High-Temperature Corrosion of Refractory Materials in Biomass and

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High-temperature corrosion of refractory materials in biomass and waste combustion- Method development and tests with alumina refractory exposed to a K2CO3-KCl mixture Na Li, Emil Vainio, Leena Hupa, Mikko Hupa, and Edgardo Coda Zabetta Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b01123 • Publication Date (Web): 05 Jul 2017 Downloaded from http://pubs.acs.org on July 6, 2017

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High-temperature corrosion of refractory materials in biomass and waste combustion- Method development and tests with alumina refractory exposed to a K 2CO3-KCl mixture

Na Li [email protected] Johan Gadolin Process Chemistry Centre, Åbo Akademi University, Åbo/Turku, Finland

Emil Vainio [email protected] Johan Gadolin Process Chemistry Centre, Åbo Akademi University, Åbo/Turku, Finland

Leena Hupa* (Corresponding author) Professor Address: Åbo Akademi University Johan Gadolin Process Chemistry Centre Biskopsgatan 8 20500 Åbo, Finland Phone: +358 2 215 4563 Fax: +358 2 215 4915 (Inorganic Laboratory) E-mail: [email protected] Mikko Hupa [email protected] Johan Gadolin Process Chemistry Centre, Åbo Akademi University, Åbo/Turku, Finland

Edgardo Coda Zabetta Amec Foster Wheeler

ACS Paragon Plus Environment

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High-temperature corrosion of refractory materials in biomass and waste combustion- Method development and tests with alumina refractory exposed to a K 2CO3-KCl mixture

Na Li, Emil Vainio, Leena Hupa*, Mikko Hupa, Edgardo Coda Zabetta Johan Gadolin Process Chemistry Centre, Åbo Akademi University, Åbo/Turku, Finland Amec Foster Wheeler

Abstract When burning biomass and waste fuels, alkalis and chlorine may form corrosive vapours and deposits, which can cause corrosion of boiler refractories. A laboratory method for studying refractory deterioration at temperatures and conditions relevant for biomass and waste fuel combustion was developed. Alumina refractory samples were exposed to a K2CO3-KCl mixture at 800°C and 1000°C for 168h. Different approaches to analyze alkali penetration into the refractory material were tested. The infiltration of potassium into the refractory was determined by horizontal and vertical line scan analyses, and area analyses of the sample cross-section using SEM-EDXA. A descriptive analysis of alkali infiltration mechanisms was obtained by applying Xphase spectral imaging software. Additionally, XRD analyses of the cross-section were made before and after exposure to detect new phases. The method gives detailed information of the alkali penetration and the mechanism behind it, and can be used to estimate and compare the performance of refractories in various biomass and waste combustion conditions.


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Keyword：biomass/waste combustion; alkaline deposits; refractory corrosion

1. Introduction The utilization of renewable energy sources, such as biomass, recycled wood, and municipal waste-derived fuels, in large-scale boilers has become increasingly popular, especially in Europe. Combustion or co-combustion of biomass reduces the use of fossil fuels and may be an alternative to increase self-sufficiency of power generation. The EU has set as a target to get at least 27% of the total energy from renewables by 2030.1 Biomass is one important energy source to meet this target. The properties and chemical composition of biomasses differ a lot from fossil fuels, and some of the minor constituents or impurities may cause challenges in their use in power production. Especially, alkalis present in biomass may contribute to many problems, such as, slagging, fouling, and corrosion of boiler materials.2-4 These problems will reduce the boiler efficiency and may result in unscheduled shut-downs and costly repairs.5-9 Corrosion problems are often related to the release of inorganic elements, such as Na, K, S, and Cl, to the gas phase.10,11 The vaporized compounds will undergo chemical reactions and may form aggressive ash deposits in the boiler.10 The presence of KCl in deposits lowers the first melting temperature of the ash and has the potential to form a sticky ash, which leads to increased deposit formation. The sticky ash deposits enhance the corrosion and fouling of the boiler materials.12 These problems may be solved by using fuels with high sulphur content, by sulphating the potassium chloride to potassium sulphate (Reaction 1). This reaction is desired since K2SO4 has a lower corrosion potential than KCl. Several studies dealing with alkali chloride and alkali salt mixture induced corrosion of metal alloys used in boilers have been published during the past years.13-18 A laboratory method was developed to study the corrosion of boiler steels by deposited ashes at high temperatures.14,19 In this method, the


