High-temperature corrosion behavior of S30432 in high-efficiency ultra-supercritical boiler burning low-alkali and high-sulfur coal

1 Introduction

The global electricity consumption in 2021 increased by 6 % and coal-powered electricity production reached 10, 350 TWH accounting for 31 % of the total energy consumption (China Electricity Council 2021). Moreover, the total electricity consumption and coal-powered electricity production in China increased by 10.3 % and 9 %, respectively (National Energy Administration 2021). Although the installed capacity of coal-powered power plants accounts for less than 50 % of the total power plant capacity in China, the power generation exceeds 60 % of the total power generation, which even reaches 75 % of the total power consumption at peak hours (China Federation of Electric Power Enterprises 2021). Considering further promotions in China’s industrialization and urbanization, the power demand will inevitably continue to grow. It is worth noting that although wind and solar power plants have developed rapidly in the past few decades for the “carbon neutrality”, these renewable power plants can not meet the rapidly increasing power demands of social development. Accordingly, coal power is still expected to play an important role as a stabilizer of China’s power supply.

Currently, carbon reduction is a central process in coal-powered plants, which is carried but mainly using high-efficiency ultra-supercritical technology, which has been significantly improved in the past two decades (Financial Association. National Development and Reform Commission 2021). Boiler, steam turbine, and generator are the major equipment of coal-fired power units. The main function of a boiler is to heat up water with the heat generated by coal combustion, making it into steam with a certain pressure and temperature. Improving the initial steam parameters of the generating unit helps to increase the efficiency of the Rankine cycle in the thermal system and significantly reduce the energy consumption (Wang and Gong 2007). The main steam pressure of the 620 °C boiler is 29.3 MPa, the main steam temperature is 605 °C, and the reheat steam temperature is 623 °C. For the conventional ultra-supercritical boiler, the main steam pressure is 26.25 MPa g, the main steam temperature is 605 °C, and the reheat steam temperature is 603 °C. Compared with conventional ultra-supercritical boilers, the higher main steam pressure and reheat steam temperature of 620 °C boiler can greatly improve the efficiency of power unit and reduces the coal consumption by about 3.5 g per kilowatt-hour. Therefore, the 620 °C boiler is also known as high-efficiency ultra-supercritical boiler. 620 °C and secondary reheat power units have been applied in batch at present. Currently, a 630 °C generating unit is under construction as the national power demonstration project in China, and 650 °C and 700 °C generating units are at the design stage (Electric Power Planning and Engineering Institute 2016; Gao et al. 2014; Guo et al. 2014; Jiang and Huang 2012; Lin et al. 2011; Mao 2013; Wang et al. 2017; Xu and Zhou 2012). Further investigations reveal that when high-sulfur coal is burnt in the boiler of a high-efficiency ultra-supercritical generating unit, sulfur-induced corrosion (SIC) occurs on the high-temperature parts and tube bursts, resulting in an unplanned shutdown of the unit. This issue is of significant importance in China due to the rich high-sulfur coal resources accounting for 33 % of the coal resources (Feng et al. 2016), the large quantities of high-sulfur coal imports, and the continuously development coal-burning units. Compared with conventional power coal, high-sulfur coal with the same calorific value is cheaper. For example, the price of high-sulfur bituminous coal in Yulin, China is about 100 yuan per ton lower than that of Shenhua bituminous coal. “National Development and Reform Commission” and “National Energy Administration of China” jointly issued the “national implementation plan for the transformation and upgrading of coal-fired power units”. Accordingly, new coal-fired power units with a coal consumption of less than 270 g per kilowatt-hour, wet cooling units with a design coal consumption of more than 285 g per kilowatt-hour, and air cooling units with a coal consumption of more than 300 g per kilowatt-hour are not allowed to be built (China National Development and Reform Commission 2021). To meet these requirements, it is necessary to use high-efficiency ultra-supercritical technology in coal-fired power units.

