Closed loop operation of power plant equipment can result in concentration of selected ions. In order to illustrate the concentration phenomina that occurs in closed loop operation, an analogy is being made to a flue gas desulfurization system in this paper. The information presented in this paper was obtained on the closed loop operation of z flue gas desulfurization (FGD) system is described with all streams In and out of the closed loop defined and monitored. The composition of the coal remained uniform during the test period of 60 days. The water balance of the FGD system was maintained closed loop during the test period. Closed loop FGD operation means the only water leaving the FGD system is through evaporation in the cooling and saturating of the flue gas and the10 SS of water in the Solids which are removed from the FGD system.
Closed-loop operation of large systems on fossil-fueled power plants under controlled conditions are difficult to establish and to maintain. In order to illustrate the concentrations effects on monitored chemical ions in a boiler closed-loop operation an analogy is presented to an actual closed-loop operation of a flue gas desulfurization system. This was the first closed-loop FGD system maintained for 60 continuous days in the coal-fired power industry.
In order to establish the closed-loop operation the power output of the boiler was maintained at a constant load while burning an uniform coal . The flue gas flow through the absorbers during this study was held constant at 400,000 acfm. And additional gas flow was bypassed around the FGD system. This constant operation condition, while unusual, promoted a uniform increase in the concentrations of the various ions in the FGD system.
After the flue gas left the boiler it was pulled through an induced draft fan. The discharge from the induced draft fan raised the pressure of the flue gas. After leaving the induced draft fan the flue gas entered a venturi rod scrubber.
The venturi rod scrubber contained a recirculating limestone slurry. The solids in the limestone slurry were approximately 15 percent. When the flue gas contacted the limestone slurry, a portion of the sulfur dioxide in the flue gas was physically transferred to the limestone slurry. Once the sulfur dioxide was physically in the limestone slurry, it began to be adsorbed into the water in the slurry. In the process of the sulfur dioxide being absorbed into the water it was converted to the bisulfite ion. In the process of the limestone dissolving into water it had formed the bicarbonate ion. A reaction then occurred between the bisulfite and the bicarbonate ion resulting in the formation of calcium sulfite hemihydrate. Actually, the calcium sulfite hemihydrate that formed was a solid solution containing calcium sulfate in the calcium sulfite hemihydrate crystals.
In addition to removing sulfur dioxide from the flue gas, several other ions were adsorbed into the limestone slurry. Some of these ions included: chlorides, fluorides, nitrates, sodium, potassium, and numerous others.
Samples were taken of the coal and limestone being consumed during this period of chemical testing. The coal samples were analyzed by the utility. The limestone samples were analyzed by the university performing the FGD chemical analysis.
All of the coal burned in. the boiler came from a mine owned by the utility and was washed at the mine prior to shipment to the power plant. All of the limestone came from the same seam of one quarry. The limestone contained soluble magnesium carbonate. The addition of magnesium to the FGD slurry resulted in increased removal of sulfur dioxide from the treated flue gas.
