Most laboratory corrosion tests simulating waterwall corrosion in coal-fired boilers indicate corrosion rates of carbon and low alloy steels well below those recently found in boilers retrofitted with staged low NOx burner systems, especially those found in supercritical boilers. In this paper we studied the effect of boiler deposits on waterwall corrosion. It was found that FeS rich deposits can increase corrosion rates up to tenfold, under conditions in which iron sulfide can decompose to corrosive gas species. This generally occurs under oxidizing, mildly reducing or alternately oxidizing and reducing conditions. The actual presence of reduced sulfur species was confirmed in separate tests.
Waterwall corrosion has been an occasional problem in coal-fired boilers for as long as such boilers have been used in the industry. The basic failure mechanisms and perceived root causes have been described in great detail in EPRI report TR-105261, Volume 2, Chapter 18 by Dooley and McNaughton (~). Since a significant increase in the extent and severity of waterwall corrosion has occurred since the introduction of low NOx burner systems, especially those featuring overfire airports (OFA's), it is useful to briefly review the generally accepted understanding of the root causes of the wastage.
Under normal, oxidizing operating conditions low alloy or carbon steel waterwalls are protected from rapid wastage by the formation of an iron oxide, usually Fe304 scale. 3 Fe + 2 02 ---~ Fe304 (eq. 1)
The scale thus formed is dense, impermeable to gases and strongly adheres to the tube. Such a scale grows slowly and the growth rate decreases with time. Thus, the corrosion loss by oxidation is so low that metal loss becomes generally negligibly small and tube life is not determined by fireside corrosion. Since the scale strongly adheres to the tube, it is also not easily removed by normal sootblowing operations or mechanical and thermal cycling. The presence of SO2 in the fluegas may increase the porosity and decrease the strength of the scale slightly, but has no major effect on service life.
When the fluegas contacting the waterwall does not contain excess oxygen, and contains significant amounts of CO, sulfur in the coal may be partially converted into H2S instead of SO2. Under such conditions there is also some H2 present in the fluegas and the reactions FeS2 + CO + H20 --+ FeS + H2S + CO2 (eq. 2) S(org) + H2 ~ H2S (eq. 3)
become possible. When H2S is present in the fluegas it will preferentially react with iron in the waterwall tubes to form FeS.
Fe + H2S ---) FeS + H2 (eq. 4)
Already formed Fe304 may also be transformed to FeS.
Fe304 + 3 H2S + CO ~ 3 FeS+ 3 H20 + CO2 (eq. 5)
Thus depending on the relative amounts of H2S, SO2, CO and CO 2 present in the fluegas, the scale formed on the steel may consist of iron oxide (Fe304), mixtures of Fe304 and FeS or nearly pure FeS. The strength and adherence of the scale decrease with increasing FeS content, while its growth rate and permeability increase significantly. The result is increased metal wastage, which becomes the dominant factor in tube life. The weak, sulfur rich scale is also more easily removed by sootblowing or thermal cycling, which may further increase metal wastage.
The major concerns are how high the metal wastage can be for a given boiler condition or low NOx burner system, and what is practically possible to reduce high wastage rates.
From experience in coal gasifiers, it is known that corrosion rates of low alloy steels under extremely reducing conditions, i.e., CO = 30-60%, H2S 2000-10,000 ppm, are a function of the H2S content of the