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High-temperature corrosion due to lead chloride mixtures simulating fireside deposits in boilers firing recycled wood
Fuel Processing Technology 167 (2017) 306–313
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Research article
High-temperature corrosion due to lead chloride mixtures simulating fireside deposits in boilers firing recycled wood Hanna Kinnunen a,b,⁎, Daniel Lindberg b, Tor Laurén b, Mikko Uusitalo a, Dorota Bankiewicz b, Sonja Enestam a, Patrik Yrjas b a b
Valmet Technologies, Lentokentänkatu 11, PO Box 109, FI-33101 Tampere, Finland Johan Gadolin Process Chemistry Centre, c/o Laboratory of Inorganic Chemistry, Åbo Akademi University, Piispankatu 8, 20500 Turku, Finland
a r t i c l e
i n f o
Article history: Received 30 May 2017 Received in revised form 12 July 2017 Accepted 12 July 2017 Available online xxxx Keywords: High temperature corrosion Lead potassium chloride Waste wood combustion Superheater Furnace wall
a b s t r a c t One of the biggest operational concerns in recycled wood combustion, is the risk for formation of low melting, corrosive deposits. The deposits present on low-temperature heat transfer surfaces (material temperature b 400 °C) are composed of alkali metals, chlorine, sulphur, heavy metals or, as is often the case, a mixture of these. K2SO4 is commonly regarded as non-corrosive, but there have been indications that K2SO4 may worsen the PbCl2 induced corrosion. Consequently, a more detailed study with these compounds was of very high interest. This paper reports the results obtained from 24-hour isothermal laboratory corrosion tests with PbCl2 mixed with either K2SO4 or SiO2. The tests were carried out at 350 °C using low alloy steel (16Mo3). The interaction between PbCl2 and K2SO4 was investigated in a furnace with a temperature gradient. As a result, a mixture of PbCl2 and K2SO4 is more corrosive than PbCl2 mixed with SiO2. Corrosion was noticed below the deposit's first melting temperature. However, for a mixture of FeCl2, KCl and PbCl2, the first melting temperature is below 350 °C which could explain the high oxidation rate observed below the first melting temperature of the deposit. A solid phase or a mixture of phases with the composition of K3Pb2(SO4)3Cl was observed in the tests with SEM/EDX for the first time. © 2017 Elsevier B.V. All rights reserved.
1. Introduction Corrosion problems in heat transfer surfaces are limiting the rise of the steam temperatures in power boilers. Alkali chlorides are usually the main reason for corrosion of the hottest superheaters and they have been shown to be corrosive at material temperatures above 450 °C [1,2]. Alkali chloride induced corrosion has been a widely-investigated topic over the years and different corrosion mechanisms, as well as remedies, have been suggested [1–9]. One of the suggested corrosion mechanisms is so-called active oxidation, where gaseous chlorine diffuses through the oxide layer and reacts with the steel, forming metal chlorides. The formed volatile metal chlorides diffuse outwards through the oxide layer; when the O2 partial pressure is high enough, the metal chlorides are oxidised releasing chlorine gas, which in turn partly diffuses back to the steel-oxide interface [3,4]. Another suggested mechanism is that chloride salts form eutectic melts which dissolve iron from the steel [2,5]. The third theory proposes that hydrogen chloride and oxygen are absorbed to the steel surface followed by further dissociation into H+, Cl− and O2– ions. In this process water vapour evaporates ⁎ Corresponding author at: Valmet Technologies, Lentokentänkatu 11, PO Box 109, FI33101 Tampere, Finland. E-mail address: [email protected] (H. Kinnunen).
http://dx.doi.org/10.1016/j.fuproc.2017.07.017 0378-3820/© 2017 Elsevier B.V. All rights reserved.
and at the same time Cl− ions migrate to the oxide grain boundaries forming metal chlorides [6]. The increased use of recycled wood containing fuels has resulted in corrosion related challenges on low-temperature heat transfer surfaces, such as furnace walls and primary superheaters. Corrosion on these cooler surfaces is caused by heavy metal chlorides or by a combination of heavy metal and alkali chlorides [5,9–12]. Typical deposits on heat transfer surfaces with a material temperature below 400 °C are composed of a mixture of alkali metals (sodium and potassium), chlorine, sulphur and heavy metals. The first melting temperature of these types of deposits can drop below 350 °C. According to laboratory measurements, lead chloride (PbCl2) has been found to be corrosive as a pure PbCl2 salt and when mixed with potassium sulphate (K2SO4) or potassium chloride (KCl) [13,14]. A couple of possible corrosion mechanisms of such deposits are presented in the literature [5,10–12,15]. Laboratory investigations with synthetic salts on a low alloy steel have shown that corrosion caused by heavy metal chlorides initiated formation of metal chlorides and that the corrosion could be enhanced by the presence of a melt [5,10–12,15]. In a full-scale boiler, a combination of potassium and lead was shown to be corrosive at water wall temperatures (b400 °C) when using an Inconel 625 type alloy. In the same study, a low alloy steel was suggested to have been attacked by hydrogen chloride [11]. Talus et al. [16] proposed that
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lead and lead oxide (PbO) react with iron chloride (FeCl2), forming PbCl2, which was then combined with alkali chlorides. It has also been shown that PbCl2 causes accelerated corrosion by forming chromates with stainless steel [17]. The role of zinc in the corrosion process is still unclear. Zinc is often present in waste based fuels [18] and zinc chloride (ZnCl2) has been shown to be corrosive and also to lower the first melting temperature of a deposit [14,19,20]. However, in the bottom part of the furnace where the gas composition is more reducing than oxidising, zinc is most likely to condense as metallic zinc. When moving upwards in the furnace, to the secondary and tertiary air levels, the atmosphere becomes more oxidising. In an oxidising atmosphere, zinc is oxidised to zinc oxide (ZnO) at temperatures above 300 °C [10]. ZnO has been found to be significantly less corrosive than ZnCl2 [14]. Thus, this study focuses on PbCl2. Sulphur is a well-known remedy against alkali chloride induced high-temperature corrosion. It has been shown that co-combustion with a sulphur-rich fuel or using sulphur containing additives, will convert harmful alkali chlorides to less harmful alkali sulphates [1,3,7–9]. Pure alkali sulphates do not cause corrosion below 500 °C [21]. The influence of sulphur on heavy metal chloride induced corrosion is, however, not yet fully understood. Combustion studies of sewage sludge together with waste wood suggested that sewage sludge reduces the amount of chlorides in the superheater deposits, and also observed a reduction of corrosion in furnace walls [22]. The purpose of this study was to identify the corrosivity of PbCl2 mixed with K2SO4 in greater detail. PbCl2 was also mixed with silicon dioxide (SiO2) in order to pinpoint the effect of K2SO4.
2. Experimental 2.1. Corrosion tests Isothermal corrosion furnace tests were carried out to study and compare the different corrosivities between two PbCl2 containing synthetic salt mixtures. The experiments were performed using a commercial low alloy ferritic steel: EN10216-2 16Mo3. The selected steel is widely used as wall panel and low-temperature superheater material in industrial boilers. Table 1 presents the standard elemental composition of the steel in weight-%. The test specimens were approximately 20 × 20 mm coupons with a thickness of 5 mm. The steel samples were ground with ethanol using SiC paper with a final grit of 1000 and then cleaned in an ultrasonic bath. Before the tests, the coupons were pre-oxidised in a furnace for 24 h at 200 °C. Afterwards, each coupon was covered with a salt mixture (0.25 g/specimen) consisting of either pure K2SO4 or 50 wt-% PbCl2 mixed with either 50 wt-% K2SO4 or 50 wt-% SiO2. SiO2 was selected as the other salt component due to its low reactivity and to have equal amounts of PbCl2 as weight-% in both salts. The chemical compositions of the salt mixtures are presented in Table 2. The PbCl2-K2SO4 salt mixture was pre-melted at 500 °C for 30 min and ground afterwards to ensure adequate mixing of the salts. The samples were exposed in a horizontal tube furnace for 24 h at 350 °C in ambient air. After the corrosion tests, the samples were placed in a mould, cast in epoxy and cut through the middle to reveal the crosssection. The cross-sectional surfaces were polished in kerosene using SiC paper with a final grit of 2500 and were then cleaned in petroleum
Table 1 The composition of the test steel in weight-%. The composition is informed according to the EN 10216-2 standard.
