Corrosion Testing using Oxy-Fuel Combustion for Ultra-Supercritical Steam Boilers
Conventional fossil-fueled power plants have something in common with the steam engines that powered the first locomotives, the candles that illuminated medieval cathedrals, and even the cooking fires of the Paleolithic Age. They all use air in the combustion process. But instead of air, Office of Research and Development scientists are looking for ways to use nearly pure oxygen in the combustion process. This method, called oxy-fuel combustion, reduces nitrogen oxide emissions. It also produces a largely unadulterated stream of carbon dioxide (CO2). “You want to capture the CO2,” explains ORD researcher Gordon Holcomb. “It is much simpler to do that without all of the nitrogen you get from air firing.”
The environment inside a steam boiler includes coal ash and flue gas. The ash and gas react with the stainless steel boiler tubes (in this case the Fe-9Cr alloy T91) to form oxide scales. One aim of alloy design is for the oxide scales that form to act as a barrier to further degradation of the boiler tube. In this polarized light image, the oxide scale is providing some protection, but is porous and is seen to be dissolving into the coal ash.
Holcomb and his research partners recently investigated how tubing for advanced ultra-supercritical (USC) steam boilers holds up when it is exposed to oxy-fuel combustion at 700 degrees Celsius (°C). Conventional boilers operate at around 600 °C, but “there’s a big push by the Department of Energy (DOE) to raise that up to 760 °C,” says Holcomb. “You get a big increase in efficiency by doing that.” This team found that the oxy-combustion conditions did not cause boiler tube alloys to corrode any faster, including the nickel-based alloy—Inconel 617—currently planned for advanced USC steam boilers. “In the past, most fossil fuel boilers for steam power generation have been made of iron-based materials,” says ORD researcher Joe Tylczak. “DOE’s goal of 760 °C steam temperatures for boilers makes use of nickel-based materials, like Inconel 617, for parts of the boiler a necessity. There is insufficient knowledge about nickel alloys’ corrosion resistance at these conditions to know what its useful lifetime will be. We are trying to collect this information.”
To gain these and other insights, Holcomb has collaborated with many scientists, including National Energy Technology Laboratory colleagues Tylczak, Sophie Bullard, Bret Howard, and Casey Carney. Other collaborative partners include: physical metallurgy professor David Laughlin and postdoc Adam Wise from Carnegie Mellon University; Gerald Meier, a professor of mechanical engineering and materials science at the University of Pittsburgh (Pitt); and Brad Lutz, a Pitt graduate student.
The team presented its findings in March 2014 at the Fireside Coal, Air, and Oxy Fired Corrosion Testing R&D conference in Chicago, Illinois.
Contact: Gordon Holcomb, 541-967-5874
NETL’s Corrosion Erosion Facility Tests Materials in Severe Environments
According to NETL researcher Joe Tylczak, NETL’s Severe Environment Corrosion Erosion Research Facility (SECERF) is hot. “We have a capability that is unique in NETL—and semi-unique in the world.” There aren’t many labs where “people can take a whole blend of different gases, put them in a high-temperature environment, and test materials in it,” but that’s just what SECERF makes possible.
Researcher Joe Tylczak prepares a sample for testing in the SECERF.
The lab’s furnaces subject materials to intense heat so that scientists can see how they behave at different temperatures and in the presence of as many as 11 different gases. Because SECERF mimics the highly corrosive or erosive conditions found in a power plant, it helps researchers see how materials would behave in one without disrupting an actual plant’s operations.
Located at NETL’s Albany site, SECERF gives scientists a way to safely and conveniently run experiments involving toxic, flammable gases that would be incompatible with equipment found in other labs. The lab features a safety system that detects gas leaks both inside and outside of the lab’s six research modules.
Generally, SECERF has been used to support advanced combustion research, including investigations into oxy-fuel combustion, but its applications don’t end there. “We’ve done work for hydrogen membranes,” adds Tylczak, “trying to investigate how they react to hydrogen sulfite in the gas. We’ve been doing work with [gasifier] refractories to look at why thermocouples are failing.” In addition, onsite Office of Research and Development researchers at SECERF have tested the durability of solid-oxide fuel cell sensor materials.
The range of experiments that SECERF supports is evidence that the lab isn’t tied to a particular project. “We seem to be continuously doing modifications to support different work,” says Tylczak. “For instance, we’re in the midst of making a modification so we can do work to further the understanding of the interaction of molten slags with refractory materials.” Definitely keeps it interesting.
Contact: Joe Tylczak, 541-967-5849