SEM Investigation of Superheater Deposits from Biomass-Fired Boilers

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Energy & Fuels 2004, 18, 378-384

SEM Investigation of Superheater Deposits from Biomass-Fired Boilers Peter Arendt Jensen,*,† Flemming J. Frandsen,† Jørn Hansen,† Kim Dam-Johansen,† Niels Henriksen,‡ and Steffen Ho¨rlyck§ CHEC, Department of Chemical Engineering, Technical University of Denmark, Building 229, DK-2800 Lyngby, Denmark, Elsam A/S, Overgade 45, DK-7000 Fredericia, Denmark, and ENERGI E2 A/S, Lautruphøj 5, DK-2750 Ballerup, Denmark Received April 24, 2003. Revised Manuscript Received November 20, 2003

Straw is used as fuel in relatively small-scale combined heat and power producing (CHP) grate boilers in Denmark. The large content of potassium and chlorine in straw greatly increases the deposit formation and corrosion of the superheater coils, compared to boilers firing coal. In this study, mature superheater deposit samples were extracted from two straw-fired boilers, Masnedø and Ensted, with fuel inputs of 33 MWth and 100 MWth, respectively. SEM (scanning electron microscopy) images and EDX (energy dispersive X-ray) analyses were performed on the deposit samples. Different strategies are adopted to minimize deposit problems at the two boilers. At Masnedø the final superheater steam temperature is 520 °C, no soot blowing of the superheaters is applied and a relatively large superheater area is used. At Ensted, an external wood-fired superheater is used in order to obtain a final steam temperature of 542 °C, while the steam exit temperature of the straw-fired boiler is 470 °C. The mature Masnedø deposit had a thickness of 2 to 15 centimeters and consisted of three distinct main layers. The thick intermediate layer was depleted in chlorine but rich in Si, K, and Ca. This Masnedø intermediate layer was probably generated by in-situ reaction between KCl and Si-rich ash particles, which leads to release of chlorine-containing gases. The innermost layer contained many sublayers of mainly iron oxide, KCl, and K2SO4. The Ensted deposit had a maximum thickness of a few centimeters. The intermediate Ensted layer consisted of melted KCl with inclusions of Ca- and Si-rich particles, and the innermost layer was an iron oxide next to a potassium sulfate layer. Compared to deposits formed on a probe during short-time experiments, the mature superheater deposits contained larger dense inner sublayers of pure KCl and K2SO4. The present study indicates that the innermost layer of the superheater deposits expands by condensation of KCl, even when the deposit has a thickness of several centimeters.

1. Introduction To reduce CO2 emissions and substitute fossil fuels biomass is used as fuel in power generating boilers. In Denmark, straw is used in small-scale specially designed grate-fired boilers, generating both power and heat (CHP boilers). The large alkali content of herbaceous crops such as straw severely increases the deposit and corrosion problems, compared to boilers applying low-alkali biomass or coal fuels.1-5 The largest corrosion * Corresponding author. Tel: +45-4525-2800. Fax: +45-4588-2258. E-mail: [email protected]. † Technical University of Denmark. ‡ Elsam A/S. § ENERGI E2 A/S. (1) Frandsen, F.; Nielsen, H. P.; Jensen, P. A.; Hansen, L. A.; Livbjerg, H.; Dam-Johansen, K.; Hansen, P. F. B.; Andersen, K. H.; Sørensen, H. S.; Larsen, O. H.; Sander, B.; Henriksen, O. H.; Simonsen, P. Deposit and Corrosion in Straw and Coal-Straw Co-fired Utility Boilers. Impact of Mineral Impurities in Solid Fuel Combustion; Gupta et al., Eds.; Kluwer Academic/Plenum Publishers: New York, 1999; pp 272-283. (2) Baxter, L. L.; Miles, T. R.; Miles, T. R., Jr.; Jenkins, B. M.; Richards, G. H.; Oden, L. L. Transformation and Deposition of Inorganic Material in Biomass Boilers. Second International Conference on Combustion for a Clean Environment. Lisbon, Portugal, July, 1993. (3) Jensen, P. A.; Stenholm, M.; Hald, P. Deposit Investigation in Straw-Fired Boilers. Energy Fuels 1997, 11 (5), 1048-1055.

