Investigation of Corrosion Characteristics of High-Sodium High

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Cite This: Energy Fuels XXXX, XXX, XXX−XXX

Investigation of Corrosion Characteristics of High-Sodium HighChlorine Lignite during Circulating Fluidized Bed Combustion Xiaobin Qi,†,‡ Guoliang Song,*,†,‡ Shaobo Yang,†,‡ Zhao Yang,†,‡ and Qinggang Lyu†,‡ †

Institute of Engineering Thermophysics, Chinese Academy of Sciences, Beijing 100190, China University of Chinese Academy of Sciences, Beijing 100049, China

‡

ABSTRACT: Corrosion experiments were conducted in a 0.4 T/D circulating fluidized bed (CFB) test rig with the high-sodium high-chlorine lignite as fuel to investigate the corrosion phenomena and mechanisms through the air-cooled probes in furnace and tails, respectively. Three alloy materials involving four metal elements (Al, Cr, Ni, and Fe) were used to compare their anticorrosion characteristics with the thermodynamic equilibrium calculation software Factsage 6.1 as an auxiliary tool. Experimental results indicate that the probes exposed to the high-temperature flue gas in the furnace suffered severe corrosion, which was primarily caused by gaseous corrosive media, including HCl (g), NaCl (g), and Cl2 (g), while the probes in tails underwent the almost negligible corrosion arising from the deposited NaCl crystal on the top surface of the probe. The protective oxidation scale formed on the outer surface of alloy was the key to corrosion resistance rather than the internal metal with high quality. Despite different alloy elements matching different corrosive environments for its optimal anticorrosion, the Al2O3 was found most effective to resist the chlorine-induced corrosion in this study. In accordance with the Gibbs free-energy change (Delta G) of corrosion reactions, the increasing temperature failed to reduce the difficulty of corrosion in essence. The accelerated corrosion at a higher temperature was mainly ascribed to the improvement of the diffusion of the gaseous corrosive species and gaseous reaction products (metal chlorides). At a low-level wall temperature, to a certain extent the influence of gas temperature and alloy materials on corrosion can be ignored.

1. INTRODUCTION Corrosion is a very common ash-related issue during fuel (such as coal, biomass, and hazardous waste) processing.1−3 In boilers, the corrosion will result in the heat absorption deviation of metal heating surfaces. In severe cases, the leak and explosion of heating tubes might be caused due to the local high temperature.4 This not only badly affects the safe and stable operation of boiler units but also dramatically increases the overhaul workload and maintenance cost. For circulating fluidized bed (CFB), more corrosion problems have to be faced because of its better adaptability to different fuels. Furthermore, larger-scale CFB boilers with more capacity have been developed in recent years to improve the utilization efficiency of fuels,5,6 which indicates heating surfaces have to undergo the high-temperature corrosion. For almost all biomass and hazardous waste, corrosion is inevitable because they contain a lot of corrosive ingredients. Therefore, worldwide, an enormous amount of research effort goes into the corrosion caused by biomass and hazardous waste. During biomass combustion, the chlorine including Cl2 (g), HCl (g), and alkali chlorides (mainly referring to KCl) in the gaseous, aerosol, molten, and solid state, is responsible for this problem.7−9 This corrosion happens with the constant oxidation of the alloy element, so it is called active oxidation corrosion.10 Furthermore, due to the regeneration of aggressive medium during corrosion, the chlorine-induced corrosion has the feature of repeatability.9 Hence, the schemes of biomass combustion were closely scrutinized, and more and more schemes were proposed and used through cofiring with coal.11−13 Since wastes usually contain large amounts of destructive alkalis (sodium and potassium),14 in the waste incinerator, more © XXXX American Chemical Society

corrosive species will be generated at high temperature, in addition to the chlorine-based aggressive species mentioned above. In many cases, the damage of alloy in the incinerator is caused by the hot corrosion due to the low melting-point fused salts (sulfate and chloride salts) in tube deposits.15,16 The molten sulfate and chloride salts containing heavy metals such as lead and zinc in deposits would further enhance the rate of corrosion.17 Another terrible corrosive medium is HF (g) in the flue gas because of its high corrosivity to silica-containing refractory.18 Therefore, the superalloy treated with special coating is necessary to resist the corrosion of heating surfaces during waste incineration.19 However, the corrosion caused by coal is weaker by comparison. The corrosion in coal combustion frequently involves oxygen, sulfur, and carbon.20−23 For the coal-fired boilers, corrosion often occurs in the high-temperature regions such as the water-cooled wall and the low-temperature regions such as the air preheater and economizer. The high-temperature corrosion on the water-cooled wall was widely reported,24 and it was ascribed to the nearby reducing atmosphere when the lowNOx emission technology (staged combustion technology and low oxygen combustion method) were used.25 The reducing environment resulted in the presence of gaseous H2S and FeS around these walls, which could penetrate the dense iron-bearing layer on the alloy surface and cause this type corrosion.26,27 In the low-temperature regions, apart from the corrosion mechanisms mentioned above, the dew point corrosion plays a significant Received: October 25, 2017 Revised: November 18, 2017 Published: November 19, 2017 A

