Heating Rate Regimes Research Articles

Effect of heating rate on the kinetics of limestone calcination

This paper reports on the effect of heating rates between 0.1 °C/s and 205.0 °C/s on the thermodynamics and kinetics of limestone calcination in air. These heating rates are orders of magnitude higher than those used in the thermogravimetric analysers that have been reported previously (0.1 °C/s to 0.8 °C/s). It was found that the activation energy, E, is reduced by 59%, from 205.0 kJmol−1 to 84.5 kJmol −1, when the heating rate is increased from a low heating rate regime (<1 °C/s) to a moderate heating rate regime (99.2––205.0 °C/s). Additionally, the average reaction rate constant K¯ increases with an increase in the heating rate. As an example, for 205 °C/s, K¯ is 18.7 times higher than that for 0.2 °C/s, when approaching the same asymptotic temperature. It is also found that a higher heating rate increases the specific surface area (SSA), for the same Tas. This is caused by microfractures which are associated with decreased activation energy, and in turn increases the surface-to-volume ratios and speeds up conversion. The results highlight the importance of accounting for both heating rate and calcination time in the modelling of the calcination of limestone.Specific surface area measurements also revealed that the prolongation of calcination under moderate heating rate reduces the SSA, typically by a factor of 1.6 from 15.8 m2/g to 9.6 m2/g when the residence time tr is increased from 120 s to 240 s. Therefore, to maximize the SSA, the residence time should not be longer than that required for complete conversion.

Synergistic interactions and co-pyrolysis characteristics of lignocellulosic biomass components and plastic using a fast heating concentrating photothermal TGA system

Co-pyrolysis of lignocellulosic biomass (LCB) with plastic has gained significant attention recently. Studying the pyrolysis kinetic behavior of LCB and plastic is beneficial for developing a framework for designing and improving biofuel production. The co-pyrolysis behavior, synergistic interactions and kinetic triplet parameters of the three main LCB components, i.e., cellulose, hemicellulose and lignin, and two plastics (polyethylene terephthalate [PET] and polyvinyl chloride [PVC]) were evaluated using a fast heating concentrating photothermal TGA system. The maximum decrease in mass loss rate with increasing heating rates was observed for cellulose with PET (1.23–0.94%/°C) and with PVC (1.05–0.62%/°C). The mechanism of synergistic interaction between hemicellulose and PET proceeded with polymer degradation and conversions of monomer units into excess volatiles in the higher heating rate regime. Distributed Activation Energy Model [DAEM] and Coats-Redfern [CR]) were used to calculate the kinetic parameters. DAEM results confirmed that mixed samples required lower activation energy to start the reaction. Using the CR method, the first degradation phase showed the best synergistic effect for lowering the PET and PVC activation energy, particularly with hemicellulose and lignin.

New insights into the role of selective and volumetric heating during microwave extraction: Investigation of the extraction of polyphenolic compounds from sea buckthorn leaves using microwave-assisted extraction and conventional solvent extraction

We report a direct comparison of microwave heating and conventional heating in solvent extraction by using exactly the same reaction conditions (including heating rate) in the extraction of polyphenols from dried sea buckthorn leaves. We have for the first time decoupled the effects of bulk heating rate and mixing regime from the fundamental microwave heating mechanism. We show that although microwave selective heating can increase the yield and quality of the polyphenols extracted, if the same bulk heating rate is applied there is no difference in treatment time and therefore theoretical energy requirements of the process. The first implication of these results for process intensification is that if microwave selective heating can be enhanced in scaled up processes through electromagnetic design, the extract yield and quality may be increased further. The second implication is that conventional extraction processes could be designed to provide the same heating rate and hence treatment time as microwave extraction, but any potential energy and space savings would have to be balanced against the increase in capital cost and complexity of the equipment. That said, the very small penetration depth of microwaves into ethanol/water solvent also poses design challenges in the scale up of microwave equipment.

