Optimisation of a wet FGD pilot plant using fine limestone and organic acids

The effects of adding an organic acid or using a limestone with a fine part icle size distribution (PSD) have been examined in a wet flue gas desulphur isation (FGD) pilot plant. Optimisation of the plant with respect to the de gree of desulphurisation and the residual limestone content of the gypsum h as been the aim of the work. In contrast to earlier investigations with org anic acids, all essential process parameters (i.e. gas phase concentration profiles of SO2, slurry pH profiles. and residual limestone in the gypsum) were considered. Slurry concentrations of adipic acid in the range of 0-7 m M were employed. The overall degree of desulphurisation in the plant increa sed from 83% at 0 mM to 90% at 3 mM and the residual limestone level was re duced from 4.6 to 1.4 wt%. Increasing the slurry concentration of adipic ac id above 3 mM gave only a slightly higher degree of desulphurisation. The w et FGD model of Kill et al. (Ind. Eng. Chem. Res., 37 (1998) 2792) was exte nded to include buffer systems and verified against experimental data. Subs equently, the model was used as a tool to identify the optimal organic acid dissociation constants (as pK(a) values) and concentration levels at diffe rent operating conditions. At a holding tank pH of 5.5 and a temperature of 50 degreesC, simulations with Bryozo limestone and a monoprotic buffer sug gested that the optimum pK(a) value is between 4.5-5.5 and 5.5-6.5 with res pect to the degree of desulphurisation and the residual limestone level, re spectively. Adipic acid has pK(a) values close to these ranges (pK(1) = 4.4 0 and pK(2) = 5.41 at 50 degreesC). Changing limestone type tin the absence of organic acids) to one with a lower average particle size (i.e. from 20 to 4 mum) increased the overall measured degree of desulphurisation from 83 to 87% and reduced the residual limestone level from 4.6 to 1.3 wt%. Incre asing the holding tank pH level from 5.5 to 5.8 affected the degree of desu lphurisation and the residual limestone level only slightly. At holding tan k pH levels between 5.88 and 5.90, a high degree of desulphurisation was ob served, but the residual limestone content in the gypsum increased to somew here between 19 and 30 wt%, making this pH range unsuitable for use in a fu ll-scale plant. The investigations have shown that both the addition of org anic acids and the use of a limestone with a fine PSD can be used to optimi se wet FGD plants. (C) 2001 Elsevier Science Ltd. All rights reserved.