A flexible and stable numerical method for simulating the thermal decomposition of wood particles

Keywords: Wood drying; Wood pyrolysis; Wood combustion; Modelling; Numerics
1. Approaches to the thermal conversion of wood 1.1. Introduction Chemical processes involving a particulate phase are frequently encountered in technical applications, such as reforming, catalytic regeneration or solid fuel combustion in energy generation and waste treatment. Therefore,
information originating from individual particle processes disappears by this approach. However, this contribution deals with the development of a model to resolve the conversion process of a packed bed by describing the individual particle process with sucient accuracy. Therefore, a single particle model is developed, which can be applied to each particle of a packed bed and hence, represents the entire conversion of a packed bed as a sum of single particle processes. The single particle model is based on a system of onedimensional transient conservation equations for mass and energy to predict major properties such as temperature and species distribution inside a particle. The particles are coupled to the surrounding gaseous phase by heat and mass transfer. Due to further application in a packed bed model, the complexity of the particle model is limited. Thus, eects like fragmentation, swelling or the detailed description of the boundary layer surrounding the particle are not included. The combustion of the volatiles outside the particle is dealt with by an overlying model, which will not be discussed here. The applications mentioned above include a variety of particle processes, each characterised by speci®c kinetic parameters. In order to develop an extendable and ¯exible model describing the conversion of wood, a literature survey was conducted to collect various model approaches and identify their structures and common features. Based on chemical and physical behaviour, these can be arranged into heating, drying, various conversion processes, such as pyrolysis, gasi®cation and combustion. The processes are accompanied by species transport in the pore space of a particle due to diusion and convection in conjunction with heat transfer mainly within the solid matrix of a wood particle. The conversion is described by the conversation equations for mass and energy, which can be supplemented by a standard reaction system with the characteristic kinetic parameters expressed in activation energy, frequency factor, reaction order and enthalpy. Furthermore, a description by conservation equations does not bound the reactions either within a ®nite temperature range, so that they take place simultaneously, or in a reaction mode, such as the shrinking or the reacting core mode. This procedure provides the means for uniform treatment of the particle processes. The relevant information is stored in a data base, which through a combination of dierent processes, allows an entire particle process to be represented. Hence, a wide range of applications is covered, which is easily extendable. Wood already includes several of these processes and therefore is chosen for numerical simulation as a representative example in this study. However, applications are independent of the reaction system and the material, since they are implemented as a general concept.
Corresponding author. Tel.: +49-7247-823491; fax: +497247-824837. E-mail address: b.peters@iket.fzk.de (B. Peters).
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research into a comprehensive model of a chemically reacting packed bed will be bene®cial since basic understanding of the various processes taking place is fundamental to plant design, operation, and innovation. In order to gain a deeper knowledge of the conversion process of a packed bed, the processes may be separated for modelling, so that the in¯uence of each subprocess e.g. submodel on the whole problem can be investigated. Among these subprocesses is the thermal conversion of a packed bed as a particulate phase, which is often treated as a continuous medium by empirical correlations, analytical or numerical methods (Behrendt, 1992; Raull et al., 1998). Consequently, penstrauch, 1994; Kru
0045-6535/01/$ - see front matter Ó 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 0 4 5 - 6 5 3 5 ( 0 0 ) 0 0 2 2 0 - 4
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B. Peters, C. Bruch / Chemosphere 42 (2001) 481±490
Abstract The objective of this paper is to present a ¯exible and stable simulation method to predict the thermal conversion of wood particles. A combination of several subprocesses such as heating-up, drying, pyrolysis, gasi®cation and combustion of fuel particles of dierent properties and sizes represents the global process of thermal conversion. This approach allows for simultaneous processes e.g. reactions in time and covers the entire range between transport-limited (shrinking core) and kinetically limited (reacting core) reaction regimes. Thus, the model is applicable to simulate suciently accurate the thermal decomposition of each particle in a packed bed, of which the entire conversion is regarded as the sum of all particle processes. Eects such as fragmentation, swelling, homogeneous reactions e.g. ignition outside a particle are excluded as a tradeo between complexity and calculation time. However, a description of the particle processes by one-dimensional and transient dierential conservation equations for mass and energy is feasible to represent the above mentioned subprocesses. The particles are coupled to the gas phase by heat and mass transfer taking into account the Stefan correction due to the gas out¯ow during conversion. A general formulation of the conservation equations allows the geometry of a fuel particle to be treated as a plate, cylinder or sphere. In order to achieve a high degree of ¯exibility, the method distinguishes between data, such as kinetics or material properties and the conversion process, for which relevant data are stored in a data base for easy access and extension. The resulting modules of this subdivision are encapsulated into separate software units cast in a hierarchy of well-de®ned classes in Tools of Object-oriented Software for Continuum-Mechanics Applications (TOSCA) by object-oriented techniques. Ó 2001 Elsevier Science Ltd. All rights reserved.