In recent years, numerical modelling of combustion processes together with Computational Fluid Dynamics (CFD) has proven to be an efficient and reliable tool for the calculation and optimisation of different firing systems. In this work, a numerical model representing the relevant physical and chemical characteristics of pulverised biomass combustion is developed. A comprehensive CFD modelling framework originally developed for coal combustion is used for this purpose. The main physical-chemical steps of the combustion process—drying, pyrolysis, char conversion and homogeneous reactions—are individually evaluated taking into account the specific characteristics of biomass fuels. The corresponding numerical models are implemented into the CFD framework.
Considering the typically larger particle sizes related to biomass combustion (compared to those of coals), a comprehensive single-particle model is developed to describe the pyrolysis process. The model combines a physical model, considering transport phenomena inside the particle, and a flexible detailed multi-component chemical mechanism. The drying of the moisture embedded into the biomass particle is also included in the model, as well as intra-particle secondary pyrolysis reactions. The catalytic effect of ashes on thermal decomposition is also considered. The comprehensive single-particle model is implemented in a pre-processing step and it is subsequently incorporated into the CFD modelling framework. It offers significant improvements compared to more simplified approaches. Simulation predictions of the comprehensive model are compared with detailed experimental measurements. The model proves its ability to reproduce temperatures, release rates of gaseous species, and conversion times, in terms of absolute values and profiles. Furthermore, the sensitivity of simulation results against crucial model parameters is evaluated.
Char conversion models with specific kinetics for biomass chars are examined, including oxidation and gasification reactions. Homogeneous reactions of the volatile species released during pyrolysis and char conversion are also evaluated, including their interaction with turbulent flow.
A plausibility analysis of the developed biomass combustion model is performed using a standard test case. Model predictions are compared with experimental measurements conducted in a semi-industrial scale facility. In general, the model predictions show a satisfactory agreement with the experimental measurements in terms of absolute values and profiles of temperatures and gas concentrations. The model also proves to provide an improvement in the predictions over the previous version of the CFD framework.
The thesis was published by Shaker Verlag.
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