Gábor Várhegyi

Kinetic modeling of biomass pyrolysis

Abstract The thermal decomposition of lignocellulosic biomass materials and their major components is discussed. Thermogravimetric and DSC curves at different T( t ) heating programs were evaluated by the method of least squares. Pseudo-first order models, parallel, successive and competitive reaction schemes and complex reaction networks were employed in the modeling. The following topics are treated: thermal decomposition of cellulose at low (2 °C min −1 ) and high (50–80 °C min −1 ) heating rates; low temperature phenomena; the validity of the Broido-Shafizadeh model; effects of mineral catalysts; cellulose pyrolysis in closed sample holders; thermal decomposition kinetics of xylan, lignin and lignocellulosic plant samples.

Thermal decomposition of polypropylene in the presence of wood-derived materials

Abstract Thermal decomposition of polypropylene (PP) was studied in the presence of wood flour, lignin, cellulose and charcoal in order to understand the pyrolytic behavior of the mixture of these components occurring in waste. Thermogravimetry/mass spectrometry was applied for monitoring the weight loss and the product evolution profiles under slow heating conditions. The formation of oligomeric products was analyzed by pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS). Since the charring reactions of wood are still proceeding during the PP decomposition, the interactions between PP and char devolatilization are expected to be rather pronounced. The effect of the freshly formed pyrolytic chars on the decomposition of PP was simulated by mixing PP with pure activated charcoal. Activated charcoal had the most significant effect on the decomposition profile and the product distribution of polypropylene. In the presence of charcoal the decomposition shifted to lower temperature and the DTG curve became broader. The presence of charcoal promotes the formation of monomer and dimer. The effect of wood flour, lignin and cellulose is less pronounced. Though the decomposition starts at lower temperature in the presence of these additives and the DTG curve became broader, the temperature of the maximum decomposition rate and the char yields did not change significantly.

Kinetics of the thermal decomposition of cellulose in sealed vessels at elevated pressures. Effects of the presence of water on the reaction mechanism

Abstract The thermal decomposition of Avicel cellulose was studied by non-isothermal differential scanning calorimetry in hermetically sealed sample holders. The experimental results, which were published earlier, showed a marked catalytic effect of the water on the cellulose decomposition. Here we propose a reaction scheme containing two rate determining reactions to describe the result. The corresponding model resulted in a good fit between the experimental and the calculated data. The results indicate that the reaction starts with cellulose hydrolysis which may be followed immediately by decomposition reactions to intermediate products. The intermediates undergo further water-catalyzed decomposition reactions giving char, water and gases. The results may help in the understanding of biomass pyrolysis under experimental or industrial conditions where the thickness of the layer, the size of particles or the enclosure of the reactor keeps part of the water vapor formed during the reaction in the pores or between the particles of the decomposing material.

Thermogravimetric/mass spectrometric characterization of two energy crops, Arundo donax and Miscanthus sinensis

Abstract Two herbaceous plants, Arundo donax and Miscanthus sinensis with high biomass production potential, were studied by simultaneous thermogravimetry/mass spectrometry (TG/MS) in an inert argon atmosphere at a heating rate of 20 °C min−1. The effect of particle size, the partial removal of the minerals by acid- and water-washing, and the effect of potassium carbonate as catalyst were studied. Experiments with covered sample pans were carried out to investigate the effect of secondary tar cracking on the charcoal production. TG/MS supplied intensity profiles of the low molecular weight volatile products as a function of time or temperature. Their comparison to the weight-loss rate curves (DTG) provided information of the details of the degradation mechanism. The amounts of various volatile pyrolysis products were estimated by the integrals of the corresponding mass spectrometric intensities. The factors influencing the char yield were discussed.

Least squares criteria for the kinetic evaluation of thermoanalytical experiments. Examples from a char reactivity study

Abstract The mathematical modeling of the chemical processes is a crucial problem of the thermal analysis. Simple models with few parameters seldom can describe the real complexity of the phenomena arising during the heating of the various substances. In the case of more complex models the determination of the parameters and the validation of the model require the evaluation of carefully designed experimental series. The one-by-one evaluation of the experiments is a mathematically ill-defined problem at a larger number of unknown parameters. The non-statistical experimental errors of the thermal analysis hinder the determination of a single parameter set by the simultaneous least squares evaluation of the experiments. This paper discusses several evaluation techniques for the handling of the non-statistical errors during the least squares evaluation of experimental series. The methods are illustrated by the evaluation of oxidative thermogravimetric experiments of a lignite and a coal char.

