Corrosion in Recycled Wood Combustion—Reasons, Consequences

Feb 26, 2019 - Combustion of recycled wood leads to increased corrosion problems of furnace walls. One of the causes is known to be the elevated ...
0 downloads 0 Views 3MB Size
Download PDF
Subscriber access provided by UNIV OF TEXAS DALLAS

Combustion

Corrosion in recycled wood combustion - reasons, consequences and solutions Hanna Kinnunen, Merja Hedman, Daniel Lindberg, Sonja Enestam, and Patrik Yrjas Energy Fuels, Just Accepted Manuscript • Publication Date (Web): 26 Feb 2019 Downloaded from http://pubs.acs.org on February 26, 2019

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

Corrosion in recycled wood combustion – reasons, consequences and solutions Hanna Kinnunen*†, Merja Hedman†¤, Daniel Lindberg‡, Sonja Enestam†, Patrik Yrjas§ †Valmet

Technologies Oy, Lentokentänkatu 11, PO Box 109, FI-33101 Tampere, Finland Fortum Power and Heat Oy, Keilalahdentie 2-4, FI-02150 Espoo, Finland ‡Department of Chemical and Metallurgical Engineering, Aalto University, PO Box 11000, FI00076, Aalto, Espoo, Finland §Johan Gadolin Process Chemistry Centre, Laboratory of Inorganic Chemistry, Åbo Akademi University, Piispankatu 8, FI-20500 Turku, Finland, ¤Current:

ABSTRACT Combustion of recycled wood leads to increased corrosion problems of furnace walls. One of the causes is known to be the elevated concentrations of heavy metals and chlorine and rather low levels of sulphur in the fuel. Detailed understanding of the boiler environment, deposit formation and corrosion mechanisms are essential factors in finding the most cost-effective solutions for reducing corrosion problems for full-scale boilers. The presence and behaviour of lead in bubbling fluidized bed (BFB) boilers firing recycled wood were studied using a fine particle measurement technique and short-term deposit probe measurements. The main compounds present in the fine particles were determined with the use of ion chromatography (IC) and mass spectrometer (ICP-MS). Deposit samples were analysed with a scanning electron microscope coupled with energy dispersive X-ray spectroscopy (SEM/EDX) and with an X-ray diffractometer (XRD). The results were compared to the similar deposit samples taken from a circulating fluidized bed boiler (CFB). The analyses revealed that the major part of lead was combined with chlorine and in most cases also with potassium. Two compounds were identified (KPb2Cl5 and K2PbCl4) in the boiler environment as well as in laboratory-scale corrosion tests. Thermodynamic calculations showed also formation of these compounds. The corrosivity and the corrosion mechanism of potassium-lead-chlorides were further studied with the laboratoryscale measurements. The results showed that iron, potassium, lead and chlorine will form sintered particles together. This supports the theory that corrosion observed below the deposit’s first melting temperature could be driven by the melt formation between the deposit and the corrosion product. 1. INTRODUCTION Global warming and demand for CO2 neutrality have driven the power industry towards greener fuel solutions. Attractive fuel alternatives include different waste-derived fuels providing green energy benefits; they are widely available and cheap. However, waste-derived fuels are problematic from the corrosion perspective. In recycled wood (or waste / demolition wood) firing, high concentrations of heavy metals in combination with potassium and chlorine, produce highly corrosive conditions even at low temperature heat transfer surfaces, such as in furnace walls1-4. Both lead and zinc including compounds have been under investigations and many laboratory-scale corrosion tests have shown the corrosivity of lead and zinc chlorides5-9. However, according to thermodynamic considerations, the major part of the zinc in the oxidizing part of the furnace is in the form of solid ZnO10,11. Only low amounts of gaseous ZnCl2 is expected to be present in the furnace upper part and is, due to a low condensation temperature, predicted to condense at the cold end of the flue gas channel. In case of lead, which is expected to be present mainly as gaseous PbO and PbCl210,11, condensation of its quite stable chloride is

