The Diagnostics of Power Boilers in Terms of Their Sustainability

Abstract

Diagnosing steam pipelines is crucial because they are subjected to a water vapor environment and exhaust gases. Layers of oxides/deposits formed on steel utilized at elevated temperatures for long time periods have a significant impact on elements operating in power plants as well as in combined heat and power plants. Currently, these devices are an important topic of sustainable energy development. The aim of this work was to characterize the structure of the steel and of the oxides/deposit layer formed on the steam superheaters of power boilers and its impact on the durability of power equipment. The tests were carried out on 13CrMo4-5 steel utilized at various temperature and time parameters. In order to assess the degradation of the material, the following research methods were used: light microscopy, X-ray structural analysis, and infrared spectroscopy with Fourier transform. The use of the FTIR method in this type of diagnostics has deepened the existing analysis of oxide/sediment layers. The obtained test results showed that the kinetics of the corrosion process on steel being used for long periods at elevated temperatures is complex and depends, among others, on the element’s operating temperature, the operating time, and the flow medium.

1. Introduction

At the present time, attempts are being made to combine energy, environmental protection, and economics with the policy of sustainable development. More and more often, we are looking for opportunities to improve energy efficiency in existing energy technologies. The optimal use of available fuel, lower operating costs, and the reduction of gas emissions significantly improve the comfort of life through cleaner air. That is why it is so important to diagnose energy equipment for sustainable development.

Most of the energy produced today is provided by nuclear power and fossil fuels such as oil, coal, and natural gas [1]. However, the growing demand for energy results in an increasing use of renewable resources, which include biomass. This has led to a significant development of the wood energy sector in recent years [2]. The effects of economic development are visible by posing new challenges. The transformation of conventional energy is currently being developed. According to the literature data [3], fly ash resulting from coal combustion is a source of threat to the environment. The increasing demand for energy and the resulting environmental problems are generating worldwide interest in the development of clean and efficient alternative energy sources that use ecological processes and materials [4]. Increasing energy consumption in the industrial sector requires the adoption of sustainable energy practices. Therefore, steam pipeline networks offer the opportunity to improve energy efficiency while reducing environmental impact [5]. Therefore, among other factors, it is so important to choose the right material for steam boilers.

The chromium–molybdenum steel 13CrMo4-5 [6] is commonly used in the energy industry [7] for the construction of critical infrastructure elements, e.g., as a material for steam pipelines [8], due to its favorable mechanical properties [9]. Therefore, the creep resistance or corrosion resistance of this type of steel is extensively tested, taking into account various operating conditions, such as temperature, pressure, or environment [9]. High-temperature corrosion in coal-fired boilers fired by biomass, which is still an insufficiently researched phenomenon, causes the failure of the installation and thus economic problems [10]. High-temperature corrosion has been studied in plants burning biomass rich in chlorine (e.g., straw) and in municipal waste incineration plants [10]. Corrosion processes were also tested in plants fired by fuels with low- and medium chlorine content, such as wood chips and bark as well as qualitatively sorted wood waste [10]. However, in measurements in real-scale biomass plants, it is often not possible to vary just one parameter of interest to determine its effect on high-temperature corrosion [10]. According to the literature [11], oxidation of 13CrMo4-5 steel is the dominant mechanism in the temperature range between 450 and 550 °C. Gruber et al. showed that the basic mechanism of corrosion strongly depends on the prevailing conditions, i.e., oxidation and active oxidation induced by Cl [11]. Due to the fact that the failure of power pipelines can lead to serious consequences, the condition of the material must be intensively monitored. Issues related to the assessment of the durability of the technical condition and the trouble-free operation of steam pipelines are the priority diagnostic tasks [12]. Research is being carried out around the world to increase the corrosion resistance of steels used in the energy industry. Experimental results have shown that the coatings created in the laser cladding process create an effective barrier against the corrosion of 13CrMo4-5 steel. The coating deposition process was carried out with a nickel-based alloy powder [9]. In addition, in paper [13], an attempt was made to apply NiCoCrAlYHfSi coatings to X10CrMoVNb9-1 steel. However, the research did not show satisfactory results. Scientists have shown that the potential industrial use of spraying a NiCoCrAlYHfSi HVOF (high velocity oxygen fuel) coating on P91 steel would likely not lead to a noticeable increase in corrosion resistance. Therefore, this coating cannot be recommended for use in corrosive environments dominated by sodium sulphates and chlorides. On the other hand, Retschitzegger et al. [14] stated that steel P91 seems to be a more suitable material than 13CrMo4-5 for wood chip-fired CHP plants utilizing high flue gas temperatures above 800 °C for the superheater inlet. Among the various available research methods used to test elements after long-term operation, apart from methods for testing mechanical properties, the most frequently used are LM [8,15], SEM [9,12,15], and XRD [15,16,17].