DOI: 10.1021/acs.energyfuels.7b03288 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels role.28,29 At the temperature lower than the dew point temperature of sulfuric acid, the sulfuric acid vapor generated via the combination between gaseous SO3 and steam will condense and erode the alloy. Therefore, the cold end of air preheater often undergoes the dew point corrosion.30 Recently, with the development of the oxy-fuel combustion, the corrosion has become a common concern. As we know, the combustion mode can increase the CO2 concentration in the flue gas by constantly recycling the flue gas in order to readily capture CO2 and reduce its emissions. However, besides the high-level CO2 derived from the flue gas, the trivial but corrosive species (SO2 and HCl) are continually accumulated in furnace.31,32 Thus, compared to the conventional air combustion, an accelerated corrosion occurs in the oxy-fuel combustion mode. Recent research found the protective Cr2O3 layer can be formed under the air-firing mode, while it is easily damaged by sulfates/sulfide in deposits under the oxy-firing mode.33 When exposed to the CO2 atmosphere, the alloy would undergo the oxidation via the reaction CO2 + M = CO + MO, where M stands for Fe or Cr.34 This might indicate that ferrous iron oxides would be preferentially formed in the oxy-fuel combustion rather than the protective ferric iron oxides. Although extensive research about corrosion have been performed, to date, few studies have been reported on the high-chlorine coal. As the lignite in Xinjiang, China, with a very large reserve, was found and exploited, more and more attention was paid to the severe slagging and fouling characteristics caused by the high-level sodium in coal.35−37 Additionally, for the special coal mined from the Shaerhu district, its high-chlorine highsodium feature implies corrosion is another sensitive issue during combustion, besides slagging and fouling. Due to the extremely different coal property from other fuels, a different corrosion behavior might be present for this special coal. At present, further work about this type of corrosion is needed because it has not received adequate attention. In this study, a high-chlorine high-sodium lignite was used as fuel to investigate the corrosion characteristics in a 0.4 T/D CFB test rig, with the thermodynamic equilibrium calculation as an auxiliary tool, which is our subsequent research about this special coal.38−43 The air-cooled temperature-controlled probe was used to simulate the metal heating surfaces in actual boilers. Two corrosion situations were tested in the high-temperature furnace and middle-low-temperature tails, respectively, imitating the high-temperature and middle-low-temperature heating surfaces. To determine the appropriate anticorrosion alloy, three kinds of alloy (06Cr25Ni20, 06Cr19Ni10, and aluminum-coated 06Cr19Ni10) were compared. Additionally, the influence of temperature on corrosion was discussed.

Table 1. Properties of SEHc proximate analysis (wt %, air-dry basis) fixed carbon volatile ash water LHV (MJ/kg) ultimate analysis (wt %, dry basis) C H O N S Cl

43.86 30.46 14.66 11.02 17.93

57.92 2.65 22.17 0.65 0.12 1.279 Ash compositions (wt %)