16Mo3
C [wt-%]
Si
Mn
Cr
Mo
Ni
Others
0.12–0.20
≤0.35
0.40–0.90
≤0.30
0.25–0.35
≤0.30
Al, P, S
307
Table 2 The chemical compositions of the salt mixtures in weight- and mol-%. Salt mixtures [weight-%]
Salt 1 Salt 2 Salt 3
Salt mixtures [mol-%]
PbCl2
K2SO4
SiO2
PbCl2
K2SO4
SiO2
– 50 50
100 50 –
– – 50
– 38.5 17.8
100 61.5 82.2
–
ether and an ultrasonic bath. The prepared samples were analysed with Scanning Electron Microscope/Energy Dispersive X-ray (SEM/ EDX) to measure the thickness of the oxide layer and to identify various chemical elements in the layer. Corrosion products were identified using X-ray images, and the oxide layer thickness was determined using scanning electron microscope (SEM) backscatter images. Several SEM images were combined to form a panoramic picture of the whole cross-section. The resulting images were digitally enhanced based on differences in the contrast. An example of the treatment stages of a typical SEM panoramic picture is presented in Fig. 1. After the panoramic images were coloured, the thickness of the oxide layer was determined for each vertical line of pixels and was recalculated to μm. The method has also been described by WesténKarlsson [23]. The corrosion layer was determined by the thickness of the oxide layer for each line, and the corrosion attack is expressed as the mean thickness of the oxide layer. A gradient furnace test was carried out to study the interaction between PbCl2 and K2SO4 particles. A ring-shaped carbon steel sample was covered with a salt mixture containing 10 wt-% (6.5 mol-%) of PbCl2 and 90 wt-% (93.5 mol-%) of K2SO4. No pre-melting of the salt components was performed. The particle diameter was 100–500 μm for K2SO4 and 10–30 μm for PbCl2. The salts were exposed in the gradient furnace for 24 h using a steel temperature of 310 °C and a gas temperature of approximately 800 °C. The method has been described by Lindberg et al. [24]. The main equipment used in the temperature gradient corrosion experiments was a corrosion probe placed into a tube furnace. The corrosion probe consists of a probe and a protective tube. The protective tube surrounds the probe except for a window exposing part of the steel sample rings to the furnace environment. The outer protective tube was mounted on the air-cooled probe in order to reduce the cooling effect from the probe, and thereby to decrease the need for heating the tube furnace, resulting in more stable temperatures. The inner probe has two removable sample rings, which are equipped with thermocouples. The temperature of one of the test rings on the probe is regulated with a proportional-integral-derivative controller (PID controller) adjusting the flow of the cooling air. The temperature of the other test ring is monitored and logged during the test run. Typically, a difference of 5–10 °C is formed between the rings. The typical furnace set temperature is 980 °C, which gives a gas temperature of around 800 °C about 1 cm above the deposit, when the probe temperature is around 300–500 °C. Typical temperature gradients across the salt deposits are around 50–100 °C/mm in the radial direction. Additional thermocouples can be installed on the outside of the protective tube to measure the temperature between the sample probe and the alumina tube in the furnace. The inner probe is the same as used by Bankiewicz et al. [25]. Roughly 0.5 g of the salt mixture is applied on top of each ring on an area of about 10 mm × 20 mm. It corresponds to a thickness of about 5 mm of salt prior to the experiment. The edges of the application area on the rings are surrounded by a protective paste to hold the salt in place in case of melting of the salts. For the experiments, the probe is inserted into a cold tube furnace and both the probe and the furnace are heated to their set temperatures. As stable temperatures were reached, the experiment was run for 24 h, and
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Fig. 1. The colouring stages of an SEM image. The stages are used for determination of the oxide layer thickness.
subsequently cooled to room temperature. After cooling, the salt deposits on the rings were cast in epoxy, and subsequently removed from the probe. The rings were cut and polished to get a cross section of the ring and the salt deposit for SEM/EDX analysis, similar to the process for the samples from the isothermal tests. More detailed images and schematic drawings of the experimental setup are reported by Lindberg et al. [24]. 2.2. DSC/TGA-experiments Differential scanning calorimetry/thermogravimetric analyses (DSC/ TGA) was used to study the melting temperatures of binary or ternary mixtures of KCl, PbCl2, and FeCl2. A TA Instruments SDT Q600 simultaneous DSC/TGA apparatus was used in the experiments. The reagents for the mixtures were anhydrous pro analysis KCl, PbCl2, and/or FeCl2; about 15 mg of the mixtures were added to an alumina cup for the experiments. The experiments were done in nitrogen with a flow rate of 100 ml/min; the heating/cooling rate was 10 °C/min. After the initial heating to the maximum temperature, two cooling/heating cycles were made. The maximum temperature was 400 °C for the experiments with KCl, and 500 °C for the PbCl2-FeCl2 mixture. The maximum temperature was kept low to ensure minimal vaporization of the salts but high enough to achieve a completely molten mixture based on pre-calculations. The results from the first heating ramp were not considered, due to the heterogeneous nature of the unmelted samples. In the subsequent cooling/heating cycles, the sample mixture can be assumed to be homogenised and close to the expected phase equilibria. For simple DSC peaks, the onset temperature was considered to be the relevant transition temperature. For complex peaks, the peak or inflection temperatures were also considered. These temperatures typically coincide with the liquidus of the system. 3. Results In this chapter, the corrosion results from the isothermal tests will be presented, as well as the interactions of the salt components in the temperature gradient test. The results will be further discussed in the next chapter (Chapter 4). In the first isothermal test, which was done with
pure K2SO4, no corrosion was observed at 350 °C and the oxide layer was below 1 μm. This was expected as no increased oxidation was found at a higher temperature either [21]. 3.1. Isothermal corrosion test with PbCl2 mixed with SiO2 Isothermal corrosion test with 50 wt-% of PbCl2 mixed with 50 wt-% of SiO2 showed only an occasional and very thin (b 10 μm) iron oxide layer. Both, SiO2 and PbCl2 particles, are clearly seen as separate particles in the cross-section (Fig. 2). The grey particles in the figure are SiO2 particles and the white particles are PbCl2 particles. The compositions of the different particles were verified with point analyses as presented in Fig. 3. 3.2. Isothermal corrosion test with PbCl2 mixed with K2SO4 A continuous oxide layer was detected in the test with PbCl2 mixed with K2SO4. The average oxide layer thickness was about 20 μm clearly showing increased oxidation (Fig. 4). More detailed point analyses are presented in Fig. 5. An iron oxide layer covered the steel surface (point number 1) as presented in Fig. 5. Above this, another, thinner, iron oxide layer with residues of chlorine was detected (point 2). The white areas (points 3 and 4) were composed of lead, chlorine, potassium, oxygen, and also molybdenum in point 4. These layers are mixed phases and it is difficult to conclude which compounds are present based on the SEM/EDX analyses. Several greyish areas were noticed within the deposit particles. It seems that several different phases were formed due to the interactions between PbCl2 and K2SO4 since the compositions of the greyish areas varied significantly. One area consisted mainly of KCl with residues of PbCl2 (point 5) while another area had a composition corresponding to a K3Pb2(SO4)3Cl compound (point 6). A similar compound, Na3Pb2(SO4)3Cl, was identified and reported by Enestam et al. [26] in ash deposits from recovered waste wood boilers, and is also naturally occurring as the mineral caracolite. Point 7 had an elemental composition corresponding to a K2PbCl4 compound, while the dark grey areas were concluded to be K2SO4 (point 8).
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Fig. 2. An SEM/EDX image of the cross-section of the 50 wt-% PbCl2 + 50 wt-% SiO2 exposed sample. Only an occasional and thin oxide layer is seen on the steel surface. The grey deposit particles are SiO2 particles and the white particles are PbCl2.