ACS Paragon Plus Environment

Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 25

possible already at somewhat higher temperatures in the upper part of the boiler. Alipour et al.12 suggested that furnace wall corrosion was due to a compound including Pb, K and Cl, while zinc, if present, has been found as sulphates13, which are not expected to be corrosive at these conditions. According to these findings, the research in this paper is focused on lead. Understanding the corrosion mechanism and its causes and consequences are important in finding remedies for corrosion. The most optimal solutions for recycled wood fired boilers can be found by understanding the whole path from fuel to stack. Thus, a careful investigation has been done to understand the initial causes for the increased corrosion often seen in recycled wood fired boilers and to find the most suitable materials. This paper presents state-of-the-art knowledge of the work carried out in the field of recycled wood combustion. 2. EXPERIMENTAL 2.1. Fine particle measurements The fine particle measurements were carried out in a 63MWth bubbling fluidized bed (BFB) boiler. The fuel used was 100% recycled wood which was fed to the furnace from the front wall. Fine particle measurement locations were situated at different levels of the right/front corner of the furnace: before and after the secondary air feed and before and after the tertiary air feed, Figure 1. The secondary air is fed into the boiler from the front and rear walls and the tertiary air from the right and left walls. The estimated flue gas temperature is 950-990 °C in the first three measurement locations. After the tertiary air, the temperature drops to around 815 °C and in the superheater pass the flue gas temperature is approximately 690 °C. The fine particle measurements were carried out by VTT Technical Research Centre of Finland. The gaseous condensable compounds were forced to form aerosols on impactor plates, which were further analysed by mass and composition described by Vainikka14. Two parallel impactor samples, for water leaching and for acid leaching, were collected. K-, N-, Ca-, Zn- and Pb- concentrations were determined with an inductively coupled plasma mass spectrometer (ICP-MS) while SO42and Cl- were determined with ion chromatography (IC) from the water-leached samples. The same method was used for the acid leached samples. Fuel samples were also collected during the measurement period.

Figure 1. Cross-section of a BFB boiler with fine particle and deposit probe measurement locations. 2.2. Probe measurements Short-term (2h) deposit probe samples were collected after the secondary air feeding and from the superheater pass, Figure 1. The purpose of the short-term probe measurements was to identify the initial deposits coming in contact with the heat transfer surfaces. Samples were taken with an air-cooled probe, Figure 2. The probe ring temperature can be adjusted with cooling air ACS Paragon Plus Environment

Page 3 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

to the desired temperature and monitored with a thermocouple drilled into the material ring and connected to the regulator unit. Both an uncooled (Tfg ~950 °C) and a cooled sample (Tmat = 260 °C) were collected after the secondary air. In addition, a deposit sample with material temperature of 350 °C was taken from the superheater pass.

Figure 2. Principle of an air-cooled probe. Air is used to cool down the material temperature of the deposit sample ring. The sampling time was 2 hours. After the measurements, the deposit compositions were determined with a scanning electron microscope equipped with energy dispersive X-ray spectroscopy (SEM/EDX) and with X-ray diffractometer (XRD). The deposit probe ring was made of austenitic stainless steel (SA-210 TP310HCbN) to avoid possible corrosion reaction between the deposit and the steel. The results were compared to similar probe deposit samples collected previously from a 120MWth circulated fluidized bed (CFB) boiler firing 100% recycled wood15. In the CFB boiler, the deposits were collected from the hot flue gas zone (Tfg ~ 800 °C) after the cyclone and from the cooler flue gas zone (Tfg ~ 490 °C) after the boiler bank. The probe temperature was adjusted to 360 °C in both cases. 2.3. Laboratory-scale corrosion tests Not all combinations of materials, temperatures and atmospheres can be studied in full-scale environments and thus, laboratory-scale experiments are a useful option when specific corrosion mechanisms are to be studied. Short-term probe measurements help identify the deposits forming on tube surfaces. This knowledge can be further used in laboratory corrosion oven tests with synthetic salt mixtures. Several laboratory-scale corrosion tests have been carried out in an isothermal tube furnace using carbon steel (EN10216-2 P265GH), low alloy steels (EN10216-2 16Mo3 / 10CrMo9-10) and a nickel-based alloy (Alloy 625). The exposure times varied from shorter term testing (8h) up to 168-hour (7-day) tests. All the tests were carried out in ambient air with varying temperatures and salt mixtures, Table 1. The specimen coupons used in the horizontal tube furnace had a size of 20x20 mm with a thickness of 5 mm. They were first ground in ethanol with a 1000 grit SiC paper and then cleaned in an ultrasonic bath. Before the exposures, the coupons were pre-oxidized in a furnace for 24 h at 200 °C. Afterward, each coupon was covered with a salt mixture (0.25 g/specimen). Table 1. Summary of the laboratory-scale corrosion tests and the main alloying elements of the tested steels.