Reaction kinetics in thermal analysis. Part 1. The sensitivity of kinetic equations to experimental errors. A mathematical analysis

Abstract The frequent publication of contradicting or meaningless kinetic parameters and the resulting criticism of the “ill-conditioned nature” of non-isothermal reaction kinetics led the authors to an examination of the sensitivity of kinetic parameters to experimental errors. Using simple mathematical deductions, conditions were given at which about 10% precision of the kinetic parameters can easily be achieved. To obtain a graphic picture about the information content of a thermoanalytical curve and the effect of the systematic measurement errors, mathematical relationships were deduced to show the dependence of the kinetic parameters on the formal (geometric) characteristics of the thermoanalytical curves.

Integration of the rate constant and linearization of the kinetic equations in non-isothermal reaction kinetics

Abstract The theory of reaction rates gives approximations of the type A T b e − E/RT for rate constants. In thermal analysis the case where b = 0 is used in kinetic calculations. However, if b is regarded as a variable, treatment of the non-isothermal kinetic equations is not more complicated than in the special case of b = 0. In this paper Pade approximations, described in the literature of the special functions, and Legendres continued fractions are proposed for evaluation of the A ∫ T b e − E/RT d T integrals. The Coats—Redfern types methods for the determination of the kinetic parameters are discussed analysing the errors of approximation involved in their deduction. On this basis a modified parameter estimation scheme is proposed.

Kinetics of the thermal decomposition of cellulose under the experimental conditions of thermal analysis. Theoretical extrapolations to high heating rates

Abstract Thermobalance-mass spectrometer (TG-MS) experiments carried out by the authors in a period of 8 years are reviewed and analyzed. Celluloses and lignocellulosic biomass samples were studied. The data are evaluated by the method of least squares. The results indicate that a single rate-controlling reaction step dominates the kinetics of the cellulose decomposition at low heating rates (2–20°C/min) provided that the heat and mass transport problems are experimentally eliminated and the amount of catalytic impurities is reduced by dilute acid or hot water washing treatments. The kinetic parameters obtained from the experiments with different cellulose and biomass samples evidenced only ca 8% scattering. A simple explanation is given for the kinetic compensation effect observed. Theoretical extrapolations are presented to predict the behavior of extremely small, idealized cellulose samples at high heating rates.

Kinetic evaluation of non-isothermal thermoanalytical curves in the case of independent thermal reactions

Abstract A least squares curve-fitting method was developed for the following thermo-analytical problem: “Find the kinetic parameters and the unknown initial amounts of the reactants from non-isothermal thermoanalytical curves in the case of two or more independent or quasi-independent thermal reactions”. From a numerical point of view this problem differs from the non-linear least squares techniques used in other areas of reaction kinetics. The special difficulties which have arisen in such calculations were eliminated by parameter transformations and by separating the linear and non-linear parts of the problem. The method can be applied at any T(t) functions. Thermo-analytical curves differing in temperature program can be evaluated simultaneously.

Thermogravimetric-mass spectrometric study on the low temperature oxidation of coals

Abstract Four coal samples of different rank and four coals of extremely high mineral matter content were exposed to air to study low temperature oxidation (weathering) processes under laboratory conditions. The reactivity of the coals was compared by measuring the oxygen consumption and the mass gain at 80°C for 96 hours. It was concluded that both the rank and sulphur content has an influence on the reactivity towards oxygen. Thermogravimetry combined with mass spectrometry (TG-MS) was used to investigate the weathering induced changes in the coal structure. Low temperature oxidation resulted in an increased yield of carbon dioxide, water, acetic acid, carbonyl sulphide and sulphur dioxide during thermal decomposition. Furthermore, decreased intensity of aliphatic hydrocarbons, alkylphenols and hydrogen sulphide was observed in the mass spectra of oxidized samples. These observations are explained by the formation of oxygen-containing functional groups on aliphatic CH groups and by the reaction of phenolic OH groups leading to cross-linking. TG-MS experiments show that both the pyritic and organic sulphide groups are attacked by oxygen resulting in an increased abundance of oxidized forms of sulphur.