Utilising Thermodynamic Equilibrium Calculations to Model Potassium Capture by Aluminosilicate Additives in Biomass Combustion Plants. (15th May 2023)

• Record Type: Journal Article
• Authors: de Riese, Thorben; Fendt, Sebastian; Spliethoff, Hartmut
• Is Part Of: Fuel. Volume 340(2023)
• Publication Date: 2023-05-15
• Subjects: Biomass -- Aerosols -- Corrosion -- Additives -- Potassium -- Kaolin; Fuel -- Periodicals; Coal -- Periodicals; Coal; Fuel; Periodicals; 662.6
• DOI: 10.1016/j.fuel.2023.127591
• Languages: English
• ISSNs: 0016-2361

Abstract

Highlights: The chemical composition of additives plays an important role on their capture capacity if non-excessive amounts of additive are used. Slightly SiO2 -rich compositions are preferred for higher temperatures, where KAlSi2 O6 becomes the dominant form of K-Al-Si. KOH shows the most favourable tendencies compared to KCl and K2 SO4 . Fe and Ti have little impact on the thermodynamic state of potassium in the particles, while Mg and especially Ca lead to slight reductions in capture capacity in many relevant compositions. Low amounts of captured K in the additive can lead to significant shares of slag in the particle. The information about this slag can be used to model surface area degradation. Increasing the ratio of aluminium to silicon drastically reduces the slagging tendency and thus increases the thermal stability of the additive particle. The K capture ceiling of a particle can be calculated with the knowledge of the surrounding gas phase and the particle temperature. An appropriate ceiling function fit is shown. With these kinds of approaches based on thermodynamic principles, the accuracy and robustness of additive capture models can be improved. 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.