Analysis of pyrolysis and gasification reactions of hydrothermally and supercritically upgraded low-rank coal by using a new distributed activation energy model

Hydrothermal water treatments and supercritical (SC) water treatments of a lignite were performed to examine the feasibility of upgrading low-rank coals. The treatment below 400°C was found to be effective enough to keep high gasification reactivity at high temperature, as well as to suppress spontaneous combustion. The pyrolysis and gasification behaviors of raw and pretreated coals were examined by thermogravimetry (TG). The kinetic analysis was carried out based on a new distributed activation energy model (DAEM) presented by Miura [K. Miura, Energy & Fuels, (12), 864–869 (1998).]. According to this method, thermogravimetric curves measured at two or more different heating rates were needed to obtain the activation energy distribution function f(E) of a given coal sample. It was found that in the case of pyrolysis, the peak values of f(E) curves for upgraded coal samples are nearly 300 kJ/mol, whereas, the peak value of f(E) curve for their parent coal is about 200 kJ/mol. In the case of gasification, where only single reactions occur, the application of this new DAEM can give the changes of activation energy during reaction. Some interesting results occur, which may hint at some changes in the rate-controlling step of reaction or in the physical structure of coal during gasification.     

Pyrolysis of wheat straw in a thermogravimetric analyzer: Effect of particle size and heating rate on devolatilization and estimation of global kinetics

The influence of different parameters such as particle size, initial weight of the sample, and heating rate on the devolatilization of wheat straw particles have been studied using thermogravimetric analysis. In addition, the variations in proximate analysis with different particle sizes of wheat straw have also been investigated. Results show that the curves corresponding to the third stage of pyrolysis differ for variations in particle size, initial weight, and heating rate of the pyrolysis process. A devolatilization model with three parallel nth-order reactions has been considered to determine the global kinetic parameters using thermogravimetric data. The kinetic parameters such as activation energy (kJ/mol), frequency factor (1/min), and order of the reaction for the three stages considered in devolatilization model were E₁ = 69, E₂ = 78, E₃ = 80; k₀₁ = 2.57 × 10¹², k₀₂ = 3.97 × 10⁷, k₀₃ = 3.17 × 10⁶; and n₁ = 2.3, n₂ = 0.65, n₃ = 2.7, respectively. It was noted from the order of the reaction that the second stage of the pyrolysis curve corresponds to the degradation of cellulose and hemicellulose, and the third stage to the lignin degradation.     

Free-radical propagation and termination kinetics of the butyl acrylate dimer studied by pulsed laser polymerization techniques

The propagation and termination rate coefficients for bulk polymerization of the butyl acrylate dimer (BA dimer) are determined by pulsed laser techniques. The rate coefficient for propagation, k_p, is deduced for temperatures from 20 to 90 °C via the pulsed laser polymerization-size exclusion chromatography (PLP-SEC) method at pulse repetition rates between 1 and 10 Hz. The Arrhenius parameters were found to be: E_A(k_p) = (34.2 ± 1.0) kJ mol⁻¹ and A(k_p)/L mol⁻¹ s⁻¹ = (1.08 ± 0.49) × 10⁷ L mol⁻¹ s⁻¹. The termination rate coefficient, k_t, has been measured via SP-PLP-ESR, single pulse-pulsed laser polymerization in conjunction with time-resolved electron spin resonance detection of radical concentration. The resulting Arrhenius parameters as deduced from the temperature range −15 to +30 °C are: E_A(〈k_t〉) = (22.8 ± 3.7) kJ mol⁻¹ and log(A/L mol⁻¹ s⁻¹) = 10.6 ± 1. The chain-length dependence of k_t was studied at 30 °C. For short chains a significant dependence was found which may be represented by an exponent α = 0.79 in the power-law expression k_t(i) = k_t⁰i^−α.     

Theory of square-wave voltammetry of quasireversible electrode reactions using an inverse scan direction

The standard rate constant of a simple electrode reaction Ox + ne⁻ ↔ Red, in which both Ox and Red are solution soluble, can be determined by the variation of frequency in the square-wave voltammetry with inverted scan direction: log k_s = log f₀^1/2 + log D^1/2 − 0.60 ± 0.01. In this equation log f₀ is the abscissa of the intersection of straight lines E_p,2 = a log f + b and E_p,2 = E⁰, where E_p,2 is the potential of the second peak of the net response, E⁰ is the standard potential, a = 2.3RT/2(1 − α)nF, b = E⁰ − 2a log k_s + a log D − 0.0353/(1 − α)n and D is a common diffusion coefficient.     

