1114
Energy & Fuels 1999, 13, 1114-1121
Lab-Scale Investigations of High-Temperature Corrosion Phenomena in Straw-Fired Boilers Hanne Philbert Nielsen, Flemming J. Frandsen,* and Kim Dam-Johansen Department of Chemical Engineering, Technical University of Denmark, Building 229, DK-2800 Lyngby, Denmark Received January 5, 1999
The corrosion of superheater tube material in straw-fired boilers was investigated in the laboratory. Metal test elements of boiler tube steel (X20CRMV121 and AISI 347FG) were covered with synthetic (KCl and/or K2SO4) and real deposits, and exposed to a synthetic flue gas (6 vol % O2, 12 vol % CO2, 400 ppmv HCl, 60 ppmv SO2, balance N2) in 550 °C electrically heated ovens. Exposure times from 1 week to 5 months were used. The corrosion of the metal test elements was, in general, quite uniform, and the corrosion products consisted mainly of oxides of iron and chromium. All test elements covered with KCl suffered from minor internal attack, and some elements had severe pits with chlorine found in the pit. A dense layer of potassium sulfate and iron oxide was found adjacent to the metal oxide layers on all the metal test elements covered with a deposit containing KCl. The layer had a characteristic structure, with iron oxide threads in a dense potassium sulfate matrix. A mechanism for chlorine corrosion is suggested. The mechanism is based on gaseous chlorine attack coupled with a fast sulfation of KCl to K2SO4 in a melt of KCl, K2SO4, and iron compounds formed adjacent to the metal. Aspects of the sulfation of potassium chloride to potassium sulfate are discussed in the paper.
Introduction The design of biomass-fired power plants with increased steam temperatures and pressures raises hightemperature corrosion concerns. Deposition problems such as slagging and fouling of heat-transfer surfaces caused by the inorganic constituents of the biomass fuels are well-known,1-3 whereas problems with corrosion of superheater tubes traditionally have been avoided by keeping the steam temperatures below 450 °C. The corrosion of superheater tubes is closely related to deposition and is a function of fuel, operation, and boiler parameters. In particular, the presence of potassium chloride (KCl) in the deposits is believed to play a major role in the corrosion observed in biomass-fired boilers. The presence of KCl in the deposit can cause selective chlorine corrosion of chromium and iron, leaving a nickel-skeleton behind. Whether the presence of potassium chloride itself can cause the corrosion or whether it is the sulfation of potassium chloride to potassium sulfate on the superheater tube that causes the corrosion is still to be determined.4 Selective chlorine corrosion can cause very high corrosion rates compared to the corrosion rates of superheater alloys in coal-fired * Corresponding author. Fax: +(45) 45 88 22 58. E-mail: chec@ kt.dtu.dk. (1) Miles, T. R.; Miles, T. A., Jr.; Baxter, L. L.; Bryers, R. W.; Jenkins, B. M.; Oden, L. L. Alkali Deposits Found in Biomass Power Plants. A Preliminary Investigation of Their Extent and Nature. Summary Report for National Renewable Energy Laboratory, 1617 Cole Boulevard, Golden, CO 80401-3393, 1995. (2) Jensen, P. S.; Stenholm, M.; Hald, P. Deposition Investigation in Straw-Fired Boilers. Energy Fuels 1997, 11, 1048-1055. (3) Michelsen, H. P.; Larsen, O. H.; Frandsen, F. J.; Dam-Johansen, K. Deposition of High-Temperature Corrosion in a 10 MW Straw Fired Boiler. Fuel Process. Technol. 1998, 54, 95-108.