The aim of this paper was to extend the research methodology for the diagnosis of long-term exploited steel (oxides formation) using the Fourier-transform infrared spectroscopy (FTIR) technique. FTIR is an analytical method used to study the structure of individual molecules and the composition of molecular mixtures [18], which we will apply to assess the chemical composition of oxides formed on various substrates [19,20,21].

The main research objective of this work was the use of the FTIR method for the diagnostics of power plants. In addition, attempts were made to compare this method with other methods used in the assessment of devices operating in power circuits to aim at sustainability.

2. Materials and Methods

The investigated materials were 13CrMo4-5 steel samples taken from three steam superheaters of power boilers. Steam superheaters are used to increase the temperature and to transport flowing superheated steam before it enters the steam turbine [22]. The operating parameters are presented in

Table 1. Operating parameters of the tested steel.

Sample	Temperature, °C	Time, h
1—from the outside	375	120,000
1—from the inside	375	120,000
2—from the outside	375	220,000
2—from the inside	375	220,000
3—from the outside	540	220,000
3—from the inside	540	220,000

. Low-alloy chrome–molybdenum steels are often used for power boiler components [23]. The structure of the steel as well as the resulting oxide layer was tested, both from the outside and the inside of the pipe (Figure 1). For this purpose, samples with dimensions of 10 mm × 10 mm were taken from the pipe. The specimens were made transversely to the material axis. The preparation of the metallographic specimen consisted of grinding and polishing. Selected samples were etched in 5% nitric acid. The test samples were made in the metallographic laboratory at the Department of Materials Engineering of the Czestochowa University of Technology.

The microscopic examinations were performed using a light microscope Olympus GX41, while the measurement of surface roughness was conducted using a digital microscope VHX-7000 (Keyence, Mechelen, Belgium) using a Gaussian filter. On the basis of the tests carried out, the following parameters were determined: S_a (arithmetic mean of height), S_z (maximum height), S_p (height of the highest hill), S_v (depth of the lowest depression), S_q (rms height), S_sk (skewness), S_ku (kurtosis). The analyses of the phase composition were conducted using X-ray diffraction (XRD) and IR spectrophotometry with Fourier transform. The XRD measurements were carried out on a diffractometer 3003T/T from SEIFERT. In order to reduce the fluorescence radiation of iron, CoK radiation (1.79026 Å) was used. The X-ray tube was operated at 30 kV and 40 mA. The XRD patterns were collected in the 2θ range between 15° and 120° with an angular step of 0.1°. The FTIR measurements were performed on a Nicolet IS50 FT-IR spectrometer with a deuterated triglycine sulfate (DTGS) detector. In this study, the ATR (attenuated total reflectance) method was used. The samples were put onto a diamond crystal. Before sample measurement, a background spectrum was collected. All spectra were measured in the range between 4000 cm⁻¹ and 400 cm⁻¹ with 4 cm⁻¹ spectral resolution and using 32 scans. Recordings were performed by OMNIC^TM 9.2 (Thermo Fisher Scientific, Waltham, MA, USA) software. Baseline correction was conducted using OPUS 7.0 software.