rates appear on deposit-covered superheaters, and in order to minimize the corrosive attack, Danish strawfired boilers use relatively low superheater steam temperatures in the range of 440 to 540 °C. Wheat and barley straw used in the Danish power plants contains typically 1.0 wt % K, 0.4 wt % Cl, and 0.15 wt % S.6 Besides potassium, the most abundant elements in the straw fuel ash are normally Ca and Si.6 In straw-fired grate boilers, a large fraction of the fuel ash (70 to 90%) is removed from the boiler as bottom ash, and therefore never reaches the superheaters.7 However, by visual inspection of the boiler chamber it can be seen that some large Si-rich particles are lifted by the flue gas and reach the superheaters. These (4) Miles, T. R.; Miles, T. R., Jr.; Baxter, L. L.; Bryers, R. W.; Jenkins, B. M.; Oden, L. L. Boiler Deposits from Firing Biomass Fuels. Biomass Bioenergy 1996, 10 (2-3), 125-138. (5) Baxter, L. L.; Miles, T. R.; Miles, T. R., Jr.; Jenkins, B. M.; Milne, T.; Dayton, D.; Bryers, R. W.; Oden, L. L. The Behavior of Inorganic Material in Biomass-Fired Power Boilers: Field and Laboratory Experiences. Fuel Process. Technol. 1998, 54, 47-78. (6) Sander, B. Properties of Danish Biofuels and the Requirements for Power Production. Biomass Bioenergy. 1997, 12 (3), 177-183. (7) Stenholm, M.; Jensen, P. A.; Hald, P. The Fuel and Firing Characteristics of Biomass - Combustion Trials. Final Report, EFP Project No. 1323/93, 1996. (In Danish.)

10.1021/ef030097l CCC: $27.50 © 2004 American Chemical Society Published on Web 01/28/2004

Superheater Deposits from Biomass-Fired Boilers Table 1. Minimum Melting Temperature of Some K- and Cl-Containing Components and Mixturesa

a

component

minimum melting temp (°C)

KCl K2SO4 KCl + K2SO4 KCl + FeCl2 K2O + SiO2 K2O‚4SiO2 + CaO‚SiO2 KCl + K2SO4 + Fe2O3

774 1059 694 355 750 740 577

Data from refs 16 and 26.

particles fall to the floor in the second boiler pass, and are therefore not collected in the flue gas filters. The bottom ash is typically Si-rich, and SEM investigations have revealed that K Ca silicate components account for 34-84 wt % of the bottom ash in straw-fired grate boilers.8,9 Potassium-containing species, as KCl and KOH, are to some degree released to the gas phase from the primary grate combustion zone in the bottom of the boiler and then enter the freeboard. When the flue gas downstream is cooled to 700 to 800 °C, aerosols rich in KCl and K2SO4 are generated.10 The fly ash containing aerosols and entrained ash particles are collected in the flue gas filter. The fly ash is very rich in KCl8 and the elements K and Cl constitute typically 44 to 84 wt % of the fly ash.7 Deposit formation on heat transfer surfaces by fuel ash species can be generated by inertial impaction (most important for particles larger than 10 µm), thermophoresis, and direct condensation of gas-phase species.11 The morphology and tenacity of the deposits can later be changed by chemical reactions and physical transformations in the deposit. The initial deposition on surfaces by straw biomass fuels is generated by direct condensation of KCl, when the deposit grows further, impaction of larger Si- and Ca-containing particles also contributes significantly to the deposit growth.3,4,12 The rate of deposit formation on boiler heat transfer surfaces increases if the ash particles transported to the surfaces, or the deposit outer surface, are partly molten. The large content of K in straw causes the ash, generated by the combustion process, to contain many species with low evaporation and melting temperatures (see Table 1), and it has been shown that increased potassium content in the straw results in an increased deposit formation rate.3 Deposit in the boilers contains large concentrations of K and Cl in addition to some Si, Ca, and S.3,13 It has also been observed that deposit in areas with a (8) Frandsen, F.; Nielsen, H. P.; Hansen, L. A.; Hansen, P. F. B.; Andersen, K. H.; Sørensen, H. S. Ash Chemistry Aspects of Straw and Coal-Straw Co-firing in Utility Boilers. Paper presented at the 15th Annual International Pittsburgh Coal Conference. PA, USA, September, 1998. (9) Sørensen, H. Computer-controlled scanning electron microscopy (CCSEM) analysis of straw ash. Paper presented at the Engineering Foundation Conference, Kona, Hawaii, November, 1997. (10) Christensen, K. A.; Stenholm, M.; Livbjerg, H. The Formation of Submicron Aerosol Particles, HCl, and SO2 in Straw-Fired Boilers. J. Aerosol Sci. 1998, 29 (4), 421-444. (11) Baxter, L. L. Ash Deposition During Coal and Biomass Combustion: A Mechanistic Approach. Biomass Bioenergy. 1993, 4 (2), 85102. (12) Jenkins, B. M.; Baxter, L. L.; Miles, T. R.; Miles, T. R., Jr.; Oden, L. L.; Bryers, R. W.; Winther, E. Composition of Ash Deposits in Biomass Fueled Boilers: Results of Full-scale Experiments and Laboratory Simulations. Paper presented at 1994 International Summer Meeting Sponsored by ASEA. Kansas City, Kansas, June, Paper No. 946007, 1994.

