Development of Metallic Hot Gas Filters

Successful development of metallic filters with high temperature oxidation/corrosion resistance for fly ash capture is a key to enabling advanced coal combustion and power generation technologies. Compared to ceramic filters, metallic filters can offer increased resistance to impact and thermal fatigue, greatly improving filter reliability. A beneficial metallic filter structure, composed of a thin-wall (0.5mm) tube with uniform porosity (about 30%), is being developed using a unique spherical powder processing and partial sintering approach, combined with porous sheet rolling and resistance welding. Alloy choices based on modified superalloys, e.g., Ni-16Cr-4.5Al-3Fe (wt.%), are being tested in porous and bulk samples for oxide (typically alumina) scale stability in simulated oxidizing/sulfidizing atmospheres found in PFBC and IGCC systems at temperatures up to 850° C. Recent "hanging o-ring" exposure tests in actual combustion systems at a collaborating DOE site (EERC) have been initiated to study the combined corrosive effects from particulate deposits and hot exhaust gases. New studies are exploring the correlation between sintered microstructure, tensile strength, and permeability of porous sheet samples. Support is gratefully acknowledged from DOE-FE (ARM) through Ames Lab contract No. W-7405-Eng-82. INTRODUCTION: The Annual Energy Review 2001, released by the US Department of Energy’s Energy Information Administration (EIA), reported that a total of 3,602 billion kilowatts of electricity were consumed in 2001 by the residential, commercial and industrial sectors of the US economy. The review also forecasts that by the year 2020 the demand for electricity could increase by approximately 45% more than the current level. Assuming an average plant size of 300 megawatts, a quick calculation indicates that nearly 1,400 new power-generating plants will need to be constructed to meet the projected demand for electricity. The EIA report also projects that coal will remain a dominant fuel source for the electricity generated throughout the forecasted period. Advanced coalfired power generation systems have been developed to meet the continuously increasing demand for electricity in an efficient and environmentally aware manner. These systems are typically based on either pressurized-fluidized bed combustion (PFBC) or integrated gasification combined cycles (IGCC). Demonstration plants of both systems have been built and evaluated around the world. Compared to conventional coal burning processes, both have demonstrated the ability to achieve higher efficiencies while maintaining very low emission levels. However, both systems rely on delivering particulatefree gas to the downstream system components such as heat exchangers and turbines. Current techniques to remove flue-gas particles include the use of ceramic “candle” filters designed to remove airborne particles 1μm in diameter and larger. Under normal operating conditions, porous rigid ceramic filter elements are periodically back flushed with a pulse of compressed gas to clear trapped “fly ash” particulate to reestablish and maintain extended filtration service. However, studies have shown that the durability of the existing ceramic filters is unacceptably low, and prone to failure when operated in systems that utilize periodic back flushing during operation. Oakey [1] concluded that ceramic filter elements have limitations that are primarily due to their inherent brittleness, long-term microstructural instabilities, and poor thermal fatigue resistance. Metallic filters based on the Fe3Al (iron aluminide) intermetallic compound have been developed as an alternative material for the ceramic hot gas filters. Compared to SiC and Al2O3 ceramic filter materials, those based on iron aluminide have demonstrated adequate corrosion resistance to the gas environments produced by combustion processes. Unfortunately, the iron aluminides are relatively brittle at ambient temperature and their strength at 850° C is not sufficient to resist creep elongation. The Ni-Cr-Al-Fe filter material developed at Ames Laboratory has been shown to offer significant benefits over both the ceramic and iron aluminide materials [2]. The Ni-Cr-Al-Fe alloy exhibited the ability to maintain a nearly equivalent resistance to corrosion as the iron aluminide in initial simulated PFBC sulfidizing/oxidizing corrosion tests at 850° C for up to 1000 hrs. Also, the 850° C yield strength of the bulk Ni-Cr-Al-Fe material was observed to be at least 4 times that of a bulk iron aluminide casting. Subsequent work at Ames Laboratory has demonstrated that through selective alloy design the bulk corrosion resistance (weight change) of the Ni-Cr-Al-Fe alloy can be improved to become comparable to the corrosion resistance of the commercially available Fe3Al alloy when tested in a simulated PFBC gas environment [2,3]. This study evaluated the corrosion resistance of selected metal alloys exposed to an oxidizing/sulfidizing gas environment similar to the environment developed during the IGCC combustion process. RESULTS and DISCUSSION: CORROSION TESTING: The IGCC environment is oxidizing to aluminum but not to iron. As a consequence, the environment is categorized as being “reducing”. Relative to the high temperature oxidizing/sulfidizing PFBC environment, the IGCC system produces an aggressive reducing gas environment at a relatively low temperature, permitting test alloys to include chromia-forming alloys. Also included were Ni-and Fe-based alloys modified with Al+Mo and the Ni-Cr-Al-Fe-base alloys that were found earlier to be highly resistant to the higher temperature (850° C) simulated PFBC conditions (2). An iron aluminide (Fe-15.8 wt/o Al-2.2wt/o Cr-0.2wt/o Zr) sample was also tested to provide a baseline for comparison. A corrosion test, simulating IGCC conditions, was conducted on the alloys listed in Table 1. Prior to testing, each specimen was pre-oxidized in flowing argon (PO2 ̃ 10 -6 atm) gas at 1000°C for 24 hrs. During the experiment, bulk and porous coupon specimens of NiCr-Al-Fe and other alloys were exposed to a simulated IGCC gas environment having an approximate composition (vol.%) of N2 – 24 CO – 5 CO2 – 5 H2O – 14 H2 – 1.3 CH4 – 20 ppm H2S for 1000 hrs. at a test temperature of 650° C. The weight change of each sample was recorded every 250 hrs after being cooled to room temperature to simulate an IGCC system shut down. While at room temperature the samples were repositioned on the sample tray to minimize local furnace variations. Commercial alloys (wt.%) Commercial Name Test Designation Scale Type Fe 15.8Al 2.2Cr – 0.2Zr Fe3Al alumina Fe 22.0Cr 20Ni 18Co 3Mo + (others) Haynes 556 Fe-Cr-Ni-Co chromia Ni 16Cr 4.5Al 3Fe + (others) Haynes 214 Ni-Cr-Al-Fe alumina Ni 30Co 28Cr 3.5Fe 2.7Si + (others) Haynes HR-160 Ni-Co-Cr-Fe chromia Modified alloys (wt.%) Ni 16Cr 9.0Al 3Fe Ni-Cr-2xAl-Fe alumina Ni 16Cr 13.5Al 3Fe Ni-Cr-3xAl-Fe alumina Ni 16Cr 9.0Al 3Fe 20Mo Ni-Cr-2xAl-Mo Al-Mo oxide Fe 17.6Mo 9.4Al Fe-Mo-Al Al-Mo oxide Table 1. Test alloys An overview of the weight change data for the cast alloys is shown in Figure 1. As can be seen, the weight change experienced by these cast alloys are, for the most part, relatively similar. However, the Fe-Mo-Al and the Ni-Co-Cr-Fe alloys appear to react quite differently to this environment compared to the other cast alloy coupons. During the initial 250 hrs. of testing, the Fe-Mo-Al alloy demonstrated a positive weight change, while the Ni-Co-Cr-Fe alloy demonstrated a negative weight change. The weight change during the remainder of the test for either alloy was apparently small. Formatted: 0 250 500 75