Study of pyrolysis kinetics of oil shale

The pyrolysis experiments on oil shale samples from Fushun, Maoming, Huangxian, China, and Colorado, USA, were carried out with the aid of thermogravimetric analyzer (TGA) at a constant heating rate of 5 °C/min. A kinetic model was developed which assumes several parallel first-order reactions with changed activation energies and frequency factors to describe the oil shale pyrolysis. The kinetic parameters of oil shale pyrolysis were determined on the basis of TGA data. The relationship between the kinetic parameters was further investigated and the correlation equations of x_∞-E and ln A-E were obtained. These equations show that the final fractional conversion of each parallel reaction, x_∞(j), can be expressed as an exponential function of the corresponding activation energy. The plot of ln A-E for different reactions becomes a straight line. These relationship equations can provide important information to understand the pyrolysis mechanism and to investigate the chemical structure of oil shale kerogen.     

More studies on the PVOH-LiH2PO4 polymer system

Polymer electrolytes based on poly(vinyl alcohol) (PVOH) and lithium dihydrogen-phosphate (LiH₂PO₄) with molar ratio of x = 0.07, 0.10 and 0.14 were prepared in order to investigate the mechanism of ionic motion. Admittance spectroscopy measurements were used to study electrical conductivity relaxation on both anhydrous and hydrated samples in the 5 Hz to 13 MHz frequency range and temperatures ranging from 25 to 150 °C. The conductance, G, shows dispersion above a crossover frequency, f_p. This behavior is typical of systems in which correlated ionic motions in the bulk material are responsible for ionic conductivity. For hydrated samples, results reveal that the temperature dependence of the dc-conductivity, σ₀ and the characteristic frequency, f_p, shows Arrhenius-type behavior with the same energy, E_σ. However, for anhydrous conductivity, a Vogel-Tamman-Fulcher (VTF) behavior is shown for both σ₀(T) and f_p(T), with the same pseudo activation energy, B and B_σ, respectively, thus indicating that they are correlated with chain mobility.     

Transesterification of leather tanning waste to biodiesel at supercritical condition: Kinetics and thermodynamics studies

Catalyst-free transesterification of leather tanning waste with high free fatty acid (FFA) content at supercritical condition was reported in this work. The experiments were performed in batch system at various temperatures (250–325 °C) under constant pressure of 12 MPa and methanol/fatty oil molar ratio of 40:1 for reaction time of 2–10 min. Kinetic modeling of formation of fatty acid methyl esters (FAMEs) that incorporate reversible esterification and non-reversible transesterification simultaneously was verified. The proposed semi-empirical model was fitted against kinetic experimental data over temperature range studied. The kinetic parameters (i.e. k′_TE, k′_E, and k_E′) were determined by nonlinear regression fitting. Thermodynamic activation parameters of the reactions were evaluated based on activation complex theory (ACT) and the following results are obtained: ΔG³ > 0, ΔH³ > 0, and ΔS³ < 0. The activation energy (E_a) of transesterification, forward and reverse esterification reactions was 36.01 kJ/mol, 28.38 kJ/mol, and 5.66 kJ/mol, respectively.     

Free volume in poly(n-alkyl methacrylate)s from positron lifetime and PVT experiments and its relation to the structural relaxation