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relatively high flue gas temperature contains a relatively lower amount of Cl.3 Deposit formed on air-cooled probes kept at a temperature of 510 °C contains 8-13 wt % of Cl at a flue gas temperature of 830 to 870 °C and 14-22 wt % of Cl at a flue gas temperature of 650 °C. Thermodynamic equilibrium calculations on straw ash under combustion conditions can give important information on the possible transformation of the Kcontaining species, and calculations have been presented in several papers.14-16 At temperatures below 700 °C, potassium is predicted to appear mainly as solid KCl, K2SO4, and K2O‚SiO2. At temperatures between 680 °C and 780 °C, all KCl is predicted to evaporate to the gas phase, and at temperatures exceeding 1200 °C the potassium mainly appears as gas-phase KOH and KCl and liquid-phase K2O‚SiO2. It is predicted that potassium will preferentially appear as potassium chloride if sufficient chlorine is present and not as potassium silicate in the full temperature range. Up to temperatures of 1000 °C, potassium sulfate is predicted to be thermodynamically more stable than potassium chloride. The thermodynamic calculations did not include potassium bound to char or components of potassium, calcium, and silicon since no thermodynamic data was available for these components. Potassium does not appear in large quantities as potassium sulfate in the straw, which means that potassium sulfate has to be generated by reactions between KCl or KOH and SO2. In straw-fired boilers, most potassium sulfate is probably generated in the gas phase in the boiler chamber.16 Characteristic potassium sulfate layers have been observed in superheater deposits near the metal surface,14 where they are possibly generated by reactions between SO3 and KCl. The SO3 may arise from catalytic oxidation of SO2 on iron oxide.16 Corrosion of the superheaters at high steam temperatures is a severe problem in straw-fired power plants. The KCl-containing deposits cause corrosion of superheater coils. Superheater corrosion rate measurements on three Danish straw-fired boilers, the Rudkøbing, the Masnedø, and the Ensted boilers, have been performed by Montgomery et al.17 at a range of different steam temperatures. The measurements were conducted by (A) cutting a piece of the standard superheater (Masnedø and Ensted), (B) using air-cooled probes (Rudkøbing), or (C) using a steam-cooled test superheater (Masnedø). In all the tests, the same type of steelsTP347swas included. Average corrosion rates of approximately 0.8 mm/1000 h, 0.3 mm/1000 h, and 0.02 mm/1000 h were measured at steam temperatures of, respectively, 570 (13) Michelsen, H. P.; Frandsen, F.; Dam-Johansen, K.; Larsen, O. H. Deposit and High-Temperature Corrosion in a 10 MW Straw-Fired Boiler. Fuel Process. Technol. 1998, 54, 95-108. (14) Hansen, L. A.; Nielsen, H. P.; Frandsen, F. J.; Dam-Johansen, K.; Ho¨rlyck, S.; Karlsson, A. Influence of Deposit Formation on Corrosion at a Straw-Fired Boiler. Fuel Process. Technol. 2000, 64, 189-209. (15) Nielsen, H. P.; Baxter, L. L.; Sclippab, G.; Morey, C.; Frandsen, F. J.; Dam-Johansen, K. Deposition of Potassium Salts on Heat Transfer Surfaces in Straw-Fired Boilers: A Pilot-scale Study. Fuel 2000, 79, 131-139. (16) Nielsen, H. P. Deposit and High-Temperature Corrosion in Biomass-Fired Boilers. Ph.D. Thesis. Technical University of Denmark, Department of Chemical Engineering, 1998. (17) Montgomery, M.; Karlson, A.; Larsen, O. H. Field test corrosion experiments in Denmark with biomass fuels. Part 1: Straw firing. Mater. Corros. 2002, 53, 121-131.