SiO2 Al2O3 Fe2O3 CaO MgO TiO2 SO3 P2O3 K2O Na2O

41.98 17.59 6.76 19.39 2.49 1.08 1.82 0.18 0.66 4.38

Before each experiment, SEHc was air-dried, crushed, and screened in the particle-size range of 0.1−1.0 mm. The quartz sand also underwent the same preparation, and the final particle-size range is 0.18−0.71 mm. 2.2. Test System and Corrosion Probes. Combustion experiments of SEHc were carried out in a bench-scale self-heated CFB test rig, and the schematic diagram is illustrated in Figure 1. The CFB is composed of a riser with the internal diameter of 150 mm and height of 4.4 m, a cyclone and a loop seal. More detailed description about this test system was introduced in previous references.38−40,42 Anticorrosion characteristics of three types of air-cooled probe consisting of 06Cr25Ni20, 06Cr19Ni10 and aluminum-coated 06Cr19Ni10, respectively, were tested in positions A, B, and C along the flue gas direction (see Figure 1). 06Cr25Ni20 can be used for the components against corrosion and high temperatures because of its excellent corrosion resistance and mechanical properties at high temperatures. 06Cr19Ni10 is also famous for its anticorrosion. The compositions of the probes are shown in Table 3. In each position, the three probes with the same structure were distributed evenly on the cross section. As depicted in Figure 2, all probes were uniformly designed except for the length of probe in different positions. One thermocouple was embedded in the top wall of the probe to measure its wall temperature. In this paper, the probes were marked as X (Y), X represents the material of probe, such as 06Cr25Ni20, 06Cr19Ni10, and 06Cr19Ni10−Al, and Y represents the position of the corresponding probe such as A, B, and C. For example, the probe composed of aluminum-coated 06Cr19Ni10 in position A is expressed by 06Cr19Ni10−Al (A). 2.3. Preparation of the Aluminum Coating. The method of thermochemical reaction was used to prepare the aluminum coating; the main procedures were as follows. The slurry used was prepared by superfine aluminum power (200 mesh) and ultrapure water, and its property met the requirements of this test. Higher quality slurry can be gained by mixing aggregate (superfine aluminum power) and agglomerant (the water solution of phosphates).44 Meanwhile, the metallic matrix was pretreated by sanding and cleaning. The prepared slurry was uniformly brushed on the matrix. Then, the coated probe was dried in the shade at room temperature until the solidification of slurry. After that, the probe continued to undergo the high-temperature curing in a temperature-programmed muffle furnace. The designed temperature programmer proceeded as below: (1) room temperature ∼120 °C

2. EXPERIMENTAL SECTION 2.1. Fuel and Bed Material. A high-sodium high-chlorine coal mined from Shaerhu district was used as experimental coal, which is named as SEHc. As shown in Table 1, the chlorine content in coal is up to 1.279%, and the Na2O content in coal ash is 4.38%. The sulfur content in coal is only 0.12%, which indicates it belongs to the low-sulfur coal. Furthermore, other alkali and alkaline earth metals (AAEMs) and iron in SEHc ash cannot be ignored, such as the 0.66% K2O, 2.49% MgO, 19.39% CaO, and 6.76% Fe2O3. Significantly, the refractory species (SiO2 and Al2O3) contents are relatively higher compared to AAEMs, accounting for 41.98% and 17.59%, respectively, which might relieve the ash-related issues caused by the reaction between coal ash and bed material.41 Hence, the SiO2-based quartz sand, of which chemical compositions are shown in Table 2, was selected as the bed material for this special coal. B


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Energy & Fuels Table 2. Chemical Composition of the Used Bed Material component

CaO

SO3

Na2O

MgO

Al2O3

SiO2

Fe2O3

TiO2

K2O

content %

0.02

−

0.22

−

1.74

95.64

0.13

0.09

0.53

Figure 1. Schematic diagram of the CFB test system.

Table 3. Compositions of Probes Tested in This Study chemical compositions (%) metal specimen

C

Si

Mn

P

S

Ni

Cr

06Cr19Ni10 06Cr25Ni20

≤0.08 ≤0.25

≤0.75 ≤1.5

≤2 ≤2

≤0.045 ≤0.045

≤0.03 ≤0.03

8.0−10.5 19−22

18−20 24−26

Figure 2. Arrangement of corrosion probes and schematic diagram. at 2 °C/min, (2) last for 1 h at 120 °C, (3) 120−700 °C at 5 °C/min, (4) last for 2 h at 700 °C, and (5) natural cooling. Actually, before the final preparation solution, the powders of aluminum hydroxide and aluminum oxide were also tried as aggregate, but the effect was not satisfactory. 2.4. Test Conditions. Under the steady working condition, the highest temperature along the riser (usually T5 or T6) was controlled at 950 (±10) °C. In accordance with the statistical data, the average coal

feed rate was 7.35 kg/h and the experimental air was 48.85 m3/h. The fluidization air velocity was about 3.13 m/s in the riser, and the cooling air flow rate was set at 3.0 m3/h for each probe. The actual excess air coefficient was 1.207, which was very close to the designed value, 1.2. The steady condition lasted for 8 h. 2.5. Analysis and Calculation. Ashes collected from positions P1, P3, and P4, which were named as bottom ash, circulating ash, and fly ash in this study, respectively, were characterized by an X-ray fluorescence C


