Model simplifications on biomass particle combustion

Sousa, Nelson; Azevedo, João L. T.

http://hdl.handle.net/10400.1/9211

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Authors

Sousa, Nelson

Azevedo, João L. T.

Abstract(s)

Particle combustion modeling has been widely studied, although some considerations on the conversion processes are yet to be clarified. Thermal decomposition and final yield are addressed differentially in the literature, giving attention to the heating rate or to the peak temperature. It is assumed that the volatized gases are expelled immediately from the particle, neglecting homogeneous reactions, and in more simplified models that the particle is thermal and chemically uniform.This study presents an analysis of the modeling specifications required in the simulation of the conversion of a single biomass particle. This comprehensive approach shows the model requirements for more exact outcomes regarding the biomass particle size. The thermal model is discretized in concentric layers and may be used in cylindrical or parallelepiped geometry. Biomass decomposition is described by a competitive reaction model, justified by the analysis of the published experimental data. Temperature distribution is studied inside the particle as well as the gas flow, following a Darcy equation. Gas flow considers the diffusivity of oxygen to the interior of the particle and its reaction with the combustible gases and char.The validation of the numerical code is accomplished by the conversion of a 50 mm diameter wood trunk in a hot gas stream, measuring the mass loss and the temperature along the combustion process.Parametric tests were carried out to investigate the conversion of tars and oxidation of gases within the particle in typical combustion conditions. For biomass particles with a diameter more than 50 mm, the oxidation of combustible gases is considerable inside the particle, and therefore it is important to modulate the gas flow. Regarding the tar decomposition, the reactions involved are significant for particle diameters larger than 20 mm. In typical combustion conditions, small particles such as wood pellets, all properties can be considered uniform. The number of layers in the model can also be encountered considering the pyrolysis number.This study concludes that some process modulations are irrelevant for small particles. This is important when an expeditious method is required for the implementation in a multiparticle numerical code. (C) 2016 Elsevier Ltd. All rights reserved.