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oxide/corrosion layer thickness is determined based on the panorama SEM technique. Detailed analysis of the composition of the corrosion layer gives relevant information on the corrosion mechanisms of metal surfaces induced by salts deposits. 4KCl(g,s) + 2SO2(g) + O2(g) + 2H2O(g) → 2K2SO4(g,s) + 4HCl(g)

(1)

Besides different steel qualities, also refractory materials are used in combustion devices. Due to their excellent thermal and chemical properties, refractories are utilized, e.g., in the lining of the boiler wall, and in the bottom grid of biomass and waste fired fluidized bed boilers. Alkalis present in biofuels may challenge the durability of the refractory materials. The alkaline ashes may infiltrate into the porous refractory and give rise to its chemical and physical deterioration. As the infiltration depth and alkali concentration increase, cracking and spalling may result in severe degradation of the refractory. Corrosion of refractory and ceramic components in combustion, chemical, metallurgical and related process industries has been reported previously.20-22 In general, the corrosion involves different phenomena such as element penetration, homogeneous and heterogeneous reactions. Oxidation-reduction reactions may also be present. Other parameters, like refractory texture, porosity, composition of corrosive slag or salts, the atmosphere and temperature have to be taken into account when describing the mechanisms of the corrosion. Earlier studies on the corrosion of refractories have mainly focused on the interactions between the refractory material and coal-slag or metallurgical slag. Typically, the interactions were characterized in laboratory scale using slag or alkaline salts in crucible or cup tests in static conditions,23-26 or as post mortem examination of the refractories used in full-scale boilers.27-30 Some results from these studies have also been verified with thermodynamic calculations. The temperatures used in these tests were mostly above 1000 ºC, i.e. much higher than typical material temperatures in fluidized bed biomass boilers. Typically, the corrosion behaviour has been characterized at the top surface of the refractory. In more recent studies, cross sections of samples have also been


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examined, but no detailed results of the infiltration of the slag or salts have been presented.26,31,32 In this work, the laboratory thin film test was used, in which the top-side of the refractory is coated by a salt layer and exposed at a certain temperature.33 Different approaches to estimate the penetration of the salts and the interaction of salts/refractory interface in conditions typical for biomass combustion are discussed. The goal was to develop a laboratory-scale method to examine the alkali attack on various refractories at various combustion conditions. A commercial refractory, alumina brick, was exposed for 168h at 800°C and 1000°C, i.e. temperature range relevant for the bottom of fluidized circulating bed boilers, to a synthetic ash of K2CO3 and KCl. Detailed analyses of the alkali infiltration into the refractory were conducted. As a starting point, the experiments were done utilizing our method for measuring high temperature corrosion of metals.14,19 However, the method was further developed to better reflect the conditions and characteristics of refractory surfaces, to be able to predict chemical deterioration of refractories at given conditions and thus to estimate the suitability of a particular refractory exposed to different combustion environments.

2. Experimental In this work, a laboratory testing method for obtaining detailed information of alkali infiltration into refractory materials was developed. Ideally, such method should succeed to utilize short-term experiments to compare and predict the performance of refractory materials in full-scale boilers burning biofuels and waste derived fuels. The conditions in the laboratory testing method correspond to typical temperatures in the lower furnace of a fluidized bed biomass boiler. In this testing procedure, the effect of temperature gradients along the refractory wall thickness and abrading effect by the boiler particles are not taken into account.


