Understanding Corrosion Mechanisms in Oxy-Fired Systems Bruce A. Pint Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, TN 37831-6156 E-mail:
[email protected]; Telephone: (865) 576-2897; Fax: (865) 241-0215 Sebastien Dryepondt Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, TN 37831-6156 E-mail:
[email protected]; Telephone: (865) 574-4452; Fax: (865) 241-0215 Ying Zhang Dept. of Mechanical Eng, Tennessee Technological Univ., P.O.Box 5014, Cookeville, TN 38505-0001 E-mail:
[email protected]; Telephone: (931) 372-3265; Fax: (931) 372-6340 ABSTRACT Replacing air with oxygen in coal-fired boilers (i.e. oxy-firing) combined with flue gas recirculation is a leading strategy to concentrate CO2 and assist in carbon capture and sequestration. A significant area of concern is the fireside corrosion with oxy-firing due to the higher CO2 levels in the combustion gas and potentially higher SOx and H2O levels. In order to investigate this complicated issue, laboratory experiments are being conducted with and without synthetic ash to assess the potential effect of oxy-firing on fireside corrosion rates. The initial results of this project focus on commercial and model Fe-base alloys at 600°C. Without ash, a 50%H2O-50%(CO2-0.15O2) environment was the most aggressive condition, requiring higher Cr contents than 100% H2O or Ar-50%CO2. With the specimens covered in ash, several gas compositions were examined, including different levels of H2O and SO2 to simulate various oxy-firing strategies. Results also are presented for several laser-clad coating compositions for protecting tubes. An additional task is examining the effect of environment on mechanical properties. Initial work studied Ni-base alloys in steam at 800°C and found little effect of steam on the creep rupture life of alloy 230 but a 35% decrease for alloy 740. INTRODUCTION Using oxygen rather than air for coal-fired boilers is a leading concept for enabling large-scale carbon capture and storage.1,2 However, there is considerable concern about the effect of oxy-firing on fireside corrosion rates and several studies have already considered the effect of higher CO2, H2O and SO2 levels.3-5 The fireside environment with combustion gas and ash chemistry dependent on the coal composition is a difficult environment to simulate, including oxidizing and reducing “micro-climates”.6 In order to better understand the effect of oxy-firing on the fireside corrosion mechanism, laboratory simulations are being conducted both with and without synthetic coal ash. In the initial stages of this project, the experiments are focusing on 600°C, where ferritic-martensitic (FM) steels are the primary candidates. (Future experiments will cover higher temperatures.) Along with candidate alloy specimens, model Fe-Cr and Fe-Cr-Ni specimens also were exposed to gain a better understanding of the effect of composition on the reaction rates. In addition to the fireside experiments, a few steam-side experiments also are being conducted. Relevant to current operating conditions, coupons were exposed for up to 5,000h at 550°C in 17bar steam. To understand the effect of steam on creep properties, an in-situ creep rig was constructed and initial experiments were conducted on Ni-base alloys at 800°C and compared to ex-situ results from creep specimens exposed in steam prior to creep testing.
EXPERIMENTAL PROCEDURE For the gas-only oxidation experiments, a range of Fe- and Ni-base alloy substrates (~19 x 10 x 1.5 mm) with compositions listed in Table 1 were exposed at 600°C in 500h cycles to three environments: Ar50%(CO2-0.15O2), 100% H2O and H2O-50%CO2-0.1%O2. The specimens were heated and cooled to temperature in an Ar environment. Oxidizing gas was introduced when the tube furnace had reached temperature. Specimen mass changes (±0.02 mg/cm2 accuracy) were measured after exposure using a Mettler Toledo model AG245 or XP205 balance. The fireside corrosion experiments were primarily conducted with Fe- and Ni-based alloy rod specimens (6mm diameter x ~25mm long) in a porous alumina crucible with 9g of synthetic ash (30%Fe2O3-30%Al2O3-30%SiO2-5%Na2SO4-5%K2SO4) covering the specimen. Additional experiments were conducted on 309 and 8020 (Table 1) weld overlay specimens (~20 x 12 x ~1.5 mm) provided by an industrial partner that were removed from a steel tube substrate. These specimens were exposed in the same atmosphere with and without ash coverage. The exposure was at 600°C for 500h with an air-firing gas of N2-16%CO2-10%H2O-3%O2-0.15%SO2 and oxy-firing gas of CO2-5%N2-32%H2O-3%O2-0.45%SO2, additional oxy-firing experiments were conducted with only 10%H2O or 0.15%SO2. After exposure, the ash was removed before weighing. For characterization, specimens were metallographically sectioned and polished. In some cases, Cu plating was used to protect the scale. RESULTS AND DISCUSSION GAS-ONLY OXIDATION EXPERIMENTS Figure 1a shows the mass gain data for the 550°C 17 bar steam exposures as a function of alloy Cr content. There were multiple specimens of each alloy so in some cases there are multiple data points for the same alloy, especially at the shorter times. Surprisingly, the mass gain for Gr.22 (2.3Cr) was not higher than for Grades 91, 92 or 122 with 8-11%Cr. However, scale spallation could affect these values. At higher Cr
Table I. Alloy chemical compositions (mass% or ppmw for S) determined by inductively coupled plasma and combustion analyses.
Material
Fe
Ni
Cr
Co
Mn
Si
Other
Gr.22 95.5 0.23 2.3 0.01 0.55 0.13 0.9Mo, 0.2Cu Gr.315 93.4 0.14 2.9 0.01 0.33 0.26 1.7W,0.7Mo,0.2V Gr.91 89.7 0.13 8.3 0.01 0.34 0.13 0.8Mo,0.3V Gr.92 87.2 0.13 9.2 0.01 0.45 0.16 1.8W,0.2V,0.5Mo MARB13 82.9 0.01 9.5 3.4 0.51 0.75 2.6W,0.2V,0.01B MARB22 82.9 0.01 9.4 3.4 0.51 0.76 2.6W,0.2V,0.02B Gr.122 84.7 0.35 10.6 0.02 0.67 0.12 1.9W,0.2V,0.4Mo SAVE12 83.4 0.27 9.6 2.6 0.43 0.08 3.0W,0.3V 410SS 86.9 0.11 11.8 0.02 0.52 0.37 Fe-15Cr 85.1 < 14.8 < < < Fe-20Cr 80.3 < 19.7 < < 0.01 347HFG 66.0 11.8 18.6 0.23 1.5 0.39 0.8Nb,0.1V,0.2Mo 310HCbN 51.3 20.3 25.5 0.34 1.2 0.31 0.4Nb,0.1V,0.1Mo 309* bal. 13.6 22.6 0.8 0.6 8020* 80 19.3 230 1.2 60.1 21.9 0.14 0.49 0.37 14.1W,1.2Mo,0.3Al 617(CCA) 0.6 55.9 21.6 11.3 0.02 0.12 8.6Mo,1.3Al,0.4Ti 740 1.9 48.2 23.4 20.2 0.32 0.45 0.8Al,2.0Ti,2.1Nb < indicates below the detectability limit of