3.3. Interaction of the salt components The reaction between K2SO4 and PbCl2 may result in the formation of a molten phase consisting of the following components: K2SO4, lead sulphate (PbSO4), KCl and PbCl2. The reported minimum eutectic temperature of the K2SO4-PbSO4-KCl-PbCl2 system is 403 °C according to Dombrovskaya [27]. The reported stable solid phases are the pure K2SO4, PbSO4, KCl and PbCl2, and the double salts KPb2Cl5, K2PbCl4, K2Pb(SO4)2 and K2Pb2(SO4)3. In addition, the high temperature modification of K2SO4 can dissolve up to 25 mol-% PbSO4. The compound K3Pb2(SO4)3Cl was not observed by Dombrovskaya. The point analyses 3–7 in Fig. 5 clearly show that the salt particles consisting of PbCl2 and K2SO4 have reacted with each other, since both lead and potassium were found at the same location. However, this could have already occurred during the pre-melting stage (performed at 500 °C for 30 min). To confirm the interaction between PbCl2 and K2SO4 particles without pre-melting, a temperature gradient furnace test was carried out using the same two salts. The material temperature was 310 °C and the gas temperature was approximately 750–800 °C in the test. Fig. 6 presents the cross-section of the salt mixture after the exposure. The uppermost layer of the deposit consists of pure, partly sintered K2SO4 with some of the particles containing rims consisting of the K3Pb2(SO4)3Cl phase. The particles at the hottest temperatures do not have any Pb-containing rims, suggesting that the PbCl2 that has not
reacted with the K2SO4 in the deposit, has vaporized and been transported into the furnace. A white layer enriched in lead occurs at around 1.6 mm from the ring surface. SEM/EDX analysis shows that the white layer consists of mainly K2PbCl4, but also some sulphatephases, specifically the proposed K3Pb2(SO4)3Cl phase. Based on the phase diagram of Dombrovskaya [27], the lowest-melting eutectic point of the K2SO4-PbSO4-KCl-PbCl2 system (403 °C) and a eutectic or peritectic point at 426 °C are close to or on a potential tie-line between K2PbCl4 and K3Pb2(SO4)3Cl. The composition of the invariant points suggests that K2PbCl4 would be the main phase that crystallizes from the eutectic or peritectic melts. It is therefore plausible that the dense Pbrich layer at around 1.6 mm from the ring surface corresponds to solidified eutectic melt at around 400–430 °C. Typical temperature gradients in the gradient tests, the measured gas temperature above the deposit, and the distance of the layer from the ring surface support this explanation. Small crystals consisting only of lead, chlorine and oxygen also exist in the molten layer. These may be various lead-oxychlorides [28]. The close-up of this area in Fig. 6 shows that rims surround the outer layer of K2SO4 particles. The outer layer of rims is mainly K2PbCl4, whereas the inner layer consists of K-Pb-sulphates or the K3Pb2(SO4)3Cl phase. The rim shape suggests that the gas phase has been involved in some of the reactions. KCl and PbCl2 have been identified in the deposit close to the steel surface. The above-mentioned phases have not been identified with X-ray diffraction. However, the SEM/EDX analyses of the various phases are
Fig. 3. Point analyses of the salt mixture 50 wt-% PbCl2 + 50 wt-% SiO2 after the exposure. The elements with concentrations below 1 atomic-% are marked as residue, “res.”, in the table.
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Fig. 4. An SEM/EDX image of the cross-section of the sample exposed to 50 wt-% PbCl2 + 50 wt-% K2SO4. The oxide layer can be seen on the steel surface as a grey, continuous line. The average oxide layer thickness was 20 μm.
shown in Fig. 7 and compared to the theoretical composition based on the K-Pb-S-Cl atomic distribution. The EDX-analyses do not differentiate between solid crystalline phases and amorphous phases or mixtures with very fine-grained microstructure. The identified phases correspond the phases observed by Dombrovskaya, except for KPb2Cl5, which was not observed in the present study and K3Pb2(SO4)3Cl, which was not identified by Dombrovskaya. It is possible that K3Pb2(SO4)3Cl does not correspond to a single phase but instead to a very fine-grained mixture corresponding to a multicomponent eutectic composition. However, the composition does not correspond to any of the eutectic points or cotectic lines in the liquidus projection published by Dombrovskaya. It is clear from the analyses that the interaction between K2SO4 and PbCl2 can lead to the formation of several complex phases, either through melting reactions or by interactions among gaseous components. Solidsolid interactions are also possible, but are typically rather slow.
4. Discussion The isothermal laboratory tests conducted at 350 °C, showed that the corrosivity of PbCl2 increases when mixed with K2SO4 whereas no increased oxidation was detected when PbCl2 was mixed with SiO2. A negligible oxide layer growth of low-alloy steel was also reported by Bankiewicz et al. with pure PbCl2 at 350 °C [14]. The most probable reason for the increased oxidation when both PbCl2 and K2SO4 are present, is the formation of a mixture consisting of lead, potassium and chlorine. The temperature gradient furnace test clearly verified that reactions take place between the PbCl2 and K2SO4. Based on the literature, the melting point of PbCl2 is 501 °C and the first melting temperature of a 50 wt-% PbCl2 and 50 wt-% K2SO4 mixture is 426 °C [27]. The reactions between K2SO4 and PbCl2 can produce mixtures that have a first melting temperature at 403 °C, which is still ~50 °C above the test temperature and does not entirely explain the increased oxidation at 350 °C.
Fig. 5. Point analyses of the salt mixture 50 wt-% PbCl2 + 50 wt-% K2SO4 after the exposure. Concentrations below 1 at-% are marked as residue, “res.”.
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311
Fig. 6. A cross-section of the salt mixture exposed in the gradient furnace. More detailed distribution of the elements is presented in the magnified images.
FeCl2 has been suggested to be involved in the corrosion reaction and to form low melting mixtures with chlorides in the deposit, thus causing increased oxidation [5,29–31]. FeCl2 is known to be a reaction product of chlorine induced corrosion with iron oxide forming steels. The minimum melting temperature for a KCl-FeCl2 mixture is 355 °C [32]. Since no similar data have been found for the FeCl2-KCl-PbCl2 system, a solidus projection was calculated with FactSage and the results are presented in Fig. 8. The calculation was made using FactSage version 7.1 [33] and using the FTsalt database for molten salt systems. The ternary FeCl2-KCl-PbCl2 system has not been evaluated and compared with experimental data. The binary KCl-FeCl2 system has been evaluated by Robelin et al. [34], and the binary KCl-PbCl2 has been optimised but not published, based on the documentation of FactSage. However, the thermodynamic properties are similar to the experimental study
and thermodynamic evaluation of Gabriel & Pelton [35]. The binary PbCl2-FeCl2 system has not been evaluated and the liquid is assumed to be ideal in the thermodynamic database. Ferrari measured the phase equilibrium of the PbCl2-FeCl2 phase diagram [36]. The calculated PbCl2-FeCl2 phase diagram predicts a solidus temperature of 452 °C, whereas the experimental determination of Ferrari and Colla gives solidus temperatures around 415 °C, but varying between 400 and 422 °C. Therefore, some uncertainties in the predicted solidus temperature of the FeCl2-KCl-PbCl2 system are to be expected as the ternary data are extrapolated from binary systems, and the PbCl2-FeCl2 system shows some variations between the calculated and the previously measured data. In order to get additional verification of the melting temperature of the FeCl2-KCl-PbCl2 system, a synthetic mixture was made and analysed with DSC/TGA. The mixture composition was chosen to coincide with
Fig. 7. Point analyses of phases in the temperature gradient sample of K2SO4 and PbCl2 recalculated as the atomic ratios of potassium, lead, sulphur and chlorine. Theoretical compositions of the various phases are given as the last analysis of the grouping. Mixture compositions are not shown in the analyses.