ACS Paragon Plus Environment

Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 25

After the experiment, the deposit samples were cast in epoxy resin and cut to reveal a cross-section. The cross-section was analysed and characterized using SEM/EDX. The procedure is further described by Bankiewicz et al.7. The threshold temperature for increased corrosion was defined after the tests for each material type in part 1 of the experiments, while the corrosion mechanism was further studied in part 2. 2.4. Thermodynamic considerations Thermodynamic calculations were performed to study the behaviour of gaseous lead including compounds and their condensation. Calculations were performed with FactSage 7.2. The thermodynamic databases used in the calculations were the FTsalt databases for the alkali salt phases, and the FactSage pure substance database for the gas components and other solid compounds. In addition, thermodynamic data not available in the commercial Factsage databases were used for the interaction of alkali salts with calcium and magnesium compounds in the molten salt phase. Thermodynamic data for PbCl2, ZnCl2 and FeCl2 from the FTSalt databases was also combined with the other data for molten salts. Parts of these data have been published by Lindberg and Chartrand16 and discussed in the review paper of Lindberg et al.17. Fuel data gained from the BFB full-scale measurements, Table 2, was used as input for the calculations. The calculations were carried out assuming the prevailing conditions as oxidizing, thus considering locations after the secondary air feed in the boiler. New gaseous KPbCl3 and NaPbCl3 phases were added to the database. Thermodynamic data of KPbCl3 and NaPbCl3 was fitted to reproduce experimentally measured partial pressures of KPbCl3 and NaPbCl3 in equilibrium with molten KCl-PbCl2 and NaCl-PbCl2, respectively18. The calculations are based on unpublished KPbCl3 and NaPbCl3 thermodynamic evaluations. 3. RESULTS 3.1. Origin of lead Recycled wood is composed of different construction and demolition work residues and it may also contain pieces of furniture and wood pallets. These wood residues could include different surface treatment agents such as paints, lacquers and siccatives, which are known to include harmful elements like lead, zinc and chlorine19-21. Thus, for e.g. the concentrations of lead may be high. Fuel samples were collected and analysed during the probe measurements from both the BFB and the CFB boilers presented in Chapter 2.2. Despite of the fact, that elemental variations at times might be considerable, the fuel compositions were very similar to one another, Table 2. Table 2. Analyses of the recycled wood fired in the BFB and CFB boilers.

ACS Paragon Plus Environment

Page 5 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

The chlorine content was high in both fuel mixtures, although being much higher in the BFB case. High chlorine together with elevated alkalis and low sulphur, predicts formation of alkali chlorides for both cases. A part of the sulphur will also be captured by calcium, which is clearly higher in the CFB case. The amounts of lead and zinc are high as expected but are surprisingly similar between the two analysed recycled wood fuels despite of the fact that variation may be high. 3.2. Presence and form of lead The presence of lead in the furnace was studied with fine particle measurements. The concentrations of water-soluble elements in the collected fine particle samples are presented for each of the measurement location, Figure 3. Chlorides and sulphates are known to be in watersoluble form if present in the sample22. Elements found in the fine particle fraction < 1 µm have been in the form of gaseous compounds at the sampling location23. As the gaseous compounds can condense and deposit on heat transfer surfaces, the focus was set to this fine particle fraction. The fine mode cut size of the measurement method used was 1.6 µm.

ACS Paragon Plus Environment

Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 25

Figure 3. Concentrations of water-soluble elements in fine particle samples collected from the measurement locations. The thickness of each ribbon describes the element concentration. When considering the fractions < 1.6 µm, alkali metals (Na and K), chlorine and lead are the main elements present in the lower furnace. The concentrations of sulphate and zinc are negligible until the tertiary air. The concentration of sulphate increases as chlorine concentration decreases after the tertiary air indicating that sulphation of chlorides has taken place. The concentration of lead is at its highest between the secondary and tertiary airs. As illustrated in Figure 3, which presents the watersoluble fraction of the elements, the main part of the alkalis and lead are bound with chlorine since sulphates were not present in these measurement points. Zinc was also found in the water-soluble fraction between the secondary and tertiary airs but in clearly smaller concentrations than lead. Lead in other forms than chloride can be calculated by decreasing the water-soluble fraction from the acid-soluble fraction. Based on the results, the major part of lead (over 70%) is in form of chlorides, Figure 4.

Figure 4. Form of lead at different measurement locations based on the fine particle measurements.

