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.