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°C, 530 °C, and 440 °C. Most of the test superheater corrosion rates measured at Masnedø were a little above the average rate measurements, except at high steam temperatures (around 570 °C) where all measured rate data on the different boilers was similar. Few rate measurements were conducted at the Ensted boiler, but they were in good agreement with the average level. Recent comparisons of measured superheater corrosion rates on straw-fired boilers18 revealed that an increased flue gas temperature significantly increased the corrosion rate, even when the steam temperature level was maintained. It was also observed that measurements performed with probes and test superheaters generally showed larger corrosion rates than measurements performed on the boiler superheaters. It is evident that the corrosion rate in straw-fired boilers is very high at high superheater temperatures. However, the corrosion mechanism in straw-fired boilers is not completely understood today.16-19 It has been proposed that the sulfation of potassium chloride near the metal surface generates HCl(g) or Cl2(g) that reacts with the superheater tube removing Fe and Cr.14,16 In some cases, molten phases may appear and increase the corrosion rate.16,18 Reactions between KCl and Fe2O3 may also generate Cl2(g) that promotes corrosion.17 It is proposed that after some time a stable dense layer of potassium sulfate is generated next to the superheater tube surface, the release of gaseous Cl species is reduced, and the corrosion rate is thus lowered.14,16 Improved knowledge of the deposition and corrosion processes on superheaters is necessary so that the steam temperature can be increased in future straw-fired boilers. Even though several studies of deposit generation and corrosion in straw-fired boilers have been conducted, knowledge of the detailed chemistry and morphology of superheater deposits is limited.14,20 In this study, mature deposit samples were collected from two straw-fired grate boilers operating at different superheater conditions. The deposit samples were investigated by SEM (scanning electron microscopy) images and EDX (energy dispersive X-ray) analysis. 2. Collection of Deposits Deposit samples were collected from the two strawfired boilers at Masnedø and Ensted shown in Figure 1. The main characteristics of the two boilers are summarized in Table 2. The Masnedø straw-fired boiler is used for combined heat and power generation (CHP). Straw is fed to the boiler chamber by means of two screw feeders. The straw enters the boiler on a small stationary grate, where combustion is initiated, and moves further onto a vibration grate. At the Ensted boiler four screw feeders are used to push the straw into the boiler. The bottom of the boiler consists of an inclined vibration grate. The maximum fuel inputs of the boilers are 33 (18) Montgomery, M.; Biede, O.; Larsen, O. H. Corrosion Investigation at Maribo Sakskrbing Combined Heat and Power Plant, Part I. Technical University of Denmark, Department of Manufacturing Engineering, 2001. (19) Nielsen, H. P.; Frandsen, F. J.; Dam-Johansen, K.; Baxter, L. L. The Implication of Chlorine-Associated Corrosion on the Operation of Biomass-Fired Boilers. Prog. Energy Combust. Sci. 2000, 26, 283298. (20) Montgomery, M.; Henriksen, N. Corrosion Investigation of Specimens Exposed at Ensted Straw-Fired Power Plant. Technical University of Denmark, DTU report, May, 1999.

Jensen et al.

Figure 1. Sketches of the Masnedø and Ensted straw-fired grate boilers. Table 2. Main Characteristics of the Masnedø and Ensted Boilers fuel maximum fuel input maximum exit superheater steam temperature normally applied stoichiometric air-to-fuel ratio

Masnedø

Ensted

straw 33 MWth 520 °C

straw 100 MWth 470 °C

1.3

1.25

Table 3. Conditions Where Deposit Samples Were Collected and Number of Deposit Samples Investigated by SEM/EDX Masnedø flue gas temperatures superheater steam temperature soot blowing where samples was collected samples including innermost layer (A) samples including intermediate layer (B) samples including outermost layer (C)

Ensted

960-1019 °C 387 to 521 °C

below 900 °C 389 to 470 °C

not applied Test SH, SH2, SH3

1 time per 8 h SH1, SH3

5

1

3

4

2

3

MWth (Masnedø) and 100 MWth (Ensted). Different strategies are adopted to minimize superheater deposit and corrosion problems in the boilers. At Masnedø the final superheater temperature is 520 °C, the boiler has a relatively large superheater area, and soot blowing is never applied. At Ensted an external wood-fired superheater is used to obtain a final steam temperature of 542 °C, while the straw boiler steam exit temperature is limited to 470 °C. The positions of the superheaters where it was possible to gain access and collect superheater deposit are shown in Figure 1. The local conditions in those areas of the two boilers are summarized in Table 3. Both boilers can to some extent use fuels other than straw; however, for several months before the deposit sampling only straw fuel was burned. The flue gas temperatures shown in Figure 1 and Table 3 were measured previously with a suction pyrometer.14,21,22 The gas temperature upstream of the superheaters at Masnedø was (21) Van der Lans, R. P. Gas Concentrations and Temperature Measurements during Straw and Biomass Co-firing in the Grate Furnace of Masnedø Combined Heat and Power Plant. Technical University of Denmark, Department of Chemical Engineering, CHEC Report No. 9914, 1999.

