A comparative kinetic study on the pyrolysis of three different wood species

doi:10.1016/S0165-2370(03)00065-2

Journal of Analytical and Applied Pyrolysis

Volumes 68–69, August 2003, Pages 231-249

https://doi.org/10.1016/S0165-2370(03)00065-2 Get rights and content

Abstract

The catalytic effect of pH-neutral inorganic salts on the pyrolysis temperature and on the product distribution was studied by fractionated pyrolysis followed by GC/MS and GC/FID and by thermogravimetric analysis (TGA) of cold-water-washed hornbeam wood. Sodium and potassium chloride have a remarkable effect on the pyrolysis temperature and on the product distribution, whereas calcium chloride only changes the low temperature degradation of hornbeam wood and the product distribution is nearly unchanged compared with water-washed hornbeam wood. All studied potassium salts (KCl, KHCO₃, and K₂SO₄) decrease the amount of levoglucosan the order of magnitude being dependent on the anion: chloride has a more pronounced effect than sulphate, and sulphate a more pronounced effect than bicarbonate. The thermal degradation of three different wood species (hornbeam, walnut and scots pine) was investigated by analysis of thermogravimetric/mass spectrometric pyrolysis. Commonly used model substances for the main components of wood, like xylan, pure cellulose or filter pulp, were found to be unreliable for the evaluation of formal kinetic parameters that are able to describe the pyrolysis of wood. A method for the individual evaluation of formal kinetic parameters for the main components of wood was used, that uses specific ion fragments from lignin degradation products to study the lignin degradation. Coniferous lignin is thermally more stable than deciduous lignin, and the latter produces smaller char yields. The differences in wood species mainly result in different degradation rates for the lignin and for the early stages of the hemicellulose degradation.

Introduction

To decrease the amount of CO₂ emission from energy conversion, biomass as a renewable and CO₂-neutral resource has gained great interest. Especially wood and agricultural residues like straw are widely distributed and easily accessible at relatively low costs. Of these lignocellulosic materials wood is favourably used because of its higher density (higher energy content per volume), lower amount of ash, and of its very low amount of nitrogen. In principle, there are two ways to release heat and energy from wood: direct combustion or thermochemical conversion into gases and liquids, that can be used in gas turbines or diesel engines. Also, upgrading of wood to quality fuels such as methanol or hydrogen, or production of fine chemicals is a research topic. In all these cases, the knowledge of the kinetics of the devolatilisation of wood is essential, because pyrolysis is always the first step in any gasification or combustion process. However, the mechanisms and the kinetic data for wood pyrolysis are still unknown to a large extent because of the complexity and the varying physical and chemical properties of wood.

Wood as a major representative of biomass consists mainly of cellulose, hemicellulose, and lignin. Its thermal decomposition as performed in thermogravimetric analysis (TGA) of small samples reveals two decomposition regimes, which are attributed to the decomposition of cellulose and hemicellulose [1]. The peak in the decomposition rate at lower temperatures can be associated with pyrolysis of hemicellulose and the peak at higher temperatures is associated with cellulose decomposition. A peak due to lignin decomposition can not be observed. Therefore, the integral result of the merged peaks does not promote a reliable analysis of the decomposition kinetics of the major constituents. Assuming that cellulose, hemicellulose and lignin decompose independently, pretreatment in conjunction with separation of the major components may allow investigating into the decomposition of the components. However, commonly applied separation techniques to reduce the complex structure of wood include extensive depolymerisation and structural changes, which may lead to kinetic parameters that do not represent the decomposition of wood. In recent studies [2], [3], [4], [5], it was found that using model substances (such as xylan, Avicel cellulose, filter pulp, or Klason lignin) as substitute compounds can lead to errors for wood pyrolysis when described with formal kinetic parameters obtained from these substitute compounds.

From earlier studies [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], it is well known that the addition of inorganic salts to wood samples result in a wide variety of changes in the pyrolysis process. The main intent of the early studies (e.g. [9], [14]) was, to investigate the fire-retardant characteristics of organic and inorganic chemicals for the pyrolysis and combustion of wood. Today, the influence of inorganic salts on the pyrolysis of wood is studied with multiple aims: e.g. to increase the yield of charcoal, to develop analytical pyrolysis as a quantitative method for the analysis of pulps [10], to dispose heavy metal containing wood waste [16], to increase the yield of valuable fine chemicals from biomass pyrolysis, and to provide a model that comprises the influence of inorganic salts (ash) on the pyrolysis kinetics.

The objective of this study is to investigate the influence wood's naturally occurring inorganic salts on the temperature of pyrolysis and on the product distribution, and to determine the decomposition kinetics of the three wood species, hornbeam (Carpinus betulus), walnut (Juglans regia), and scots pine (Pinus sylvestris) by incorporating the thermal degradation kinetics of the main components (lignin, hemicellulose, and cellulose). The kinetic measurements were performed by online thermogravimetry/mass spectrometry (TG/MS) [17] and by isothermal measurements [18], [19]. The comparison of the thermal degradation behaviour of the main components of the different wood species allows to identify differences and similarities of the main components during pyrolysis. Also, a better understanding of the relationship between the chemical structure and the pyrolysis behaviour of the individual components can be achieved.

Section snippets

Material

The wood species used in this study are hornbeam (C. betulus), walnut (J. regia), and scots pine (P. sylvestris), each obtained from a single log from the region of Karlsruhe. The bark free wood was ground to a size range less than 250 μm. The composition of the material is given in Table 1. Prior to the experiments, the samples were dried 3 h at 105 °C. Each wood sample was also washed with cold-water to reduce the amount of inorganic salts and treated with dilute sulphuric acid (4 wt.%) at 100

Thermal decomposition of wood

Fig. 1 shows the measured TG curves and the negative first derivatives of the thermogravimetric (DTG) curves of differently treated hornbeam wood (heating rate β=10 °C min⁻¹). The DTG curve of untreated and water-washed wood reveals two decomposition regimes: The peak at higher temperatures is mainly due to the decomposition of the cellulose and the shoulder, at lower temperatures, can be attributed to the decomposition of the hemicellulose. A peak due to the lignin degradation cannot be

Conclusion

Inorganic salts have a strong influence on the temperature of pyrolysis as well as on the product distribution. The alkaline metal chlorides studied, strongly decrease the temperature of pyrolysis, whereas calcium chloride mainly influences the low temperature degradation regime. The influence on the pyrolysis temperature is also dependent on the anion. The influence is increasing in the following row: bicarbonate<sulfate<chloride. In the case of the product distribution, this trend can only be

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