Modeling of inherent [formula omitted] capture in coal particles during combustion in fluidized bed

Abstract

The process of inherent sulfur capture in coal particles during combustion in fluidized bed was modeled. The significance of the process is in fact that a part of sulfur may be retained in ash, decreasing emission of sulfur oxides. The process, which is also known as sulfur self-retention, is influenced by parameters that depend on coal characteristics and combustion conditions. The interest for this process was increased with the introduction of fluidized bed combustion technology because of its favorable temperatures and other conditions. The presented model is based on a model of porous char particles combustion under fluidized bed conditions, and the changing grain size model of sulfation of the CaO grains dispersed throughout the char particle volume. The phenomena of sintering, reduction of the produced CaSO 4 with CO and thermal decomposition of the produced CaSO 4 were incorporated in the model, allowing for the different reactivities of various forms of calcium. A temperature-dependent relation for the CaO grain radius takes sintering into account. Reductive and thermal decomposition were taken into account by the corresponding reaction rate constants of the Arrhenius type. The reactivity of the calcium forms in coal was considered by different initial radius of the CaO grains. A comprehensive parametric analysis of the model was done. The model was verified by the experimental results for three low-rank Serbian coals. Samples of different particle size (4.0–7.0, 7.0–10.0 and 10.0–13.0 mm) for each coal were combusted under fluidized bed conditions at three temperatures: 750, 800 and 850 ∘ C . The maximal values of inherent sulfur capture at 800 ∘ C were obtained. The comparison with the experimentally obtained results showed that the model can adequately predict the levels of the obtained values of sulfur self-retention efficiencies, as well as the influence of temperature, coal type and coal particle size.