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Table 4. Chemical Analysis of Straw and Deposits Samplesa sample

Si

Ca

K

P

Na

Mg

Al

Fe

Cl

S

molar ratio: K/(2S + Cl)

Masnedø strawb Masnedø fly ash outer deposit layer Masnedø (C) intermediate deposit layer Masnedø (B) intermediate deposit layer Ensted (B) inner deposit layer Ensted (A)

20.4 2.5 16.0 17.0 11.8 7.9

7.5 2.7 7.2 6.6 6.0 3.4

16.7 38.3 23.0 23.0 29.5 33.5

1.8 1.7 1.9 2.5 1.5 1.0

0.5 0.8 0.8 0.7 0.5 0.7

1.2 0.3 1.1 1.1 1.0 0.6

0.4 0.1 0.7 0.3 0.6 0.4

0.2 2.4 0.4 0.2 0.3 0.6

9.0 26 6.3 0.1 13.0 6.1

2.2 4.1 4.8 6.6 5.0 11.2

0.99 1.23 1.42 1.12 0.99

wt % of ash in dry straw 6.64

a Element concentrations as weight % in ash. b Mean value of 5 samples extracted the five weeks before the plant stop. All elements were measured directly on dry straw with ICP-AES (Inductive Coupled Plasma-Atomic Emission Spectroscopy) and then calculated to the ash basis.

Figure 2. Pictures of the deposit-covered superheater coils at Masnedø.

seen to fluctuate strongly. As no soot blowing was applied at Masnedø, and as both the flue gas and the steam temperatures were larger at Masnedø, the local deposit temperature must be larger at Masnedø compared to that of Ensted. At Masnedø, deposit samples from both the standard superheater and a test superheater were collected. However, the test superheater was operated at the same steam temperatures as the standard superheater, in the areas where deposits could be collected. Pictures of the superheater coils covered by deposit at Masnedø are shown in Figure 2. While the boiler walls were almost clean, the superheater tubes were completely covered by a 2-15 cm thick layer of dark brown deposit. The deposit contained three distinct main layers: a thin dark brown outer layer (C), a thick white porous intermediate layer (B), and a complex thin inner layer that contained several sublayers (A). The Ensted superheater deposit was very different (see Figure 3). It had a maximum thickness of a few centimeters and a completely white outer surface. However, again three distinct main layers could be observed. The chemical composition of straw from the feeding line, deposit samples, and fly ash of the Masnedø boiler and deposit samples of the Ensted boiler are shown in Table 4. The chemical analyses of the straw were performed directly on the dry fuel and the weight percentages in the ash were calculated. This was also done with Cl and S even though these elements are partly released during laboratory ashing. The straw ash analysis is very similar to an average Danish wheat straw,6 with the most abundant elements being Si, K, (22) Van der Lans, R. P. Gas Concentrations and Temperature Measurements in the Straw-Fired Grate Furnace of SH Energy A/S. Technical University of Denmark, Department of Chemical Engineering, CHEC Report No. 9914, 1999.

Figure 3. Pictures of the deposit-covered superheaters coils at Ensted.

Cl, and Ca. The composition of the Ensted straw was not analyzed, but it is probably reasonably similar to that of the Masnedø straw. A few comments to the chemical analysis of the ash samples will be provided here while the detailed SEM analysis on the deposit samples will be discussed in the next section. The calculated molar ratio K/(2S + Cl) of deposits and fly ash shown in Table 4 indicates whether all K can be bound as KCl and K2SO4 (the value is then 1) or some K is present as other species (the value is then above 1). The Masnedø fly ash consists mainly of KCl and K2SO4. The high content of iron in the fly ash probably originates from low-temperature corrosion in the flue gas channel or filter. Compared to the fly ash, the outermost Masnedø deposit (C) is enriched with Si and Ca, but it has a composition similar to that of the straw ash. The intermediate Masnedø deposit (B) has a high content of K but is totally depleted of Cl. The intermediate Ensted deposit (B) is reasonably similar to the intermediate Masnedø deposit, but with a lower Si-content and a much larger Cl-content. The Ensted inner deposit (A) is very rich in K2SO4. The composition of the mature superheater deposits shown in Table 4 was compared with the composition of 1 to 15 h probe deposits from previous investigations on straw-fired boilers.3,7,13 The Masnedø straw fuel ash and fly ash have a composition similar to that of the straw ash and fly ash of the probe investigations. Except for the intermediate deposit layer from Masnedø, all the deposit compositions shown in Table 4 are within the range of compositions observed in the probe investigations. The very low Cl content of the Masnedø intermediate superheater deposit is not observed in any of the probe deposits. 3. SEM and EDX Analyses The collected deposit samples were cast in epoxy and polished, without being exposed to water, to prevent any

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Figure 6. SEM image of the inner layer (A) of the Masnedø deposit. Figure 4. SEM Images of the outermost layer (C) of the Masnedø superheater deposit.