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Energy & Fuels spectrometer (XRF, XRF-1800, Japan) to obtain their chemical compositions. The corrosion probes were tested by a thermal field emission scanning electron microscope with an energy dispersive spectrometer (TFSEM/EDS, JSM-7001F/INCA X-MAX, Japan/ England) to obtain the corrosion behaviors of metallic matrix by observing the microstructure of the cross section as well as its element mapping. Before examination, a series of preparations should be done for the probes in turn, such as slicing, solidification by epoxy resin, sanding and polishing, and coating with Pt. Apart from the above characterization methods, the thermodynamic equilibrium calculation software Factsage 6.1 was also used as an important aid to provide more information on the release of sodium/ chlorine in SEHc and corrosion reactions during combustion.

the aluminum coating or not), thermal conductivity (06Cr19Ni10 or 06Cr25Ni20), and arrangements (horizontal or vertical). 3.2. High-Temperature Corrosion in Furnace. The microstructure of the cross-section of the probe 06Cr19Ni10 (A) and its element mapping are exhibited in Figure 5. It is clearly observed that a spalled layer was found in the interface between deposited ash and alloy matrix. Element mapping results imply that the spalled layer is dominated by alloy elements (Fe and Cr), Na, Cl, and O. Such a phenomenon has been reported and considered as the sign of corrosion.34 It is certain that the three elements (Na, Cl, and O) were responsible for the corrosion as they were concentrated in the spalled layer. The EDS results (Table 4) reveal that abundant Ca, O, Cl, S, and Na were located in the deposited ash. Interestingly, both Na and Cl diffused inward, O concentrated in the interface, whereas Fe, Cr, and Ni performed the trend of diffusing outwardly. The enrichment of O in the interface indicates the oxidation of metallic element occurred. The oxidation was achieved via the reactions M+Cl2 → MCl2 and 3MCl2+2O2 → M3O4+3Cl2, where M stands for Fe and Cr in this paper. As this oxidation was accompanied by chlorine-induced corrosion, it was called active oxidation corrosion.10 The formed metallic chlorides migrated outwardly and would react with O2 that permeated inward, resulting in the retention of metallic elements in forms of oxides. This process finally led to the sign of the outward diffusion of Fe, Cr, and Ni. Actually, the dense oxidation layer initially formed under the action of O2 played a protective role on the alloy surface,21 but it is certain that the layer was destroyed under this corrosive environment. The regenerated oxidation layer was porous and failed to prevent the permeation of corrosive media. Obviously, the high concentration of Na and Cl in the interface indicate that the corrosive species also included NaCl besides Cl2. However, the very low content of Cl in ash samples shown in Figure 6 reveals that the corrosive Cl-bearing species preferentially stemmed from the flue gas rather than the solid ash particles. Since the present study failed to determine gaseous Cl/Na-bearing species, the combustion of SEHc was calculated for a reference to determine the actual Na/Cl-bearing species and their amounts in flue gas. The computing results given in Figure 7 confirm a big difference in the releasing behavior between Na and Cl, although they might exist in raw coal in the form of NaCl.46 For example, at 950 °C, almost all Cl was released in forms of gaseous HCl and NaCl, while the release rate of Na was 24.2% and the NaCl vapor was the only releasing form. To some degree, the computing results agree with the test results shown in Figure 6. The different releasing behaviors between Na and Cl were ascribed to the reactions between NaCl and minerals in coal ash (SiO2 and Al2O3) at high temperature, which generated solid sodium silicates or aluminosilicates and gaseous HCl.47,48 Furthermore, the excess O2 could oxide HCl into Cl2 via 4HCl(g) + O2(g) → 2H2O(g) + 2Cl2(g),9 resulting in more Cl2 in the high-temperature flue gas. It can be deduced, therefore, that the corrosive species present in the deposited ash were indeed derived from gaseous species via physical condensation or chemical reactions. Besides, at the high gas temperature, the formation of liquid/molten phases contributed to the metal wastage.22 Overall, the corrosion reactions in the furnace caused by NaCl and HCl can be summarized by the following reactions R1−R7.7−9,49

3. RESULTS AND DISCUSSION 3.1. Temperature Conditions around the Probes. The flue gas temperature around the corresponding probe as a function of time is illustrated in Figure 3. As we can see, under the steady condition, the three probes in positions A, B, and C have to suffer the gas temperatures of 940, 850, and 650 °C for 8 h, respectively.

Figure 3. Variation of flue gas temperature in positions A, B, and C with time.

