Aims and methods in non-isothermal reaction kinetics

The majority of the works dealing with non-isothermal kinetics assumes only one kinetic differential equation, uses linearization techniques and do not check the fit between the simulated and the experimental data. It is not clear from the literature why we need kinetic evaluations at all in this field. Due to this controversial situation, the author has outlined his views on the aims and methods of the non-isothermal kinetics in this paper. Accordingly, the goal of the kinetic evaluation is to obtain better, more informative results from the experiments. If realistic models are used, numerous unknown parameters have to be determined during the evaluation. Since the most important errors of thermal analysis are not random, the laws of the mathematical statistics do not offer means to find the best set of model parameters. Nevertheless, the simultaneous evaluation of a series of thermoanalytical experiments by the method of least squares aims directly at the description of the sample behavior in a wide range of experimental conditions and helps the determination of a large number of unknown parameters. The outlined considerations are supported by examples from the work of the author and his coworkers. As a comparison, a statistical survey is given on those papers that were published in journals specialized for thermal analysis, thermochemistry and pyrolysis in 2006 and contained the term “kinetic” or “kinetics” in their titles

Biomassza tüzelőanyagok optimális hasznosítását elősegítő alapkutatás termikus analízis segítségével = Fundamental research by thermal analysis for the optimal utilization of biomass fuels

Kísérleti biomassza ültetvények termékeit, különböző fafajtákat, valamint biomasszából készült faszenet vizsgáltunk termogravimetrai, termogravimetria -tömegspektrometriai valamint pirolízis-gázkromatográfia-tömegspektrometriai mérések segítségével. A munka a hőbomlás valamint a kontrollált hőprogram mellett végzett égetés folyamatainak mélyebb megismerésére irányult. A folyamatokat befolyásoló tényezők közül az alábbiak szerepéről nyertünk az eddigieknél részletesebb ismereteket: a lignin hőbomlása; az extrahálható komponensek szerepe; továbbá a fakéreg szerepe a fakérget is tartalmazó biomassza anyagok termikus tulajdonságaiban. Részletes reakciókinetikai leírásmódokat dolgoztunk ki, melyeket nagyobb méréssorozatok egyidejű, legkisebb négyzetek elve szerinti kiértékelésére alapoztunk. A reakciókinetikai kiértékelés és modellezés a lejátszódó folyamatok pontosabb megértését szolgálta. Emellett segített az egyes minták közötti hasonlóságok és különbségek felderítésében is. | We studied the products of experimental biomass plantations as well as various wood samples and charcoal produced from biomass by thermogravimetry, thermogravimetry - mass spectrometry and pyrolysis - gas chromatograpy - mass spectrometry. A deeper insight was obtained into the partial processes of devolatilization and temperature-programmed combustion. We clarified several aspects of lignin devolatilization and the role of the extractives and bark in the thermal behavior of biomass samples. Detailed kinetic modeling was carried out. It was based on the least squares evaluation of larger series of experiments and helped in a more accurate understanding of the processes. The kinetic modeling helped also in discerning differences and observing similarities between the various biomass samples

Computer processing of thermogravimetric - mass spectrometric and high pressure thermogravimetric data. Part 1. Smoothing and differentiation

High quality data smoothing is frequently required in the thermal analysis. Though the mathematical methods for smoothing are well known, the selection of the proper smoothing parameters cannot be based on statistical checks alone in thermal analysis. Many times a compromise must be found between the effective removal of the various experimental error components and the distortion of the curves by too strong smoothing. The following topics are discussed from a practical point of view: (i) the determination of the derivative thermogravimetric (DTG) curves at low sample masses; (ii) the elimination of the noise and flutter from the results of high-pressure TG experiments; (iii) the evaluation of the noisy, low intensity mass spectrometric signals arising from the minor volatile products of decomposition or oxidation. The performance of the methods is shown in two applications: (1) Evaluation 0.25 – 0.5 mg mass loss steps in high-pressure thermogravimetry; (2) Study of NOx formation by atmospheric pressure thermogravimetry – mass spectrometry during the temperature programmed combustion of 0.4 mg coal char. Keywords: Thermogravimetry – mass spectrometry, high-pressure thermogravimetry, smoothing, differentiation, DT