ACS Paragon Plus Environment

Page 7 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

3.3. Behaviour of lead in the deposits 2-hour deposit samples were collected from the 100% recycled wood firing BFB boiler. The results were compared with the samples collected earlier from the 100% recycled wood firing CFB boiler15. The wind side of the deposits were analysed with the SEM/EDX, Figure 5. Only the most interesting elements from the corrosion point of view are presented in the Figure. The rest of the elements (O, Mg, Al, Si, P, Ti, Cr, Fe, Ni and Cu) are not presented here. Besides, the XRD analysing method was used to identify the compounds present, Table 3.

Figure 5. Elements characterized with SEM/EDX from the wind side of the short-term deposit probe samples. Table 3. Chlorine containing compounds identified with XRD.

In both boilers, alkali chlorides (NaCl and KCl) were identified with XRD from all the cooled deposit samples. NaCl was also identified from the uncooled sample. The share of chlorine was remarkable in each of the analysed deposit samples. The share increased with increasing flue gas temperature and decreased with increasing material temperature. Lead was bound with potassium and chlorine, as KPb2Cl5 or (KPbCl3)3·H2O. However, higher intensity was detected for KPb2Cl5. (KPbCl3)3·H2O was not detected from the BFB sample. Instead, a small amount of PbCl2 as well as PbO2 were detected. The share of lead decreased when moving from the cooler material temperatures to the warmer surfaces and no lead was found from the uncooled sample. The share of lead decreases along with the decreasing flue gas temperature. Zinc was found together with sodium or potassium, as Na2ZnCl4·H2O from the CFB probe sample and as K2ZnCl4 in the case of BFB. The share of zinc was clearly emphasized in the samples with lower material temperatures. The share of zinc in the deposit sample with Tmat 262 °C was over 20 wt-%. This could indicate that zinc tends to condense at lower surface temperatures than lead. Zinc

ACS Paragon Plus Environment

Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 25

was also found from the uncooled sample indicating that there might be compounds that have come to the surface with some other mechanism than condensation (for e.g. molten or partially molten particles). However, the composition of those particles could not be identified with XRD. 3.4. Corrosivity of lead in the deposits Lead has been found to form a mixture with potassium and chlorine according to several publications4,5,15,24-27. The results showed that PbCl2 reacts with alkali chlorides and forms either K2PbCl4 or KPb2Cl5. PbCl2 may also reacts with alkali sulphates forming a caracolite-type K3Pb2(SO4)3Cl compound24. Thus, a threshold temperature for lead chloride induced corrosion was tested with a small amount of lead chloride (5 wt-%) and higher share of potassium sulphate (95 wt%) in a synthetic salt mixture. Tests were carried out at different temperatures with low alloy 16Mo3 and 10CrMo9-10 steels and with a higher alloy nickel-based steel (Alloy 625), Figure 6. In earlier studies, nickel-based alloys have shown to be more resistant against lead attack12.

Figure 6. Corrosion test results in search for a threshold temperature for low alloy 16Mo3 and 10CrMo9-10 steels and for nickel-based Alloy 625. Corrosion can be assumed severe when the oxide layer thickness exceeds 20 µm28. According to the test results, a threshold temperature for 16Mo3 was noticed to be 325 °C when salt mixtures includes 5 wt-% of PbCl2. This is already below the first melting temperature of the salt mixture (the lowest T0 for a PbCl2-K2SO4 system is 403 °C)29. A threshold temperature for the low alloy 10CrMo9-10 steel was a bit higher, 350 °C, probably due to the small chromium alloying. No corrosion was noticed with Alloy 625, which was tested only at the two highest temperatures, 350 and 375 °C. 16Mo3 was not tested at 375 °C. 3.5. Corrosion mechanism In many corrosion studies, increased oxidation has been observed even though the test temperatures have been below the deposit’s first melting temperature2,5,15,24,30. The corrosion mechanism seems to follow the chlorine-induced corrosion mechanism with low alloy steels in the presence of lead chlorides2,3,5,15. Gaseous chloride gas (Cl2) diffuses through the oxide layer and reacts with the steel, forming metal chlorides in the metal-oxide interface. The formed metal chlorides diffuse outwards through the oxide layer and become oxidized releasing Cl2 again. Increased oxidation, even below the deposit’s first melting temperature, could be explained by molten phase formation between the deposit and the corrosion product3,26,27. This was further studied with laboratory-scale tests. A test salt composed of equal amounts of FeCl2, PbCl2 and KCl was used. FeCl2, an intermediate corrosion product according to the chlorine-induced corrosion mechanism, was added to the salt mixture to verify possible reactions with the other salt components. The tests were carried out in two different temperatures, below (300 °C) and above (340 °C) the first melting temperature of a FeCl2-PbCl2-KCl mixture (T0=312 °C)24. The exposure time was 8 hours.