Free volume data from positron annihilation lifetime spectroscopy (PALS) experiments are combined with a Simha-Somcynsky (S-S) equation of state analysis of pressure-volume-temperature (PVT) data to model free volume contributions to structural mobility in a series of poly(n-alkyl methacrylate)s. From the PALS data the glass transition temperature, T_g, decreases (from 382 to 224 ± 5 K) and a given mean free volume is observed at lower temperatures as the side-chain length increases (going from methyl- to hexyl-). This is evidence of an internal plasticization whereby the side-chains reduce effective packing of molecules. By comparing PALS and PVT data, the hole number per mass unit, N_h′, is calculated using different methods; this varies between 0.54 and 0.86 × 10²¹ g⁻¹. It is found that the extrapolated free volume becomes zero at a temperature T₀′ that is smaller than the Vogel temperature T₀ of the α-relaxation. The α-relaxation frequencies can be fitted by the free volume theory of Cohen and Turnbull, but only when the free volume V_f is replaced by (V_f − ΔV) where ΔV( = E_f(T₀ − T₀′), E_f is the thermal expansivity of V_f) varies between 0.060 and 0.027 ± 0.003 cm³/g, decreasing with side-chain length, apart from poly(n-hexyl methacrylate) where ΔV increases to 0.043 ± 0.003 cm³/g. One possible interpretation of this is that the α-relaxation only occurs when, due to statistical reasons, a group of m or more unoccupied S-S cells are located adjacent to one another. m is found to vary between 8 and 2 for poly(methyl methacrylate) and poly(n-butyl methacrylate), respectively. We found that no specific feature in the free volume expansion was consistently in coincidence with the dynamic crossover.     

Synthesis and characterization of the LDH hydrotalcite-pyroaurite solid-solution series

A layered double hydroxide (LDH) hydrotalcite-pyroaurite solid-solution series Mg₃(Al_xFe_1 − x)(CO₃)_0.5(OH)₈ with 1 − x = 0.0, 0.1……1.0 was prepared by co-precipitation at 23 ± 2 °C and pH = 11.40 ± 0.03. The compositions of the solids and the reaction solutions were determined using ICP-OES (Mg, Al, Fe, and Na) and TGA techniques (CO₃²⁻, OH⁻, and H₂O). Powder X-ray diffraction was employed for phase identification and determination of the unit cell parameters a_o and c_o from peak profile analysis. The parameter a_o = b_o was found to be a linear function of the composition. This dependency confirms Vegard's law and indicates the presence of a continuous solid-solution series in the hydrotalcite-pyroaurite system. TGA data show that the temperatures at which interlayer H₂O molecules and CO₃²⁻ anions are lost, and at which dehydroxylation of the layers occurs, all decrease with increasing mole fraction of iron within the hydroxide layers.Features of the Raman spectra also depend on the iron content. The absence of Raman bands for Fe-rich members (x_Fe > 0.5) is attributed to possible fluorescence phenomena.Based on chemical analysis of both the solids and the reaction solutions after synthesis, preliminary Gibbs free energies of formation have been estimated. Values of ΔG°_f(hydrotalcite) = − 3773.3 ± 51.4 kJ/mol and ΔG°_f(pyroaurite) = − 3294.5 ± 95.8 kJ/mol were found at 296.15 K. The formal uncertainties of these formations constants are very high. Derivation of more precise values would require carefully designed solubility experiments and improved analytical techniques.     

The kinetic study of the curing reaction of mono- and di-epoxides obtained during the reaction of divinylbenzene and hydrogen peroxide with acid anhydrides

In this paper, the non-isothermal differential scanning calorimetry (DSC) was employed to investigate the cure process and to determine the kinetic parameters of the curing reactions of mono- and di-epoxides with maleic and glutaric anhydrides. The epoxides were obtained during the epoxidation process of commercially available divinylbenzene by using 60% hydrogen peroxide as the oxidant in the presence of organic solvents and magnesium oxide as the catalyst. It was found that the cure process of epoxides with maleic anhydride was described through higher values of enthalpy of polymerization (ΔH_R) and lower temperatures of the cure initiation (T_onset), the peak maximum temperature (T_max) and the final cure temperature (T_end). It can be considered to accelerate the rate of reaction and lead to an excellent network structure when maleic anhydride was used as curing agent. The kinetic analysis was firstly computed using a model free-estimation of the activation energy (Friedman, Ozawa-Flynn-Wall methods) and then the multivariate non-linear regression with a 6th degree Runge-Kutta process in a modified Marquardt procedure was employed to calculate the corresponding kinetic parameters (E_i, n_i, A_i) using the nth-order reaction f(α). The unbranched three-step process of the nth-order reaction f(α) for each step was the best to describe the cure process of mono- and di-epoxide with acid anhydrides. The determined values of the activation energies were in the range 64.7-105.2 kJ/mol for epoxides/glutaric anhydride system and 64.7-82.7 kJ/mol when maleic anhydride was used as hardening agent.     