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The conditions in the laboratory test mimic the outer layer of a refractory wall in a boiler, and thus the temperature gradient in the refractory is not large. 2.1. Refractory material and preparation of the samples Commercial alumina refractory based on bauxite was used as the test material. The refractory material comprised of two structurally distinct phases: large aggregate grains which are bonded together by fine grained matrix. The dominant compounds in bauxite based refractory are alumina (Al2O3) and silica (SiO2) with the presence of some other oxide traces. The chemical composition and properties of the refractory are presented in Table 1. More detailed information about the minerals or compounds in the refractory are presented in the results part. Table 1. Chemical composition and physical properties of the alumina refractory Chemical composition (wt%)

Property

Al2O3

83.0

Bulk density

2800 kg/m3

SiO2

11.0

Apparent porosity

12.0 %

Fe2O3

1.5

Maximum grain size

5.0 mm

unspecified

Rest

Thermal conductivity 1.7 W/m·K

In Figure 1, the potassium distribution in the refractory before exposure is presented. The figure is based on 600 spots analyzed by SEM-EDX. This figure shows that less than 0.2 wt% of potassium was detected in 90% of the spots in the unexposed refractory. The mean potassium content in the unexposed refractory was 0.16 wt% with a standard deviation of 0.08. In the line scan determination of potassium penetration after exposure, potassium contents higher than 0.2 wt% were taken into account.


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Figure 1. The distribution of the potassium content detected in the refractory before exposure. Samples of the size 20×20×10 mm were cut from a precast refractory brick and ground/polished with 320 grid silicon carbide grinding paper to give a flat and smooth surface. After grinding/polishing, the samples were washed three times with ethanol in ultrasound bath and dried in an oven at 50 ºC. A synthetic ash was used in the experiments. This synthetic ash was prepared by mixing K2CO3 and KCl in a molar ratio of 9:1, melting the mixture for homogeneity, followed by quenching and crushing. The synthetic ash particles were sieved to give a 53-250 µm size range fraction. This ash mixture has previously been used in corrosion studies to simulate a simplified ash deposit which forms in the lower furnace in biomass combustion.14 The salt exposure was accomplished using two different procedures. First, a heat resistant paste (Bostik heat seal 1200) was applied on the edges of the top surface of the refractory sample. This fire sealant contained 10-30 wt% of sodium silicate The paste was applied to prevent the salt from flowing off the sample and to protect the refractory material beneath the paste from salt attack. This method has been used when studying the influence of ashes on high temperature corrosion of metals.14,19 The salt (0.25 g) was put on the surface surrounded


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by the protective paste. In the second procedure, refractory sample without using paste was covered with 0.25 g of salts. The salt was shaped as a tablet and was placed on the top of refractory by pressing it with a glass rod to obtain better contact adherence. The tests were run in ambient air at 800 ºC and 1000 ºC for 168h, equivalent to one week, in an electrically heated laboratory furnace. 168h exposure has proven to be sufficient for metal corrosion studies.14,19 The temperatures were chosen to give typical temperature ranges the refractory may experience in biomass boilers. After the exposure, the samples were cooled to room temperature. Loose salt residue at the sample surface was removed by pressurized air. The samples were cut vertically through the middle to obtain the cross sectional surface. The remnants of the cutting oil used as a lubricant in the sawing were cleaned thoroughly by alternating between an ultrasound bath in petroleum ether and a vacuum until all the oil was dissolved or drained out of the pores in the refractory. A carbon evaporator (BalTec CED 030) was used to create vacuum. The cross-sections of the samples were polished with 320 and 600 SiC papers to obtain smooth surfaces for the analyses.

2.2 Analysis methods Phase compositions in the refractory samples before and after exposure to the alkali salt were identified by X-Ray Diffraction Analysis (X’pert by Philips, Cu Kα radiation). Scanning Electron Microscope with an Electron Dispersive X-ray Analyser (FEG-SEM, LEO 1530 by Zeiss/ EDXA, Vantage by Thermo Electron Corporation) was used to study the microstructure of the cross-sections. The infiltration depth of potassium into the refractory was determined by EDX analysis of the cross-section in three different ways as shown in Figure 2: i) area analysis, ii) vertical line analysis, and iii) horizontal line analysis. In the area analysis, eight areas (200 x 100 µm) at each selected depth from the surface (0 mm, 0.5 mm, 1 mm… 6 mm) were studied. In the line scan analyses the potassium content in hundreds of


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spots were measured along the lines. In the vertical line analysis, the composition was measured along six vertical lines to a depth of 6 mm from the surface as shown in Figure 2. Each line was composed of 200 spots. One line analysis (line 3) was performed in the middle of the refractory and four parallel lines at the distances of 2 mm (lines 2 and 4) and 4 mm (lines 1 and 5) were selected on both sides of the middle line. In addition, one line analysis (line 6) was carried out at the edge of the cross-section underneath the protective paste surrounding the salt. In the horizontal line analysis, the composition of the refractory at several depths was determined in the middle part of the cross section, underneath the salt. Each line had a length of 2.5 mm and was composed of around 500 spots.

