Gypsum crystal degradation in wet Flue Gas Desulphurisation Plants
Multi-scale and/or multi-disciplinary approach to process-product innovation
Analysis of Environmental Issues (T3-3b)
Keywords: FGD, SO2 emissions, Particle size distribution, Breakage, Particle technology, Population balance modelling
Introduction
The combustion of fossil fuels such as coal, oil and gas covers a significant amount of the worlds present energy demand and is expected to do so for the coming years [1]. Due to the sulphur content of coals and oils, acidic compounds such as sulphur dioxide (SO2) and to a lesser extent sulphur trioxide (SO3), will be released during combustion. Various Flue Gas Desulphurisation (FGD) technologies may be used to limit the release of these compounds. The majority of the worldwide installed FGD capacity consists of the wet scrubber FGD technology using forced oxidation to produce gypsum [2].
CaCO3 (s) + SO2 (g) + 2 H2O (l) + ½O2 CaSO4×2H2O (s) + CO2 (g) (1)
The sales potential of the produced gypsum depends on parameters such as the particle size distribution (PSD), the moisture content and the impurity content. This study consists of experimental as well as theoretical investigations of the gypsum crystallisation (crystal formation and growth) in a wet FGD plant. Experiments performed on a wet FGD pilot plant will be supplemented by measurements from FGD units at full-scale power plants. Based on the experimental results it is the aim of this study to develop population balance equations for the prediction of gypsum PSD based on operating conditions. Initial experiments have been focused on the mechanical degradation of gypsum crystals in a wet FGD pilot plant.
Crystal degradation in a wet FGD pilot plant
The extent of crystal degradation has been investigated by subjecting samples of gypsum slurry to the mechanical stresses caused by stirring, recirculation and air injection for up to 600 hrs in a wet FGD pilot plant. The development in PSD as a function of time was monitored by analysing slurry samples with a Malvern Mastersizer S (laser diffraction measurements). No significant degree of degradation took place at short exposure times, but longer exposure times caused a decrease in the volume fraction of particles above approximately 25 μm and a corresponding increase in the volume fraction of particles below this size. The increased fraction of small particles may cause a reduced dewatering potential of the gypsum. Conventional models, assuming a single multiple piece breakage mechanism, have been unable to describe the experimentally observed changes in PSD. A new model based on multiple breakage mechanisms has been verified against experimental data.
References
[1] International Energy Agency (IEA) – World energy outlook 2004.
[2] H.N. Soud – Developments in FGD, IEA Coal Research, London 2000.
Presented Tuesday 18, 08:45 to 09:05, in session Analysis of Environmental Issues (T3-3b).