3. Results and Discussion

The results of the structural analysis of the investigated materials are shown in Figure 2. The results show that 13CrMo4-5 steel, after a service life of 120,000 h at T = 375 °C, shows a slightly degraded ferritic-pearlite structure. Occasional carbide precipitations appear both inside the grains and along their boundaries (Figure 2a). Increasing the working time of the element by 100,000 h already causes a destruction of the structure, which is shown in Figure 2b. A significant number of carbide precipitates, located mainly in the grain boundaries, were observed. These boundaries are no longer as clear as after 120,000 h of operation. In addition, occasional etching occurs, not only around carbide precipitates but also around sulfides. According to the literature [12], after a certain period of pipeline operation, structural changes are revealed, combined with the processes of structural degradation. On the other hand, increasing the temperature by 165 °C with a working time of 220,000 h caused huge changes in the structure (Figure 2c). The pearlite has completely degraded. A ferritic structure with a lot of carbide precipitates is practically visible. Carbides occur both inside grains and within grain boundaries. They are quite large in size. The places where the carbides occurred form so-called chains. Etching around these precipitates was also observed. The etching sites resemble single creep micropores, which are located mainly in grain boundaries. Decomposition of bainite or pearlite is quite a dangerous phenomenon, which can lead to material failure. Therefore, efforts should be made to identify potential areas of increased risk of failures before allowing operation for the next period [12]. Brodecki et al. [24] and Tuz et al. [25] also used light microscopy to observe the structure of steels operating at elevated temperatures for long periods of time.

Jackiewicz et al. [8] described structures obtained by heat treatment simulating the degradation conditions in industrial environments. The obtained material exhibited a ferrite–carbide structure, with clearly visible grains of primary pearlite, in which there was an effect of lamellar structure decomposition. Paper [12] presents the structure of the material of a steam pipeline elbow (an area at the place of a crack and outside the damaged area), made of 13CrMo4-5 steel after 300,000 h of operation in conditions of overheated steam. The microstructure studies show that in the undamaged areas the depletion of the structure is the same as in the base material, which corresponds to a partial decomposition of the bainite/pearlite areas. In the places where cracks were found, the bainite/pearlite decomposes, and carbide precipitates coagulate with clear microcracks. The results of cross-sectional examinations showed significant differences in the thicknesses of the oxide/sediment layer (Figure 3). In all three cases, a ferritic structure with carbide precipitations was observed directly at the oxide/steel interface. This phenomenon occurs regardless of whether it is the flue gas inflow side or the water vapor flow side. However, differences in the thicknesses of the ferritic structure occur. The higher the temperature and the longer the working time of the element, the wider the area of the ferritic structure. In addition, the density of the precipitations also increases. It was also observed that the thicker the oxide/deposit layer, the greater the tendency for the layer to crack. As oxides grow on any metal or alloy surface, a layer of stress develops, causing cracking, which ultimately promotes spalling. The development of stresses in the layer may also result from a thermal expansion mismatch between the oxide and the steel. This phenomenon occurs primarily during high-temperature oxidation [26]. In addition, an increased number of precipitations was observed directly from the flue gas inflow side. The same tendency was observed in the formation of the oxide/precipitate layer. In addition, with operating parameters of 540 °C and 220,000 h, corrosion progressing along the grain boundaries was also observed, both from the side of the steam flow and the flue gas inflow, although the influx of exhaust gases intensifies this process. Steels utilized in the energy industry must withstand the harsh conditions to which they are subjected through long-term exposure to oxidizing or reducing atmospheres. Paper [27] presents the results of tests carried out to examine the effect of 1% H₂S in 99% Ar at a pressure of 1 bar on the corrosion resistance of low-alloy steels used in coal-fired power plants. Based on the conducted research, the researchers found that these steels in sulfur-rich atmospheres do not show protection due to the formation of porous, thick, sulfide scales. The authors showed that during the sulfurization of steel in the temperature range of 450–500 °C for 100 h, Fe_1-XS scales are formed.

The results of the XRD phase analysis and the FTIR analysis are shown in Figure 4 and Figure 5, respectively. As the XRD studies have shown, the dominant oxide phases present in both the inner and outer oxide layers are hematite (Fe₂O₃) and magnetite (Fe₃O₄). According to data sheet 01-079-0007, hematite, having the R-3c space group, consists of 69.94% iron and 30.06% oxygen by weight, corresponding to 40% Fe and 60% O atomically. The crystal data: a = 5.029Å, b = 5.029Å, c = 13.736Å, α = 90°, β = 90°, and γ = 120° indicate a rhombohedral arrangement. The peaks originate mainly from planes (104), (110), and (116). In the case of magnetite, it is a cubic system with a space group Fd-3 m. The parameters for Fe₃O₄ are a = b = c = 8.491Å and α = β = γ = 90°. According to data sheet 01-089-0951, the main peaks come from the (311), (220), and (440) planes. The chemical composition of magnetite is 72.36 wt% Fe and 27.64 wt% O, which corresponds to 42.86 at% iron and 57.14 at% oxygen.