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Fig. 8. Solidus projection for FeCl2 – KCl – PbCl2 system. Temperatures shown as °C. Black circles indicate experimental mixture compositions.
the region with the lowest predicted solidus temperature of the FeCl2KCl-PbCl2 system. In addition, to verify the accuracy of the phase equilibrium measurements, mixtures of KCl and FeCl2, as well as PbCl2 and FeCl2, with compositions close to the eutectic point of the respective systems were analysed and compared with calculated and experimental data. The measurement points are marked with black circles in Fig. 8. Table 3 gives the compositions of the mixtures in weight-% and mol-% and the measured solidus, liquidus, and solid phase transition temperatures. The measured and the calculated solidus temperatures for the KClFeCl2 and FeCl2-KCl-PbCl2 mixtures coincide very well. The experimental solidus and liquidus temperatures for the PbCl2-FeCl2 mixture are considerably lower than the predicted results, assuming ideal mixing in the liquid phase. The measured solidus and liquidus temperatures, however correspond very well with the results of Ferrari & Colla. The measured solidus temperature of the KCl-FeCl2 mixture is predicted accurately by the calculations, whereas the liquidus temperature is slightly higher in the experiment. However, the measured liquidus at 372 °C could also be an overlapping signal from a predicted peritectic temperature of 374 °C in the KCl-rich region. The measured thermal events at 273 and 323 °C in the KCl mixtures are most likely attributed to solid phase transitions of K2FeCl4 and KFeCl3, based on the thermodynamic evaluation of Robelin et al. The measured solidus temperature of the FeCl2-KCl-PbCl2 is 311 °C and is predicted to be 312 °C. However, due to the discrepancy in the binary PbCl2-FeCl2 system between the calculated and measured melting temperatures, it would be recommended to perform a thermodynamic evaluation of the binary PbCl2-FeCl2
system. In addition, experimental studies and a thermodynamic evaluation for the ternary FeCl2-KCl-PbCl2 is recommended for more accurate predictions over a larger compositional range. However, this is outside the scope of the present study. As marked in Fig. 8, the melting temperatures vary between 312 and 334 °C depending on the composition. This might explain why increased oxidation was seen below the deposit's first melting temperature. The elements present closest to the iron oxide layer in the sample exposed to the 50 wt-% PbCl2 and 50 wt-% K2SO4 salt mixture included lead, iron, potassium, chloride and oxygen (Fig. 4). In the same figure, an iron oxide layer above the steel surface included small amounts of chlorine, indicating that corrosion follows the already-known chlorine induced corrosion mechanism by the formation of FeCl2. A corrosion mechanism is suggested, in which a potassium lead chlorine mixture initiates the corrosion reaction by the formation of FeCl2, which in turn further accelerates the corrosion by lowering the melting temperature via formation of a low melting mixture including iron, potassium, lead and chlorine. 5. Conclusions Laboratory corrosion tests were carried out in ambient air at 350 °C exposing low alloy steel to PbCl2 salt mixed either with 50 wt-% of SiO2 or 50 wt-% of K2SO4. The mixture with K2SO4 was more corrosive compared to the mixture with SiO2. Actually, no increased oxidation was noticed with PbCl2 mixed with SiO2. PbCl2 mixed with K2SO4 salt produced a thick and continuous oxide layer in a mere 24 h. Several different
Table 3 Compositions of the mixtures in weight-% and mol-% and the measured solidus, liquidus, and solid phase transition temperatures. KCl
FeCl2
PbCl2
KCl
27.9 38.9 32.9
72.1 – 15.5
– 48 31.2
[mol-%] Mixture 1 Mixture 2 Mixture 3
– 61.1 51.6
FeCl2
PbCl2
Tsolidus
15 52 33.8
85 – 35
[weight-%]
Tliquidus
Ttrans
Tsolidus
Experimental [°C]
Calculated [°C]
412 350 311
273, 323 273
452 351 312
420 372
Tliquidus
491 351 341
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phases with varying amounts of lead, potassium, chlorine and sulphur were detected from the cross-section of the sample. The interaction between the two salt components in the deposit was verified with a gradient furnace test. This interesting finding indicates that the usually noncorrosive K2SO4 can increase the chloride induced corrosion at relatively low material temperatures (~350 °C), if it comes in contact with PbCl2. A solid phase or a mixture of phases with the composition K3Pb2(SO4)3Cl was observed with SEM/EDX in the isothermal and temperature gradient tests. The phase is similar to the mineral caracolite (Na3Pb2(SO4)3Cl), which has been observed previously in boiler deposits. However, additional characterisation of the new phase is needed in future work. The formation of a low melting eutectic mixture including FeCl2-KClPbCl2 has melting temperatures below 350 °C and could possibly accelerate the corrosion once started. The melting behaviour of the ternary mixture was measured with DSC/TGA and confirmed with thermodynamic predictions for the first time. Nevertheless, it is concluded that the initiation of the corrosion takes place due to the K2SO4 mixed with PbCl2. The proposed low melting mixture formation could explain the increased oxidation below the deposit's first melting temperature. Acknowledgements Linus Silvander, Jaana Paananen and Peter Backman from Åbo Akademi are gratefully acknowledged for carrying out the laboratory and analysis work. Additional funding from the Academy of Finland (Decision no. 266384 and no. 269277) for one of the co-authors (D. Lindberg) is greatly appreciated. References [1] K. Salmenoja, Field and Laboratory Studies in Chlorine-induced Superheater Corrosion in Boilers Fired with Biofuels, Åbo Akademi, Turku, Finland, 2000. [2] H.P. Nielsen, F.J. Frandsen, K. Dam-Johansen, L.L. Baxter, The implications of chlorineassociated corrosion on the operation of biomass-fired boilers, Prog. Energy Combust. Sci. 26 (2000) 283–298, http://dx.doi.org/10.1016/S0360-1285(00)00003-4. [3] H.J. Grabke, E. Reese, M. Spiegel, The effects of chlorides, hydrogen chloride, and sulphur dioxide in the oxidation of steels below deposits, Corros. Sci. 37 (1995) 1023–1043, http://dx.doi.org/10.1016/0010-938X(95)00011-8. [4] A. Zahs, M. Spiegel, H.J. Grabke, Chloridation and oxidation of iron, chromium, nickel and their alloys in chloridizing and oxidizing atmospheres at 400–700 °C, Corros. Sci. 42 (2000) 1093–1122, http://dx.doi.org/10.1016/S0010-938X(99)00142-0. [5] M. Spiegel, Corrosion in molten salts, Mater. Sci. Mater. Eng. 1 (2010) 316–330, http://dx.doi.org/10.1016/B978-044452787-5.00019-6. [6] N. Folkeson, J. Pettersson, C. Pettersson, L.-G. Johansson, E. Skog, B.-A. Andersson, S. Enestam, J. Tuiremo, A. Jonasson, B. Heikne, J.-E. Svensson, Fireside corrosion of stainless and low alloyed steels in a waste-fired CFB boiler; The effect of adding sulphur to the fuel, Mater. Sci. Forum 595–598 (2008) 289–297 (http://dx.doi.org/ 192.89.97.1-05/00/00,00:24:35). [7] J. Pettersson, C. Pettersson, N. Folkeson, L.-G. Johansson, E. Skog, J.-E. Svensson, The influence of sulphur additions on the corrosive environment in a waste-fired CFB boiler, Mater. Sci. Forum 522–523 (2006) 563–570, http://dx.doi.org/10.4028/ www.scientific.net/MSF.522-523.563. [8] M. Aho, P. Yrjas, R. Taipale, M. Hupa, J. Silvennoinen, Reduction of superheater corrosion by co-firing risky biomass with sewage sludge, Fuel 89 (2010) 2376–2386, http://dx.doi.org/10.1016/j.fuel.2010.01.023. [9] R. Backman, R. Khalil, D. Todorovic, Ø. Skreiberg, M. Becidan, F. Goile, A. Skreiberg, L. SØrum, The effect of peat ash addition to demolition wood on the formation of alkali, lead and zinc compounds at staged combustion conditions, Fuel Process. Technol. 105 (2013) 20–27, http://dx.doi.org/10.1016/j.fuproc.2011.04.035. [10] S. Enestam, Corrosivity of hot Flue Gases in the Fluidized bed Combustion of Recovered Waste Wood, in Department of Chemical Engineering, Åbo Akademi, Turku, Finland, 2011. [11] Y. Alipour, P. Henderson, P. Szakálos, The effect of a nickel alloy coating on the corrosion of furnace wall tubes in a waste wood fired power plant, Mater. Corros. 65 (2014) 217–225, http://dx.doi.org/10.1002/maco.201307118. [12] G. Sorell, The role of chlorine in high temperature corrosion in waste to energy plants, Mater. High Temp. 14 (1997)http://dx.doi.org/10.1080/09603409.1997. 11689546.