Dynamic relaxation characteristics of Matrimid polyimide

The dynamic relaxation characteristics of Matrimid^® (BTDA-DAPI) polyimide have been investigated using dynamic mechanical and dielectric methods. Matrimid exhibits three motional processes with increasing temperature: two sub-glass relaxations (γ and β transitions), and the glass—rubber (α) transition. The low-temperature γ transition is purely non-cooperative, and displays an identical time-temperature response to both the dynamic mechanical and the dielectric probes with a corresponding activation energy, E_A = 43 kJ/mol. The β sub-glass transition shows a more cooperative character as assessed via the Starkweather method. Comparison of dynamic mechanical and dielectric data for the β process suggests that the dynamic mechanical test (E_A = 156 kJ/mol) is sensitive to a broader, more strongly correlated range of sub-glass motions as compared to the dielectric probe (E_A = 99 kJ/mol). Time-temperature superposition was used to establish mechanical master curves across the glass-rubber (α) relaxation, and these data could be described using the Kohlrausch-Williams-Watts function with an exponent value, β_KWW = 0.34. The corresponding shift factors were used as the basis of a cooperativity plot for the determination of dynamic fragility. The relation between fragility index (m = 115) and β_KWW for the Matrimid polyimide was in good agreement with the wide correlation reported in the literature.     

Structure, phase transition and electric properties of poly(vinylidene fluoride-trifluoroethylene) copolymer studied with density functional theory

The geometry, energy, internal rotation, vibrational spectra, dipole moments and molecular polarizabilities of poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE)) of α- and β-chain models were studied with density functional theory at B3PW91/6-31G(d) level and compared with those of the poly(vinylidene fluoride) (PVDF) homopolymer. The chain length and the trifluoroethylene (TrFE) concentration were examined to discuss the copolymer chain stabilities, chain conformations and electric properties. The asymmetrical internal-rotation potential energy curve shows that the angles for the g and g′ conformations in the α-chain (tg and tg′) models are 53° and −70°, respectively, and the β-chain (ttt) conformation is a slightly distorted all-trans plane with dihedral angle at 177°. The energy differences, E_β − E_α(g) and E_β − E_α(g′), between the β- and the α-conformation are 2.1 and 7.8 kJ/mol, respectively. These values are smaller than that in PVDF (8.4 kJ/mol), suggesting that the β-conformation in the copolymer will be more stable than in PVDF. The energy barriers for β → α(g) and β → α(g′) transitions are 16.2 and 5.8 kJ/mol, respectively. The former is almost twice of the energy barrier in PVDF by 8.2 kJ/mol and the latter is slightly smaller (by 2.4 kJ/mol) than that in PVDF. The respective energy barriers for α(g) → β and α(g′) → β transitions are 18.3 and 13.6 kJ/mol compared with the value 16.3 kJ/mol in PVDF. The asymmetrical energy barriers may be one of the reasons for the copolymers with 0.5-0.6 (mole fraction) VDF exhibiting complicated phase transition behavior. The conformation of α-chain P(VDF-TrFE) exhibits from a helical (containing higher TrFE) to a nearly beeline (containing lower TrFE). This behavior is different from that in the PVDF and the nearly beeline conformation might be responsible for the increasing crystallizability. The helical might also be associated with the complicated phase transition behavior and the larger lattice strain in the P(VDF-TrFE)s with higher TrFE concentration. The energy difference per monomer unit between the β- and α-chain decreases with increasing TrFE content. The ideal β-chain is curved with a radius of about 30 Å, which is similar to that in PVDF. The chain curvature and the TrFE content will affect the dipole moment contribution per monomer. The chain length and TrFE content will not significantly affect the mean polarizability. The calculations indicated that there are some additional characteristic vibrational modes that may be used in identification of the α- or β-phase P(VDF-TrFE)s with different TrFE contents.     