Hagarowa et al. in [22] studied the oxidation behavior of martensitic 9Cr steel reinforced with boron and MX nitrides. This steel was exposed for 1000 h at temperatures of 600 and 650 °C in an atmosphere consisting of air and 10% water vapor. Researchers showed that the outer steel oxide layer, after 1000 h of exposure at 600 °C, consisted mainly of Fe₂O₃ and a small amount of Cr₂O₃. Increasing the temperature by 50 degrees resulted in the occurrence of hematite, as before. However, in addition to hematite, the formation of a phase rich in chromium and Fe₃O₄ was also observed. The inner oxide layer consisted of (FeMnCr)₂O₄ and Cr₂O₃ for T = 600 °C. However, increasing the temperature to 650 °C caused the internal oxide layer to consist mainly of iron–chromium–manganese spinel (FeMnCr)₂O₄. On the other hand, the authors in [28] found that the scale formed on steel containing 5% chromium consisted of three layers. The inner layer was Fe₃O₄; the middle layer consisted of Fe₂O₃. However, the outer layer contained Fe₃O₄ and a small amount of Fe₂O₃.

The XRD phase analysis was further supplemented with FTIR tests. FTIR analysis in connection with XRD analysis is often used to determine the phase composition of materials [29].

Figure 5a shows FTIR spectra from the inside and outside in sample 1, where the same vibrations were noticed only in the case of Fe-O vibrations (~470 cm⁻¹, ~570 cm⁻¹) [30] and C-H (~2900 cm⁻¹). Additional peaks from the inside sample 1 originating from Cr-O bands (731 cm⁻¹) [31], Zn-O (943 cm⁻¹) [32], and C-H (~1300–1400 cm⁻¹) were noticed. The band corresponding to O-H vibrations appears mainly as a characteristic peak in the areas 3600–3750 cm⁻¹. This peak was observed for sample 1 (from the inside), while from the outside of sample 1, a peak corresponding to Al-O vibrations around 782 cm⁻¹ was visible [33]. In this case, a vibration peak belonging to the SiO₂ groups was observed. This peak in the spectrum is visible at ~1100 cm⁻¹. In both the inside and outside sample 2, peaks corresponding to Fe-O were observed around ~470 cm⁻¹, ~570 cm⁻¹ [30], and C-H (~2900 cm⁻¹). In paper [34], researchers using the FTIR method identified iron and oxygen-based compounds such as Fe₂O₃ and Fe₃O₄. Moreover, in outside sample 2, peaks originating from Al-O vibration (739 cm⁻¹) [33], S=O (1045 cm⁻¹) [35], and O-H (~3600–3700 cm⁻¹) were observed, Figure 5b. In many countries, power units are fired with coal containing varying amounts of sulfur in organic and inorganic forms [27]. The formation of H₂S depends on the sulfur content, conditions during exposure to high temperature, and volatile substances in the coal. When burning coal containing sulfur, sulphide scale may form on high-temperature elements. These types of deposits show much less protection compared to oxide deposits. Coal containing sulfur produces rigorous reducing conditions during combustion. Such an atmosphere is susceptible to the formation of highly aggressive hydrogen sulfide, which leads to accelerated sulfidation, which in turn leads to rapid corrosion degradation of low-alloy steels [27]. In both sample 3s (Figure 5c), from the inside and outside, peaks originating from Fe-O vibrations (~470 cm⁻¹, ~570 cm⁻¹) [30], Si-O-Si bonds (~960 cm⁻¹) [36], C-H (~2900 cm⁻¹), and O-H (~3600–3700 cm⁻¹) were observed. Furthermore, from the inside sample 3, an additional peak around 730 cm⁻¹ corresponding to Cr-O vibrations was visible [31], while from the outside sample 3, second peaks from Si-O-Si bonds (1216 cm⁻¹) [36] were noticed. This line comes from asymmetric stretching vibrations. The modern development of the energy sector is aimed at reducing harmful emissions, e.g., carbon dioxide [37]. Therefore, it is very important to diagnose power plant equipment in terms of formed deposits.