Figure 4 gives the wall temperature of the probe as a function of time. It is observed that the wall temperatures of the probes in position A slightly increased with time, while the wall temperatures reduced for those in tails (positions B and C). This variation of wall temperature can be ascribed to the ashrelated issues (ash deposition and corrosion) occurring on the probes. For the probes in position A, the high-temperature corrosion was responsible for the increasing wall temperature, which will be discussed later. This increasing trend with time was considered as the foretaste of the local high temperature, even the leak and explosion of heating tubes.4 However, the decreasing wall temperatures in positions B and C were primarily ascribed to the ash deposition. In our previous study, severe ash deposition easily occurred in these regions.39 The ash deposition would increase the thermal resistance and reduce the thermal conductivity.45 Additionally, under the action of cooling air, a great drop in the wall temperatures was observed compared to the gas temperatures, such as the wall temperature range of 500−600 °C for the position A, 180−250 °C for the position B, and below 200 °C for the position C. It is noted that the three wall temperatures in position A followed the order of 06Cr19Ni10 > 06Cr19Ni10−Al > 06Cr25Ni20 (from high to low) whereas 06Cr25Ni20 > 06Cr19Ni10 > 06Cr25Ni20−Al for position B. The difference might be caused by their different surface treatments (spraying

6HCl(g) + M 2O3 → 2MCl3 + 3H 2O D

(R1)


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Figure 4. Variation of wall temperature of the probes with time.

Figure 5. Microstructure of the cross-section of the probe 06Cr19Ni10 (A) and its element mapping.

4NaCl(g) + Cr2O3 + 2.5O2 → 2Na 2CrO4 + 2Cl 2 or 2NaCl(g) + Fe2O3 + 0.5O2 → 2NaFeO2 + Cl 2 (R2)

6HCl(g) + 2M + 1.5O2 → 2MCl3 + 3H 2O

(R3)

2NaCl(g) + 2M + 2O2 → Na 2M 2O4 + Cl 2

(R4)

Cl 2 + M → MCl 2

(R5)

3MCl 2 + 2O2 → M3O4 + 3Cl 2

(R6)

3.3. Deposited-Ash-Induced Corrosion in Tails. Clearly, the interface between deposited ash and alloy matrix as illustrated in Figure 8 was smooth and completed, implying the absence of obvious defects and cracks on the surface of the probe. E


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Energy & Fuels Table 4. Element Distribution on the Cross Section Shown in Figure 5 element content (%) zone

Na

O

Al

Si

S

Cl

K

Ca

Cr

Fe

Ni

1 2 3 4

3.14 0.7 3.11 0.82

20.88 18.39 28.12 27.51

3.1 0.71 0 0.35

4.77 4.54 1.29 1.41

5.6 0 0.7 0.53

10.32 1.89 7.78 0.3

0.34 0.22 0.05 0.6

27.93 1.79 1.33 0.73

0.48 4.32 15.42 16.24

4.12 19.67 37.57 43.57

0.76 1.76 4.54 6.61

film was not destroyed, although the gas temperature around the probe was still high, ∼850 °C. It is believed that the low-level wall temperature (170−250 °C) was responsible for it, which was also proved elsewhere.35 A new picture emerged when zone 4 in Figure 8 was magnified by 2000 times, just as depicted in Figure 9. In the amplified SEM picture, one gap could be observed between the deposited ash and the top surface of the probe. Irregular crystal particles adhered onto the surface of the probe were rich in Na and Cl via EDS mappings, which indicates that NaCl was the main occurrence mode. Undoubtedly the NaCl crystal started to deposit on the probe surface at the initial deposition stage via coagulation. It can be inferred that corrosion still happened in view of the high concentration of Fe, Cr, and Ni found on the side of deposited ash, although it was weak. Apart from the above corrosion mechanisms (R1∼R6), the corrosion caused by the coagulated NaCl crystal (R7 and R8) was dominant.8,49

Figure 6. Chemical compositions of bottom ash, circulating ash, and fly ash.

4NaCl(s) + Cr2O3 + 2.5O2 → 2Na 2CrO4 + 2Cl 2 or 2NaCl(s) + Fe2O3 + 0.5O2 → 2NaFeO2 + Cl 2 (R7)

2NaCl(s) + Cr + 2O2 → 2Na 2CrO4 + Cl 2

(R8)

The generated Cl2 via R7 and R8 to some degree enhanced the active oxidation corrosion, resulting in the presence of volatile chlorides in the interface. The outward diffusion of chlorides led to the formation of the gap but which was limited by the low wall temperature and hardly further expanded. 3.4. Anticorrosion Characteristics of Different Metals. The corrosion process was usually accompanied by oxidation, and O2, Cl2, and CO2 acted as oxidizer at high temperatures.9,34 As mentioned before, a protective oxidation film can be formed on the surface of alloy under the action of O2, reducing the wastage of alloy. The oxidation caused by Cl2 will result in the formation of porous oxidation scale due to the generation of volatile chlorides, which accelerates further corrosion. When CO2 is used as the oxidizer (3CO2 + 4M → 2M2O3 + 3C, or

Figure 7. Main gaseous phases containing sodium and chlorine and the release rate of sodium and chlorine during SEHc combustion.