The boiler is mainly composed of the components such as economizer, water wall, superheater, reheater, etc. These components are assembled from steel tubes. During the operation of the boiler, the working medium (water or steam) flows inside the tubes of each component, while the flue gas flows outside the tubes. The problems of burning high-sulfur coal in a high-efficiency ultra-supercritical boiler mainly originate from high-temperature sulfur corrosion in the superheater, reheater, and water wall tube (Mao 2013). The main objective of this work was to investigate the high-temperature sulfur corrosion phenomenon in the superheater and reheater. When the coal ash carried by the flue gas adheres to the high-temperature superheater or reheater, the composite sulfates formed in the coal ash will be in molten state, which has a strong corrosive effect on the alloy. The high-temperature superheater and reheater subjected to corrosion may not meet the strength requirement, resulting in tube burst which can cause the leakage of high-temperature steam, leading to boiler shutdown. In a 300 MW sub-critical boiler that burns coal with 1.53 % sulfur content, high-temperature sulfur corrosion after 8000 h of operation reduces the radiant heating surface by 3 mm (Lang and Li 2019). Li studied a 350 MW supercritical boiler (Datang International Shizhu Power Generation Co., China) that burns coal with 3.62 % content sulfur and found that sulfur-induced corrosion mainly occurs in the coking area of the platen superheater in the upper part of the furnace (Li 2009). Mao analyzed a 600 MW supercritical boiler burning coal with 2 % sulfur content and found that tubes of the platen superheater burst frequently caused by high-temperature sulfur corrosion (Mao et al. 2019). Although remarkable developments have been achieved, sub-critical and supercritical boilers burning high-sulfur coal still have operating problems, and let alone the ultra-supercritical boilers. Studies show that the corrosion of high-temperature superheater and reheater is mainly caused by the sulfate attack (Borio et al. 1978; Cain and Nelson 1961). For some materials, the high-temperature corrosion rate mainly depends on the metal wall temperature, the chemical composition of coal ash that sticks to the wall, and the concentration of SO₂ in the chamber atmosphere (Bakker 1989; Borio and Hensel 1972). Moreover, it was found that chromium is an effective element to increase resistance to high-temperature corrosion (Blough and Stanko 1997; Kihara et al. 1984; Plumley et al. 1979; Stringer 1987; Weele et al. 1994).

Currently, considering steel strength, steam oxidation resistance, and cost, high-temperature superheaters and reheaters of high-efficiency ultra-supercritical boilers are usually made of S30432 and TP310HCbN, which are typical austenitic materials with a chromium content of 17–19 % and 24–26 %, respectively. The corrosion resistance of S30432 and TP310HCbN in high-alkali coal ash and high-sulfur atmosphere was studied in (Liu et al. 2022). This study mainly focuses on the corrosion resistance of S30432 under low-alkali coal ash and high-sulfur atmosphere. Low-alkali coal refers to the relatively lower alkali metal content in coal ash. The so-called low-alkali coal ash studied in this paper refers to the lower alkali metal content in coal ash compared to previous studies (Liu et al. 2022). Our published work (Liu et al. 2022) has shown that S30432 has good corrosion resistance to high-alkali coal ash at temperatures not exceeding 650 °C, while its corrosion resistance to high-alkali coal ash is poor at temperatures greater than 650 °C. This is mainly due to the formation of more composite sulfates in high-alkali coal ash. The composite sulfates are almost in molten state at the temperatures higher than 650 °C, which causing more severe corrosion to the metal.