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Henderson, The analysis of furnace wall deposits in a lowNox waste wood-fired bubbling fluidised bed boiler, VGB PowerTech 12 (2012) 96–100. [18] F. Jones, D. Bankiewicz, M. Hupa, Occurrence and sources of zinc in fuels, Fuel 117 (2014) 763–775, http://dx.doi.org/10.1016/j.fuel.2013.10.005. [19] P. Vainikka, D. Bankiewicz, A. Frantsi, J. Silvennoinen, J. Hannula, P. Yrjas, M. Hupa, High temperature corrosion of boiler waterwalls induced by chlorides and bromides. Part 1: occurrence of the corrosive ash forming elements in a fluidised bed boiler co-firing solid recovered fuel, Fuel 90 (2011) 2055–2063, http://dx.doi.org/ 10.1016/j.fuel.2011.01.020. [20] W.M. Lu, T.J. Pan, K. Zhang, V. Niu, Accelerated corrosion of five commercial steels under a ZnCl2–KCl deposit in a reducing environment typical of waste gasification at 673–773 K, Corros. Sci. 50 (2008) 1900–1906, http://dx.doi.org/10.1016/j.corsci. 2008.03.004. [21] B.-J. Skrifvars, R. Backman, M. Hupa, K. Salmenoja, E. 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Fuel Processing Technology journal homepage: www.elsevier.com/locate/fuproc
Research article
High-temperature corrosion due to lead chloride mixtures simulating fireside deposits in boilers firing recycled wood Hanna Kinnunen a,b,⁎, Daniel Lindberg b, Tor Laurén b, Mikko Uusitalo a, Dorota Bankiewicz b, Sonja Enestam a, Patrik Yrjas b a b
Valmet Technologies, Lentokentänkatu 11, PO Box 109, FI-33101 Tampere, Finland Johan Gadolin Process Chemistry Centre, c/o Laboratory of Inorganic Chemistry, Åbo Akademi University, Piispankatu 8, 20500 Turku, Finland
a r t i c l e
i n f o
Article history: Received 30 May 2017 Received in revised form 12 July 2017 Accepted 12 July 2017 Available online xxxx Keywords: High temperature corrosion Lead potassium chloride Waste wood combustion Superheater Furnace wall
a b s t r a c t One of the biggest operational concerns in recycled wood combustion, is the risk for formation of low melting, corrosive deposits. The deposits present on low-temperature heat transfer surfaces (material temperature b 400 °C) are composed of alkali metals, chlorine, sulphur, heavy metals or, as is often the case, a mixture of these. K2SO4 is commonly regarded as non-corrosive, but there have been indications that K2SO4 may worsen the PbCl2 induced corrosion. Consequently, a more detailed study with these compounds was of very high interest. This paper reports the results obtained from 24-hour isothermal laboratory corrosion tests with PbCl2 mixed with either K2SO4 or SiO2. The tests were carried out at 350 °C using low alloy steel (16Mo3). The interaction between PbCl2 and K2SO4 was investigated in a furnace with a temperature gradient. As a result, a mixture of PbCl2 and K2SO4 is more corrosive than PbCl2 mixed with SiO2. Corrosion was noticed below the deposit's first melting temperature. However, for a mixture of FeCl2, KCl and PbCl2, the first melting temperature is below 350 °C which could explain the high oxidation rate observed below the first melting temperature of the deposit. A solid phase or a mixture of phases with the composition of K3Pb2(SO4)3Cl was observed in the tests with SEM/EDX for the first time. © 2017 Elsevier B.V. All rights reserved.
1. Introduction Corrosion problems in heat transfer surfaces are limiting the rise of the steam temperatures in power boilers. Alkali chlorides are usually the main reason for corrosion of the hottest superheaters and they have been shown to be corrosive at material temperatures above 450 °C [1,2]. Alkali chloride induced corrosion has been a widely-investigated topic over the years and different corrosion mechanisms, as well as remedies, have been suggested [1–9]. One of the suggested corrosion mechanisms is so-called active oxidation, where gaseous chlorine diffuses through the oxide layer and reacts with the steel, forming metal chlorides. The formed volatile metal chlorides diffuse outwards through the oxide layer; when the O2 partial pressure is high enough, the metal chlorides are oxidised releasing chlorine gas, which in turn partly diffuses back to the steel-oxide interface [3,4]. Another suggested mechanism is that chloride salts form eutectic melts which dissolve iron from the steel [2,5]. The third theory proposes that hydrogen chloride and oxygen are absorbed to the steel surface followed by further dissociation into H+, Cl− and O2– ions. In this process water vapour evaporates ⁎ Corresponding author at: Valmet Technologies, Lentokentänkatu 11, PO Box 109, FI33101 Tampere, Finland. E-mail address: [email protected] (H. Kinnunen).
http://dx.doi.org/10.1016/j.fuproc.2017.07.017 0378-3820/© 2017 Elsevier B.V. All rights reserved.
and at the same time Cl− ions migrate to the oxide grain boundaries forming metal chlorides [6]. The increased use of recycled wood containing fuels has resulted in corrosion related challenges on low-temperature heat transfer surfaces, such as furnace walls and primary superheaters. Corrosion on these cooler surfaces is caused by heavy metal chlorides or by a combination of heavy metal and alkali chlorides [5,9–12]. Typical deposits on heat transfer surfaces with a material temperature below 400 °C are composed of a mixture of alkali metals (sodium and potassium), chlorine, sulphur and heavy metals. The first melting temperature of these types of deposits can drop below 350 °C. According to laboratory measurements, lead chloride (PbCl2) has been found to be corrosive as a pure PbCl2 salt and when mixed with potassium sulphate (K2SO4) or potassium chloride (KCl) [13,14]. A couple of possible corrosion mechanisms of such deposits are presented in the literature [5,10–12,15]. Laboratory investigations with synthetic salts on a low alloy steel have shown that corrosion caused by heavy metal chlorides initiated formation of metal chlorides and that the corrosion could be enhanced by the presence of a melt [5,10–12,15]. In a full-scale boiler, a combination of potassium and lead was shown to be corrosive at water wall temperatures (b400 °C) when using an Inconel 625 type alloy. In the same study, a low alloy steel was suggested to have been attacked by hydrogen chloride [11]. Talus et al. [16] proposed that
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lead and lead oxide (PbO) react with iron chloride (FeCl2), forming PbCl2, which was then combined with alkali chlorides. It has also been shown that PbCl2 causes accelerated corrosion by forming chromates with stainless steel [17]. The role of zinc in the corrosion process is still unclear. Zinc is often present in waste based fuels [18] and zinc chloride (ZnCl2) has been shown to be corrosive and also to lower the first melting temperature of a deposit [14,19,20]. However, in the bottom part of the furnace where the gas composition is more reducing than oxidising, zinc is most likely to condense as metallic zinc. When moving upwards in the furnace, to the secondary and tertiary air levels, the atmosphere becomes more oxidising. In an oxidising atmosphere, zinc is oxidised to zinc oxide (ZnO) at temperatures above 300 °C [10]. ZnO has been found to be significantly less corrosive than ZnCl2 [14]. Thus, this study focuses on PbCl2. Sulphur is a well-known remedy against alkali chloride induced high-temperature corrosion. It has been shown that co-combustion with a sulphur-rich fuel or using sulphur containing additives, will convert harmful alkali chlorides to less harmful alkali sulphates [1,3,7–9]. Pure alkali sulphates do not cause corrosion below 500 °C [21]. The influence of sulphur on heavy metal chloride induced corrosion is, however, not yet fully understood. Combustion studies of sewage sludge together with waste wood suggested that sewage sludge reduces the amount of chlorides in the superheater deposits, and also observed a reduction of corrosion in furnace walls [22]. The purpose of this study was to identify the corrosivity of PbCl2 mixed with K2SO4 in greater detail. PbCl2 was also mixed with silicon dioxide (SiO2) in order to pinpoint the effect of K2SO4.
2. Experimental 2.1. Corrosion tests Isothermal corrosion furnace tests were carried out to study and compare the different corrosivities between two PbCl2 containing synthetic salt mixtures. The experiments were performed using a commercial low alloy ferritic steel: EN10216-2 16Mo3. The selected steel is widely used as wall panel and low-temperature superheater material in industrial boilers. Table 1 presents the standard elemental composition of the steel in weight-%. The test specimens were approximately 20 × 20 mm coupons with a thickness of 5 mm. The steel samples were ground with ethanol using SiC paper with a final grit of 1000 and then cleaned in an ultrasonic bath. Before the tests, the coupons were pre-oxidised in a furnace for 24 h at 200 °C. Afterwards, each coupon was covered with a salt mixture (0.25 g/specimen) consisting of either pure K2SO4 or 50 wt-% PbCl2 mixed with either 50 wt-% K2SO4 or 50 wt-% SiO2. SiO2 was selected as the other salt component due to its low reactivity and to have equal amounts of PbCl2 as weight-% in both salts. The chemical compositions of the salt mixtures are presented in Table 2. The PbCl2-K2SO4 salt mixture was pre-melted at 500 °C for 30 min and ground afterwards to ensure adequate mixing of the salts. The samples were exposed in a horizontal tube furnace for 24 h at 350 °C in ambient air. After the corrosion tests, the samples were placed in a mould, cast in epoxy and cut through the middle to reveal the crosssection. The cross-sectional surfaces were polished in kerosene using SiC paper with a final grit of 2500 and were then cleaned in petroleum
Table 1 The composition of the test steel in weight-%. The composition is informed according to the EN 10216-2 standard.