Figure 2. Three different analysis methods used to study the infiltration of potassium into the refractory: i) area analysis (black rectangle), ii) vertical line analysis (black dotted lines), and iii) horizontal line analysis (red dotted lines). Semi-quantitative chemical analysis of the refractory was performed by analysing areas/spots using SEM-EDXA, and the average potassium content at different depths was calculated. For


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the horizontal line analysis, the distribution of elements at a certain depth was additionally determined. Areas (two yellow rectangles 2.5 x 3.5 mm) in the middle of the cross-sections, shown in Figure 2, were analysed using SEM backscatter electron mode to identify the distribution and relative amounts of the elements. Furthermore, the distribution of various phases in the crosssection was identified and visualized by applying Xphase spectral imaging software (Thermo Scientific, NORAN System 7). The mechanism of the potassium infiltration into the refractory was based upon comparing the potassium content in various phases of certain Al/Si molar ratios together with the XRD analyses. Some area/spot analyses at certain depths were also carried out to support the phase analysis and further to confirm the mechanism of the interaction between the salts and refractory.

3. Results and discussion 3.1. Surface appearance after salt exposure Surfaces of the refractory samples before and after the salt exposure with and without protective paste at 800 ºC and 1000 ºC are shown in Figure 3. The structure of the commercial refractory was very heterogeneous, consisting of light coloured matrix phase and dark grey and brown aggregate particles before the exposure (Fig. 3 a). After the thermal treatment at 800 ºC, a residual layer of salt is seen in the middle of the samples (Fig. 3 b and c). The salt particles seem to have sintered and partly molten during the exposure. The calculated solidus temperature for the synthetic ash is 631 ºC and the liquidus temperature is 866 ºC. The calculations were performed with the thermodynamic software package Factsage version 6.4,34 utilising the FTpulp database, which is based on the thermodynamic assessment of Lindberg et al..35 Furthermore, the calculated melt fraction at 800 ºC is about 35 wt%. After the exposure at 1000 ºC, no residual salt is seen at the surface of the sample without the paste


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(Fig. 3 e). However, some changes in colour can be seen. The reason for the difference between 800 ºC and 1000 ºC is that at 1000 ºC the temperature is well above the liquidus temperature for the synthetic ash. In contrast, a transparent thin layer covering the top surface was formed on the sample with paste (Fig. 3 d). This suggests that reactions occurred among synthetic ash, refractory and paste at high temperatures.

Figure 3. Images of the samples: 1) before exposure (a); 2) after exposure to KCl-K2CO3 at 800 ºC with protective paste (b), and without protective paste (c), and at 1000 ºC with protective paste (d) and without protective paste (e). 3.2. Infiltration of potassium in the refractory 3.2.1 X-ray mapping


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The distribution and relative amounts of aluminium, silicon, and potassium in the middle of the cross-section of the refractory exposed to the KCl-K2CO3 synthetic ash at 800°C are shown in Figure 4. The colour intensities in the images correspond to the content of the elements; the brighter the corresponding colour, the higher amount of the particular element. Aluminium is the main element both in the large aggregate particles and the fine grains embedded in the matrix. Silicon mainly exists in the matrix phase. Higher relative amounts of potassium were found at the surface and in the aggregate particle interfaces close to the surface. In the potassium rich surface layer some alumina was also detected. This suggests that at 800 ºC, potassium salts could partly start to be molten and easily infiltrate into the matrix phase of the refractory because of their porosity. But some residues at the surface of the refractory also start to react with the aggregate particles to form potassium containing alumina particles with the composition K2O·nAl2O3. In literature, formation of beta-alumina or potassium aluminate has been suggested between potassium vapour and alumina refractories.36 Further away from the surface, K was observed mainly in the Si-rich matrix phase. However, potassium did not react to a larger extent with the finer alumina grains. This may be attributed to the lower concentration of potassium below the surface and on the slow reaction with alumina to form beta-alumina.