ACS Paragon Plus Environment

Page 9 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

Figure 7. A SEM/EDX image of the cross-section of the exposed steel and its corrosion layer with five point analyses (300 °C).

Figure 8. A SEM/EDX image of the cross-section of the exposed steel and its corrosion layer with five point analyses (340 °C). A narrow iron chloride ribbon was found just above the steel surface in both samples, Figures 7 and 8. Above the iron chloride was an iron oxide layer, which was much thicker at the higher test temperature (30 µm vs. 140 µm). However, part of the FeCl2 salt may have oxidized at the beginning of the tests, causing partly the increased oxide layer. Lead including compounds were noticed within and above the iron oxide layer with the lower test temperature, Figure 7. The white areas in Figure 7 seem to include all elements from the original salt mixture (Fe, K, Pb and Cl) indicating that these elements will interact and form a melt or sintered particles. At the higher test temperature, no particles including lead could be found within the iron oxide layer, Figure 8. Instead, lead including particles were located above the iron oxide layer and only small residues of iron could be detected from those particles. 3.6 Thermodynamic considerations of the form of lead Thermodynamic calculations can be used to understand the chemical reactions taking place during combustion processes. They produce information about stable phases, condensation, melting properties and deposition of potential elements and compounds. The principle of the calculations is Gibbs free energy minimization. Stable phases and condensation of zinc and lead species during recycled wood combustion have been studied by Enestam et al.10,11. At oxidizing conditions PbCl2

ACS Paragon Plus Environment

Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 25

is stable until ~700 °C and oxidizes to PbO at higher temperatures. Two phases are expected to be present on tube surfaces; K2PbCl4 and KPb2Cl5. There are couple of formation routes for K2PbCl4 and KPb2Cl5 as described by Niemi et al.25. An important conclusion was that neither of these two compounds exists in gas phase, so the initial compounds in the furnace must be something else. Formation routes for K2PbCl4 and KPb2Cl5 were either via PbCl2 (g/l/s) and KCl (g/s) or via KPbCl3 (g) and KCl (g/s). As KPbCl3 is a new phase and may change the behaviour and stability of the PbCl2 studied earlier10,11, KPbCl3 phase was added to the thermodynamic database and formation of KPbCl3 was studied, Figure 9. Similar gaseous phase exists for sodium, and NaPbCl3 was also considered in the calculations. However, in condensed fractions, it is found separately as NaCl and PbCl225.

Figure. 9. The main gaseous and condensed Pb-phases according to the thermodynamic calculations. The calculations suggest the formation of KPbCl3 between 400 and 900 °C. Also, NaPbCl3 is suggested to form approximately within the same temperature area but smaller concentration. Majority of the lead is still suggested to be in the form of PbCl2. The maximum share of PbCl2 is around 400 °C and is slightly decreased above this temperature. In contrast, formation of KPbCl3 begins at 400 °C and the amount increases until ~600 °C. The steeper drop in the concentration begins at ~750 °C when the formation of PbO if more emphasized. Small concentrations of PbCl is present above 800 °C. Lead compounds are suggested to be fully oxidized to PbO at ~1200 °C. In the case of condensed phases, lead will be present in heat transfer surfaces as KPb2Cl5, K2PbCl4, PbCl2, or as PbSO4 according to the calculations. KPb2Cl5 will be stable in the cooler surface temperatures, up to 285 °C. K2PbCl4 is suggested to form to surfaces with temperatures between 285-365°C. Above this temperature, the condensed form will be PbCl2 and the calculations indicates that the highest condensing temperature for solid PbCl2 is ~440 °C. Small concentration of liquid PbCl2 and PbSO4 is suggested to condense in the multicomponent molten salt phase between 600 and 700 °C. 4. DISCUSSIONS AND CONCLUSIONS The form and behaviour of lead-including compounds have been widely studied with field- and laboratory-scale measurements and with thermodynamic calculations. The results have shown that lead will form a compound together with chlorine and potassium, either K2PbCl4 or KPb2Cl5. These two compounds were also found with fine particle measurements from different parts of the boiler and with short-term probe measurements as well as in laboratory-scale tests. It

