long-term isothermal TGA studies revealed that the highest application temperature for [bmim][BF4] was 513 K. Moreover, the temperature dependence of the equilibrium vapor pressure and the standard enthalpy of vaporization of [bmim][BF4] were also investigated by TG-based transpiration. In the temperature range of 503-543 K, the relationship between the equilibrium vapor pressure (p(subscript e)) of [bmim][BF4] and temperature was l(subscript gpe)=(16±1)+(-6.85±0.25)×10^3/T. The standard enthalpy of vaporization (△(subscript vap)H(superscripat Ө)) of [bmim][BF4] was found to be(131±5) kJ•mol^(-1).

Study on the distillation rates of LiCl–KCl eutectic salt under different vacuum conditions

A study on the distillation rate of LiCl–KCl eutectic salt under different vacuums from 0.5 to 50 Torr was performed by using thermogravimetric (TG) method. A distillation rate of the order of 10 −4–10 −5 mol cm −2 s −1 was obtainable at temperatures of 1200–1300 K and vacuums of 5–50 Torr. Based on the non-isothermal TG data, model distillation flux equations could be derived as a function of temperature. Pure gas-phase and gas–liquid interfacial resistances at different vacuum conditions were evaluated from the comparison of experimental vaporization fluxes with the maximum flux obtained from the kinetic theory of gas. The difference between interfacial mass transfer coefficients and gas-phase ones increases with the temperature. Gas-phase resistance is much greater than that of the phase transition between condensed and gas phases at tested vacuum conditions of 0.5–50 Torr.

Kinetic analysis of a thermal dechlorination and oxidation of gadolinium oxychloride

Kinetics of a thermal dechlorination and oxidation of gadolinium oxychloride (GdOCl) originating from a molten salt process was investigated under various oxygen partial pressures by using a non-isothermal thermogravimetric (TG) analysis. The results of isoconversional analysis of the TG data suggests that the dechlorination and oxidation of GdOCl follows a single step reaction and the observed activation energy was determined as 137.7±4.1 kJ mol−1. The kinetic rate equation was derived for a conversion of the GdOCl with a linear-contacting boundary reaction model. The power dependency for oxygen and the pre-exponential factor was determined as 0.306 and 1.012 s−1 Pa−0.306, respectively.

Kinetic study of a thermal dechlorination and oxidation of neodymium oxychloride

This study investigated the kinetics of a thermal dechlorination and oxidation of neodymium oxychloride (NdOCl) by using a non-isothermal thermogravimetric (TG) analysis under various oxygen partial pressures. The results of the isoconversional analysis of the TG data suggests that the dechlorination and oxidation of NdOCl follows a single step reaction and the observed activation energy was determined as 228.3 ± 6.1 kJ mol −1. A kinetic rate equation was derived for a conversion of the NdOCl into Nd 2O 3 with a power law model, f( α) = 2/3 α −1/2. The oxygen power dependency and the pre-exponential factor were determined as 0.315 and 3.55 × 10 5 s −1 Pa −0.315, respectively.

COMBUSTION KINETICS OF POLYETHYLENE TEREPHTHALATE

The combustion kinetics of poly(ethylene terephthalate) (PET) was studied by the dynamic model which accounts for the thermal decomposition of polymer at any time. The kinetic analysis was performed by a conventional nonisothermal thermogravimetric (TG) technique at several heating rates between 10 and 40 K/min in air atmosphere. The thermal decomposition of PET in air atmosphere was found to be a complex process composed of at least two stages for which kinetic values can be calculated. The combustion kinetic analysis of PET gave apparent activation energy for the first stage of , with a value of for the second stage. To verify the effectiveness of the kinetic analysis method used in this work, the kinetic analysis results were compared with those of various analytical methods. The kinetic parameters were also compared with values of the pyrolysis of PET in nitrogen atmosphere.