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Figure 4. SEM image and X-ray maps of Al, Si and K in the refractory after exposure to KClK2CO3 at 800°C for 168h. 3.2.2 Area and line scan anlysis Detailed information of the potassium infiltration into the refractory was obtained by SEMEDX analyses of the compositions in several areas and spots along horizontal and vertical lines of the cross section, shown in Figure 2. The potassium content from the area analysis as a function of distance to the surface is presented in Figure 5. It should be pointed out that the average potassium content takes into account both aggregate particles and the matrix. The potassium content decreased almost exponentially from around 11 wt% at the surface to 0.8 wt% at 1 mm from the surface. From 1 mm to 4 mm the potassium content steadily decreased and only negligible amounts were measured at 4 mm. As can be seen in Figure 5, the paste did not fully protect the area beneath it, and some potassium was detected beneath the paste. This is due to diffusion of potassium along grain boundaries and pores in the refractory. This suggests that the paste does not give similar protection to salt attack as in the corrosion studies with metal samples14,19 and that the areas under the paste cannot be used as uncorroded reference. However, Figure 4 suggests that potassium is mainly found in the matrix phase. Thus, the potassium content in the matrix phases could be higher than the average value suggested in Figure 5.


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Figure 5. Potassium content (wt%) as a function of distance to surface after exposure to KClK2CO3 at 800 °C for 168h. The potassium content analysed in areas according to Figure 2. The results from the vertical line scan analyses are shown in Figure 6. The potassium content is given as a mean value of groups of 15 consecutive spots measured from the surface for each vertical line. Spots containing less than 0.2 wt% potassium are excluded from the calculations, since the refractory itself contained some potassium. The potassium content was high, around 15 wt% close to the surface, but decreased to around 4 wt% at the depth of 1 mm. The vertical line analysis was assumed to give more accurate measurement of the potassium penetration compared with the area analysis in Figure 5, because the line analysis only takes into account the spots located in the matrix phase. As can be seen in Figure 6, the potassium content varied somewhat between the different lines. This was probably attributed to local differences in the refractory microstructure.


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Figure 6. Potassium content (wt%) measured along the vertical lines, presented in Figure 2, as a function of distances from the surface. Line 6 was underneath the protective paste. Figure 7 compares mean values of potassium in the matrix phase obtained from the horizontal line analyses (without protective paste) and the values from vertical line analyses (with protective paste). It clearly shows a gradient in the potassium content at the surface layers from 21 wt% at the surface to 7 wt% at the depth of 1 mm. At the depth of 3 mm the potassium content was 0.7 wt%. The values appear in a similar range as the values from the vertical line scan analysis. The paste does not affect the potassium infiltration at 800°C. These results from the parallel tests also prove that the usability and reproducibility of the line scan analysis are good. The values obtained from the vertical line scan analysis are limited by the locations, whereas the results from the horizontal line scan analysis gives more information at a certain depth.


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Figure 7. Potassium content (wt%) as a function of distance to surface after exposure to KClK2CO3 at 800°C. For the vertical line analyses all spots containing ≥ 0.2 wt% K are given, while the horizontal line analyses give the mean content in the matrix phase. The scanned lines are presented in Figure 2. In Figure 8, the distribution of potassium content is shown at 2 mm and 3 mm from the surface after exposure to KCl-K2CO3 at 800°C. The figure is based on SEM-EDX analyses of 500 spots along horizontal lines at 2 and 3 mm depth. The distribution curve at 2 mm is more flat than at 3 mm, and the spots with higher values of potassium content distribute widely at 2 mm. This indicated that at 2 mm, the potassium penetrates unevenly into refractory. At the depth of 3 mm, the potassium content is more uniform. Figure 9 shows the potassium, alumina, and silicon analysed in the spots at 2 mm and 3 mm. It clearly shows a correlation between the potassium content and the ratio of Si/Al. The spots with high potassium content have higher ratios of Si/Al, while spots with no potassium have higher Al contents. Thus, this indicates that potassium has reacted with the matrix phase where the Si content is higher.