Figure 5. SEM images of the intermediate layer (B) of the Masnedø deposit. (I) Image shows the high porosity of the deposit. (II) Image shows a small area with no pores using a large magnification.

dissolution, recrystallization, and removal of potassium salts. Later, SEM (scanning electron microscopy) images and EDX (energy dispersive X-ray) analyses were performed on the samples. Each deposit sample was classified into an inner (A), an intermediate (B), and an outer main layer (C). In Figures 4 to 8, SEM images of some of the cast samples are shown. In all cases, the images are oriented so that the metal tube is placed in the bottom of the figures. The uniform dark gray color seen in the SEM pictures is the casting mass, while the light gray is the deposit material. The outermost layer (C) of the Masnedø deposit is shown in Figure 4. This layer was very porous and had an overall composition similar to that of the straw ash (Table 4). It contained many Si- and Ca-rich particles glued together by KCl and some K2SO4 that have been molten. The outer surface of the deposit was dark brown, so it was proposed that it contained a large content of unburned carbon. However, chemical analysis of some outermost layers showed a maximum carbon content of 0.2 wt %. In Figure 5, SEM images with a low and a high magnification of the intermediate layer (B) are presented. The layer contained some very large pores, but was quite dense between the pores, which indicates that the layer has been molten. Most of this layer was totally depleted of Cl but rich in Si, Ca, and K. The layer seemed to contain mainly two phases, one totally melted (with a high K-content) and one partly melted (with a high Ca-content). The two intermediate deposit samples investigated by SEM were similar. The chemical composition of this intermediate layer resembled bottom ash.

Figure 7. SEM image of the inner layer (A) of the Masnedø deposit.

Examples of the innermost main layer (A) at Masnedø are given in Figures 6 and 7, and some important observations based on the SEM images of this layer are listed in Table 5. The collected inner sample layers contained several sublayers and were quite variable, but they were always very rich in K, S, and Cl. It was not possible to observe any systematic variation in the inner layer as a function of the steam temperature. Figure 6 shows a sample consisting of an iron-rich inner layer, a dense KCl layer, a large crack, a layer rich in Cr and Fe, a layer of K2SO4, a layer of dense KCl, and a layer of KCl flakes. In Figure 7, an inner main layer with a different structure is presented. Here are two Fe-rich sublayers separated by a thick dense KCl layer, which also contains some K2SO4. This sample is probably generated by spallation of the outer iron-rich layer (shedding of the protective iron oxide on the metal tube), and then subsequent generation of the dense KCl layer, by condensation of KCl gas beneath the iron oxide layer. Of the five Masnedø samples of the inner main layer, investigated by SEM, four samples included an innermost sublayer very rich in iron. We believe that in those cases, some of the protective iron oxide scale of the superheater tube was included in the sample. Of the samples including the iron oxide scale, three had KCl on top of the scale, one had K2SO4, and one had both KCl and K2SO4. In several cases, Fe-rich layers were observed at some distance from the inner surface, indicating spallation of the iron oxide scale. In most cases, a characteristic porous sublayer of KCl flakes (Figure 6) was present. The total thickness of the innermost layers (A) dominated by the elements Fe, K, Cl, and S varied from 1 to 4.5 millimeters. Figure 8 gives an example of all three layers of the Ensted superheater deposits. It is possible to show all three layers in one SEM picture because the Ensted

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Table 5. Some Characteristics of the Sublayers in the Innermost Layer (A) of Masnedø Superheater Deposits

sample number 1 2 (Figure 6) 3 (Figure 7) 4 5

innermost layer is Fe-rich yes yes yes yes no

innermost layer or layer next to the Fe-rich layer KCl KCl KCl K2SO4 KCl + K2SO4

Figure 8. SEM image of all three Ensted deposit layers (A,B,C). Table 6. Common Characteristics of the Mature Superheater Deposits from the Masnedø and Ensted Boilers Based on the Present Study and the Investigations Conducted by Hansen et al.14 and Montgomery et al.20 A. inner layer

B. intermediate layer

C. outermost layer

The sublayers of the innermost layer were very rich in the elements Fe, K, S, and Cl. In most cases was observed a dense layer on top of the iron oxide scale, then a porous layer of KCl flakes, and then a dense layer of mainly KCl. The observed intermediate layer consisted always of an innermost sublayer of dense KCl with inclusion of silicate and calcium-rich particles. In one case (the present Masnedø investigation), a porous layer rich in K, Si, and Ca but depleted of Cl appeared. A very porous layer containing many Siand C-rich particles glued together by melted KCl and K2SO4.

