In limestone-gypsum flue gas desulfurization systems, the consumption of a desulfurization agent (lime) is controlled by using online pH meters. Of great concern in the pH measurement is heavy lime scaling on the pH electrode. To ensure accurate measurements, frequent cleaning of the electrodes with acid is required, adding to both maintenance workload and cost.
The EXA AUTO CLEAN chemical cleaning system automates the acid cleaning process, which not only saves both time and expense but also ensures precise pH measurement over long periods.

-	Improves the efficiency of a limestone-gypsum flue gas desulfurization system
-	Ensures stable, continuous pH measurement
-	Reduces operating costs
-	Eliminates manual cleaning

The limestone-gypsum flue gas desulfurization consists of two processes: absorption and oxidation.

Absorption process
Reaction formula; Ca(OH)2 + CO2 -> CaCO3 + H2O, 2CaCO3 + 2SO2 + 1/2H2O -> 2CaSO3·1/2H2O + CO2
In this process, it is important to promote the reaction of calcium (Ca) and to achieve a target desulfurization rate. The absorption rate is affected by the pH, concentration, and temperature of liquid and the liquid/gas ratio. When the pH of the absorbing solution is below 5, calcium hydrogen sulfite (Ca(HSO3)2) is generated. Ca(HSO3)2 is converted into sulfur dioxide (SO2) in a reversible reaction, thereby reducing the desulfurization rate. When the pH of the absorbing solution is 7 or higher, carbon dioxide (CO2) in the flue gas reacts with calcium (Ca) to produce calcium carbonate (CaCO3). This increases lime consumption. Since the pH has an influence on the subsequent oxidation process, the pH of the absorbing solution should be kept to between 5 and 6.

Oxidation process
Reaction formula: CaSO3·1/2H2O + 1/2O2 + Aq -> CaSO4·1/2H2O + Aq
In this process, calcium sulfite (CaSO3·1/2H2O) generated in the absorption process is oxidized with air to calcium sulfate (CaSO4) under a pressure of approximately 500 kPa and a temperature of 50 to 80 °C. The pH of the absorbing solution in the absorber is approximately 6. It should be kept at around 4 by adding acid from the cooling tower.

Field Data
Deposits of crystalline substances (e.g., CaCO3) on electrodes in limestone-gypsum flue gas desulfurization systems decrease the accuracy of pH measurements. The cleaning of the electrodes and the reduction of maintenance time and cost are key points to consider when selecting a pH meter for the system.

	pH System with Chemical Cleaning	General pH Meter
Cleaning	"Automatic acid cleaning: 2 or 3 times/day, user programmable Manual acid cleaning: once/day"	"Manual acid cleaning: approx. monthly"
Calibration	Manual calibration: weekly	"Manual 2-point calibration: weekly"
Other maintenance	Replenishment of chemical tank: approx. every 2 months	-

Acid cleaning is done with a 4% hydrochloric acid solution

Utilities
Power supply:		100 V AC, 50/60 Hz
Power consumption:		approx. 60 VA
Air source: pressure:		300 to 950 kPa
	Maximum consumption:	approx. 20 L/min
Chemical cleaning solution consumption:		approx. 100 mL
	Tank effective capacity:	approx. 17 L
Notes
-	An organic solvent cannot be used for cleaning.
-	Deterioration of materials in chemical solution piping and air piping When the system is installed in a location where it is exposed to direct sunlight, the polyethylene resin piping will last approximately one year. (It is recommended that fluoro resin piping be used.)

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