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Figure 8. Distribution of potassium at a depth of 2 mm and 3 mm from the surface after exposure to KCl-K2CO3 at 800°C


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2 mm 8

1.4 K

Si/Al

7

1.2 1

5 0.8 4 0.6 3

Si/Al molar ratio

K content (wt%)

6

0.4

2

0.2

1

0

0 0

50

100

150

200

250 300 Spots analyzed

350

400

450

500

3 mm 8

1.4 K

Si/Al

7

1.2

6

1

5 0.8 4 0.6 3

Si/Al molar ratio

K content (wt%)

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

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0.4

2

0.2

1 0

0 0

50

100

150

200 250 Spots analyzed

300

350

400

450

Figure 9. K and molar ratio of Si/Al at a depth of 2 mm and 3 mm from the surface in the horizontal line scan analyses, after exposure to KCl-K2CO3 at 800°C. ACS Paragon Plus Environment

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3.3. Characterization of the mechanisms of alkali infiltration The potassium distribution curves in Figure 9 clearly show that potassium is present mainly in spots which have relatively high Si/Al ratio. Thus, the microstructure of the refractory plays an important role in the infiltration mechanism. The microstructure of the refractory was studied from cross-sections with Xphase maps. This technique enables detailed composition analysis of areas and gives these as phases indicated by different colours. These coloured maps of the distribution of the different phases in the sample cross-section can be used to formulate the mechanisms of the infiltration of ions into the refractory. X-Ray diffractograms of the alumina refractory surface also show the crystal phases present before and after exposure at 800 and 1000°C in Figure 10. XRD results indicate that the alumina refractory before exposure consisted of corundum, mullite, SiO2 as cristobalite and quartz, and calcium aluminum silicate (2CaO·Al2O3·SiO2) crystals. According to literature,37 some amorphous phase can also be present; however, this cannot be verified by XRD. Some new phases were identified on the surface of the refractory exposed at 800°C. These were: beta-alumina

(K2O·nAl2O3,

n=9~11),

kalsilite

(K2O·Al2O3·2SiO2)

and

leucite

(K2O·Al2O3·4SiO2). Some residual potassium carbonate was also detected. After the exposure the corundum peaks decreased in intensity and the mullite peaks disappeared. After exposure to the K2CO3-KCl synthetic ash at 1000°C, kaliophilite (K2O·Al2O3·2SiO2), SiO2, and Al2O3 were detected on the surface (Figure 10). Similar potassium aluminium silicate compounds have been reported when exposing alumina silicate bricks to potassium vapour or slag.20,23,38 New phases were only observed at the upmost surface layer. In this work, XRD analyses carried out on the cross section at the depths of 0.5 mm and 1 mm in the refractory after exposure did not indicate any new phases compared with the original material, although the line scan analyses showed that the


