Emma Jakab

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 milled wood lignins studied by thermogravimetry/mass spectrometry

Thermogravimetry/mass spectrometry has been applied to characterize 16 milled wood lignins isolated from grasses, softwoods and hardwoods. The samples were thermally decomposed in inert atmosphere using a heating rate of 20°C min−1. The effect of acetylation as well as ZnCl2 and NaCl as catalysts on the thermal decomposition has been studied. The weight loss and evolution profiles of the most abundant degradation products were monitored as a function of temperature. Correlation has been found between the intensity of several products and the compositional parameters of lignins determined by wet chemical methods. The modification of OH groups by acetylation changed the decomposition patterns, but the acetic acid produced by thermolysis had no catalytic effect on the formation of monomers. The evolution profiles of H2O and CO2 from the original and acetylated lignins proved that free OH groups promote the scission of COOH groups. The additives ZnCl2 and NaCl have different influence on the product distribution. NaCl promotes dehydration, demethoxylation and recombination of the primarily formed radicals, although the maximum of the evolution profiles is not shifted significantly. However, water and formaldehyde formation shifts to 60–80°C lower temperature in the presence of ZnCl2 indicating the occurrence of ionic decomposition mechanisms.

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.

Thermogravimetry/mass spectrometry study of six lignins within the scope of an international round robin test

Thermogravimetry/mass spectrometry was applied to characterize six lignins prepared for an international round robin test. The sample set includes a mildly isolated lignin, and technical lignins prepared by steam explosion, Alcell, Indulin and Sucrolin processes. The samples were thermally degraded in an argon atmosphere using a heating rate of 20 °C min−1. The weight loss and the evolution profiles of the thermal decomposition products of low molecular mass were monitored. It was found that the intensity and the evolution profile of the products (especially water, formaldehyde, methane and methanol) reflect the severity of the isolation procedure and the origin of the lignin. Correlations have been observed between the abundance of volatile products and the type and amount of functional groups. The terminal CH2OH groups decompose by the release of both water and formaldehyde, as demonstrated by the relationship between the aliphatic hydroxyl group content and the formaldehyde as well as the water evolution. The dependence of the methane yield on the methoxyl group content provided evidence that the scission of methoxyl groups results in the formation of methane as well as methanol. The correlations found allow the assignments of the gaseous products to functional groups.

Effect of metals, metal oxides, and carboxylates on the thermal decomposition processes of poly (vinyl chloride)

Abstract In this work, pyrolysis–mass spectrometry and pyrolysis–gas chromatography/mass spectrometry (Py–GC/MS) was applied to investigate the influence of some metals (aluminium, iron and zinc), metal oxides (aluminium, titanium, copper and iron) and carboxylates (zinc and tin) on the thermal decomposition processes of PVC. The metals and the oxides were not mixed with the polymer in order to observe the effect of the surface contacts only. The evolution of the volatile thermal decomposition products has been monitored by mass spectrometry using low energy ionisation. Metals (aluminium, zinc, iron) and oxides of enough large metal ion radius (ferric oxide and titanium dioxide) reduced the onset temperature of dehydrochlorination, admittedly by attracting chlorine, weakening the C–Cl bonds in PVC. Depressed HCl formation was found in those cases when chlorides could formed (iron, zinc, Ca/Zn carboxylate, cupric oxide and titanium dioxide). Benzene formation was hindered by the studied metals, oxides, and carboxylates as far as they are forming chlorides with HCl. The promotion of benzene evolution observed on alumina (and to a lower extent on titanium dioxide) is assumed to be the result of a facilitated hydrogen exchange on the oxide surface, necessary for the detachment of benzene from a dehydrochlorinated PVC segment. The second step of PVC thermal decomposition is also shifted to a lower temperature by the metals tested and by transition metal oxides. This effect is explained by the easier cleavage of the polyenic chain at segments getting contacted to metal surfaces or to transition metal ions. Apparently, fast pyrolysis of PVC is similarly influenced by the materials studied, as the product yield data of Py–GC/MS are consistent with that of pyrolysis/mass spectrometry.