ACS Paragon Plus Environment

Page 11 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

seems that the K2PbCl4 and KPb2Cl5 are likely to form in the deposits in different parts of the boiler. They were found from the furnace to the boiler bank area. It seems that understanding the formation and behaviour of these potassium-lead-chloride compounds is essential when finding corrosion protection solutions for recycled wood fired boilers. K2PbCl4 and KPb2Cl5 can be formed either via PbCl2 (g/l/s) and KCl (g/s) or via KPbCl3 (g) and KCl (g/s)25. Thermodynamic calculations confirmed that at least part of the gaseous lead chloride could be in the form of KPbCl3. Calculations of condensed phases confirmed the presence of K2PbCl4 and KPb2Cl5 and their temperature dependence. In the calculated case, the solid lead chloride is able to condense up to temperatures of 440 °C. Laboratory corrosion exposures with PbCl2 and K2SO4 including salt mixture showed that corrosion started already at 325 °C with low alloy 16Mo3 steel, which is already below the first melting temperature of the salt mixture. This could be explained by the formation of a lowmelting eutectic mixture with the formed iron chloride and the deposit. The results from the laboratory experiments supported this theory and particles including Fe, Cl, K and Pb were found. Thus, using materials with lower iron content could inhibit the formation of FeCl2 and provide better corrosion resistance. This was also proved with the laboratory corrosion experiments in search for a threshold temperature. The severe corrosion of low alloy 10CrMo910 steel started at 350 °C and nickel-based alloy did not corrode at 375 °C. In addition, many publications have shown that nickel-based material is a good option against heavy metal chloride-induced corrosion and thus, this material is widely used as a furnace wall coating material in waste-derived fuel fired boilers1,3,4,12. ACKNOWLEDGEMENTS Linus Silvander and Jaana Paananen from Åbo Akademi University and Dan Boström from Umeå University are gratefully acknowledged for carrying out the laboratory and analysis work. Special thanks also to Vattenfall, Jordbro Värmeverk, for enabling the full-scale measurements. REFERENCES (1) Daniel, P.L.; Barna, J.L.; Blue, J.D. Furnace-wall corrosion in refuse-fired boilers. National Waste Processing Conference, 1986. http://www.seas.columbia.edu/earth/wtert/sofos/nawtec/1986-National-Waste-ProcessingConference/1986-National-Waste-Processing-Conference-22.pdf (2) Viklund, P. High temperature corrosion during waste incineration. Licentiate thesis, KTH Chemical Science and Engineering, Stockholm, Sweden, 2011. https://www.divaportal.org/smash/get/diva2:410556/FULLTEXT02.pdf (3) Sorell, G. The role of chlorine in high temperature corrosion in waste-to-energy plants. Material at High Temperatures, 1997, 14 (3), 137-150. http://dx.doi.org/10.1080/09603409.1997.11689546 (4) Alipour, Y.; Henderson, P. Corrosion of furnace wall materials in waste-wood fired power plant. Corrosion Engineering, Science and Technology, 2015, 50 (5), 355-363. http://dx.doi.org/10.1179/1743278214y.0000000228 (5) Talus, A.; Norling, R.; Wickström, L.; Hjörnhede, A. Effect of lead content in used wood fuel on furnace wall corrosion of 16Mo3, 304L and alloy 625. Oxidation of metals, 87, 2017, 813-824. http://dx.doi.org/10.1007/s11085-017-9727-3