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potassium penetrated to a depth of about 3 mm. This suggests that the concentration of the potassium penetrated into the refractory were too low to be detected with XRD. Figure 11 shows the phase map of the cross-section of the refractory before and after exposure to the K2CO3-KCl synthetic ash at 800°C. The chemical compositions in various phases were determined. Before the exposure (Figure 11a), the phases given by grey colour had an Al2O3:SiO2 molar ratio greater than 2. These grey areas show large aggregate particles and also finer alumina particles embedded in the matrix. Some CaO, TiO2 and Fe2O3 are also detected in the aggregate particles. The composition suggests that these particles consist mainly of a mixture of corundum and mullite. The matrix phase given by the purple colour has an Al2O3:SiO2 molar ratio of 0.7-0.9. This molar ratio suggests that the matrix consists of SiO2 and mullite. In the matrix phase areas with high content of CaO marked with light blue colour are seen. According to the diffractogram, CaO is present as 2CaO·Al2O3·SiO2 in the refractory. Phase analysis based on oxide composition of the light blue area suggests the presence of a mixture of corundum and calcium aluminium silicate. Finally, the green coloured areas in the matrix phase were identified to have an Al2O3:SiO2 molar ratio of 0.3. Comparing this ratio with the XRD analysis suggests that these areas consist of SiO2 and some mullite. The Xphase map and corresponding composition analysis of the cross-section after the exposure at 800°C are given in Figure 11b. The oxide composition analysis of the coloured areas clearly shows potassium in the matrix phase. Highest content of potassium was analysed in the red-coloured areas close to the surface. The areas marked with bright red colour, areas with high SiO2 and K2O contents, are seen at the surface and in the matrix, especially along the aggregate particles. The phase analysis of these red coloured areas indicates that potassium has reacted with silica and mullite to potassium aluminium silicates, most likely to kalsilite and leucite as given by the XRD analyses of the surface layers. The potassium oxide


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balance suggests that all potassium has not reacted with the refractory components but is left as a salt residue on the surface. Since no potassium chloride but only potassium carbonate was identified in the diffractogram, the salt residue is most likely potassium carbonate. Areas marked with the lighter red colour contain more Al2O3 than the bright red coloured areas. Comparing the composition of these areas with the phases identified with XRD suggests that potassium has reacted with free α-alumina to form β-alumina (K2O·nAl2O3). Finally, the potassium content in the matrix decreases with increasing distance to the surface as shown by the areas marked with yellow and blue colours.

M u llite A l2O 3

B efore exposure

SiO 2 2 C a O .Al 2 O 3.SiO 2

20

30

40

50

60

o

20

70

K 2C O 3 β − A l2O 3 K a lsilite L e ucite

K C l-K 2 CO 3 -800 C

Intensity

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Energy & Fuels

30

40

50

60

70

o

K C l-K 2 CO 3 -1000 C K ao lio p hilite

20

30

40

50

60

70

2θ

Figure 10. X-Ray diffractograms from the surface in the alumina refractory before and after exposure at 800°C and 1000°C.


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Color

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Composition (mol%) Al2O3

SiO2

K2O

CaO

TiO2

Fe2O3

70

28

-

2

0

0

64

30

-

1

5

2

Purple

41

53

-

5

0

0

Light blue

44

30

-

27

0

0

Green

21

76

-

3

0

0

Grey

Composition (mol%) Color

Al2O3

SiO2

K2O

CaO

TiO2

Fe2O3

Grey

76

19

0

1

3

1

Red 1

43

19

38

0

0

0

Red 2

83

6

11

0

0

0

Yellow

37

52

5

5

0

0

41

48

5

6

1

0

Dark blue

47

24

4

25

0

0

Light blue

47

22

0.5

30

0

0

Purple

44

50

0

5

1

0

Green

33

62

0

4

0

0

Figure 11. Distribution of different phases identified from elemental maps by Xphase in the alumina refractory: (a) cross-section before, and (b) cross-section underneath the salt after exposure to KCl-K2CO3 at 800°C for 168h.

Figure 12 shows SEM images of the alumina refractory after the exposure and selected EDX area analyses. It can clearly be seen that potassium has reacted with the matrix phase and no potassium can be found in the aggregate particles. Potassium can penetrate deeper into the refractory in cracks. This is illustrated in Figure 12b. High potassium contents can be found near cracks deeper down in the refractory. Thus, liquid potassium salts can more easily infiltrate through the porous part and cracks in the brick.


