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Lab-scale Investigation of Deposit-induced Chlorine Corrosion of Superheater Materials under Simulated Biomass-firing Conditions. Part 1: Exposure at 560 °C† Simone C. van Lith,‡ Flemming J. Frandsen,*,‡ Melanie Montgomery,§,|,⊥ Tommy Vilhelmsen,| and Søren A. Jensen⊥ CHEC Research Centre, Department of Chemical and Biochemical Engineering, Technical UniVersity of Denmark, Building 229, DK-2800 Lyngby, Denmark, Materials Science and Engineering, Department of Mechanical Engineering, Technical UniVersity of Denmark, Building 204, DK-2800 Lyngby, Denmark, Vattenfall AS, FynsVærket, Odense, Denmark, and DONG Energy, Copenhagen, Denmark ReceiVed February 2, 2009. ReVised Manuscript ReceiVed April 14, 2009
Deposit-induced chlorine corrosion was studied under well-controlled laboratory conditions, simulating the conditions in straw-fired boilers and boilers cofiring coal and straw. This was done by exposing pieces of superheater tube (TP 347H FG) covered with synthetic deposits of known Cl content to gas mixtures simulating straw-firing and cofiring of coal and straw, at 560 °C (1040 °F), for 3 days. The corroded specimens, and the reacted deposits, were studied in detail using a scanning electron microscope to determine the corrosion rate, investigate the chemistry and morphology of the corrosion attack, and study the sulfation behavior. Besides the gas compositions, various parameters were studied systematically. Most specimens suffered some internal attack, mostly by selective corrosion and in some cases by grain boundary attack. In all experiments with KCl and KCl-SiO2 deposits, the corrosion products consisted of an oxide scale, containing oxides of Cr and Fe, and on top of that a characteristic mixed layer of iron oxide threads in a potassium sulfate matrix. However, the thickness and shape of this layer was found to be strongly dependent on the experimental conditions. An increase of the percentage of KCl in the deposit resulted in a more uniform and deeper internal corrosion attack. The presence of HCl in the flue gas did not seem to be essential for chlorine-induced corrosion to occur, when a deposit containing KCl was present, but it enhanced the corrosion rate. The degree of sulfation of KCl in the deposits after exposure was quantified by wet chemical analysis and was shown to be dependent on many parameters, including the SO2 concentration in the gas flow, the concentration of KCl in the deposit, and the SiO2 and KCl particle sizes in the deposit. No simple relation was observed between the degree of sulfation in the deposit and the depth of internal attack or the thickness of the oxide or mixed layer. Whereas SiO2 particles were found to be chemically inert with respect to the flue gas and the corrosion attack, CaO particles reacted with HCl from the flue gas, and the resulting CaCl2 played an important role in the corrosion mechanism. As a result, the corrosion rate was strongly enhanced when CaO was present in the deposit, instead of SiO2.
Introduction To meet the objectives of reducing CO2 emissions, the Danish power plants are encouraged to increase the utilization of biomass for power generation. Straw is utilized because it is widely available in Denmark, but it causes fly ash and deposits with a very high content of K and Cl. As a result, final steam temperatures have to be kept low in straw-fired boilers, in order to minimize superheater corrosion due to the presence of chlorine-rich deposits. Other options to avoid chlorine corrosion are cofiring with coal or oil, or using additives. Cofiring of coal with straw gives the possibility to run plants with similar steam data as coal-fired plants, that is, with a much higher electrical efficiency, but with reduced CO2 emissions. Superheater corrosion rates are lower in cofired boilers, since the deposits † Impacts of Fuel Quality on Power Generation and Environment. * Corresponding author. E-mail:
[email protected]; fax: +45 45 88 22 58; phone: +45 45 25 28 83. ‡ CHEC Research Centre, Technical University of Denmark. § Materials Science and Engineering, Technical University of Denmark. | Vattenfall AS. ⊥ DONG Energy.
contain a higher fraction of K2SO4 compared to straw-fired boilers, due to a higher content of sulfur in coal. Chlorine corrosion is also observed in cofired boilers, depending on the fuel mix. However, in many cases other corrosion mechanisms appear, such as sulfidation, oxidation, and hot corrosion due to sulfate deposits.1 There have been many full-scale investigations over the past couple of decades, which has provided insight into the corrosion process and choice of materials. However, the plant conditions (temperature, deposit, and gas composition) are difficult to control, meaning that it has been difficult to obtain precise data on the exact influence of the relevant parameters on the corrosion rate in these investigations. Lab-scale corrosion tests have the advantage that the conditions are rather well-controlled compared to full-scale tests. Many authors have reported lab-scale tests in which the influence of SO2 and/or HCl in the gas flow2-7 (1) Montgomery, M.; Vilhelmsen, T.; Jensen, S. A. Mater. Corros. 2008, 59 (10), 783–793. (2) Sroda, S.; Tuurna, S. Mater. Corros. 2006, 57 (3), 244–251. (3) Kalivodova, J.; Baxter, D.; Schu¨tze, M.; Rohr, V. Mater. Corros. 2005, 56 (12), 882–889.