Effects of Biomass Particle Size on Slow Pyrolysis Kinetics and Fast Pyrolysis Product Distribution

Thermochemical conversion of lignocellulosic biomass is a promising technique to produce biofuels and intermediates. The effects of important parameters such as biomass particle size, shape, composition, heating rate, and residence time on the kinetics of devolatilization and bio-oil composition need to be understood thoroughly in order to successfully scale up the process. Pyrolysis of mixed wood sawdust of eight different particle sizes (26.5–925 µm) is conducted at nine different heating rates (0.5–100 °C min−1) in a thermogravimetric analyzer, and at fast heating rates (~10,000 °C s−1) in analytical pyrolyzer coupled with gas chromatograph/mass spectrometer. The apparent activation energies (Eα) evaluated by isoconversional Friedman method in the very slow (0.5–3 °C min−1), slow (5–20 °C min−1) and medium heating rate regimes (50–100 °C min−1) were 153–203, 174–251 and 286–380 kJ mol−1, respectively. The yield of phenolics and linear hydrocarbons decreased, while the production of gases like CO and CO2 increased with particle size during fast pyrolysis. High yield of aromatics was obtained with medium sized particles (362.5, 512.5 μm). This study demonstrates that Eα decreases and increases with particle size in the very slow and slow heating regimes, respectively, which is attributed to the effect of particle shape that induces mass transfer limitations in the transport of volatiles, and intraparticle thermal gradients that induce tar decomposition reactions.

Introduction to frictional pyrolysis (FP) – An alternative method for converting biomass to solid carbonaceous products

In the biomass sector, technologies like carbonization and torrefaction have been utilized for the production of solid carbonaceous biofuels and materials. In the framework of this manuscript a novel method for production of solid carbonaceous materials is introduced and is defined from now on as frictional pyrolysis. It uses only the application of pressure and friction whereas no external heat transfer is needed for the propagation of the process. This novel method is compared with torrefaction in order to assess its potential to process corn stover which has strongly bounder water and high content of deciduous xylan. Mass balances have been implemented for both technologies. Characterization of the products has been done by means of Simultaneous Thermal Analysis and Elemental analysis. Frictionally pyrolyzed corn stover has higher recovered mass yield, higher recovered energy yield and fixed carbon content than torrefied corn stover. Although external energy is provided by means of an internal combustion engine the net energy content of the final solid yield contained 92.2% of the input energy. The differential scanning calorimetry analysis showed that under the same heating rate regime and in oxygen-rich environment, the frictionally pyrolyzed corn stover had more exothermic decomposition than the torrefied material.

Effect of heating rate on the kinetics and mechanism of crystallization in amorphous Se85Te10Pb5 glasses

The kinetic parameters of amorphous Se85Te10Pb5 glasses were studied for nonisothermal and predicted isothermal conditions. Differential scanning calorimetry (DSC) was used to study the kinetics of the amorphous-to- crystalline transformation at heating rates in three regimes (3–15, 20–50 and 60–99Kmin−1). Analysis of the obtained data shows that the activation energy of crystallization is not constant but varies with the degree of conversion and, hence, with temperature; thus, the variation depends on the heating rate. The Avrami exponents indicate that the mechanism of crystallization varies from one- to two- to three-dimensional growth at low, medium and high heating rates, respectively. The reduced reaction model was observed to follow the Avrami–Erofeev mechanism, but varies from A1.5 to A2 to A3 for the different heating-rate regimes. A good fit was achieved between experimental and predicted sigmoidal curves for the crystallization fraction with increasing temperature.

A Comparative Analysis of the Kinetic Experiments in Polyethylene Pyrolysis

A review of literature has been conducted to survey the kinetic data of low-density polyethylene (LDPE) pyrolysis. The review reveals large variations in the reported global kinetic parameters. The cause of variation has been identified to be the difference in the experimental techniques, including thermogravimetric analysis (TGA) and non-TGA methods. Even within the nonisothermal TGA data, large variations have been observed at heating rate of 20 K/min, while the variations are insignificant at lower heating rate regimes (2–10 K/min), indicating the influence of heat/mass transfer resistance controlling the kinetics. Detailed analysis revealed that most of the current techniques are unable to capture all the relevant data necessary for estimating the kinetic parameters of the aforementioned process. The outcome of this review work thrusts the need for a better experimental technique to estimate the kinetic parameters of complex reactions, such as polymer pyrolysis.