ACS Paragon Plus Environment

Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(6) Bankiewicz, D.; Enestam, S.; Yrjas, P.; Hupa, M. Experimental studies of Zn and Pb induced high temperature corrosion of two commercial boiler steels. Fuel Processing Technology, 105, 2012, 89-97. http://dx.doi.org/10.1016/j.fuproc.2011.12.017 (7) Bankiewicz, D.; Yrjas, P.; Hupa, M. High temperature corrosion of superheater tube materials exposed to zinc salts. Energy & Fuels, 23, 2009, 3469-3474. http://dx.doi.org/10.1021/ef801012z (8) Bankiewicz, D.; Alonso-Herranz, E.; Yrjas, P.; Laurén, T.; Spliethoff, H.; Hupa, M. Role of ZnCl2 in high-temperature corrosion in a bench-scale fluidized bed firing simulated waste wood pellets. Energy & Fuels, 25, 2011, 3476-3483. http://dx.doi.org/10.1021/ef200674k (9) Sánchez Pastén, M.; Spiegel, M. High temperature corrosion of metallic materials in simulated waste incineration environments. Materials and Corrosion, 57, 2006, 192-195. http://dx.doi.org/10.1002/maco.200503909 (10) Enestam, S.; Mäkelä, K.; Backman, R.; Hupa, M. Evaluation of the condensation behavior of lead and zinc in BFB combustion of recovered waste wood. Fuel Processing Technology, 105, 2013, 161-169. http://dx.doi.org/10.1016/j.fuproc.2011.09.002 (11) Enestam, S.; Mäkelä, K.; Backman, R.; Hupa, M. Occurrence of zinc and lead in aerosols and deposits in the fluidized-bed combustion of recovered waste wood. Part 2: Thermodynamic considerations. Energy & Fuels, 25, 2011, 1970-1977. http://dx.doi.org/10.1021/ef101761w (12) Alipour, Y.; Henderson, P.; Szakálos, P. The effect of a nickel alloy coating on the corrosion of furnace wall tubes in a waste wood fired power plant. Materials and corrosion, 65, 2014, 217-225. http://dx.doi.org/10.1002/maco.201307118 (13) Vainikka, P.; Bankiewicz, D.; Frantsi, A.; Silvennoinen, J.; Hannula, J.; Yrjas, P.; Hupa, M. 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 (14) Vainikka P. Occurrence of bromine in fluidised bed combustion of solid recovered fuel. Academic dissertation, VTT Publications 778, Espoo, Finland, 2011. https://www.vtt.fi/inf/pdf/publications/2011/P778.pdf (15) Kinnunen, H.; Hedman, M.; Engblom, M.; Lindberg, D.; Uusitalo, M.; Enestam, E.; Yrjas, P. The influence of flue gas temperature on lead chloride induced high temperature corrosion. Fuel, 196, 2017, 1-11. http://dx.doi.org/10.1016/j.fuel.2017.01.082 (16) Lindberg, D.; Chartrand, P. Thermodynamic Evaluation and Optimization of the (Ca + C + O + S) System. The Journal of Chemical Thermodynamics, 41, 2009, 1111-1124. http://doi.org/10.1016/j.jct.2009.04.018 (17) Lindberg, D.; Backman, R.; Chartrand, P.; Hupa, M. Towards a Comprehensive Thermodynamic Database for Ash-Forming Elements in Biomass and Waste Combustion Current Situation and Future Developments. Fuel Processing Technology, 105, 2013, 129-141. http://doi.org/10.1016/j.fuproc.2011.08.008