Furthermore, no distinct regularity of element distribution was found on the cross section by EDS. Compared to the corrosion in the furnace (position A), the extent of corrosion in the tails (position B) seems weaker, or at least the protective oxidation

Figure 8. Microstructure of the cross section of the probe 06Cr19Ni10 (B) and element distribution by EDS. F


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Figure 9. Microstructure of the zone 4 in Figure 8 and its element mapping.

3CO2 + 2M → M2O3 + 3CO), the gaseous product (CO) and carburization lead to a poor corrosion resistant of the formed oxidation scale. Take the oxidation of Al at 600 °C for example, the Gibbs free-energy change (Delta G) of the reaction between O2 and Al (1.5O2 + 2Al → Al2O3) is −1402.2 kJ/mol. The Delta G becomes −1082.6 kJ/mol when the reaction 3Cl2 + 2Al → 2AlCl3 happens. For the CO2 case (3CO2 + 4M → 2M2O3 + 3C and 3CO2 + 2M → M2O3 + 3CO), the Delta G is −808.5 kJ/mol and −781.7 kJ/mol, respectively. On the basis of the minimum principle of Gibbs free energy, generally, under the same conditions, the reaction with a lower Delta G is inclined to occur. Hence, it can be inferred that the reaction priority of three oxidants is O2 > Cl2 > CO2. This also implies that a protective oxidation scale caused by O2 was initially dominant during SEHc combustion. For different alloy materials, their priorities in oxidation reactions certainly differ.33 In this study, the oxidation reactions between O2 and alloy elements were calculated in the system of Al−Cr−Fe−Ni-O2 with different O2 amounts in order to determine the most possible alloy oxides in the protective oxidation scale. In this system, Al, Cr, Fe, and Ni were set at 1 mol, whereas the O2 amount varied from 0 to 3.2 mol. The computing results are present in Figure 10.

It is found that ten stages took place as the O2 amount increased: (I) Al is oxidized, 2Al + 1.5O2 → Al2O3; (II) Cr is oxidized, 2Cr + 1.5O2 → Cr2O3; (III) Fe is oxidized to ferrous Fe and restructured with Cr2O3, 2Fe + 2Cr2O3 + O2→ 2FeCr2O4; (IV) Fe is sequentially oxidized to ferrous Fe and restructured with Al2O3, 2Fe + 2Al2O3+O2 → 2FeAl2O4; (V) ferrous Fe in the form of FeAl2O4 and Ni are oxidized and restructured inside the system, 3Ni + 3FeAl2O4 + 2O2→ 3(NiO)(Al2O3) + Fe3O4; (VI) ferrous Fe in the form of Fe3O4 and Ni are oxidized and restructured inside the system, 2Fe3O4 + 3Ni + 2O2 → 3(NiO)(Fe2O3); (VII) Ni is sequentially oxidized, 2Ni + O2 → 2NiO; (VIII) ferrous Fe in the form of FeCr2O4 is oxidized and restructured with NiO, NiO + 2FeCr2O4 + 0.5O2 →2Cr2O3 + (NiO)(Fe2O3); (IX) ferrous Fe in the form of FeCr2O4 is oxidized and restructured inside the system, 2FeCr2O4 + (NiO)(Cr2O3) + 0.5O2 → 3Cr2O3 + (NiO)(Fe2O3); (X) no change happens with the further increasing O2 amount. Overall, the above analysis indicates the priority of four alloy elements oxidized by O2 and obeys the order of Al > Cr > Fe (from Fe to ferrous Fe) > Ni > Fe (from ferrous Fe to ferric Fe). Therefore, the formation sequence of oxides in the protective oxidation scale is Al2O3, Cr2O3, FeO, NiO, and Fe2O3, and this sequence is found almost unaffected by temperature. G


