• The use of alternative absorbents
• The influence of additives (like adipic acid) on removal efficiency  
• The optimum amount of oxidation air needed
• The risks of limestone blinding by fluoride complexes
• The influence of supply water quality
• The gypsum quality as a function of several process parameters.  

Alternative absorbents
The FGD pilot plant is used to test the influence of alternative limestone on the SO2 removal efficiency and the gypsum quality. Based upon a large variety of tests a computer model has been developed (in AspenPlus) to calculate the effect on performance of alternative absorbents in the full-scale FGD plant. 

High SO2  loads
Additives are attractive to be applied at high SO2 load conditions. The influence of additives like formic acid and adipic acid on the SO2 removal efficiency, the gypsum quality and the waste water have been investigated. The SO2  removal efficiency as a function of the additive concentration, the consumption of different additives as function of the FGD process conditions, the related influence on the gypsum quality and performance of the waste water treatment plant have been defined. 

Oxidation air
In the limestone gypsum process, forced oxidation is generally used to obtain a complete oxidation of sulphite into sulphate (gypsum). However, in the flue gases a large excess of O2 is already present compared to the O2 needed for complete oxidation of sulphite. In the FGD pilot plant it was demonstrated successfully to reduce the amount of oxidation air required for complete sulphite oxidation by using special additives. The results have been evaluated by demonstration at a full scale Dutch FGD plant.

Limestone blinding
The blinding of limestone particles by alumina-fluoride complexes is a major concern to many operators of FGD plants: in this particular case the addition of extra limestone does not increase the pH and the SO2 removal efficiency, but will lead to off-spec gypsum. In the FGD pilot plant a number of tests have been carried out to better understand the blinding phenomenon. The results have been implemented in the operating procedures that are followed by Dutch power plants.

Supply water quality
The influence of the composition of the supply water on the performance of the FGD process has been studied. It has been concluded, that the composition of the supply water can seriously influence the SO2 removal efficiency and especially the gypsum quality. KEMA specifies the supply water quality requirements for different applications in the FGD process, as mist eliminator cleaning, lime preparation and gypsum rinsing.

Gypsum quality
During most of the FGD pilot plant tests, the parameters that influence the gypsum quality were examined in detail. The quality of the gypsum is influenced by:

• The quality of the absorbent
• The quality of the supply water
• The composition of the flue gas and
• The fine tuning of the dewatering equipment.