systems. Substituting the alloy with a more corrosion resistant and more expensive alloy tends to have marginal effect on this type of corrosion.5,6 Selective chlorine corrosion has until now been observed in straw-fired low-temperature plants, such as grate-fired and fluid-bed boilers. In a recent investigation, superheater tubes at elevated temperatures (simulated by air-cooled probes) in a straw-fired boiler showed an increased corrosion rate at steam temperatures above 490 °C in general and above 520 °C in particular. The corrosion rates were found to be 2-20 times higher than those observed during pulverized coal combustion. The nature of the corrosion attack changed with temperature, and selective chlorine corrosion was found, for the austenitic materials, at steam temperatures above 490 °C.6 No selective chlorine corrosion was observed when straw was co-fired with coal in suspension-fired (4) Nielsen, H. P.; Frandsen; F. F.; Dam-Johansen, K.; Baxter, L. L. Chlorine-Induced High-Temperature Corrosion of Superheater Tubes in Biomass-Fired BoilerssA Literature Study. Progress Combust. Sci., submitted. (5) Fujikawa, H.; Maruyama, N. Corrosion Behavior of Austenitic Stainless Steels in High Chloride-containing Environment. Mater. Sci. Eng. 1986, A120, 301-306. (6) Larsen, O. H.; Henriksen, N. Ash Deposition and High-Temperature Corrosion at Combustion of Aggressive Fuels. Proceedings from Power Plant Chemical Technology, Kolding, Denmark, 4-6 September, 1996, 7.1-7.18. (7) Henriksen, N.; Larsen, O. H.; Blum, R.; Inselman, S. Hightemperature Corrosion when Co-Firing Coal and Straw in Pulverized Coal Boilers and Circulating Fluidized Bed Boilers. VGB Conference, Corrosion and Corrosion Protection in Power Plants, Essen, November 29 and 30, 1995. (8) Hansen, P. F. B.; Lin, W.; Dam-Johansen, K.; Henriksen, N. Can Superheater Corrosion during Co-Combustion of Straw and Coal in a CFB-Boiler be Reduced, CFB-5 preprints, Fifth International Conference on Circulating Fluidized Beds, May 28-June 1, 1996, Beijing, People’s Republic of China, 1996.
10.1021/ef990001g CCC: $18.00 © 1999 American Chemical Society Published on Web 10/22/1999
Lab-Scale Studies of High-Temperature Corrosion in Boilers
Energy & Fuels, Vol. 13, No. 6, 1999 1115
Table 1: Experimental Conditions for Lab-Scale Corrosion Experiments test of different deposits
test of rate law deposit
KCl
exposure time
1 week
KCl K2SO4 eutectic mixture of KCl-K2SO4a deposit from Rudkøbing CHP no deposit 3 months
1, 2, 3, 5 months 550 °C X20 AISI 347 simulated straw combustion
550 °C X20 AISI 347 simulated straw combustion
temperature metals flue gasb
Figure 1. Experimental setup.
a mol % KCl. b Composition of synthetic flue gas is shown in Table 3.
Figure 2. Reactor with quartz sledge for metal test elements. Metal test element.
Table 2: Composition of Deposit from Rudkøbing CHP in wt % (the remaining part of the deposit is oxygen)
Table 3: Composition (wt %) of Iron-Based Alloys
composition in wt %
Si
K
Ca
Mg
P
S
Cl
10.2
33.8
4.7
0.8
0.62
1.9
23.7
boilers with straw-shares up to 20% (energy basis). For more information on this topic please refer to Michelsen et al. (1998),3 Nielsen et al. (1998),4 Larsen and Henriksen (1996),6 Henriksen et al. (1995),7 and Hansen et al. (1995).8 Experiments In this investigation, corrosion measurements were performed in the laboratory to investigate the corrosion of superheater tubes in straw-fired boilers under welldefined conditions. Metal test elements covered with deposits were exposed to a synthetic flue gas in electrically heated ovens. Experiments were conducted to determine the corrosion rate for metal test elements with KCl deposit and to test the influence of different deposits on the metal test elements. An overview of the two test series is shown in Table 1. The deposits investigated included potassium chloride, potassium sulfate, a eutectic mixture of potassium sulfate and potassium chloride (26.3 mol % K2SO4), and a deposit collected at a 10 MWth straw-fired grate boiler (Rudkøbing CHP) (Table 2). In addition, metal test elements without deposit were exposed as a reference. Experimental Method The experimental equipment consisted of a gas mixing panel and two electrically heated ovens (Figure 1). The deposits were placed on top of the boatlike metal test elements at the start of an experiment. The metal test elements were placed on specially designed quartz sledges (Figure 2) and inserted into quartz reactors in the horizontal ovens. The metal test elements were cut out of real boiler tube material and thus had a rounded boatlike shape. The test elements were approximately 20 mm long, 10 mm wide, and had a thickness of 3 mm (Figure 2). Two commercial superheater alloys were used for the test elements: the ferritic ×20CRMOV121 (X20) and the austenitic AISI 347FG (AISI 347). The metal test elements were not oxidized prior to exposure.
metal
Abr.
Fe
C
Cr Ni Mo
Si
Mn
Other
X20CRMOV121 X20 bal. 0.20 12 0.5 1.0
