CFD modeling of a small-scale cyclonic combustor chamber using biomass powder

Abstract

Thermal energy in the form of heat generally is produced and provided through thermal plants by combustion using coal, a type of fuel which impacts the environment severely. The use of biomass to provide heat and power as an alternative fuel is growing and can reduce the percentage use of the fossil fuels. However, the performance of the combustion systems with biomass as fuel needs evaluation, which is not an easy task. Modeling a fluidized biomass chamber involves the simulations of both solid-air combustion within the fluidized bed, and at the same time the gas phase reactions into the combustion chamber. Commercial CFD software packages usually have strong built-in capacities that facilitate gas phase modeling. The current study presents the computational modeling of a horizontal cyclonic combustor chamber burning biomass powder using ANSYS FLUENT software in conjunction with experimental biomass data. The mathematical models selected for the simulation were described as well as the boundary conditions. The obtained results are presented through velocity profiles and vectors, temperature fields and species mass fractions distribution. The temperature fields showed a maximum close to 1600 K, while mass fraction of species have demonstrated that, with the combustible and initial conditions set, the combustion reaction occurs completely at 0.55 m, leaving the remaining 1.5 m long for radial heat transfer in gas phase and eventually for a heat transfer to the chamber wall (in the boiler case). This extra length is directly related to the NOx formation, increasing the residence time of the product combustion gases at higher temperatures. The main mechanism of NOx formation was identified as the thermal mechanism.