An experimental investigation of the effects on plant performance of converting spray dry absorption product (SDAP) to gypsum in a wet flue gas desulphurisation (FGD) pilot plant, based on the falling film principle, has been conducted. At concentrations up to 300 g SDAP/l (feed tank basis), no scaling or plugging of pipes was observed. The SO2 removal efficiency increased from 69 to 76.5% when increasing the concentration of SDAP in the feed tank from 0 to 300 g/l. At the same time, the average limestone slurry feed rate decreased from 1.62 to 0.53 l/h as a consequence of the residual content of Ca(OH)(2) and CaCO3 in the SDAP. The compounds, CaCl2.2H(2)O NaF, CaF2 and Al(OH)(3) were added, in various concentrations, to the feed tank to investigate the effects of Cl-, F-, and Al3+ on the limestone reactivity. The degree of desulphurisation (in the presence of 200 g SDAP/l) increased from 75 to 78% with an increase of Cl- concentration from 0 to 30 g/l and simultaneously the residual limestone content of the gypsum increased from 2.8 to 15.8 wt%. The effects on SO2 removal efficiency of adding F- and Al3+ to the system were within the experimental uncertainty. However, the residual limestone content of the gypsum increased from 11.7 to 19.9 wt% with an increase of F- concentration from 0 to 800 mg/l, and from 19.9 to 24.2 wt% with an increase of Al3+ concentration from 0 to 378 mg/l. The investigation has shown that SDAP can be used as a sorbent in wet FGD plants and the sulphite converted to gypsum, but the observed increase in the residual limestone level of the gypsum may give some problems in utilization of the product. (C) 2002 Elsevier Science Ltd. All rights reserved.