Most investigations on the high-temperature corrosion resistance of these two materials have mainly focused on high alkali coal ash, short-term laboratory tests, and field tests (Blough and Stanko 1997; Kihara et al. 1984; Plumley et al. 1979; Stringer 1987; Weele et al. 1994). More specifically, the long-term test of low-alkali coal ash has not been carried out yet. Compared with a sub-critical and supercritical boiler, a high-efficiency ultra-supercritical boiler has higher quality steam and higher metal temperature at the heating surface, resulting in aggravated high-temperature sulfur corrosion. Liu et al. (2022) studied the high-temperature sulfur corrosion resistance of these materials. However, the research mainly focused on the high-alkali coal ash attack and proposed an effective material selection method for high-temperature superheaters and reheaters of high-efficiency ultra-supercritical boilers burning high sulfur coal. More specifically, S30432 and TP310HCbN were selected for the heating surfaces with metal wall temperature less than 650 °C and higher than 650 °C, respectively. Since the price of TP310HCbN is about twice that of S30432, it unavoidably increases the capital investment of boilers. There is high demand in China for high-efficiency ultra-supercritical boilers capable of burning high-sulfur and low-alkali coal. In terms of strength, S30432 could have been used in high-temperature superheaters and reheaters.

Using more S30432 with lower cost in boilers burning high-sulfur and low-alkali coal can greatly reduce the boiler costs. The main objective of the this work was to study the high-temperature sulfur corrosion resistance of S30432 in high-efficiency ultra-supercritical boilers burning high-sulfur and low-alkali coal. The obtained results were compared with that of high-alkali coal ash corrosion (Liu et al. 2022). Moreover, the on-site measured data in a 700 MW high-efficiency ultra-supercritical boiler at 620 °C is used to verify the results. This study is expected to provide a guidance for the material selection of high-temperature superheaters and reheaters in high-efficiency ultra-supercritical boilers burning high-sulfur and low-alkali coal.

2 Materials and methods

3 Results

3.1 Corrosion kinetic curves

The corrosion kinetic curves of S30432 specimens coated with low-alkali coal ash are shown in Figure 1. It is observed that the weight of S30432 specimens slightly increase during the experiment. Moreover, as the volume fraction and temperature of SO₂ increase, the corrosion rate increases slightly. The severest working condition is of 700 °C and SO₂ volume concentration of 0.35 %, where the weight of S30432 specimens increased by only 1.08 mg cm⁻² after 2000 h. By comparing the corrosion kinetics curves of the S30432 specimen with low-alkali coal ash and high-alkali coal ash, it is found that the specimen with low-alkali coal ash has a slight weight gain, while the specimen with high-alkali coal ash has a remarkable weight loss.

Figure 1:

Corrosion kinetics curves of S30432 steel.

3.2 Surface corrosion products

Figure 2 shows the XRD diffraction spectra of S30432 coated with low-alkali coal ash after 2000-h experiment. It is observed that the surface corrosion test products under various working conditions are mainly Fe and Cr oxides, and there are a small amount of coal ash components SiO₂ and Al₂O₃. Moreover, the matrix peak at 700 °C is weaker than that at 650 °C, while the oxides diffraction peak is stronger at 700 °C than that at 650 °C. This may be attributed to the increase in the metal oxidation reaction rate with the increase in temperature.

Figure 2:

X-ray diffractions spectra of S30432 steel after 2000 h of the experiment.

3.3 Micromorphology and composition of corrosion products

3.3.1 Surface micromorphology and composition of corrosion products

The surface micromorphology of S30432 coated with low-alkali coal ash after 2000 h of the experiment is presented in Figure 3. Furthermore, the energy spectrum analysis results of corresponding regions are presented in Table 4. It is found that under a working temperature of 700 °C and an SO₂ volume fraction of 0.35 %, corrosion products are slightly peeled off. However, since the generation rate of oxidation products is greater than the peeling-off rate, the specimen weight increases gradually. Meanwhile, the peeling phenomenon of corrosion products does not occur in other working conditions. In all working conditions, the corrosion products are mainly metal oxides. In some working conditions, carbon, silicon, and aluminum particles from coal ash particles that are embedded in the corrosion layer.

Figure 3:

Surface micromorphology of specimens.

Table 4:

EDS results of corroded areas in Figure 3 (wt%).

