Advanced Energy Systems — Advanced Combustion
Conventional coal-fired power plants utilize steam turbines to generate electricity, which operate at efficiencies of 35–37 percent. Operation at higher temperatures and pressures can lead to higher efficiencies, resulting in reduced fuel consumption and lower emissions. Higher efficiency also reduces CO2 production for the same amount of energy produced, thereby facilitating a reduction in greenhouse gas emissions. When combined, oxy-combustion comes with an efficiency loss, so it will actually increase the amount of CO2 to be captured. But without so much N2 in the flue gas, it will be easier and perhaps more efficient to capture, utilize and sequester.
NETL’s Advanced Combustion Project and members of the NETL-Regional University Alliance (NETL-RUA) will conduct laboratory through pilot-scale tests of advanced oxycombustion capture technologies that show, through engineering and systems analysis studies, continued achievement toward the goal of 90 percent CO2 capture at no more than a 35 percent increase in electricity cost.
Advanced Combustion Research Overview
Experience with steam boilers has provided information on existing boiler alloys, but limited data is available at higher pressure and temperatures, or with alloys not used in existing boilers.. The Advanced Combustion Project addresses fundamental issues of fire-side and steam-side corrosion in oxy-fuel combustion environments. NETL’s advanced ultra-supercritical (A-USC) steam autoclave provides a unique capability to examine long-term steam oxidation as a function of pressure, including A-USC conditions.
Oxy-combustion Environment Characterization
Laboratory and field testing will address fire-side corrosion issues during oxy-fuel combustion with emphasis on design considerations, such as flue gas recycle paths and the use of staged combustion burners. To better understand the effect of geometry of the laboratory specimen on scale morphology and exfoliation behavior, research has been pursued on the effects of oxide scale constraints (i.e., comparing tube sections with flats), atmospheres (steam, moist air, dry air), and pressure. Scale morphologies from the lab tests will be compared with utility exposures to assess the importance of each test variable for acceptance of laboratory data in scale exfoliation models with time spent on understanding the variation that exists between the two approaches.
Results of this work will provide information on the performance of materials in oxy-fuel combustion environments, which is necessary to select the most cost effective materials for boilers and steam turbines. Research on steam-side corrosion addresses the effect of pressure on steam oxidation in A-USC conditions for test alloys and determines the role of each oxidant in mixed-oxidant conditions. Evaluating the effect of pressure on steam oxidation is important because this aspect is poorly understood for existing boiler alloys and there is no industrial experience for existing alloys in A-USC steam. At 700°C, increases in CO2, H2O, and SO2 that are associated with oxycombustion were found to have little effect on corrosion loss (see Figure 1). The scale morphologies consisted of an inner oxide scale of iron and chromium oxides, an outer oxide scale of iron oxide, and an oxide deposition zone of iron oxide that had dissolved from the outer oxide and deposited within the ash (see Figure 2).
Figure 1. Metal loss for T91, TP347 and alloy617 exposed at 700°C in oxidative
atmosphere with thick ash cover, simulating air-fired and
three (3) oxy-fired superheater/reheater conditions.
Figure 2. Commercial boiler tube alloy T91 after exposure at 700°C for 240 hours in Oxy,
FGD with 20 percent H20.
High pressure steam oxidation tests were conducted on a variety of candidate alloys for A-USC components at 670°C and 267 bar for 293 hours in flowing steam (and these were subsequently compared to an atmospheric pressure test at the same temperature and of the same duration). Five of six Ni-base alloys in the first high pressure steam exposure test campaign showed a small, yet significant, increase in mass gain upon exposure. SEM analyses showed this increase to be due to thicker oxide scales. These oxide scales, however, still consisted primarily of chromia. The sixth Ni-base alloy, alloy 625, showed an extremely large increase in mass. This is best seen using low magnification light microscopy, Figure 3. The scale formed at 1 bar was chromia; however, the scale formed at 267 bar had a metal content similar to the base alloy composition. For this alloy, at this temperature and pressure, it seems that it is harder to establish a protective Cr scale. While alloy 625 did not have the lowest Cr content of the Ni-base alloys, it combined a relatively low Cr content with low Al content. Even at low concentrations, Al has been shown to enhance chromia scale formation and improve oxidation resistance.
Figure 3. IN625 after exposure to steam at 670°C for 293 hr at 1 bar (a) and 267 bar
(b). The 1 bar scale was chromia; the 267 bar scale had a metal content similar to the alloy.
The marker is 50 μm.
Alloy Manufacturing and Process Development
Research is aimed at designing alloys and developing manufacturing processes that can be practically and economically utilized to produce full-scale components for deployment.
Work in alloy design and large-scale castings and forgings has focused on alloy design to produce large-scale heat resistant Ni-based and steel castings as well as Ni-based and steel forgings. Collaboration with industry partners has been pursued to facilitate the supply and process development of large-scale Ni-based superalloy castings for power plant applications.
NETL has developed a unique computational homogenization heat treatment approach based on the dendrite arm spacing distance in a casting that allows for a desired degree of homogenization in that particular casting based on the heat treatment facilities available at the foundry. This heat treatment step can be individualized for the specific casting of interest and facilitates full development of cast article mechanical strength, especially if strengthened with a second phase precipitate like γ′in nickel superalloys. Consequently, a range of castings have been homogenized in the commercial sector using this approach. The following three castings have been made from Haynes 282 and subsequently homogenized to using the schedule shown to reduce segregation and improve mechanical properties and long-term microstructure stability.
Flowserve Step Block (1000#): 1100°C/6 h + 1200°C/48 h.
Special Metals ESR/VAR (10,000#): 1133°C/4 h + 1190°C/8 h + 1223°C/30 h.
NETL has also been working on developing superior heat resistant 9-12 percent Cr ferritic/martensitic steels with creep life better than existing commercial alloys. Research to date has led to CPJ-7, a ferritic/martensitic steel that could be used for both boiler and steam turbine components.
Impact and Benefits
Advances in oxy-combustion technology will be instrumental in improving power generation efficiency while lowering the amount of CO2 produced and concentrating the CO2 effluent streams, making them suitable for carbon capture and sequestration. The Advanced Combustion Project represents an integrated, cross-functional approach and will lead to a better understanding of environmental and mechanical behavior of alloys.
The development and refinement of an algorithm for combining microstructural features in a casting related to dendrite arm spacing and the utilization of computational modeling tools built around thermodynamics and kinetics has allowed substantial improvement in the stability of heat resistant alloys for high temperature energy applications. In particular the complete homogenization of steels and nickel-base alloys ameliorates the development of undesirable phases in these alloys that robs them of long-term strength by reducing and/or eliminating residual segregation in the alloys after casting and prior to thermomechanical processing and subsequent heat treatment. This better stabilizes the resultant microstructure and delays temperature and stress related evolutionary changes.