Utilising Thermodynamic Equilibrium Calculations to Model Potassium Capture by Aluminosilicate Additives in Biomass Combustion Plants

Abstract

When it comes to the modelling of solid aluminosilicate additive particles, many current attempts are still using a high-level approach, leaving out the mechanistic detail on a particle level to reduce model complexity. This paper shows different ways in which thermodynamic equilibrium calculations can help to understand the processes inside additive particles and to generate sub-models for detailed CFD-calculations. First, the influence of the additive's chemical composition (Al/Si ratio and traces of Ca, Mg, Fe, Ti) on its maximum capture value is investigated. Afterwards, two sub-models for the behaviour of additives in combustion chambers are presented: one model for the calculation of the loss in surface area inside a particle, as well as a model for the in-situ determination of an additive particle's capture ceiling function. The results show that the chemical composition of the additive plays an important role in determining the capture of potassium from flue gases. High Al/Si ratios lead to higher thermal stability of the particles, while simultaneously reducing the amount of potassium that is captured in equilibrium. This knowledge can help improve models of solid additive particles and help optimise their use in thermal power plants.