Mwema et al. in [19] used this method when testing thin aluminum layers deposited on stainless steel (316L) substrates at a variable radio frequency (RF) power by magnetron sputtering at a constant substrate temperature of 90 °C. Researchers found that the narrow bands between 500 and 2000 cm⁻¹ can be attributed to the stretching vibration mode of Al-O and C-O. In turn, Ghaith et al. [20] treated this type of steel with a laser irradiation process using hydroxyapatite–titanium oxide. The coatings were assessed, among others using ATR-FTIR techniques. Studies have shown the formation of a strong absorption band at 1036 cm⁻¹, which is attributed to the (PO₄)³⁻ group. Also, the absorption bands at 1090 and 965 cm⁻¹ are assigned to (PO₄)³⁻. The band observed at 1432 and 1550 cm⁻¹ is caused by bond vibrations in the CO₃²⁻ group. These bonds are attributed to the apatite crystal lattice and calcium carbonate. In turn, the absorption band at 650 cm⁻¹ indicates Ti–O–Ti vibrations. Chen et al. [21] applied the FTIR tests to a nanocomposite deposited on Q235 carbon steel. The nanocomposite (TiO₂ + graphene oxide + polyaniline) was synthesized by in situ oxidation and used as a filler on epoxy resin. The O–H stretching vibration peak at 3400 cm⁻¹ has been shown to decrease when TiO₂ is mixed with graphene oxide and even disappears when polyaniline is added. The flat peak at about 700 cm⁻¹ is visible in all spectra and corresponds to the vibration peak of Ti–O–Ti and Ti–O–C, which is the characteristic absorption band of TiO₂. FTIR analysis was also used to confirm the phase change from Fe₃O₄ to α-Fe₂O₃ during the post-synthesis heat treatment [38]. The absorption peak observed at 567 cm^–1 belongs to the stretching vibration mode of Fe–O bonds in Fe₃O₄ for the as-prepared powder. On the other hand, the annealed powder shows two absorption peaks at 470 cm⁻¹ and 548 cm⁻¹, attributed to Fe–O vibrations in α-Fe₂O₃. Fourier-transform infrared (FTIR) spectroscopy has also been used to analyze the functional groups of the graphene oxide [39]. Characteristic GO infrared peaks include prominent O-H, C=O, and C-O absorption peaks near 3198 cm⁻¹, 1608 cm⁻¹, and 1177 cm⁻¹. In addition, in paper [40], Chen et al. studied the structure of a flat SiO₂/ITO/sapphire/stainless steel emitter using FTIR. Researchers showed that this structure has a selective emission with a high emissivity of 0.8 in the wavelength range of 1–1.6 μm at 1000 °C.

The authors of paper [41] tested 13CrMo4-5 steel and found that two basic sublayers can be distinguished in the examined layer, namely sediments and products. Products that occur right next to sediments contain spheroidal sediment particles. Sediments can be the “place” and “source” of compounds that can influence the corrosive degradation of pipes. This destruction may be caused by reaction with products and/or diffusion or penetration through pores. Moreover, researchers found that the sediments consisted mainly of compounds containing SiO₄⁴⁻ groups (silicates and aluminosilicates). However, the products mainly included Fe₂O₃ and Fe₃O₄ oxides. The sediment sublayer was not homogeneous. It was characterized by the presence of mainly very small particles, several micrometers in diameter. The deposits were dominated by O, Si, and Al. Additionally, there were also elements such as Na, Mg, S, K, Ca, Ti, and Fe. The fine-spherical crystals were fly ash, which is a collection of chemically active and inactive crystallites. The passage of coal dust through the high temperature zone in the combustion chamber causes the ash grains to become vitrified. Therefore, the unburned components of SiO₂ and Al₂O₃, as well as MgO, K₂O, and CaO, remain inside the grains, forming a eutectic subjected to melting, and then solidify in the form of glass [41].

The obtained roughness results showed (Figure 6,

Table 2. Roughness measurements.