ACS Paragon Plus Environment

Page 12 of 25

Page 13 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

(18) Karpenko, N. V.; Sysoev, S. V. Tensimetric study of the dissociation of mixed chlorides of sodium (or potassium) and lead(II). Vestnik Leningradskogo Universiteta, Seriya 4: Fizika, Khimiya, 1, 1974, 83-8. (19) Edo, M.; Björn, E.; Persson, P-E.; Jansson, S. Assessment of chemical and material contamination in waste wood fuels – A case study ranging over nine years. Waste Management, 49, 2016, 311-319. http://dx.doi.org/10.1016/j.wasman.2015.11.048 (20) Krook, J.; Mårtensson, A.; Eklund, M. Metal contamination in recovered waste wood used as energy source in Sweden. Resources, Conservation and Recycling, 41, 2004, 1-14. http://dx.doi.org/10.1016/S0921-3449(03)00100-9 (21) Strömberg, B.; Svärd, S. Bränslehandboken. Värmeforsk, Stockholm, 2012. http://www.sgc.se/ckfinder/userfiles/files/sokmotor/Rapport1234.pdf (22) Zevenhoven-Onderwater, M. Ash forming matter in biomass fuels. Academic dissertation, Åbo Akademi University, Turku, Finland, 2001. http://users.abo.fi/mzevenho/portfolj/publikationer/PhD%20MZ.pdf (23) Valmari, T. Potassium behavior during combustion of wood in circulating fluidized bed power plant. Academic dissertation, Helsinki University of Technology, Helsinki, Finland, 2000. https://www.vtt.fi/inf/pdf/publications/2000/P414.pdf (24) Kinnunen, H.; Lindberg, D.; Laurén, T.; Uusitalo, M.; Bankiewicz, D.; Enestam, S.; Yrjas, P. High-temperature corrosion due to lead chloride mixtures simulating fireside deposits in boilers firing recycled wood. Fuel Processing Technology, 167, 2017, 306-313. http://dx.doi.org/10.1016/j.fuproc.2017.07.017 (25) Niemi, J.; Kinnunen, H.; Lindberg, D.; Enestam, S. Interactions of PbCl2 with alkali salts in ash deposits and effects on boiler corrosion. Energy and Fuels, 32 (8), 2018, 8519-8529. http://dx.doi.org/10.1021/acs.energyfuels.8b01722 (26) Viklund, P.; Hjörnhede, A.; Henderson, P.; Stålenheim, A.; Pettersson, R. Corrosion of superheater materials in a waste-to-energy plant. Fuel Processing Technology, 105, 2013, 106112. http://dx.doi.org/10.1016/j.fuproc.2011.06.017 (27) Spiegel, M. Corrosion in molten salts. Materials Science and Materials Engineering, 1, 2010, 316-330. http://dx.doi.org/10.1016/B978-044452787-5.00019-6 (28) Bankiewicz, D. Corrosion behaviour of boiler tube materials during combustion of fuels containing Zn and Pb. Academic dissertation, Åbo Akademi University, Turku, Finland, 2012. http://urn.fi/URN:ISBN:978-952-12-2747-9 (29) Dombrovskaya, N.S. Double Decomposition in the Absence of a Solvent. XXXVI. Reversible Reciprocal System of Potassium and Lead Chlorides and Sulfates, 1938. (30) Skrifvars, B.J.; Backman, R.; Hupa, M.; Salmenoja, K; Vakkilainen, E. Corrosion of superheater steel materials under alkali salt deposits, Part 1, The effect of salt deposit composition and temperature. Corrosion Science, 50, 2008, 1274-1282. https://doi.org/10.1016/j.corsci.2008.01.010

ACS Paragon Plus Environment

Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 1. Cross-section of a BFB boiler with fine particle and deposit probe measurement locations. 256x166mm (96 x 96 DPI)

ACS Paragon Plus Environment

Page 14 of 25

Page 15 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

Figure 2. Principle of an air-cooled probe. Air is used to cool down the material temperature of the deposit sample ring. The sampling time was 2 hours. 113x61mm (96 x 96 DPI)

ACS Paragon Plus Environment

Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Table 1. Summary of the laboratory-scale corrosion tests and the main alloying elements of the tested steels. 260x106mm (96 x 96 DPI)

ACS Paragon Plus Environment

Page 16 of 25

Page 17 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

Table 2. Analyses of the recycled wood fired in the BFB and CFB boilers. 215x182mm (96 x 96 DPI)

ACS Paragon Plus Environment

Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 3. Concentrations of water-soluble elements in fine particle samples collected from the measurement locations. The thickness of each ribbon describes the element concentration. 276x230mm (96 x 96 DPI)

ACS Paragon Plus Environment

Page 18 of 25

Page 19 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

Figure 4. Form of lead at different measurement locations based on the fine particle measurements. 177x117mm (96 x 96 DPI)

ACS Paragon Plus Environment

Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 5. Elements characterized with SEM/EDX from the wind side of the short-term deposit probe samples. 266x104mm (96 x 96 DPI)

ACS Paragon Plus Environment

Page 20 of 25

Page 21 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

Table 3. Chlorine containing compounds identified with XRD. 248x76mm (96 x 96 DPI)

ACS Paragon Plus Environment

Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 6. Corrosion test results in search for a threshold temperature for low alloy 16Mo3 and 10CrMo9-10 steels and for nickel-based Alloy 625. 146x106mm (96 x 96 DPI)

ACS Paragon Plus Environment

Page 22 of 25

Page 23 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

Figure 7. A SEM/EDX image of the cross-section of the exposed steel and its corrosion layer with five point analyses (300 °C). 271x91mm (96 x 96 DPI)

ACS Paragon Plus Environment

Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 8. A SEM/EDX image of the cross-section of the exposed steel and its corrosion layer with five point analyses (340 °C). 273x89mm (96 x 96 DPI)

ACS Paragon Plus Environment

Page 24 of 25

Page 25 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

Figure. 9. The main gaseous and condensed Pb-phases according to the thermodynamic calculations. 274x95mm (96 x 96 DPI)

ACS Paragon Plus Environment