Pre-ignition laser ablation of nanocomposite energetic materials

Laser ignition of energetic material composites was studied for initiation with heating rates from 9.5 × 104 to 1.7 × 107 K/s. This is a unique heating rate regime for laser ignition studies because most studies employ either continuous wave CO2 lasers to provide thermal ignition or pulsed Nd:YAG lasers to provide shock ignition. In this study, aluminum (Al) and molybdenum trioxide (MoO3) nanoparticle powders were pressed into consolidated pellets and ignited using a Nd:YAG laser (1064 nm wavelength) with varied pulse energy. Results show reduced ignition delay times corresponding to laser powers at the ablation threshold for the sample. Heating rate and absorption coefficient were determined from an axisymmetric heat transfer model. The model estimates absorption coefficients from 0.1 to 0.15 for consolidated pellets of Al + MoO3 at 1064 nm wavelength. Ablation resulted from fracturing caused by a rapid increase in thermal stress and slowed ignition of the pellet.

Effects of pyrolysis conditions on the heating rate in biomass particles and applicability of TGA kinetic parameters in particle thermal conversion modelling

A one-dimensional single particle model is utilised to investigate the effects of radiation temperature, moisture content, particle size and biomass physical properties on the heating rate in biomass particles during pyrolysis. The model divides the particle into four layers – drying, pyrolysis, char and ash layer – corresponding to the four main stages of biomass thermal conversion. The average of the time derivative of the pyrolysis layer centre temperature weighted by the pyrolysis rate is introduced as an appropriate indicator for the heating rate in the particle during pyrolysis. The influencing parameters on the heating rate are summarised in the Biot number and the thermal time constant, to make the investigation of their effects easier. The heating rate is inversely proportional to the thermal time constant. The effect of a variation of the Biot number on the heating rate is negligible in comparison to the thermal time constant. Therefore, the thermal time constant can be sufficiently used to specify the heating rate regimes during pyrolysis. It is found that for thermal time constants of more than 50s, pyrolysis takes place in a low heating rate regime, i.e. less than 50K/min. Additionally, the heating rate during pyrolysis of various biomass types under a wide range of thermal conversion conditions has been examined, in order to classify the heating rate regime of pyrolysis in state-of-the-are combustion/gasification plants. The pyrolysis of wood dust and wood pellets is found to happen always in high heating rate regimes. Therefore, the kinetic parameters obtained by conventional TGA systems (typically with heating rates lower than 50K/min) are not applicable for them. On the contrary, the pyrolysis of wood logs always happens in low heating rate regimes, which indicates that kinetic parameters obtained by conventional TGA systems can be applied. However, pyrolysis of wood chips can undergo low or high heating rate regimes depending on their particle size. Concerning the moisture content, it can be stated that it does not strongly influence the heating rate regime of certain biomass particles.

Non-isothermal kinetics of melting and nematic to isotropic phase transitions of 5CB liquid crystal

This work reports a non-isothermal kinetics of the melting and the nematic to isotropic (N–I) phase transitions of the pentylcyanobiphenyl (5CB) liquid crystal compared with octylcyanobiphenyl (8CB) liquid crystal using calorimetric technique. Temperature scans and heating rate scans were performed for 5CB and 8CB from 280 to 333 K at various rates using differential scanning calorimetry from 0.5 to 20 K min−1. Double activation was observed for 5CB for two heating rate regimes whereas 8CB indicated single activation only. The 5CB has smaller enthalpy and entropy of the transitions and needs larger activation than 8CB. This kinetic change can be explained in terms of the length scale and mobility of the liquid crystal molecules.

On the Thermal Expansion Behavior of Polystyrene/Polyethyleneterephthalate Blend Systems: Experimental Study