16Mo3
C [wt-%]
Si
Mn
Cr
Mo
Ni
Others
0.12–0.20
≤0.35
0.40–0.90
≤0.30
0.25–0.35
≤0.30
Al, P, S
307
Table 2 The chemical compositions of the salt mixtures in weight- and mol-%. Salt mixtures [weight-%]
Salt 1 Salt 2 Salt 3
Salt mixtures [mol-%]
PbCl2
K2SO4
SiO2
PbCl2
K2SO4
SiO2
– 50 50
100 50 –
– – 50
– 38.5 17.8
100 61.5 82.2
–
ether and an ultrasonic bath. The prepared samples were analysed with Scanning Electron Microscope/Energy Dispersive X-ray (SEM/ EDX) to measure the thickness of the oxide layer and to identify various chemical elements in the layer. Corrosion products were identified using X-ray images, and the oxide layer thickness was determined using scanning electron microscope (SEM) backscatter images. Several SEM images were combined to form a panoramic picture of the whole cross-section. The resulting images were digitally enhanced based on differences in the contrast. An example of the treatment stages of a typical SEM panoramic picture is presented in Fig. 1. After the panoramic images were coloured, the thickness of the oxide layer was determined for each vertical line of pixels and was recalculated to μm. The method has also been described by WesténKarlsson [23]. The corrosion layer was determined by the thickness of the oxide layer for each line, and the corrosion attack is expressed as the mean thickness of the oxide layer. A gradient furnace test was carried out to study the interaction between PbCl2 and K2SO4 particles. A ring-shaped carbon steel sample was covered with a salt mixture containing 10 wt-% (6.5 mol-%) of PbCl2 and 90 wt-% (93.5 mol-%) of K2SO4. No pre-melting of the salt components was performed. The particle diameter was 100–500 μm for K2SO4 and 10–30 μm for PbCl2. The salts were exposed in the gradient furnace for 24 h using a steel temperature of 310 °C and a gas temperature of approximately 800 °C. The method has been described by Lindberg et al. [24]. The main equipment used in the temperature gradient corrosion experiments was a corrosion probe placed into a tube furnace. The corrosion probe consists of a probe and a protective tube. The protective tube surrounds the probe except for a window exposing part of the steel sample rings to the furnace environment. The outer protective tube was mounted on the air-cooled probe in order to reduce the cooling effect from the probe, and thereby to decrease the need for heating the tube furnace, resulting in more stable temperatures. The inner probe has two removable sample rings, which are equipped with thermocouples. The temperature of one of the test rings on the probe is regulated with a proportional-integral-derivative controller (PID controller) adjusting the flow of the cooling air. The temperature of the other test ring is monitored and logged during the test run. Typically, a difference of 5–10 °C is formed between the rings. The typical furnace set temperature is 980 °C, which gives a gas temperature of around 800 °C about 1 cm above the deposit, when the probe temperature is around 300–500 °C. Typical temperature gradients across the salt deposits are around 50–100 °C/mm in the radial direction. Additional thermocouples can be installed on the outside of the protective tube to measure the temperature between the sample probe and the alumina tube in the furnace. The inner probe is the same as used by Bankiewicz et al. [25]. Roughly 0.5 g of the salt mixture is applied on top of each ring on an area of about 10 mm × 20 mm. It corresponds to a thickness of about 5 mm of salt prior to the experiment. The edges of the application area on the rings are surrounded by a protective paste to hold the salt in place in case of melting of the salts. For the experiments, the probe is inserted into a cold tube furnace and both the probe and the furnace are heated to their set temperatures. As stable temperatures were reached, the experiment was run for 24 h, and
308
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Fig. 1. The colouring stages of an SEM image. The stages are used for determination of the oxide layer thickness.
subsequently cooled to room temperature. After cooling, the salt deposits on the rings were cast in epoxy, and subsequently removed from the probe. The rings were cut and polished to get a cross section of the ring and the salt deposit for SEM/EDX analysis, similar to the process for the samples from the isothermal tests. More detailed images and schematic drawings of the experimental setup are reported by Lindberg et al. [24]. 2.2. DSC/TGA-experiments Differential scanning calorimetry/thermogravimetric analyses (DSC/ TGA) was used to study the melting temperatures of binary or ternary mixtures of KCl, PbCl2, and FeCl2. A TA Instruments SDT Q600 simultaneous DSC/TGA apparatus was used in the experiments. The reagents for the mixtures were anhydrous pro analysis KCl, PbCl2, and/or FeCl2; about 15 mg of the mixtures were added to an alumina cup for the experiments. The experiments were done in nitrogen with a flow rate of 100 ml/min; the heating/cooling rate was 10 °C/min. After the initial heating to the maximum temperature, two cooling/heating cycles were made. The maximum temperature was 400 °C for the experiments with KCl, and 500 °C for the PbCl2-FeCl2 mixture. The maximum temperature was kept low to ensure minimal vaporization of the salts but high enough to achieve a completely molten mixture based on pre-calculations. The results from the first heating ramp were not considered, due to the heterogeneous nature of the unmelted samples. In the subsequent cooling/heating cycles, the sample mixture can be assumed to be homogenised and close to the expected phase equilibria. For simple DSC peaks, the onset temperature was considered to be the relevant transition temperature. For complex peaks, the peak or inflection temperatures were also considered. These temperatures typically coincide with the liquidus of the system. 3. Results In this chapter, the corrosion results from the isothermal tests will be presented, as well as the interactions of the salt components in the temperature gradient test. The results will be further discussed in the next chapter (Chapter 4). In the first isothermal test, which was done with
pure K2SO4, no corrosion was observed at 350 °C and the oxide layer was below 1 μm. This was expected as no increased oxidation was found at a higher temperature either [21]. 3.1. Isothermal corrosion test with PbCl2 mixed with SiO2 Isothermal corrosion test with 50 wt-% of PbCl2 mixed with 50 wt-% of SiO2 showed only an occasional and very thin (b 10 μm) iron oxide layer. Both, SiO2 and PbCl2 particles, are clearly seen as separate particles in the cross-section (Fig. 2). The grey particles in the figure are SiO2 particles and the white particles are PbCl2 particles. The compositions of the different particles were verified with point analyses as presented in Fig. 3. 3.2. Isothermal corrosion test with PbCl2 mixed with K2SO4 A continuous oxide layer was detected in the test with PbCl2 mixed with K2SO4. The average oxide layer thickness was about 20 μm clearly showing increased oxidation (Fig. 4). More detailed point analyses are presented in Fig. 5. An iron oxide layer covered the steel surface (point number 1) as presented in Fig. 5. Above this, another, thinner, iron oxide layer with residues of chlorine was detected (point 2). The white areas (points 3 and 4) were composed of lead, chlorine, potassium, oxygen, and also molybdenum in point 4. These layers are mixed phases and it is difficult to conclude which compounds are present based on the SEM/EDX analyses. Several greyish areas were noticed within the deposit particles. It seems that several different phases were formed due to the interactions between PbCl2 and K2SO4 since the compositions of the greyish areas varied significantly. One area consisted mainly of KCl with residues of PbCl2 (point 5) while another area had a composition corresponding to a K3Pb2(SO4)3Cl compound (point 6). A similar compound, Na3Pb2(SO4)3Cl, was identified and reported by Enestam et al. [26] in ash deposits from recovered waste wood boilers, and is also naturally occurring as the mineral caracolite. Point 7 had an elemental composition corresponding to a K2PbCl4 compound, while the dark grey areas were concluded to be K2SO4 (point 8).
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309
Fig. 2. An SEM/EDX image of the cross-section of the 50 wt-% PbCl2 + 50 wt-% SiO2 exposed sample. Only an occasional and thin oxide layer is seen on the steel surface. The grey deposit particles are SiO2 particles and the white particles are PbCl2.
