Acid Condensation/Heater Basket

 

Three air heater basket samples were received for failure analysis. The samples were specified as 22 gauge, It was a proprietary low-alloy corrosion-resistant steel (LACR). These baskets typically last at least 10 years or more, depending upon conditions within the unit. These failed within two years of service. Approximately a year ago, the plant started injecting activated carbon in the flue gas and calcium bromide (CaBr2) in the coal steam to help with mercury removal.

The air heater consists of four stages (1 hot, 2 intermediate and 1 cold end). The failed samples were removed from the cold end stage. The flue gas inlet temperature to the air heater is about 780 oF and exit temperature is about 290oF. The air enters about 70oF and leaves at 691oF. The cold end basket temperature triggered an alarm, sometimes suggesting the temperature may have dropped below the acid dew point corrosion point. However, with the addition of CaBr2, there may be potential for this to also be condensed or deposited. There is no SCR system in the unit for NOx control. The sulfur content in the coal is about 0.3%. There is a sootblower that cleans these baskets as needed to keep them free of ash pluggage. There is a concern that there may be a small amount of water sprayed at the beginning of the sootblower cycle from condensation in the soot blower pipe.  

SAMPLE 1 SAMPLE 2 SAMPLE 3
 
The samples were randomly named as Samples 1, 2 and 3 for identification. Sample 3 had a very light, black-colored smooth deposit on both sides. Severe corrosion was observed on all three samples.

Elemental analysis was performed on a black deposit that was scraped off the surface. Analysis was via energy dispersive spectroscopy in the scanning electron microscope (ASTM E1508-98((03)). Carbon and sulfur were determined by Combustion Analysis (ASTM E1019-08). For the anion analysis (ASTM D4327-03), the deposit was scraped from the sample, weighed, and ultrasonically shaken in warm deionized water for 30 minutes. The sample was then filtered through a 0.2 micron filter and the detectable fluoride (F-), chloride (Cl-), nitrate (NO3-), nitrite (NO2-), phosphate (PO43-) and sulfate (SO42) contents of the water-leachable portion of the sample was determined by ion chromatography (IC). Note that the ionic concentrations result from the amount of dilution of the sample, and so are not absolute numbers.

 
  

                         Deposit Weight %                         

 

 Element

 

 Sample 3  

 
 Anion 
 
ppm
 
                     
                          
 
 

Iron

 

64.6

  Sulfate(SO42-)   13740  
 

Oxygen

 

30.7

 

Bromide (Br-)

 

559

 
 

Aluminum

 

1.1

 

Chloride (Cl-)

 

93

 
 

Sulfur

 

1.0

 

Fluoride (F-)

 

78

 
 

Calcium

 

0.5

 

Nitrate (NO3-)

 

20

 
 

Chromium

 

0.5

 

Nitrite (NO2-)

 

Not detected

 
 

Manganese

 

0.5

 

Phosphate (PO43-)

 

Not detected

 
 

Silicon

 

0.3

         
 

Sodium

 

0.3

                           
 

Carbon

 

0.2

         
 

 Phosphorus 

 

0.2

         
 

 Magnesium

 

0.2