Comparison of temperature programmed char combustion in CO2 - O2 and Ar - O2 mixtures at elevated pressure

A pressurized thermobalance (TGA) system was built. The role of CO2 was studied in the controlled temperature combustion under pressurized conditions. Particular care was taken to employ low sample masses in thin layers, to avoid self-heating phenomena and ensure kinetic control. It was found that a CO2 partial pressure of 0.57MPa at a 0.6 MPa total pressure had no effect on the overall reaction rate of a temperature programmed combustion. KEYWORDS: High pressure thermogravimetry, char, combustion, effect of carbon dioxid

Thermal Decomposition of Wheat, Oat, Barley, and Brassica carinata Straws : A Kinetic Study

The slow pyrolysis of four biomass samples was studied by thermogravimetry (TGA) at different heating rates. The samples belonged to different botanical classes/genera, and their mineral matter content showed a high variation. A distributed activation energy model (DAEM) was used due to the complexity of the biomass samples of agricultural origin. The common features of their decomposition kinetics were sought by evaluating 12 experiments of four biomasses simultaneously by the method of least-squares. Two parallel DAEM reactions with a Gaussian distribution of the activation energies were sufficient for an acceptable fit between the experimental and simulated data. Common means and deviations of the activation energies were required for all four samples. The reactivity differences between the samples were expressed by the differences between the preexponential factors, while the weights of the parallel reactions described further differences between the samples. Altogether, 20 unknown model parameters were estimated from 12 experiments. When the method of least-squares was used on the mass loss rate (DTG) curves, one of the obtained partial curves showed a sharp peak with a small variation of E. This was associated with the cellulose decomposition. The other partial curve had a much wider E distribution and was assumed to include the decomposition of hemicellulose, lignin, and extractives. The evaluation of the sample mass (TGA) data resulted in wider partial peaks than the ones obtained from the DTG data. To exclude the possibility of any mathematical artifact, the evaluation was also carried out on the analytical integrals of the DTG curves

Impact of systematic errors on the determination of cellulose pyrolysis kinetics

The literature of cellulose pyrolysis well illustrates the vexing problems associated with measurements of the reaction rate of a strongly temperature sensitive substrate, and suggests the need for a round-robin study of cellulose pyrolysis by thermogravimetry. In this paper the effect of the inherent systematic errors of the TGA experiments on the apparent reaction kinetics are investigated in details

Supporting Information for Article Empirical Models with Constant and Variable Activation Energy for Biomass Pyrolysis

Models are presented for 16 different types of biomass in this Supporting Information. Their performance is illustrated by figures showing the calculated and experimental data

In Honor of Michael J. Antal

In Honor of Michael J. Anta

Empirical models with constant and variable activation energy for biomass pyrolysis

The so called “model-free” or isoconversional way of kinetic modelling was examined. In this field the available evaluation methods do not aim at an optimal fit for the experimental data. In the present work the functions of the corresponding kinetic equation were approximated by simple versatile formulas; the number of the unknown parameters was kept on reasonably low levels; and the evaluation aimed at the best fit for the experiments by the method of least squares. The considerations and methods were tested on 85 thermogravimetric (TGA) experiments which had been published earlier with different types of kinetic modelling. The experiments belonged to 16 biomass samples including woody biomass, agricultural residues and industrial wastes. The temperature programs comprised constant heating rates, stepwise heating, constant reaction rate heating (CRR), isothermal temperature programs, and a modulated temperature program. The evaluations were based on four to nine experiments for each sample. The best fit was searched for the mass loss rate curves because they reflect better the peculiarities of the pyrolysis than the integral curves. An empirical model with variable activation energy provided good fit for all experimental data. It described the experiments of a biomass sample by 11 parameters. Another model with constant E values provided rougher, but still usable approximations for the data. It allows fast numerical solutions that may be helpful in complex modelling tasks. Both models described the experiments at a variety of temperature programs by a given set of model parameters