Thermal decomposition of mixtures of vinyl polymers and lignocellulosic materials

Abstract The effect of wood, cellulose, lignin and activated charcoal on the thermal decomposition of polystyrene (PS) and polyethylene (PE) has been studied in order to investigate the thermal behavior of these materials occurring in municipal waste. Thermogravimetry/mass spectrometry and pyrolysis–gas chromatography/mass spectrometry revealed that these materials had a similar influence on PS and PE thermal decomposition under slow and fast heating conditions respectively. The effect is related to the char-forming capability of the wood-derived additives; thus cellulose had the least and pure charcoal had the greatest influence on the decomposition of the polymers studied. Polystyrene is more sensitive to the presence of additives than the two PE polymers investigated. The thermal decomposition of PS shifts to higher temperature and the product distribution changes significantly in the presence of wood-derived additives. The yield of monomer, dimer and trimer decreases and the formation of other products (e.g. toluene, ethyl benzene and α-methyl styrene) increases. The effect of additives is interpreted in terms of the free radical mechanism of the thermal decomposition of PS. Wood, cellulose and lignin have a small effect on the thermogravimetric curves of PE. Charcoal promotes the hydrogenation of the unsaturated products and the hydrogenated products evolve at higher temperature.

The effect of cations on the thermal decomposition of lignins

Abstract The thermal decomposition of three lignosulphonates, three sodium salts of a spruce milled wood lignin (MWL) and a mixture of MWL and NaCl was studied by thermogravimetry/mass spectrometry (TG/MS). The TG curves and the evolution profiles of degradation products showed that the lignosulphonate cations (H + , NH + 4 , Na + ) play a key role in the decomposition. The sulphonate groups degrade at low temperature and give rise to a large amount of SO 2 in the case of lignosulphonic acid and NH 4 -lignosulphonate, whereas Na-lignosulphonate releases only 10–20% of the theoretical yield of SO 2 . The catalytic effect of sodium was studied on spruce MWL in detail. Sodium enhances dehydration, demethoxylation, decarboxylation and char formation, whereas it decreases the yield of organic volatiles and CO. Tentative reaction routes are proposed for the rationalization of the influence of sodium on the thermal decomposition of lignins.

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.

Thermal decomposition of low-density polyethylene in the presence of chlorine-containing polymers

Abstract The thermal decomposition of low-density polyethylene was studied on samples containing a few per cent of poly(vinyl chloride), poly(vinylbenzyl chloride) or poly(chlorostyrene), using thermogravimetry/mass spectrometry, pyrolysis-gas chromatography/mass spectrometry between 400 and 1000 °C, and FT-IR analysis of the pyrolysis tars. Reactive species evolving from the chlorine-containing polymers during their thermal degradation affect the decomposition reactions of polyethylene. Hydrogen chloride, formed from chloropolymers prior to the beginning of the PE thermal decomposition, promotes the initiating steps of thermal degradation and inhibits β-scission of macroradicals. Chlorine evolves at a relatively higher temperature, and enhances dehydrogenation reactions leading to aromatic products.

The effect of carbon black on the thermal decomposition of vinyl polymers

Abstract The effect of carbon black on the thermal decomposition of various polymers has been studied in an inert atmosphere by thermogravimetry/mass spectrometry and pyrolysis–gas chromatography/mass spectrometry. It has been established that the nature of substituents on the hydrocarbon chain of the polymers affects the thermal behavior of the mixtures with carbon black. Carbon black exhibits no influence on the decomposition of poly(methyl methacrylate) (PMMA) which has quaternary carbon atoms in the polymer chain and decomposes by depolymerization. The decomposition of polypropylene (PP) is promoted, whereas, that of polyethylene (PE), polystyrene (PS) and polyacrylonitrile (PAN) is hindered in the presence of carbon black. The char yield of PAN is increased significantly, however, carbon black has no impact on the amount of residue of non-char-forming polymers. Analysis of the pyrolysis products indicates that carbon black has influence through the chain cleavage and H-transfer reactions. The promotion of the chain scission reactions in PP is indicated by the lower decomposition temperature and the increased formation of products originating from the primary macroradicals. It appears that carbon black participates in the termination of the chain reactions, too, thus, the yield of oligomers is significantly reduced from the vinyl polymers. The increased yield of hydrogenated products also confirms that carbon black participates in the H-transfer reactions.