Cross section of a KEMCOP probe. The element Pb is present on the white areas.

Optical microscopic image of the fire side tube corrosion and deposit layer.

SEM Backscatter electron image.

Hazardous accumulation of radioactive lead on the water wall tubes in coal fired boilers

W.M.M. Huijbregts*, M.P. de Jong*, and C.W.M. Timmermans** (paper 51)

Anti Corrosion Methods and Materials Volume 47, No 5, (2000), 274-279

(Award for excellence paper of the year 2001 of Emerald Literati Club)

* Based at KEMA, Utrechtseweg 310, PO Box 9035, 6800 ET Arnhem

** Based at NRG, Utrechtseweg 310, PO Box 9035, 6800 ET Arnhem

pdf available

Summary

Dutch power companies recently have become aware of a hazard due to the presence of radioactive 210Pb in their boilers. In an investigation conducted by KEMA in the Netherlands it was discovered that 210Pb isotope accumulates within fire-side deposit layers that form on membrane waterwall tubes.

Because the measured levels of total radioactivity exceeded the governmental regulation limit of 100 Bq/g, refurbishment activities in the boiler, such as renewal of waterwall panels and/or welding repairs because of corrosion damage, are subject to governmental authorization and must be conducted under the supervision of authorised radiation protection officers.

The 210Pb material originates from decay of 238U, which is present in small amounts in coal. During combustion of the coal, heavy metals such as Pb evaporate and are present in the flue gas in the form of Pb, PbCl, PbCl2, PbS, PbS2 or PbSO4, dependent upon the gas environment and temperature. These volatile products subsequently will condense at lower temperatures on the waterwall tubes in the boiler.

In order to track the corrosion and fouling layers on the waterwall tubes, a probe was developed that is placed in the waterwall. The probe can be exchanged during full load operation of the boiler (known as the KEMCOP: KEMa COrrosion Probe). A patent is pending for the system.

Conclusions

- The corrosion of the waterwall tubes on coal fired boilers is accompanied by the deposition of heavy metal chlorides and/or sulfides, such as PbCl2 and PbS.

- A proportion of the condensed total Pb has been found to be the radioactive isotope 210Pb.

- Enrichment due to condensation behavior within wall tube deposits can increase the concentration of radioactive species, and it has been observed that several hundreds of Bequerels per gram of 210Pb can be attained.

- Thermodynamic calculations show that the amount of condensed lead can vary, dependent upon the precise firing conditions within the boiler or incinerator.

- The use of simple low-cost probes in the membrane waterwalls allows the corrosion and build-up of the radioactivity levels within deposits to be tracked easily and conveniently during normal on-load boiler operation.

- The results of this investigation have confirmed that the condensing temperature of lead compounds in oxidizing flue gas environments is relatively high, ca. 880 °C, which normally would be expected to minimize the tendency for lead condensation to take place. However, under reducing combustion conditions, especially in the presence of chloride, the lead condensation temperature decreases drastically to 490 °C and the accumulation of radioactive species within wall tube deposits is highly probable.

- Radioactive concentrations of this degree fall within the scope of statutory regulations for the control and supervision of hazardous nuclear substances, and special precautions are warranted for the protection of personnel engaged in boiler inspection and maintenance activities.

Some interesting Figures in the paper.

The thermodynamically stable constituents of lead in the reducing gas environments R1, R2 and R3. The lead condensation (dewpoint) temperatures are 680, 670 and 495 °C at 500 vppm H2S and respectively 50, 500 and 5000 vppm HCl.

Cross section of one of the deposition probes (KEMCOP) after an exposure period of 4 months. The element Pb is present on the white areas on the interface between the thick porous deposit of fly ash and the outer oxide layer. Although only low levels of total activity of the 210Pb isotope are present on the KEMCOP probe it was possible to identify the contamination with a low background n-type HpGe gamma-spectroscopy system and measure it quantitatively.