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  • Modeling biomass particle drying, devolatilization and combustion in a grate fired combustor
    Publication . Sousa, Nelson; Azevedo, João L.T.
    The paper presents the application of a numerical model to describe the evolution of a particle in a grate fired combustion system. Due to the particle dimensions, important temperature gradients exist inside the particles and reactions occur within the particles. A model using layers is used to describe the processes inside the particles. The biomass composition is expressed as fractions of moisture, unreacted biomass, char and ash. Biomass conversion is described by a competitive reaction model leading to the formation of light volatiles, tars and char. Tar is subject of secondary reactions inside the particles forming either light gases or char. Transport equations are solved for the gases within the particle including oxygen from the environment that reacts with gases or char. The model is applied to different heating rates leading to different amounts of tars and char in accordance with data from the literature. The model is also applied to simulate the combustion of a single trunk standing in a heated gas stream and comparisons are done for the temperature and mass loss.
  • Study of biomass particle combustion model to use in multi-particle simulation codes
    Publication . Sousa, Nelson; Azevedo, João L.T.
    This paper presents an analyses of the modelling specifications required in the simulation of the conversion of a single biomass particle to be used in multi-particles computer simulation codes. It considers the transport and reaction of gases, the heterogeneous combustion and heat transfer within the biomass. The biomass conversion is described by a competitive reaction mechanism specifying the composition of the volatile species and tar properties. This approach can represent the influence of the heating conditions on biomass conversion and produce acceptable compositions for the final products. Tar decomposition is considered within the particle as well as combustion reactions for all combustibles: volatile, tar and char. The study of several approaches show the level of numerical approach required to simulate the thermal conversion of the biomass particle and further reactions such as tar decomposition and oxidation of the combustibles gases within the particle. The validation of the numerical code is accomplished with the conversion of a 50 mm trunk in a hot gas stream by measuring the mass loss and temperature along the combustion process. Parametric tests are carried out to investigate the conversion of tars and oxidation of gases within the particle and it is concluded that these can be neglected for particle diameters smaller than 20 mm with an error less than 1%. For combustible particles such as wood chips and pellets with 6 mm diameter, all intern gradients may be neglected.
  • Hydrodynamic model for a biomass grate fired system
    Publication . Sousa, Nelson; Azevedo, João L.T.
    The present paper describes a hydrodynamic model for the solids motion in a grate fired combustion system. The overfed bed material is considered in a Lagrangian referential until the particles stop in a position over the bed or exit the domain. The solid material in the grate is then considered as an incompressible continuous media. The momentum balances are applied in an Eulerian referential to particle elements to calculate their velocity in the grate direction. For the conditions considered the calculated velocity of the elements increase always from the start of the grate towards the exit, so the motion in the vertical direction is always downward. This motion is calculated from continuity and the two components enable the definition of the solids flow within the bed. The application of the model for solids motion is shown to be representative of different situations that are analysed for a vibrating grate working with wood pellets. The distribution of solids in the bed is visually and computationally characterised for three situations: i) feeding particles above the bed over an inclined still grate, ii) vibrating an initial bed promoting the motion of particles and iii) vibrating the bed and feeding above the grate in order to achieve a continuous evolution. The comparison of the results show that the model provides a good representation of reality although it can be improved by adjusting model parameters. The model allows for the generation of solids flow patterns in the bed and is a base for the development of a model for grate fired combustion systems.