The effects of adding an organic acid or using a limestone with a fine particle size distribution (PSD) have been examined in a wet flue gas desulphurisation (FGD) pilot plant. Optimisation of the plant with respect to the degree of desulphurisation and the residual limestone content of the gypsum has been the aim of the work. In contrast to earlier investigations with organic acids, all essential process parameters (i.e. gas phase concentration profiles of SO(2), slurry pH profiles. and residual limestone in the gypsum) were considered. Slurry concentrations of adipic acid in the range of 0-7 mM were employed. The overall degree of desulphurisation in the plant increased from 83% at 0 mM to 90% at 3 mM and the residual limestone level was reduced from 4.6 to 1.4 wt%. Increasing the slurry concentration of adipic acid above 3 mM gave only a slightly higher degree of desulphurisation. The wet FGD model of Kill et al. (Ind. Eng. Chem. Res., 37 (1998) 2792) was extended to include buffer systems and verified against experimental data. Subsequently, the model was used as a tool to identify the optimal organic acid dissociation constants (as pK(a) values) and concentration levels at different operating conditions. At a holding tank pH of 5.5 and a temperature of 50 degreesC, simulations with Bryozo limestone and a monoprotic buffer suggested that the optimum pK(a) value is between 4.5-5.5 and 5.5-6.5 with respect to the degree of desulphurisation and the residual limestone level, respectively. Adipic acid has pK(a) values close to these ranges (pK(1) = 4.40 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%. Increasing the holding tank pH level from 5.5 to 5.8 affected the degree of desulphurisation and the residual limestone level only slightly. At holding tank pH levels between 5.88 and 5.90, a high degree of desulphurisation was observed, but the residual limestone content in the gypsum increased to somewhere between 19 and 30 wt%, making this pH range unsuitable for use in a full-scale plant. The investigations have shown that both the addition of organic acids and the use of a limestone with a fine PSD can be used to optimise wet FGD plants. (C) 2001 Elsevier Science Ltd. All rights reserved.