3.3. Interaction of the salt components The reaction between K2SO4 and PbCl2 may result in the formation of a molten phase consisting of the following components: K2SO4, lead sulphate (PbSO4), KCl and PbCl2. The reported minimum eutectic temperature of the K2SO4-PbSO4-KCl-PbCl2 system is 403 °C according to Dombrovskaya [27]. The reported stable solid phases are the pure K2SO4, PbSO4, KCl and PbCl2, and the double salts KPb2Cl5, K2PbCl4, K2Pb(SO4)2 and K2Pb2(SO4)3. In addition, the high temperature modification of K2SO4 can dissolve up to 25 mol-% PbSO4. The compound K3Pb2(SO4)3Cl was not observed by Dombrovskaya. The point analyses 3–7 in Fig. 5 clearly show that the salt particles consisting of PbCl2 and K2SO4 have reacted with each other, since both lead and potassium were found at the same location. However, this could have already occurred during the pre-melting stage (performed at 500 °C for 30 min). To confirm the interaction between PbCl2 and K2SO4 particles without pre-melting, a temperature gradient furnace test was carried out using the same two salts. The material temperature was 310 °C and the gas temperature was approximately 750–800 °C in the test. Fig. 6 presents the cross-section of the salt mixture after the exposure. The uppermost layer of the deposit consists of pure, partly sintered K2SO4 with some of the particles containing rims consisting of the K3Pb2(SO4)3Cl phase. The particles at the hottest temperatures do not have any Pb-containing rims, suggesting that the PbCl2 that has not
reacted with the K2SO4 in the deposit, has vaporized and been transported into the furnace. A white layer enriched in lead occurs at around 1.6 mm from the ring surface. SEM/EDX analysis shows that the white layer consists of mainly K2PbCl4, but also some sulphatephases, specifically the proposed K3Pb2(SO4)3Cl phase. Based on the phase diagram of Dombrovskaya [27], the lowest-melting eutectic point of the K2SO4-PbSO4-KCl-PbCl2 system (403 °C) and a eutectic or peritectic point at 426 °C are close to or on a potential tie-line between K2PbCl4 and K3Pb2(SO4)3Cl. The composition of the invariant points suggests that K2PbCl4 would be the main phase that crystallizes from the eutectic or peritectic melts. It is therefore plausible that the dense Pbrich layer at around 1.6 mm from the ring surface corresponds to solidified eutectic melt at around 400–430 °C. Typical temperature gradients in the gradient tests, the measured gas temperature above the deposit, and the distance of the layer from the ring surface support this explanation. Small crystals consisting only of lead, chlorine and oxygen also exist in the molten layer. These may be various lead-oxychlorides [28]. The close-up of this area in Fig. 6 shows that rims surround the outer layer of K2SO4 particles. The outer layer of rims is mainly K2PbCl4, whereas the inner layer consists of K-Pb-sulphates or the K3Pb2(SO4)3Cl phase. The rim shape suggests that the gas phase has been involved in some of the reactions. KCl and PbCl2 have been identified in the deposit close to the steel surface. The above-mentioned phases have not been identified with X-ray diffraction. However, the SEM/EDX analyses of the various phases are
Fig. 3. Point analyses of the salt mixture 50 wt-% PbCl2 + 50 wt-% SiO2 after the exposure. The elements with concentrations below 1 atomic-% are marked as residue, “res.”, in the table.
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Fig. 4. An SEM/EDX image of the cross-section of the sample exposed to 50 wt-% PbCl2 + 50 wt-% K2SO4. The oxide layer can be seen on the steel surface as a grey, continuous line. The average oxide layer thickness was 20 μm.
shown in Fig. 7 and compared to the theoretical composition based on the K-Pb-S-Cl atomic distribution. The EDX-analyses do not differentiate between solid crystalline phases and amorphous phases or mixtures with very fine-grained microstructure. The identified phases correspond the phases observed by Dombrovskaya, except for KPb2Cl5, which was not observed in the present study and K3Pb2(SO4)3Cl, which was not identified by Dombrovskaya. It is possible that K3Pb2(SO4)3Cl does not correspond to a single phase but instead to a very fine-grained mixture corresponding to a multicomponent eutectic composition. However, the composition does not correspond to any of the eutectic points or cotectic lines in the liquidus projection published by Dombrovskaya. It is clear from the analyses that the interaction between K2SO4 and PbCl2 can lead to the formation of several complex phases, either through melting reactions or by interactions among gaseous components. Solidsolid interactions are also possible, but are typically rather slow.
4. Discussion The isothermal laboratory tests conducted at 350 °C, showed that the corrosivity of PbCl2 increases when mixed with K2SO4 whereas no increased oxidation was detected when PbCl2 was mixed with SiO2. A negligible oxide layer growth of low-alloy steel was also reported by Bankiewicz et al. with pure PbCl2 at 350 °C [14]. The most probable reason for the increased oxidation when both PbCl2 and K2SO4 are present, is the formation of a mixture consisting of lead, potassium and chlorine. The temperature gradient furnace test clearly verified that reactions take place between the PbCl2 and K2SO4. Based on the literature, the melting point of PbCl2 is 501 °C and the first melting temperature of a 50 wt-% PbCl2 and 50 wt-% K2SO4 mixture is 426 °C [27]. The reactions between K2SO4 and PbCl2 can produce mixtures that have a first melting temperature at 403 °C, which is still ~50 °C above the test temperature and does not entirely explain the increased oxidation at 350 °C.
Fig. 5. Point analyses of the salt mixture 50 wt-% PbCl2 + 50 wt-% K2SO4 after the exposure. Concentrations below 1 at-% are marked as residue, “res.”.
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Fig. 6. A cross-section of the salt mixture exposed in the gradient furnace. More detailed distribution of the elements is presented in the magnified images.
FeCl2 has been suggested to be involved in the corrosion reaction and to form low melting mixtures with chlorides in the deposit, thus causing increased oxidation [5,29–31]. FeCl2 is known to be a reaction product of chlorine induced corrosion with iron oxide forming steels. The minimum melting temperature for a KCl-FeCl2 mixture is 355 °C [32]. Since no similar data have been found for the FeCl2-KCl-PbCl2 system, a solidus projection was calculated with FactSage and the results are presented in Fig. 8. The calculation was made using FactSage version 7.1 [33] and using the FTsalt database for molten salt systems. The ternary FeCl2-KCl-PbCl2 system has not been evaluated and compared with experimental data. The binary KCl-FeCl2 system has been evaluated by Robelin et al. [34], and the binary KCl-PbCl2 has been optimised but not published, based on the documentation of FactSage. However, the thermodynamic properties are similar to the experimental study
and thermodynamic evaluation of Gabriel & Pelton [35]. The binary PbCl2-FeCl2 system has not been evaluated and the liquid is assumed to be ideal in the thermodynamic database. Ferrari measured the phase equilibrium of the PbCl2-FeCl2 phase diagram [36]. The calculated PbCl2-FeCl2 phase diagram predicts a solidus temperature of 452 °C, whereas the experimental determination of Ferrari and Colla gives solidus temperatures around 415 °C, but varying between 400 and 422 °C. Therefore, some uncertainties in the predicted solidus temperature of the FeCl2-KCl-PbCl2 system are to be expected as the ternary data are extrapolated from binary systems, and the PbCl2-FeCl2 system shows some variations between the calculated and the previously measured data. In order to get additional verification of the melting temperature of the FeCl2-KCl-PbCl2 system, a synthetic mixture was made and analysed with DSC/TGA. The mixture composition was chosen to coincide with
Fig. 7. Point analyses of phases in the temperature gradient sample of K2SO4 and PbCl2 recalculated as the atomic ratios of potassium, lead, sulphur and chlorine. Theoretical compositions of the various phases are given as the last analysis of the grouping. Mixture compositions are not shown in the analyses.