Thermal expansion behavior defined as the percentage of dimensional change upon heating for the blend systems of polystyrene/polyethyleneterephthalate has been investigated in this study. General purpose polystyrene (PS) and high impact polystyrene (HIPS) were blended with polyethyleneterephthalate (PET) at a weight ratio of (15/85). The dimensional change of the neat and blended polymers versus temperature is measured by a thermomechanical analyzer using an expansion probe. The study investigates thermal expansion behavior of the PS/PET and HIPS/ PET blend systems under various conditions of heating rate, load, and heating regime. Higher thermal expansion values for the neat polymers and the blends were observed at lower heating rate, i.e., 5°C/min, while increasing load did not seem to affect thermal expansion behavior of the materials except for neat PET and HIPS/ PET blend system at higher temperature range, i.e., beyond Tg of PET and near softening temp of HIPS, where thermal expansion values decreased as load increased. Neat polystyrenes exhibited noticeable thermal expansion and contraction behavior when tested isothermally. The blend systems formulated and tested in this study, which consisted 15/85 wt% of both PS/PET and HIPS/PET, have proven to be more thermally stable and have better thermal expansion behaviors in both heating regimes used, i.e., nonisothermal and isothermal, than those behaviors observed for the neat polymers especially the minor phase, i.e., the polystyrenes.

Heat transfer in symmetric U-shaped microreactors for thin film calorimetry

We describe the results of two-dimensional finite difference analysis of the thermal profile, in both transient and steady state, of a symmetric U-shape designed high-sensitive nanocalorimeter. The thin film calorimeter, with a heat capacity of 100 nJ K−1 at room temperature, consists of a 180 nm thick freestanding silicon-rich nitride membrane on which thin film heaters and sensors are deposited. Simulated temperature profiles are in good agreement with in situ experimental data obtained at the heater and sensor locations. The first-order solid-to-liquid transition of indium films, from a few Å to hundreds of nm thick, was used as an experimental reference of the thermal profiles obtained from the 2D modeling. Temperature differences inside the sample region induced by the symmetric U-shape design of the Pt heaters limit the use of the nanocalorimeter to two different heating rate regimes. At low heating rates, β < 10 K s−1, especially with a thermal layer, the temperature profile is reasonably flat so that small samples can be characterized in power compensation mode. At heating rates faster than 4 × 104 K s−1 the nanocalorimeter works in adiabatic mode and measures transitions occurring in the sample directly loaded underneath the heater.

Plume heat flow in a numerical model of mantle convection with moving plates

Abstract Self-consistent numerical models are presented for a two-dimensional thermal convection with moving plates to estimate the heat flow transported by hot upwelling plumes in both internally and basally heated mantle. The plume heat flow depends on the internal heating rate and convective regimes but does not exceed 30% of basal heat flow when the plates rigidly move as observed on the Earth; the fraction of plume heat flow to basal heat flow becomes higher when the lithosphere behaves as a stagnant lid or as a mechanically not so strong plate. When estimated from the dynamic topography of plume-swells above hot upwelling plumes, the plume heat flow appears to be only a small fraction of the real plume heat flow. As a whole, the apparent plume heat flow estimated from dynamic topography remains only a few percent of the total heat flow on the surface boundary, the value observed for the Earth, even when the basal heating rate exceeds 50% of the total heating rate. Such a high fraction of basal heating rate is necessary for the amplitude of topography above hot upwelling plumes to become comparable to the value observed for the Earth, too. It is necessary to reexamine the earlier inference that the heat released from the core manifests itself as plume heat flow in the Earth.

Lack of synergetic effects in the pyrolytic characteristics of woody biomass/coal blends under low and high heating rate regimes

Pyrolytic behaviours of woody biomass/coal mixtures were investigated under both low and high heating rate conditions over a range of temperatures between 200°C and 1400°C. Results obtained from this comprehensive investigation indicated that the pyrolytic characteristics of the mixtures follow those of the parent materials in an additive manner. Therefore, under inert (non-oxidising) conditions the two fuels undergo independent thermal conversion without any chemical interactions. As such, the yield of the major pyrolysis products (e.g. volatiles and char) is proportional to the percentage of woody biomass and coal in the mixture. This confirms the hypothesis made by a number of researchers about the lack of synergistic effects in the yield of pyrolysis products from blended coal and woody biomass. However, in this study, we show that even the compositions of the gaseous products from blended samples are linearly proportional to those of their parent fuels (lack of synergistic effects). These findings can potentially help to understand and predict the behaviour of woody biomass/coal blends in practical combustion systems.

