INTRODUCTION
From 1994-1999 several protective coatings and weld overlays were evaluated in Hatfield's Ferry #2 boiler. This boiler is a universal pressure, once through, supercritical boiler rated at 550 MWe. It burns eastern bituminous coals with a sulfur content of about 2.2%. Prior to the tests the boiler was retrofitted with a low NOx cell burner system (LNCB). During the first round of the testing mainly thermal spray coatings were tested, as expected corrosion rates were relatively low (<25 mils (0.62 mm)/yr). This expectation was based on laboratory corrosion testing as well as results from field testing in the first boiler retrofitted with a low NOx cell burner system. However, corrosion rates were found to be at least 2-3 times higher than expected and the thermal spray coating provided inadequate protection. Thus a second panel containing weld overlays, and diffusion coatings in addition to the most promising thermal spray coatings was installed and tested for 2.5 yrs. Evaluation of the coatings and weld overlays confirmed that the durability of the thermal spray and diffusion coatings was generally inadequate in the most corrosive areas. Further testing is needed to confirm their suitability for less corrosive areas, although some promising results were obtained. The durability of weld overlays depended greatly on their chemical composition, but all showed measurable wear. Weld overlays with a Cr content less than 20% generally corroded quite rapidly. Weld overlays with Cr content above 20% had generally low corrosion rates. The best performing materials had projected life expectancies 2-3 times greater than that of the poorest performing material.
Long term service tests of various coatings and weld overlays were conducted in Allegheny Energy Supply Hatfield's Ferry 9Hatfield) #2 boiler, between 1994 and 1999.
Hatfield #2 is a universal pressure, once through, supercritical boiler, rated at 555Mwe. It burns an eastern bituminous coal with a fixed carbon/volatile matter ratio of about 1.38, 1.3-1.6 Nitrogen, 2.1-2.3% sulfur and about 0.10% C1. It has an opposed wall fired furnace, originally equipped with B&W cell burners. This tightly spaced burner system was originally developed by B&W during the 1960's in response to the demand for highly efficient burner designs with a high heat release. The down side of this efficient design is that it produces high levels of nitrous oxides (NOx), generally in the 0.9- 1.3 lbs/106 Btu range.
The Clean Air Act amendments, passed by Congress in 1990 required a reduction of NOx by at least 50%. In response to this requirement B&W and EPRI with DOE support, started a research program to develop a new burner system able to reduce the NOx by 50%, while minimizing the capital cost of the system as well as operational impacts, such as waterwall corrosion, large increases in unburned carbon in fly ash and CO emissions. The result of this effort was the patented low NOx cell burner system. It received the R&D 100 award in 1994, as one of the 100 best new products commercially introduced during that year. Figure 1-1 shows a schematic of the low NOx cell burner system, as configured for Hatfield boiler #2. The principal difference between it and a standard cell burner system is that the 2 burners in each cell are replaced by a single burner, closely coupled with an overfire air (OFA) or NOx port. In most cases the OFA port is located above the burner, but combustion modeling indicated that it was beneficial to reverse this order for some burners in the bottom row, to reduce the levels of CO and H2S near the sidewalls.