TG study on pyrolysis of biomass and its three components under syngas

Pyrolysis of sawdust and its three components (cellulose, hemicellulose and lignin) were performed in a thermogravimetric analyzer (TGA92) under syngas and hydrogen. The effect of different heating rates (5, 10, 15 and 20 °C/min) on the pyrolysis of these samples were examined. The pyrolysis tests of the synthesized samples (a mixture of the three components with different ratios) were also done under syngas. The distributed activation energy model (DAEM) was used to study the pyrolysis kinetics. It is found that syngas could replace hydrogen in hydropyrolysis process of biomass. Among the three components, hemicellulose would be the easiest one to be pyrolyzed and then would be cellulose, while lignin would be the most difficult one. Heating rate could not only affect the temperature at which the highest weight loss rate reached, but also affect the maximum value of weight loss rate. Both lignin and hemicellulose used in the experiments could affect the pyrolysis characteristic of cellulose while they could not affect each other obviously in the pyrolysis process. Values of k₀ (frequency factor) change very greatly with different E (activation energy) values. The E values of sawdust range from 161.9 to 202.3 kJ/mol, which is within the range of activation energy values for cellulose, hemicellulose and lignin.     

A 1-D non-isothermal dynamic model for the thermal decomposition of a gibbsite particle

A 1-D mathematical model describing the thermal decomposition, or calcination, of a single gibbsite particle to alumina has been developed and validated against literature data. A dynamic, spatially distributed, mass and energy balance model enables the prediction of the evolution of chemical composition and temperature as a function of radial position inside a particle. In the thermal decomposition of gibbsite, water vapour is formed and the internal water vapour pressure plays a significant role in determining the rate of gibbsite dehydration. A thermal decomposition rate equation, developed by closely matching experimental data reported previously in the literature, assumes a reaction order of 1 with respect to gibbsite concentration, and an order of −1 with respect to water vapour pressure. Estimated values of the transformation kinetic parameters were k₀ = 2.5 × 10¹³ mol/(m³ s) for the pre-exponential factor, and E_a = 131 kJ/mol for the activation energy. Using these kinetic parameters, the gibbsite particle model is solved numerically to predict the evolution of the internal water vapour pressure, temperature and gibbsite concentration. The model prediction was shown to be very sensitive to the values of heat transfer coefficient, effective diffusivity, particle size and external pressure, but relatively less sensitive to the mass transfer coefficient and particle thermal conductivity. The predicted profile of the water vapour pressure inside the particle helps explain some phenomena observed in practice, including particle breakage and formation of a boehmite phase.     

Immobilization of polyvinylacetate macromolecules on hydroxyapatite nanoparticles

Concentration dependence of the storage modulus, E′, was investigated for polyvinylacetate (PVAc) filled with hydroxyapatite (HAP) nanoparticles. The filler volume fraction, v_f, varied from 0 to 0.05 and the E′ and loss tangent, tan δ, were measured below neat matrix T_g at −40 °C and above neat matrix T_g at +50 °C at 1 Hz. The T_g determined as the position of the maximum on the temperature dependence of tan δ increased by 14 °C compared to the neat PVAc (39 °C) by adding 5 vol.% of HAP. At −40 °C, the observed small increase of E′ with v_f was in agreement with the prediction based on the simple Kerner equation. At +50 °C, the increase of E′ with v_f observed was an order of magnitude greater than that predicted using the simple continuum mechanics model. An attempt was made to explain the observed deviation employing the hypothesis of immobilized entanglements.     

Effect of two stage, tray and heat pump assisted-dehumidified drying on drying characteristics and qualities of dried ginger

Desorption isotherms for sliced gingers have been measured. A non-linear regression programme was used to fit four moisture sorption isotherm models to the experimental data. The Modified Halsey and Modified Oswin models gave the best fit for X_e = f(RH_e, T) and RH_e = f(X_e, T), respectively. Tray and heat pump dehumidified drying incorporated by single and two stage drying were conducted. It was found that the modified Page model was the most effective. The drying constant was fitted to drying air temperature using the Arrhenius model. Effective moisture diffusivities were determined using the drying data. The heat pump dehumidified drying incorporated by the two stage drying could reduce the drying time at 40 °C by 59.32% and increase 6-gingerol content by 6%. Quality evaluation by 6-gingerol content, rehydration ratio and ΔE* showed the best quality for dried sliced gingers in the heat pump dehumidified drying incorporated by the two stage drying at 40 °C.     

