Boiler Oxygen Scavenging 

     As a general rule of thumb, oxygen removal in boiler feedwater is necessary, but what treatment methods are used and how are they utilized?

 

     Oxygen corrosion can occur anywhere prior to the boiler, in boilers or condensate return systems and is the direct consequence of dissolved oxygen present in the feedwater. In addition to weakening the metal, this attack generates metallic oxides that create problematic deposits. And while oxygen provides a major pitting challenge on its own, additional forms of corrosion are accelerated by its presence. There are two methods of deaeration: mechanical means and internal treatment. As mechanical deaeration has been addressed in several articles, this one will focus on oxygen scavenging.

 

     Because is it not feasible to remove all dissolved oxygen by mechanical means, introduction of chemical agents are effective at removing trace amounts of oxygen by chemical reaction. The family of sulfites (whether sulfite or bisulfite, sodium or potassium) have become the most recognized workhorses and first choice for scavenging. Sodium sulfite is relatively cheap and is typically fed to not only remove oxygen, but to be sufficiently present to constitute a comfortable excess. The reaction of sodium sulfite with dissolved oxygen is as follows:

Catalyzation

     

     Sulfites are available as is or catalyzed which is recommended to speed up the oxygen scavenging reaction rate. This sodium sulfite/oxygen reaction is inhibited typically by chelants, by feedwater contaminants, or treatment chemicals. Metabisulfite when mixed with sulfite helps to lower the pH and improve solubility to catalyze the reaction. The reaction rate is often enhanced by the addition of a small quantity of cobalt salt as a catalyst. However, this catalytic action can be deactivated by several products commonly used for boiler treatment. For instance, the addition of catalyzed sulfite and any caustic product to the same feed location is likely to result in deactivation of the catalyst as the cobalt precipitates as an insoluble hydroxide. Cobalt precipitation occurs by phosphates or when pH values are above 8.3. One method to avoid deactivating the catalyst is to feed oxygen scavengers upstream of other chemical injections. Additional factors determining the rate at which this reaction takes place are the purity of the water, the residence time, location of the chemical injection, dosage, the temperature of the water, its pH, and presence/concentration of catalysts. The reaction rate for sulfite appears to be the fastest of all the chemical scavengers, followed by erythorbic acid and DEHA.

 

Diethylhydroxylamine (DEHA)

Diethylhydroxylamine (DEHA) formula

     DEHA is the most popular of the organic oxygen scavengers and its use has grown steadily. It is able to both pick up the excess oxygen and form a protective surface on both steel and copper surfaces. At elevated temperatures some side products like acetic acid, acetaldoxime and acetaldehyde may form. To compensate for the possible presence of these weak acids, sufficient neutralizing capacity must be present within the treatment program to prevent any corrosion from occurring. Another benefit from using DEHA is its capability to carry through steam and the condensate cycle which provides added protection. Using this chemical can be pricier in comparison to sulfites but is often utilized for its steamline capabilities.

 

Erythorbic Acid

Erythorbic Acid formula

     The reaction of erythorbic acid with oxygen produces water, and the main reaction is shown above. One downside to erythorbic acid is the amount required to react with oxygen. For each mole of oxygen (32g), 143g of erythorbic acid is required compared to 40g of DEHA. Another flaw is the formation of weak acids, including carbon dioxide, from its degradation.

 

Dosage & Feeding

 

NOTE: 1 ml/L dissolved oxygen = 0.0715 mg/L (or lbs oxygen/million lbs water) [0.05 x 1.43 = 0.0715]. This requires 0.58 lbs of sodium sulfite/million pounds of water [0.0715 x 8.12 = 0.58], or 4.83 lbs of sodium sulfite/million gallons of water [0.58 x 8.33 = 4.83].


     Consideration should be given to methods of introducing scavengers to achieve a max speed of reaction. Ideally, oxygen scavengers are fed continuously to provide a constant blanket of oxygen removal. To ensure that ample time for reaction is given, chemicals should be introduced as far upstream from the boiler as possible. Keep in mind that oxygen corrosion can and will occur within feedwater operations as well, especially when quality water and sufficient concentration cycles are achieved. While the goal is to remove dissolved oxygen, a sufficient residual is typically recommended to handle any unpredictable incursions. Slight over-feeding will also provide a means for testing/confirmation downstream that enough scavenger has been introduced. As mentioned earlier, oxygen scavengers will have less favorable reactions with other treatment chemicals, and combining these may reduce the speed of reaction.

 

     Of boiler water testing, one of the common parameters is to test for sulfite (SO3) in the boiler. Testing for sulfite is a method to confirm that an adequate chemical residual is present. Although less popular, I believe it is just as necessary to test sulfite in feedwater settings to monitor its performance. Where little water is used, testing for sulfite in feedwater will help confirm that chemical is being fed proportionally. A sulfite reading from a boiler in this scenario may not provide the whole picture if the DA goes periodically without the presence of a scavenger. The traditional specification with sulfite tends to go as high as 30-60 mg/L in a boiler but can be reduced with sufficient data and the use of an online dissolved oxygen analyzer. It's not unusual to see plants running DEHA scavenger as high as 1-2 mg/L (ppm) to maintain some degree of certainty. With testing for scavengers, it is necessary to follow testing procedures as not testing immediately can provide an inaccurate measurement. Increasing the time between collection and testing will result in the reduction of scavenger due to the reaction with ambient oxygen.

 

     Oxygen scavengers play an integral role in proper boiler water treatment. As mentioned, dissolved oxygen left uncontrolled can form oxygen corrosion cells that can result in localized pitting, boiler corrosion and lead to costly maintenance. With proper dosage, feeding, and monitoring techniques, these issues can be avoided.

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     Jed Kosch

 

 

List of Resources:

 

[1] Cavano, R. R. (2007). Sulfites for Oxygen Control. Scanton Accosiates. Published. http://scrantonassociates.com/soc.pdf

 

[2] Zhao, B., Li, Y., Tong, H., Zhuo, Y., Zhang, L., Shi, J., & Chen, C. (2005). Study on the reaction rate of sulfite oxidation with cobalt ion catalyst. Chemical Engineering Science, 60(3), 863–868. https://doi.org/10.1016/j.ces.2004.09.064

 

[3] Silbert, M. D. (2006). Garratt-Callahan Water Treatment and Reference Manual. Marvin Silbert & Associates.

 

[4] Boiler Water Oxygen Scavengers – Sulphite, Tannin, DEHA, Carbohydrazide. (2017). Accepta. https://accepta.com/environmental-water-wastewater-knowledge/water-treatment-knowledge/312-boiler-water-oxygen-scavengers-sulphite-tannin-deha-carbohydrazide

Sodium sulfite reaction with oxygen