deposits in some cases were very thin. The general structure seen in Figure 8 was observed in all the Ensted samples investigated by SEM. The innermost layer contained FexOy covered by a sublayer of K2SO4 and then a porous sublayer of KCl flakes. The intermediate layer contained many Si- and Ca-rich particles glued completely together by melted KCl. Two previous investigations of Masnedø (Hansen et al.14) and Ensted (Montgomery et al.20) superheater deposits focus on the influence of the innermost deposits on corrosion, but the studies also contain some information about the structure of the deposits. In Table 6, the common characteristics in agreement with all the investigations are summarized. The previous SEM investigation20 of the inner layers of the Ensted superheater deposits always revealed layers rich in S and K but with a low Cl content, next to the iron oxide scale. This is in good agreement with the present investigation. Compared to the previous Masnedø investigation14 two important differences are observed in the present investigation. The thin dense layer adjacent to the

Is shedding of iron scale observed? no yes yes no yes

Is a layer with flake particles observed? yes yes no yes yes

total thickness of inner layer with very few Si- and Ca-rich particles 3800 µm 4500 µm 1000 µm 3350 µm undetermined

innermost iron oxide consisted in the previous investigation always of K2SO4, while in the present study mainly KCl was observed, as seen in Table 5. In this study lifted iron oxide scales were observed in several of the SEM images, while lifted scales were not reported in the previous investigation.14 The cause of the differences in deposit structure is not completely clear. However, in the previous investigation the deposit samples were collected in August, while the present investigation is based on samples collected two years later in May. It is possible that the August samples are mainly generated during partial boiler load, and the May samples during full load operation, at which the flue gas temperature near the superheaters is higher. A comparison of the intermediate deposit layers (B) of Masnedø and Ensted clearly shows the differences: The Ensted intermediate deposit consists of Si- and Carich particles glued completely together by melted KCl, while the Masnedø intermediate deposit consists of a more uniform material depleted of Cl. In the SEM images the Ensted outermost layer (C) looks like the Masnedø outermost layer. 4. Discussion and Conclusions Mature deposit samples from the straw-fired Masnedø and Ensted boilers were investigated by SEM and EDX. Each deposit sample was classified into an inner (A), an intermediate (B), and an outer main layer (C). The outermost deposit layers (C) at Masnedø and Ensted looked chemically quite similar, even though they were of different colors. Only trace amounts of carbon were observed in the outer deposit layer, showing that unburnt carbon is not an important part of the deposit. The large content of KCl in the outermost layer of the Masnedø deposit is surprising. The mean flue gas temperature is above 900 °C, the outer surface temperature of the thick porous deposit must be high, and at temperatures above 780 °C most potassium chloride appears in the gas phase.16 The large KCl content must be caused by occasional surface temperatures below 780 °C, or because the KCl has condensed during the boiler shutdown period. The intermediate layer (B) at Ensted contained many Si- and Ca-rich particles glued together by melted KCl, while the intermediate deposit layer at Masnedø was different. Since the straw fuels are probably similar, the differences observed in the deposit chemistry must be caused by the higher temperature of the Masnedø deposit. The intermediate layer at Masnedø was depleted of Cl and might have been generated by a chemical reaction initiated by the high deposit temperature:

KCl + (Si, Ca) + H2O f HCl + (K, Ca, Si)

