Simplified model and simulation of biomass particle suspension combustion in one-dimensional flow applied to bagasse boilers

A simple numerical model is presented to simulate the combustion of a biomass particle in a vertical stream. Emphasis focuses on the trajectories of spherical and cylindrical particles in the furnace. Combustion is modeled in three sequential stages: drying, pyrolysis and char combustion. Biomass consumption is determined by correlations based on Arrhenius kinetics and mass transfer parameters. Pyrolysis is modeled using five first-order kinetic equations considering the following products: volatiles, char and tar. The char consumption rate is modeled by three first-order kinetic equations, considering that char reacts with oxygen, carbon dioxide and water. The model is validated by comparing the duration of each simulated stage against experimental data taken from the literature. It is validated for spherical particles of up to 5 mm in diameter using a shrinking core model and for cylindrical particles of up to 3 mm using an ash-segregated model. Particle trajectory results are presented in order to determine the geometry and functional parameters of the combustion chamber that ensure complete suspension-firing. The combustion chamber geometry and biomass distributor height are determined as a function of airflow velocity and biomass characteristics for the combustion of bagasse with moisture contents of 30%-50% and particle diameters of 0.5 mm-3.5 mm. This study also allows the airflow velocity to be determined based on the boiler dimensions and the biomass characteristics to ensure that no particle ends up on the grate. After establishing the velocity, it is possible to determine what particle size will reach the top of the chamber or burn completely in suspension.