10.1021/ef9000924 CCC: $40.75 2009 American Chemical Society Published on Web 05/26/2009
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Figure 1. Corrosion mechanism proposed by Nielsen et al.23
or deposit composition (primarily using synthetics deposits and mostly in the presence of a mixture of N2 or He, and O2)8-12 on the corrosion of superheater materials was studied. These investigations have provided valuable information on the effects of different parameters on the corrosion behavior. However, as a deposit interacts with the flue gas, it is important to include both the deposit and the flue gas in the experiments when simulating the corrosion behavior in a biomass-fired or cofired boiler. Although there are many publications that present results of corrosion tests performed in the presence of both a deposit and a flue gas simulating waste incineration conditions (which mainly involves molten salt corrosion),13-21 there are just a few that focused on biomass-firing conditions.22,23 The lab-scale corrosion study previously performed at the CHEC Research Centre by Nielsen et al.,23 in which specimens (4) S´roda, S.; Tuurna, S.; Penttila¨, K.; Heikinheimo, L. Mater. Sci. Forum 2004, 461-464, 981–988. (5) Ja¨rdna¨s, A.; Svensson, J.-E.; Johansson, L.-G. Oxid. Met. 2003, 60 (5/6), 427–425. (6) Salmenoja, K.; Hupa, M.; Backman, R. J. Inst. Energy 1999, 72, 127–133. (7) Haanappel, V. A. C.; Haanappel, N. W. J.; Fransen, T.; van Corbach, H. D.; Gellings, P. J. Corrosion 1992, 48 (10), 812–821. (8) Skrifvars, B.-J.; Backman, R.; Hupa, M.; Salmenoja, K.; Vakkilainen, E. Corros. Sci. 2008, 50, 1274–1282. (9) Li, Y. S.; Niu, Y.; Wu, W. T. Mater. Sci. Eng. 2003, A345, 64–71. (10) Otsuka, N.; Fukuda, Y.; Kawahara, Y.; Hosoda, T. Mater. Corros. 2000, 51, 236–241. (11) Spiegel, M. Mater. Corros. 1999, 50, 373–393. (12) Nakagawa, K.; Matsunaga, Y. Mater. Sci. Forum 1997, 251-254, 535–542. (13) Pettersson, R.; Flyg, J.; Viklund, P. Materials Performance in Simulated Waste Combustion EnVironments; Presented at NKM14 (14th Nordic Corrosion Congress): Copenhagen, Denmark, May 2007. (14) Moulin, G.; Weulersse, K.; Favergeon, J. Mater. Sci. Forum 2006, 522-523, 547–554. (15) Sa´nchez-Paste´n, M.; Spiegel, M. Mater. Corros. 2006, 57 (2), 192– 195. (16) Kawahara, Y. Corros. Sci. 2002, 44, 223–245. (17) Sa¨mann, N.; Spiegel, M.; Grabke, H. J. Mater. Sci. Forum 2001, 369-372, 963–970. (18) Spiegel, M. Mater. Corros. 2000, 51, 303–312. (19) Spiegel, M. Mater. High Temp. 1997, 14 (3), 221–226. (20) Spiegel, M.; Schroer, C.; Grabke, H. J. Mater. Sci. Forum 1997, 251-254, 527–534. (21) Grabke, H. J.; Reese, E.; Spiegel, M. Corros. Sci. 1995, 37 (7), 1023–1043. (22) Skobir, D. A.; Spiegel, M. Mater. Technol. 2006, 40 (6), 247–251. (23) Nielsen, H. P.; Frandsen, F. J.; Dam-Johansen, K. Energy Fuels 1999, 13, 1114–1121.
covered with KCl, a eutectic mixture of KCl and K2SO4, and fly ash from a straw-fired boiler were exposed to a gas flow simulating straw-firing, and subsequently studied by scanning electron microscopy (SEM), has given improved insight into corrosion and sulfation phenomena in straw-fired boilers. A characteristic feature of all the specimens covered with a deposit containing KCl was the presence of a dense mixed layer on top of the metal oxide scale, consisting of iron oxide threads in a matrix of K2SO4. A corrosion mechanism was suggested (illustrated in Figure 1), whereby KCl forms a melt with K2SO4 and iron components (FexOy, FeCl2) adjacent to the oxide scale. The sulfation of KCl occurs fast in the melt, and HCl or Cl2 is released close to the metal surface or the oxide scale. The chlorine gas is able to diffuse through the oxide scale to the metal surface, where it reacts with iron to form iron chlorides. The iron chlorides are thermodynamically stable at the metal/ scale interface, where the partial pressure of oxygen is very low. The volatile iron chlorides diffuse out through the scale and oxidize when they reach areas with higher partial pressures of oxygen. This causes formation of iron oxides, either in the metal oxide scale or in the dense mixed layer. The net reactions in the melt phase are that iron chloride is transformed to iron oxide, and KCl to K2SO4. This mechanism can explain the shift in corrosion behavior with temperature, which has been observed in full-scale corrosion tests.24 At low metal temperatures, solidphase sulfation of KCl is slow, and only general oxidation of the metal takes place. At metal temperatures exceeding the lowest melting temperature in the KCl/K2SO4/iron compounds system, the sulfation reaction is fast, generating a high partial pressure of Cl2/HCl. This causes accelerated oxidation and selective chlorine corrosion of the metal. Simultaneous thermal analysis (STA) of the system KCl-K2SO4-FexOy performed by Nielsen et al.23 showed that no melt was formed below 577 °C (1071 °F), but the presence of other corrosion products, such as FeCl2, may lower the melting temperature. The aim of the present work was to investigate depositinduced chlorine corrosion under well-controlled laboratory conditions, simulating the conditions in straw-fired boilers, and boilers cofiring coal and straw, and to study the effect of various parameters on the corrosion attack. This was done by exposing (24) Larsen, O. H.; Henriksen, N. Ash Deposition and High Temperature Corrosion at Combustion of AggressiVe Fuels; Proceedings from Power Plant Chemical Technology: Kolding, Denmark, Sept. 4-6, 1996.
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Figure 2. Corrosion test setup. Table 1. Composition of TP 347H FG (wt %) C
Si
Mn
P
S
Cr
Ni
Nb + Ta
Fe
0.06-0.10