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Similarly, the priority of the four alloy elements oxidized by Cl2 and CO2 are Al > Cr (from Cr to Cr2+) >Fe (from Fe to Fe2+) > Ni > Cr (from Cr2+ to Cr3+) > Fe (from Fe2+ to Fe3+), and Al > Cr > Fe (only from Fe to Fe2+), respectively. It should be noted that CO2 cannot oxidize Ni because of its weak oxidizability, so the Ni-containing alloy can be used as the heating tubes to resist the CO2 corrosion during the oxy-fuel combustion. Figure 11 confirms the different corrosion resistances among 06Cr25Ni20, 06Cr19Ni10, and aluminum-coated 06Cr19Ni10. As we can observe, the top surface of the probe 06Cr19Ni10 was spalled off, and for the probe 06Cr25Ni20, the surface erosion existed rather than the obvious spalled layer, while the top surface of the probe 06Cr19Ni10−Al still kept completed under the protection of the aluminum-sprayed coating. This signifies that the sequence of their corrosion resistances from high to low is aluminum-coated 06Cr19Ni10, 06Cr25Ni20 and 06Cr19Ni10. The results of Figure 12 that abundant Al and O were filled in the aluminum-sprayed coating suggest that an Al2O3 layer was formed on the top surface of 06Cr19Ni10 during the high temperature curing or SEHc combustion. Clearly, the Al2O3 layer prevented the alloy 06Cr19Ni10 from corrosion, although it was not very dense due to the simple spraying technology. Since 06Cr25Ni20 contains more Cr and Ni than 06Cr19Ni10 (Table 3), it is natural for 06Cr25Ni20 to have a stronger corrosion resistance than 06Cr19Ni10.50,51 However, the previous discussion indicates that Al/Cr/Ni is more easily corroded by corrosive gases (Cl2 and CO2). This means a protective oxidation scale has to be formed before corrosion, otherwise the anticorrosion of the three used alloys here follow the complete reverse order. Besides, combining experimental and computing results, it can be deduced that the corrosion resistance of Al2O3 occupies first place, Cr2O3/NiO comes second, and Fe2O3 is the worst. To prove it, corrosion reactions involved in this study were summarized in Figure 13, and the Delta G at 950 °C was also given in this figure for comparison. In this study, the reactions resulting in the corrosion of oxidation scale mainly refer to the corrosion of FeO, Fe2O3, Al2O3, Cr2O3, and NiO caused by HCl, and that of Cr2O3 caused by NaCl, which correspond to the reactions R9 and R19∼R23 in

Figure 10. Phase species in the system of Al−Cr−Fe−Ni-O2 with different O2 amounts.

Figure 11. Microstructure of the cross section of the probes in position A. H


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Figure 12. Microstructure of the cross section of the probe 06Cr19Ni10−Al (A) and its element mapping.

happens (i.e., the corrosion priorities are basically contrary to those of their corresponding oxides). This means a high quality is not necessary for the internal metal, and the key to anticorrosion is the outer alloy material. Hence, the alloy material should be selected carefully as the heating tubes in the SEHc-fired boiler. The ordinary alloy material sprayed with the anticorrosion Al2O3 coating seems the optimal case. 3.5. Influence of Temperature. Figures 14 and 15 illustrate the microstructure of the cross section of the probes in positions B and C. Compared to the case in position A, it can be judged by the smooth degree of the interface that the probes in positions B and C were almost free from corrosion. Apart from the gas−solid flow difference, the biggest difference between the two cases (in the furnace and the tails, respectively) was the temperature around the probes, which was also considered as the main reason for the big difference in corrosion degree in this study. Moreover, extensive investigations in lab-scale test equipment that successfully eliminated the interference of gas−solid flow difference also confirmed the great impact of temperature on corrosion.7,9,53 Thus, the difference mainly refers to those in the gas and wall temperature. However, for the two tail cases (in the position B and C), it can be deduced that the impact of temperature (referring to the gas temperature) could be ignored when the probes suffered a low wall temperature. Interestingly, the low wall temperature also weakened the influence of alloy species on corrosion, resulting in little difference in their microstructure. Figure 15 presents the Delta G of corrosion reactions involved in this study as a function of temperature. Unexpectedly, almost all Delta G of corrosion reactions increases with the rise of temperature, implying the steadily weakened corrosion with the increasing temperature. This is patently against the experimental results (especially Figure 14). As we know, the increasing temperature is bound to lead to a bigger difference between gas and solid phases and increase the difficulty of heterogeneous corrosion reactions. Therefore, one possibility is that a high temperature can accelerate the corrosion in other ways rather than directly reducing the Delta G. A high temperature can result in the increased movement of the gas molecules, and the molecular diffusion is just dependent on the random motion of the molecules. Hence, one reason is that the accelerated corrosion with increasing temperature results from the improvement of the diffusion of the gas reactants (Cl2 and O2) and gaseous reaction products (metal chlorides).7,54 The formation of the spalled layer, as present in Figure 5, might be ascribed to the intense diffusion of gaseous reaction products. Besides, the liquid/molten corrosive species play a role of electrolyte and accelerate the corrosion of alloy via assisting

Figure 13. Delta G of corrosion reactions at 950 °C.