Case	Point	O	Cr	Fe	C	K	Si	Mn	Al
(a) φSO2: 0.24 %, T: 650 °C	1	52.39			18.33		23.68		5.6
	2	27		51.35	19.58		2.07
(b) φSO2: 0.35 %, T: 650 °C	3	56.16		7.07			30.70		6.07
	4	33.96	2.54	51.53			6.66		5.31
(c) φSO2: 0.24 %, T: 700 °C	5	37.29	5	37.58			10.78		9.35
	6	30.59	40.39	26.34			1.57		1.11
(d) φSO2: 0.35 %, T: 700 °C	7	47.05	9.4	19.03		3.35	12.35		8.82
	8	31.01	46.85	9.26			1.72	9.63	1.53

3.3.2 Section micromorphology and composition of corrosion products

The cross-sectional morphology of specimens coated with low-alkali coal ash after 2000 h of the experiment is shown in Figure 4. Moreover, the energy spectrum analysis results of the corresponding regions are presented in Table 5. It is observed that sulfur elements appear in the corrosion products at a temperature of 700 °C and an SO₂ volume fraction of 0.35 %, indicating the occurrence of high-temperature sulfur-induced corrosion. Meanwhile, the sulfur element does not appear in the corrosion products under other working conditions. However, the sulfur element does not appear in the corrosion products of S30432 coated with high-alkali coal ash only at a temperature of 650 °C and SO₂ volume fraction of 0.2 %, while it appears under other working conditions (Liu et al. 2022).

Figure 4:

Cross-section micromorphology of the corrosion products.

Table 5:

EDS results of corroded areas in Figure 4 (wt%).

Case	Point	O	Cr	Fe	C	S	Si	Al
(a) φSO2: 0.24 %, T: 650 °C	1	29.31	30.64	20.99	19.06
	2	35.44		55.53	9.03
(b) φSO2: 0.35 %, T: 650 °C	3	23.86			12.73		39.63	23.78
	4	50.82	12.12	35.21				1.85
(c) φSO2: 0.24 %, T: 700 °C	5	51.43	48.57
(d) φSO2: 0.35 %, T: 700 °C	6	50.76	42.22		2.56	1.79	2.67

4 Discussion

The main influencing factors of metal high-temperature corrosion are metal composition, temperature, atmosphere, and components of sediments. In this study, the material and the temperature conditions are the same as that in (Liu et al. 2022), while the volume fraction of SO₂ is higher (φSO₂: 0.24 %, and 0.35 % vs. φSO₂: 0.2 %, and 0.3 % (Liu et al. 2022)), and the alkali metal content of coal ash is lower. The mass ratios of coal ash compositions are SiO₂: 28.26 %, Al₂O₃: 13.12 %, Fe₂O₃: 22.60 %, CaO: 22.76 %, Na₂O: 1.58 %, K₂O: 1.54 %, SO₃: 8.43 %, others: 0.71 % in (Liu et al. 2022). The corrosion products of S30432 specimens coated with low-alkali coal ash tested for 2000 h under various working conditions are mainly composed of elements Cr, O, Fe, as well as the embedded coal ash components Si, Al, and K (see Tables 4 and 5). A small amount of sulfur element was detected in the corrosion products only under the severest working condition (i.e. φSO₂: 0.35 %, T: 700 °C). However, for S30432 specimens, the sulfur element was detected in the corrosion products at a working temperature of 650 °C and an SO₂ volume fraction of 0.3 % (Liu et al. 2022). It is inferred that S30432 specimens coated with low-alkali coal ash are mainly oxidized at high temperatures, and the required temperature to produce high-temperature sulfur corrosion is higher than that of high-alkali coal ash.

Figure 5:

700 MW, 620 °C ultra-supercritical boiler.

High-temperature corrosion of metal can be mainly divided into two stages, including the initial incubation period and the later accelerated corrosion. The corrosion rate during the initial incubation period is generally low, and the duration varies from a few minutes to thousands of hours depending on the material and environmental conditions. The salient feature of the later accelerated corrosion stage is the increase of the corrosion rate, accompanied by a large number of peeled-off corrosion products (Stein et al. 2014). The weight increase of the studied specimens after 2000-h corrosion experiment under the severest working condition (i.e. φSO₂: 0.35 %, T: 700 °C) was only 1.08 mg cm⁻² and there is almost no severe peeled-off corrosion products. However, when the specimen coated with high-alkali coal ash and exposed to the atmosphere with an SO₂ volume fraction of 0.3 % at 700 °C, the specimen weight decreased by up to 41.8 mg cm⁻² after 2000-h corrosion, accompanied by a large amount of peeled-off corrosion products (Liu et al. 2022). This indecates that after 2000-h corrosion, the studied S30432 specimen coated with low-alkali coal ash is still in the initial incubation period, while that with the high-alkali coal ash has entered the later accelerated corrosion stage.

The primary reason for coal ash to aggravate alloy corrosion is the dissolution of oxide film on the alloy surface by molten salt (Zeng et al. 2014). Alkali metals in coal ash react with SO₂ and O₂ in the atmosphere and composite sulfates form (Birks et al. 2014). These composite sulfates were molten under various working conditions. The more alkali metal content in the coal ash, the more composite sulfate will be generated, and the effects of destructing the oxide film on the alloy surface will become more intense (Hussain et al. 2013). Table 6 presents the alkali metal content related to the generation of composite sulfate in low and high alkali coal ash. It is worth noting that the content of Na₂O, K₂O, and Fe₂O₃ affects the quanity of composite sulfate, and Fe₂O₃ intensifies the reaction as a catalyst (Li 2001). Compared with the corrosion results obtained from high-alkali coal ash (Liu et al. 2022), S30432 has better high-temperature corrosion resistance to low-alkali coal ash with the same temperature and a relatively higher SO₂ volume fraction.

The obtained results reveal that under the severest working condition (i.e. φSO₂: 0.35 %, T: 700 °C) after 2000 h, the weight of the specimen coated with low-alkali coal ash increased by only 1.08 mg cm⁻². Accordingly, the equivalent corrosion depth after 30 years and 7500 h of operation per year is 0.153 mm, which is within an acceptable range in engineering applications. It is concluded that S30432 can be used in the boiler design up to 700 °C under the condition of low-alkali coal ash and SO₂ volume fraction of 0.35 %.

5 Engineering verification

The performed experiments show that S30432 is an appropriate material for high-temperature superheater and reheater in the high-efficiency ultra-supercritical boiler with a maximum outer wall temperature of less than 700 °C. A 700 MW, 620 °C ultra-supercritical boiler has the high-temperature superheater and reheater made of the S30432 tubes with the geometries of φ45 × 10 and φ51 × 4, respectively. And the tube wall thickness accuracy is ±10 %. Observations during the bolier maintenance periods in 3 years indicated that the surfaces of high-temperature superheater and reheater tubes were not rough and there were no obvious signs of high-temperature corrosion. The wall thickness of the high-temperature superheater and reheater were measured on-site, as shown in Figure 5. The measurement results of the high-temperature superheater were 10.16 mm and 10.13 mm respectively, and the results of high-temperature reheater were 3.89 mm and 3.88 mm. The measured wall thicknesses are within the manufacturing deviation range of the design specification. This indicates that there is no obvious high-temperature corrosion of S30432 in engineering operation, which is consistent with the experimental results.

6 Conclusions

In this study, the corrosion resistance of S30432 to low-alkali coal ash was exprimentally investigated. The main achievements can be summarized as follows:

1. S30432 is mainly oxidized under the four working conditions of 650 °C, 700 °C, and SO₂ volume fraction of 0.24 %, and 0.35 %. Under the constant temperature and SO₂ volume fraction in the simulated atmosphere, the corrosion resistance of S30432 to low-alkali coal is significantly higher than that of high-alkali coal ash.

2. On-site meaurements indecate that S30432 has satisfactory resistance to high-temperature corrosion on the boiler burning low-alkali and high-sulfur coal.

3. In the design of large capacity 620 °C high-efficiency ultra-supercritical boilers burning low-alkali and high-sulfur coal, S30432 is an appropriate material for the tubes with a wall temperature less than 700 °C.