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Fig. 8. Solidus projection for FeCl2 – KCl – PbCl2 system. Temperatures shown as °C. Black circles indicate experimental mixture compositions.
the region with the lowest predicted solidus temperature of the FeCl2KCl-PbCl2 system. In addition, to verify the accuracy of the phase equilibrium measurements, mixtures of KCl and FeCl2, as well as PbCl2 and FeCl2, with compositions close to the eutectic point of the respective systems were analysed and compared with calculated and experimental data. The measurement points are marked with black circles in Fig. 8. Table 3 gives the compositions of the mixtures in weight-% and mol-% and the measured solidus, liquidus, and solid phase transition temperatures. The measured and the calculated solidus temperatures for the KClFeCl2 and FeCl2-KCl-PbCl2 mixtures coincide very well. The experimental solidus and liquidus temperatures for the PbCl2-FeCl2 mixture are considerably lower than the predicted results, assuming ideal mixing in the liquid phase. The measured solidus and liquidus temperatures, however correspond very well with the results of Ferrari & Colla. The measured solidus temperature of the KCl-FeCl2 mixture is predicted accurately by the calculations, whereas the liquidus temperature is slightly higher in the experiment. However, the measured liquidus at 372 °C could also be an overlapping signal from a predicted peritectic temperature of 374 °C in the KCl-rich region. The measured thermal events at 273 and 323 °C in the KCl mixtures are most likely attributed to solid phase transitions of K2FeCl4 and KFeCl3, based on the thermodynamic evaluation of Robelin et al. The measured solidus temperature of the FeCl2-KCl-PbCl2 is 311 °C and is predicted to be 312 °C. However, due to the discrepancy in the binary PbCl2-FeCl2 system between the calculated and measured melting temperatures, it would be recommended to perform a thermodynamic evaluation of the binary PbCl2-FeCl2
system. In addition, experimental studies and a thermodynamic evaluation for the ternary FeCl2-KCl-PbCl2 is recommended for more accurate predictions over a larger compositional range. However, this is outside the scope of the present study. As marked in Fig. 8, the melting temperatures vary between 312 and 334 °C depending on the composition. This might explain why increased oxidation was seen below the deposit's first melting temperature. The elements present closest to the iron oxide layer in the sample exposed to the 50 wt-% PbCl2 and 50 wt-% K2SO4 salt mixture included lead, iron, potassium, chloride and oxygen (Fig. 4). In the same figure, an iron oxide layer above the steel surface included small amounts of chlorine, indicating that corrosion follows the already-known chlorine induced corrosion mechanism by the formation of FeCl2. A corrosion mechanism is suggested, in which a potassium lead chlorine mixture initiates the corrosion reaction by the formation of FeCl2, which in turn further accelerates the corrosion by lowering the melting temperature via formation of a low melting mixture including iron, potassium, lead and chlorine. 5. Conclusions Laboratory corrosion tests were carried out in ambient air at 350 °C exposing low alloy steel to PbCl2 salt mixed either with 50 wt-% of SiO2 or 50 wt-% of K2SO4. The mixture with K2SO4 was more corrosive compared to the mixture with SiO2. Actually, no increased oxidation was noticed with PbCl2 mixed with SiO2. PbCl2 mixed with K2SO4 salt produced a thick and continuous oxide layer in a mere 24 h. Several different
Table 3 Compositions of the mixtures in weight-% and mol-% and the measured solidus, liquidus, and solid phase transition temperatures. KCl
FeCl2
PbCl2
KCl
27.9 38.9 32.9
72.1 – 15.5
– 48 31.2
[mol-%] Mixture 1 Mixture 2 Mixture 3
– 61.1 51.6
FeCl2
PbCl2
Tsolidus
15 52 33.8
85 – 35
[weight-%]
Tliquidus
Ttrans
Tsolidus
Experimental [°C]
Calculated [°C]
412 350 311
273, 323 273
452 351 312
420 372
Tliquidus
491 351 341
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phases with varying amounts of lead, potassium, chlorine and sulphur were detected from the cross-section of the sample. The interaction between the two salt components in the deposit was verified with a gradient furnace test. This interesting finding indicates that the usually noncorrosive K2SO4 can increase the chloride induced corrosion at relatively low material temperatures (~350 °C), if it comes in contact with PbCl2. A solid phase or a mixture of phases with the composition K3Pb2(SO4)3Cl was observed with SEM/EDX in the isothermal and temperature gradient tests. The phase is similar to the mineral caracolite (Na3Pb2(SO4)3Cl), which has been observed previously in boiler deposits. However, additional characterisation of the new phase is needed in future work. The formation of a low melting eutectic mixture including FeCl2-KClPbCl2 has melting temperatures below 350 °C and could possibly accelerate the corrosion once started. The melting behaviour of the ternary mixture was measured with DSC/TGA and confirmed with thermodynamic predictions for the first time. Nevertheless, it is concluded that the initiation of the corrosion takes place due to the K2SO4 mixed with PbCl2. The proposed low melting mixture formation could explain the increased oxidation below the deposit's first melting temperature. Acknowledgements Linus Silvander, Jaana Paananen and Peter Backman from Åbo Akademi are gratefully acknowledged for carrying out the laboratory and analysis work. Additional funding from the Academy of Finland (Decision no. 266384 and no. 269277) for one of the co-authors (D. Lindberg) is greatly appreciated. References [1] K. Salmenoja, Field and Laboratory Studies in Chlorine-induced Superheater Corrosion in Boilers Fired with Biofuels, Åbo Akademi, Turku, Finland, 2000. [2] H.P. Nielsen, F.J. Frandsen, K. Dam-Johansen, L.L. Baxter, The implications of chlorineassociated corrosion on the operation of biomass-fired boilers, Prog. Energy Combust. Sci. 26 (2000) 283–298, http://dx.doi.org/10.1016/S0360-1285(00)00003-4. [3] H.J. Grabke, E. Reese, M. Spiegel, The effects of chlorides, hydrogen chloride, and sulphur dioxide in the oxidation of steels below deposits, Corros. Sci. 37 (1995) 1023–1043, http://dx.doi.org/10.1016/0010-938X(95)00011-8. [4] A. Zahs, M. Spiegel, H.J. Grabke, Chloridation and oxidation of iron, chromium, nickel and their alloys in chloridizing and oxidizing atmospheres at 400–700 °C, Corros. Sci. 42 (2000) 1093–1122, http://dx.doi.org/10.1016/S0010-938X(99)00142-0. [5] M. Spiegel, Corrosion in molten salts, Mater. Sci. Mater. Eng. 1 (2010) 316–330, http://dx.doi.org/10.1016/B978-044452787-5.00019-6. [6] N. Folkeson, J. Pettersson, C. Pettersson, L.-G. Johansson, E. Skog, B.-A. Andersson, S. Enestam, J. Tuiremo, A. Jonasson, B. Heikne, J.-E. Svensson, Fireside corrosion of stainless and low alloyed steels in a waste-fired CFB boiler; The effect of adding sulphur to the fuel, Mater. Sci. Forum 595–598 (2008) 289–297 (http://dx.doi.org/ 192.89.97.1-05/00/00,00:24:35). [7] J. Pettersson, C. Pettersson, N. Folkeson, L.-G. Johansson, E. Skog, J.-E. Svensson, The influence of sulphur additions on the corrosive environment in a waste-fired CFB boiler, Mater. Sci. Forum 522–523 (2006) 563–570, http://dx.doi.org/10.4028/ www.scientific.net/MSF.522-523.563. [8] M. Aho, P. Yrjas, R. Taipale, M. Hupa, J. Silvennoinen, Reduction of superheater corrosion by co-firing risky biomass with sewage sludge, Fuel 89 (2010) 2376–2386, http://dx.doi.org/10.1016/j.fuel.2010.01.023. [9] R. Backman, R. Khalil, D. Todorovic, Ø. Skreiberg, M. Becidan, F. Goile, A. Skreiberg, L. SØrum, The effect of peat ash addition to demolition wood on the formation of alkali, lead and zinc compounds at staged combustion conditions, Fuel Process. Technol. 105 (2013) 20–27, http://dx.doi.org/10.1016/j.fuproc.2011.04.035. [10] S. Enestam, Corrosivity of hot Flue Gases in the Fluidized bed Combustion of Recovered Waste Wood, in Department of Chemical Engineering, Åbo Akademi, Turku, Finland, 2011. [11] Y. Alipour, P. Henderson, P. Szakálos, The effect of a nickel alloy coating on the corrosion of furnace wall tubes in a waste wood fired power plant, Mater. Corros. 65 (2014) 217–225, http://dx.doi.org/10.1002/maco.201307118. [12] G. Sorell, The role of chlorine in high temperature corrosion in waste to energy plants, Mater. High Temp. 14 (1997)http://dx.doi.org/10.1080/09603409.1997. 11689546.
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