Figure 13. In accordance with Delta G, all corrosion reactions of the oxidation scale can not occur spontaneously because of the Delta G above zero. Compared to other corrosion reactions (R10∼R18), the protective effect of the oxidation scale on the alloy is further identified as a result of their far higher Delta G than the others. Furthermore, iron oxides (FeO and Fe2O3) are easier to be corroded by HCl than other oxides (NiO, Cr2O3, and Al2O3). Despite little difference among NiO, Cr2O3, and Al2O3 in Delta G, the three oxides still obey the corrosion priority of NiO > Cr2O3 > Al2O3 (from easy to hard), which is consistent with the experimental findings. However, no oxides can be corroded by NaCl except for Cr2O3, which has also been reported elsewhere.8 This indicates that a high-Cr alloy may be corroded in the NaClrich atmosphere, although its excellent corrosion resistance was verified.9,52 Once the protective oxidation scale is destroyed and the new one is not formed in time, the internal metallic matrix will suffer the ruinous corrosion because the corrosion of metallic elements (R10∼R18) caused by Cl2, HCl, and NaCl very easily I


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Figure 14. Microstructure of cross section of the probes in positions B and C.

charge transfer.2 For the case in position A, the wall temperatures were in the range of 500−600 °C, which suggests massive corrosive species likely existed in liquid/molten state, such as NaCl (melting point is 801 °C), NaCl-Na2CrO4 (557 °C), and NaCl-Na2Cr2O7 (592 °C). If the gas or wall temperature is lower than the dew point temperature of gaseous corrosive species, the corrosion behavior of the probes in positions B and C will be different due to the dew point corrosion. The dew point temperature of HCl in flue gas can be conservatively considered as about 100 °C, which is inferred from the dew point temperature of water vapor. Hence, it is believed that the alloy was free of the dew point corrosion.

However, when the high-S coal is used as fuel rather than the high-Cl SEHc, the probes will suffer the dew point corrosion due to their low wall temperatures (150−250 °C) and the higher dew point temperature of SO3 and water vapor.

4. CONCLUSIONS In this paper, the corrosion experiments were conducted on a 0.4 T/D CFB test rig with the high-sodium high-chlorine lignite as fuel to investigate the phenomena and mechanisms through the air-cooled probes in furnace and tails, respectively. The main conclusions are listed below. (1) In the furnace, the probe composed of 06Cr19Ni10, after the exposure to the gas J


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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86-010-82543129. E-mail: [email protected]. ORCID

Guoliang Song: 0000-0002-7770-0748 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Key Research & Development Program of China, Grant 2016YFB0600202.

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REFERENCES

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Figure 15. Influence of temperature on Delta G of corrosion reactions.

temperature of ∼940 °C and wall temperature of ∼525 °C for 8 h suffered obvious corrosion. This corrosion was mainly caused by the gaseous chlorine-bearing species released from the raw coal at high temperature, including HCl (g), NaCl (g), and Cl2 (g). However, the similar probe was free of corrosion due to its low wall temperature (200−250 °C), except for the slight corrosion caused by the deposited NaCl crystal in the interface between the alloy and the deposited ash. (2) The protective oxidation scale caused by O2 was formed on the top surface of alloy before corrosion. The priority of four alloy elements (Al, Cr, Ni, and Fe) oxidized by O2 was obtained via calculation, and they obeyed the order of Al > Cr > Fe (from Fe to ferrous Fe) > Ni > Fe (from ferrous Fe to ferric Fe). The oxidation scale was the key to anticorrosion because their corrosion reactions with positive Delta G can not occur spontaneously. The computing results suggest that the ability of metallic oxides to resist corrosion follows the order of Al2O3 > Cr2O3 > NiO > Fe2O3 > FeO. Once the oxidation scale is destroyed, the internal metallic matrix will suffer the ruinous corrosion. This means a high quality is not necessary for the internal metal, and the key to anticorrosion is the outer alloy material. In these experiments, the ordinary alloy material sprayed with the anticorrosion Al2O3 coating seems the optimal case. (3) At a low-level wall temperature, the influence of gas temperature and alloy materials on corrosion was little. Although an increasing temperature accelerated corrosion in this study, it was not achieved by reducing the difficulty of corrosion in essence but improving the diffusion of the gaseous corrosive species and gaseous reaction products (metal chlorides). Besides, the dew point corrosion should be paid attention at low wall/gas temperature. K


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Investigation of Corrosion Characteristics of High-Sodium High

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