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Energy & Fuels

mol%

Al2O3

SiO2

K2O

CaO

TiO2

Fe2O3

Area 1 Area 2 Area 3, 5 Area 4, 11 Area 6 Area 7 Area 8 Area 9 Area 10

18 31 94-100 39-46 57 49 29 14 28

72 35 0-6 46-48 35 2 55 66 63

4 4 0 3-5 5 0 6 4 3

5 29 0 5-8 3 49 10 16 5

0 0 0 0-1 0 0 0 0 0

0 0 0 0 0 0 0 0 0

mol%

Al2O3

SiO2

K2O

CaO

TiO2

Fe2O3

Area 1

79

15

0

0

3

2

29-37

45-52

17-24

0

0-1

0-1

Area 4

26

50

17

7

0

0

Area 7

16

57

12

14

0

0

28-34

51-54

7-9

9-10

0

0

54

37

3

3

2

1

Area 2, 3, 5, 6

Area 8, 10 Area 9

Figure 12. BSE-SEM images of the cross-section of the alumina refractory after exposure at 800 ºC at certain depth: (a) matrix phase at 1 mm and (b) boundary between aggregate particle and matrix phase at 1.5 mm. The Xphase maps and composition analyses of the cross-sections nicely illustrated the penetration of potassium into the refractory. SEM images and EDX analyses of the areas in the refractory also further proved the accuracy and completeness of Xphase map analysis. Combining the composition analysis with the phase analysis from XRD offers a possibility to study the reaction mechanisms between the salt and the different phases present in the original refractory.


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4. Conclusion A laboratory method for studying the attack of deposited salts on refractory materials in typical biomass and waste combustion conditions was developed. The method is based on detailed analysis of the refractory before and after salt exposure using SEM-EDXA and XRD. Tests were made by exposing alumina refractory samples to a K2CO3-KCl mixture at 800°C and 1000°C. The potassium infiltration was analysed both along horizontal and vertical lines and areas of the refractory cross-section using EDXA. In the first approach, a protective paste around the salt was used to restrict the salt attack to a specific area, similarly as in studies of metal corrosion. However, the paste did not protect the refractory from salt attack beneath the paste, since potassium diffused also horizontally along the matrix phase due to the porous and heterogeneous structure of the refractory. Additionally, at 1000 ºC the protective fire sealant paste reacted with the salt. Thus, protective paste to fix the salt to the sample surface is not recommended for studies of the interactions between ceramic refractories and salt deposits. Detailed information on potassium penetration in the different phases of the refractory was obtained from the line analyses both in horizontal and vertical directions. The vertical line analyses gave a good understanding of the potassium penetration into the refractory. The horizontal line analyses gave detailed information of the distribution of potassium and the association of potassium to the refractory at certain depths. Although giving relevant information on the corrosion of the refractory, reliable line scanning required a large number of points to be analysed and classified according to the main components present for each point. Xphase maps and composition analyses enabled for rapid composition analyses of large areas of the refractory. When combining the information of the phase compositions from X-Ray analysis, the Xphase maps can be used to retrieve information on the reaction mechanisms between the refractory and the components of the salt. In this work, the attack of potassium


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Energy & Fuels

with the alumina refractory was found to take place mainly via penetration into the matrix phase to form kalsilite with mullite. In further reactions with silica, some leucite was also formed at the surface. In addition, some α-alumina reacted with potassium to β-alumina at 800°C. At 1000 ºC, kaoliophilite, the cubic form of kalsilite, formed at the top surface. The laboratory method reported in this work enables for detailed characterisation of the interactions of refractories with deposits and salts at elevated temperatures by using a 168h exposure at selected conditions. Detailed SEM-EDX analyses together with XRD give information of the potassium penetration and the reaction mechanisms behind the chemical deterioration of the refractory at given conditions, and thus the method can be used to estimate the suitability of a particular refractory exposed to different combustion environments.

Acknowledgements This work has been carried out as part of the activities of the Johan Gadolin Åbo Akademi Process Chemistry Centre. Support from the National Technology Agency of Finland (Tekes), Andritz Oy, Valmet Technologies Oy, Amec Foster Wheeler Energia Oy, UPM-Kymmene Oyj, Clyde Bergemann GmbH, International Paper Inc. and Top Analytica Oy Ab is gratefully acknowledged. This research is also partly financed by MacPlus. Funding for Emil Vainio came from the project ‘Low temperature corrosion in combustion – old problem, new approaches’ (Decision No. 289869) and is financed by Academy of Finland and is greatly acknowledged. We also want to thank Linus Silvander for carrying out the SEM/EDXA analysis.


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High-Temperature Corrosion of Refractory Materials in Biomass and

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