Long-range forces between C nanotubes and between C cages: Some polarizability bounds and scaling approximations yielding interaction energies at intermediate separations

Recent work of Schröder and Hyldgaard leads us to propose for the interaction energy per unit length ?(d, R) between two parallel C nanotubes of equal radii R at distance d, the approximate scaling property ?(d, R) = f(R)d⁻⁵I(R/d) where I is known. The function f(R) is expected to be model dependent. Contact is then made with the classic study of Girifalco et al. In addition, some results are presented for interaction energy between ‘almost spherical’ C cages. Specifically considered are (i) the HOMO-LUMO gap and (ii) bounds on polarizabilities.     

Co-combustion of rice husk with coal in a cyclonic fluidized-bed combustor (ψ-FBC)

Thailand is well-endowed with renewable energy resources. In Thailand, rice husk, a by-product of the rice-milling process and one of the most potentially sustainable cultivated biomasses, has an annual energy equivalent of 6.6 × 10⁷ GJ. Using rice husk alone, however, can be problematic, particularly if there is a deficit during the off-season. Coal, the most abundant fossil fuel, has thus been considered an appropriate supplementary fuel. This paper describes the combustion characteristics of co-firing rice husk with bituminous coal in a 120 kW_th-capacity cyclonic fluidized-bed combustor (ψ-FBC), and how excess air ratios and fuel blends impacted emissions and combustion efficiency (E_c). Overall, excess air and blending ratios did not have tremendous effects on E_c, easily achieving >97%. Radial temperature profiles revealed that vortex combustion prevailed along the combustor walls. Concurring with axial temperature profiles, axial O₂ profiles suggested that the combustion was confined chiefly to regions under the vortex ring. Despite massive CO production in the lower section, CO emissions were satisfactory (range 60-260 ppm, at 6% O₂). Due to the high bed temperatures, NO_x appeared rather high (260-416 ppm, at 6% O₂). Not only were NO_x emissions affected by coal ratio, it were also highly reliable on the operating conditions. SO₂ emissions varied directly, but not proportionally, with the sulfur content of the fuel mixture.     

Supercritical water oxidation of coal: Investigation of operating parameters' effects, reaction kinetics and mechanism

Supercritical water oxidation (SCWO) of coal was conducted in a continuous tubular reactor under various reaction conditions. Our experimental results show that the removal rate of chemical oxygen demand (COD) had no significant dependence on the temperature variations. Effect of residence time was less significant as exceeded fixed values. Free radical mechanism of SCWO reaction may be a possible explanation for the relative low conversion rate of coal at the range of tested oxygen excess. Compared with other parameters, effect of pressure was less significant. A global power-law rate expression was regressed from experimental data. The reaction orders for coal slurry and oxidant were 1.79 and 0.28 respectively. The reaction activation energy E_a was determined to be 112.3 kJ mol⁻¹, and the pre-exponential factor k₀ was 412 (mol/L)^−1.07 s⁻¹. The deviation between the model and experimental data was within ± 9%. Free radical mechanism, oxidation and hydrolysis mechanisms and phenolic hydroxyl oxidation mechanism were considered to be the possible mechanisms for the SCWO process of coal.     

Study of sawdust pyrolysis and its devolatilisation kinetics

Pyrolysis of sawdust was studied using a thermogravimetric analyser (TGA) to understand the devolatilisation process and to obtain its global kinetic parameters. The influences of particle size, initial weight of the sample and heating rate on the devolatilisation of sawdust particles have been studied. Results from proximate analysis show that smaller particle size has more ash content compared to larger particle size. The TG and derivative TG curve for variation in particle size and initial weight of the sample showed significant difference in the third stage of the pyrolysis. In addition, the pyrolysis of sawdust differed significantly for variation in heating rate. As the heating rates increased, the char yield also increased. The devolatilisation kinetics was studied considering different stages of pyrolysis. The kinetic parameters for thermal devolatilisation of the sawdust were determined through a nonlinear optimisation method of two independent parallel nth‐order reaction models. The kinetic parameters such as activation energy, frequency factor and order of the reaction for the two stages considered in the model were: E₂ = 79.53 (kJ/mol), E₃ = 60.71 (kJ/mol); k₀₂ = 1.90 × 10⁶ (1/min), k₀₃ = 1.01 × 10³ (1/min); n₂ = 0.91, n₃ = 1.78, respectively. The results show good agreement between the proposed model and the experimental data of the sawdust pyrolysis.