Crystallisation of Gypsum and Prevention of Foaming in Wet Flue Gas Desulphurisation (FGD) Plants

摘要

The aim of this project is to investigate two operational problems, which have been experienced during wet flue gas desulphurisation (FGD) operation, i.e. poor gypsum dewatering properties and foaming. The results of this work can be used for the optimization of wet FGD-plants in terms of reliability of operation and consistency of the gypsum quality obtained. This work may furthermore be of interest to other industrial systems in which foaming or gypsum crystallisation may take place. FGD is an industrial process, which removes sulphur dioxide (SO2) from flue gasses generated by fossil fuel combustion at power plants and other heavy industries, thereby abating the detrimental effects known as “acid rain”. The majority of the 680 FGD-plants installed at power plants worldwide in 1999 (2.41•105 MWe) were using the wet FGD-technology. This process absorbs ~ 99 % of the SO2 by an alkaline slurry, where it is oxidised to sulphate (SO42-) and crystallised as gypsum (CaSO4·2H2O) - a commercial product. The crystallisation process and the operating conditions under which it takes place, will determine the particle size distribution (PSD), the crystal morphology and thereby the dewatering properties. Experiments in a falling film wet FGD pilot plant have shown a strong non-linear behaviour (in a ln(n(l)) vs. l plot) at the lower end of the particle size range, compared to the well-known linear “mixed suspension mixed product removal (MSMPR)” model. A transient population balance model, fitted to experimental breakage data, was able to model an increase in the fraction of small particles, but not to the extent observed for the experimental steady state PSD. A three-parameter, size-dependent growth model, previously used in the literature to describe sodium sulphate decahydrate (Na2SO4•10H2O) and potassium aluminium sulphate (KAl(SO4)2•12H2O) crystallisation, was able to describe the experimental data, indicating a surface integration controlled growth mechanism. The PSD at three full-scale wet FGD-plants were comparable to the gypsum produced in the pilot plant. However, the crystals had fewer distinct crystals faces and more rounded tuber resembling shapes. An episode with a deterioration in gypsum dewatering properties at unit 3 of Amager Power Plant was linked to a change in crystal morphology, possibly due to adsorption of growth retarding aluminium fluoride compounds at specific crystal faces. Excessive foaming within wet FGD-plants has been associated with a range of operational problems as well as an increased degree of SO2 absorption. Foaming agents include surfactants, macromolecules (such as polymers or proteins), and finely dispersed solids. The foaming ability of particles, electrolytes and buffers, present in a wet FGD-plant, has been investigated by laboratory scale Bikerman experiments. Adipic acid, as well as a combination of small particles and an electrolyte, have been demonstrated to generate weak transient foams. Pilot plant experiments showed an increased absorption efficiency of SO2 using a foaming solution of calcium chloride (CaCl2) with small quartz particles, compared to pure non-foaming tap water.