	The Inner Side of the Wall of the Pipe (from Water Steam)			The Outer Side of the Wall of the Pipe (from Flue Gas Flow)
	Sample
	1	2	3	1	2	3
Sa/μm	19.54	21.88	24.08	37.09	39.61	44.87
Sz/μm	197.87	216.84	224.34	371.17	232.26	335.86
Sq/μm	28.02	31.00	31.10	51.73	47.02	55.24
Ssk/-	1.01	−0.71	0.28	2.25	0.39	−0.31
Sku/-	5.91	5.19	3.47	11.18	2.13	3.08
Sp/μm	114.05	97.75	119.06	306.36	106.24	142.70
Sv/μm	83.83	119.10	105.28	64.81	126.03	193.16

) that the lowest value of the S_a parameter was shown by the steel operated with the lowest parameters. Comparing this parameter, its lowest value was observed both for the inner and outer side of the pipe wall. Increasing the operating parameters of the element resulted in an increase of this parameter for the internal side by about five units. On the other hand, in the case of the outer side, an over two-fold increase in S_a was observed. For maximum height, a similar sequence was observed but only for the inner side. In the case of the outer side of the pipe wall for samples 1 and 3, the value of this parameter is above 330 μm, while for sample 2 it is significantly below this value. According to the authors of [42], the higher roughness with the maximal height S_z on the underside is caused by the incompletely removed support structure, which is more severe than the intrinsic surface roughness and is the main reason for the crack initiation. The S_q parameter, comparing the samples from the inside, remained at the same level and oscillated around the value of 30 μm. A similar tendency was observed for the outer side of the pipe wall for this parameter, with the difference that the value of this parameter oscillated around 50 units. For the parameter S_ku, along with the increase in operating parameters, a downward trend was observed on the steam inflow side. On the exhaust gas side, there was a decrease in this parameter but comparing only sample 1 to sample 2. For sample 3, an increase in S_ku by less than a unit was observed compared to sample 2. Degradation of steel power structures requires continuous diagnostic activities to maintain their reliability and safe operation. Operational loads combined with simultaneous microstructural changes occurring under the influence of high temperatures significantly accelerate the rate of damage. Therefore, it is so important to maintain the safe condition of steel pipes used in the energy industry, which are subjected to continuous operation at high temperatures [24].

Figure 7 and

Table 3. Summary of the changes of each sample after corrosion.

Sample Designation	Structure Changes	Carbide Precipitation	Corrosion Along Grain boundaries	Layer (Oxides/Deposits) Growth	Layer (Oxides/Deposits) Degradation
1—from the outside	↑	↑	-	↑↑	↑↑
1—from the inside	↑	↑	-	↑	↑
2—from the outside	↑↑	↑↑	-	↑↑↑	↑↑↑
2—from the inside	↑↑	↑↑	-	↑↑	↑↑
3—from the outside	↑↑↑	↑↑↑	↑	↑↑↑↑	↑↑↑↑
3—from the inside	↑↑↑	↑↑↑	↑↑	↑↑↑	↑↑↑

↑—intensity of the process.

summarize the results for the changes of each sample after corrosion.

4. Conclusions

Based on the research, the following statements and conclusions can be formulated:

• The degree of degradation of steam superheaters is influenced by the atmosphere surrounding a given element in energy devices. Comparing the outer wall of the pipe with the inner one, significant degradation of the oxide layers was observed from the exhaust gas atmosphere more than from the circulating medium.
• The increase in temperature and time in these elements has a more negative impact on the condition of the steel surface (comparing material from the same steel grade).
• The mechanism of high-temperature corrosion is a very complex phenomenon, depending on both the thermal and chemical conditions prevailing in the combustion area. Under normal conditions, in an oxygen atmosphere with the exhaust gases, oxide layers are formed on the steel surface, mainly based on iron, which constitute a natural passive layer of the metal and a barrier to other gaseous components of the exhaust gases. However, if this phenomenon is disturbed, in addition to the actual oxide layer, thick layers of sediment also grow, which have an unfavorable effect. In the case of heated surfaces, this leads to an increase in the temperature of the material and thus an increase in the rate of high-temperature corrosion. The test results obtained in this work showed significant destruction of the material used at the following parameters: T = 540 °C and a time of 220,000 h. Undesirable compounds in energy equipment enter device circulation or are created, among others, during assembly or construction of individual devices as well as during operation (e.g., due to leaks). The impact of contamination on the durability of these devices varies.
• In order to prevent this type of corrosion, the operating conditions of the superheater elements should be adjusted to the materials from which they are made. However, this is very difficult because one contamination may make it impossible to meet the operating conditions. That is why it is so important to diagnose these devices in order to avoid their failure in the future.