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In this reaction the Cl is released and the deposit obtains a composition very similar to that of bottom ash.8 However, equilibrium calculations indicate that potassium chloride is more stable than potassium silicate, but it is possible that components containing both K, Ca, and Si behave differently. An alternative explanation is that the Masnedø intermediate deposit layer has been generated by entrainment and subsequent deposition of bottom ash particles. However, the higher Cl content of the Masnedø outer layer indicates that the deposit in fact is generated by an on-site reaction between KCl, Si, and Ca. The large porosity of the layer could be generated by the release of HCl gas. As seen in Figure 5, the Masnedø intermediate layer is very dense between the large pores and must have been melted. A phase diagram of mixtures of K2O‚4SiO2 and CaO‚SiO2 shows a minimum melting temperature of 740 °C.26 An investigation of the melting temperatures of bottom ash (with a composition similar to that of the Masnedø intermediate deposit) from straw-fired grate boilers reveals initial melting at 750 to 800 °C.23 This indicates that the intermediate layer at least occasionally has been exposed to temperatures above 740 °C. The mature superheater deposits of both Masnedø and Ensted contained some inner sublayers (A) of very dense KCl and K2SO4. The K2SO4 may be generated on site by KCl sulfation. The present investigation of the Masnedø deposit shows in several cases KCl on top of the innermost FexOy layers, while in previous investigations of mature deposits14,20 mainly K2SO4 has been observed next to the FexOy layer. It is a question why KCl is observed adjacent to the innermost layer of iron oxide in the present Masnedø study. The reason may be occasional spallation of the innermost iron oxide scale or it may be the thick intermediate layer that prevents SO2 from entering. Occasionally, iron oxide scale spallation is in agreement with the large corrosion rates observed on the test superheaters at Masnedø.17 Compared to the mature superheater deposits, shorttime probe deposits (1 to 20 h) are quite different.13,16,24 The probe deposits generally consist of a layer of dense pure KCl with a maximum thickness of 30 µm adjacent to the metal surface, then a very porous KCl-rich layer of maximum 800 µm, and on top of this are ash particles rich in Si and Ca glued together by KCl. The KCl-rich inner layer of the mature deposits is both much denser and thicker (see Table 5) than the KCl-rich probe inner layers. This indicates that the deposits may not simply grow by collection of ash particles on the outer surface. It is possible that gas-phase KCl diffuses through the (23) Hansen, L. A.; Frandsen, F. J.; Sørensen, H. S.; Rosenberg, P.; Hjuler, K.; Dam-Johansen, K. Ashes Fusion and Deposit Formation at Straw-Fired Boilers. Impact of Mineral Impurities in Solid Fuel Combustion; Gupta et al., Eds.; Kluwer Academic/Plenum Publishers: New York, 1999; pp 342-356. (24) Frandsen, F. J.; Hansen, J.; Jensen, P. A.; Dam-Johansen, K.; Ho¨rlyck, S.; Karlsson, A. Ash and Deposit Formation in the Biomass Co-fired Masnedø Combined Heat and Power Production Plant. Paper presented at the conference: Effects of coal quality on power plant performance: Ash Problems, Management and Solutions, Utah, USA, May 2000. (25) Baxter, L. L. Influence of Ash Deposit Chemistry and Structure on Physical and Transport Properties. Fuel Process. Technol. 1998, 56, 81-88. (26) Levin, E. M.; Robbins, C. R.; McMurdie, H. F. Phase Diagrams for Ceramists; The American Ceramic Society, Inc.: Columbus, OH, 1964; Vol. I.

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deposits and condensate in the innermost layers. Equilibrium calculations have shown, that down to a temperature of 550 °C potassium chlorine can be present as a gas in the ppm range.16 This KCl condensation makes the inner deposit layers grow and densify through the time. It is a question why the dense KCl-containing layers adjacent to the tube surface appear to have been molten. The difference between the metal surface temperature and the steam temperature has been calculated to be typically 30 °C.17 This means that the largest metal surface temperatures on the Masnedø superheaters should be approximately 550 °C. The melting temperature of pure KCl is 774 °C (Table 1), which indicates that the dense KCl-rich innermost layers are probably generated by direct condensation followed by recrystallization. The porous layer of KCl flakes could possibly have been generated by differences in the thermal expansion coefficient of the deposit and the steel tube. The KCl flakes are a fracture generated by the deposit moving relative to the metal tube. Temperature fluctuations are initiated both by boiler load changes and by short-time scale flue gas temperature fluctuations. Relatively large corrosion rates have been measured on a test superheater installed in the Masnedø boiler,17 and it has also been observed that, even when superheater steam temperatures are maintained, an increased flue gas temperature seems to increase the superheater corrosion rate. It is therefore interesting that the Masnedø deposit has been kept at a higher temperature than the Ensted deposit. The mature Masnedø deposit is special in several respects. In several cases lifted iron rich layers were observed indicating spallation of the protective iron oxide, the intermediate deposit layer was depleted of chlorine, and KCl was often observed directly on top of the iron oxide scale. It is possible that the process that generates the intermediate layer acts as a chlorine donor which cause an increased corrosion. This could lead to periodic shedding of the protective iron oxide scale and generation of a new dense KCl layer on top of the tube surface. It was anticipated that by using no soot blowing on the Masnedø superheater the corrosion rate could be reduced. However, the thick porous layer of deposit reduces heat transfer,26 and a relatively large superheater area is needed. It is possible that it is better to apply regular soot blowing and thus obtain better heat transfer. However, no well documented knowledge on the influence of soot blowing on the corrosion rate exists today, so we recommend, that studies of the influence of soot blowing and flue gas temperatures on superheater corrosion in straw-fired boilers are initiated. Acknowledgment. This work is part of the CHEC research program (Combustion and Harmful Emission Control) funded by the Technical University of Denmark, Elsam A/S, Energy E2 A/S, PSO funds from Eltra A/S and Elkraft A/S, and the Danish Energy Research Program. The Danish Technological Institute performed the SEM and EDX analyses under the Center for Surface Microscopy, Micro- and Image Analysis. EF030097L