Approximate modelling of coal pyrolysis

A new approach to the modelling of the evolution of different chemical species from coal during its thermal decomposition under variable heating rate regimes was developed. The approach, which can be based on experimental data or the multiple-reaction model (MRM) with distributed activation energies, uses an Nth order reaction model where the pre-exponential factor and the apparent activation energy are functions of the rate of heating. A comparison of the predictions of the new model with the MRM was carried out. The new approach significantly cuts down the computation time with almost no loss of accuracy. The functional group pyrolysis model was examined and the parameters for the new Nth order approximation obtained considerably simplifying pyrolysis modelling. In addition, the article shows how to obtain estimates for certain parameters, which characterize chemical processes modelled by the MRM.

Pressureless Sintering of B&lt;sub&gt;4&lt;/sub&gt;C-TiB&lt;sub&gt;2&lt;/sub&gt; Ceramic Composites

Pressureless sintering of the B 4 C ceramics containing 0-25 vol.% TiB 2 phase was studied in the isothermal and constant heating rate regimes. The B 4 C-TiB 2 composites were pressureless sintered to a relative density of 97.5% at 2100°C. It was found that the heating rate had no visible effect on the density. A new approach to the constant heating rate analysis taking into account the coarsening kinetics was developed. It was shown that the sintering process is controlled by coarsening of TiB 2 particles as well as by grain boundary diffusion. The effective activation energy for sintering was estimated. The isothermal microstructural evolution at the final stage of sintering was discussed.

Rate-controlled synthesis and sintering of nanocrystalline barium titanate powder

The densification and grain size during sintering as well as particle size, of the barium titanate powder during Pechini decomposition-synthesis depends on the heating rate and this dependence originates from the kinetic competition between elementary transformation mechanisms, inherent in the two processes. The given competition is the main reason for development of the optimum mode of the mentioned thermal activated processes. As developed at the NCSU (USA) the Rate-Controlled Sintering (RCS) is known to allow the obtaining of dense and finegrained ceramics with improved properties. As developed at the IPMS (Ukraine) the Rate-Controlled Synthesis is directed to prepare nanocrystalline unagglomerated ceramic powders suitable for RCS. The extreme behavior of the BaTiO 3 particle size as a function of heating rate is established and the optimal temperature-time path is calculated and verified to obtain the best powder of 20–25 nm particles. The change in microstructure and phase composition with heating rate is considered in details. The advantages of the RCS to achieve density of 99.9% and grain size around 100–150 nm are presented in comparison with the linear heating rate regime.

Constraints upon the use of 24-ethylcholestane diastereomer ratios as indicators of the maturity of petroleum

We have measured the ratio 20 S/(20 S + 20 R), the relative abundances of the 24-ethylcholestane diastereomers, in nine pairs of closely adjacent samples of coals and shales which represent a range of geological ages and different sedimentary basins. Within some sample pairs, the values were similar, in others they were very different. The differences between the values for coal and shale members of pairs are attributed to effects of sediment matrix type upon the processes which cause the changes in the ratio. The kinetic parameters ( E and ΔS) for the changes in the 20 S/(20 S + 20 R) ratio have been estimated using previously published measurements for the Hod #1 well (shale) and Jupiter #1 well (coal), and these values were found to be very different. The process in the coal sequence has a much lower activation energy, and is therefore less responsive to differences in heating rate regimes than is the case for the shale sequence. Values of the 20 S/(20 S + 20 R) ratio in two other sequences are shown to be consistent with this explanation, and the implications of these observations for the practice of using sterane isomer ratios for assessment of maturity of sediments and crude oils are discussed.

Desorption from an immobile adlayer: Exact one-dimensional kinetics

We consider irreversible unimolecular desorption from a one-dimensional immobile unequi-librated adlayer where nearest neighbor adspecies interactions result in configuration dependent microscopic desorption rates. The master equations for this process are solved exactly . For isothermal desorption from island-forming or clustering systems with attractive interactions, we find a strong coverage dependence of the order of the desorption kinetics, and also a transition in the (very) low coverage kinetics with varying temperature, T . For the strongly clustering regime unusual scaling behavior of the effective desorption energy and of the order is elucidated. Temperature programmed desorption is considered for both attractive and repulsive interactions. In the latter case, triple peaked spectra indicating “staged desorption” are observed in the low heating rate regime.