Material selection and development of special thermocouple protective sleeves for circulating fluidized bed boilers

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1. Introduction With the development and progress of society, China’s requirements regarding environmental protection have become increasingly stringent. As a new type of combustion technology, circulating fluidized bed combustion offers significant advantages in terms of energy savings, load regulation, pollution control, and fuel adaptability. It is being used more and more widely in various fields such as the power industry, waste incineration, and the cement industry. Currently, more than 20 boiler manufacturers across the country produce conventional fluidized bed boilers and circulating fluidized bed boilers with an evaporation capacity of 20 t/h or less. To date, nearly 3,000 industrial fluidized bed boilers are in operation across the country, the highest number in the world. However, due to the special combustion mode of circulating fluidized bed boilers, the wear of vulnerable components has always been an important factor affecting their long-term safe operation. To address this issue, material scientists have done a great deal of work, proposing many meaningful ideas and solutions. Among them, the emergence and application of a large number of new materials and new processes have made it possible to solve this problem. The issue of temperature measurement using thermocouples in circulating fluidized bed boilers is currently a prominent problem that needs to be addressed urgently. Severe high-temperature erosion **reduces the normal service life of the thermocouple. Special high-temperature, wear-resistant thermocouple protective sleeves must be used to address this issue. Through research and development, significant progress has been made in their manufacturing techniques, and their cost-performance ratio continues to improve. This article discusses this issue in an attempt to draw everyone’s attention. Together, we contribute to improving the measurement standards of thermocouple technology in our country. 2 Combustion and Wear Mechanisms in Circulating Fluidized Bed Boilers Different from traditional pulverized coal boilers, in circulating fluidized bed boilers, the bed material inside the furnace rises upward along with the flue gases; it then enters the separator through the upper outlet of the furnace. There, gas and solid phases are separated from each other. The separated flue gases proceed through the upper outlet to reach the rear smoke duct of the boiler, while the separated solid particles return to the lower part of the furnace via the return valve. During the operation of a circulating fluidized bed boiler, the solid bed material containing fuel, fuel ash, limestone, and their reaction products is in continuous high-temperature circulation within a closed loop that includes the furnace, separator, material valve, and furnace. There, it undergoes efficient combustion and desulfurization reactions at temperatures of 850°C to 950°C. In addition to flowing in an external circulation within this circuit, the bed material also undergoes continuous internal circulation within the furnace under the effect of gravity. Therefore, severe wear inevitably occurs at the corresponding parts of the circulation loop. The above analysis indicates that the thermocouple protection sleeve is exposed to a high-temperature, oxidizing atmosphere inside the furnace, as well as the scouring and impact from circulating fluids; therefore, its wear should be classified as abrasive wear under high temperatures. This form of wear can be roughly divided into two categories: one is fatigue wear caused by the repeated deformation of the material during collisions ； Another type is the damage caused by the cutting action of freely moving particles on the material, known as chipping wear. The high temperature inside the furnace can alter the microstructure of the material, causing its hardness to **decrease**, which in turn increases wear. At the same time, the high-temperature oxidation of the material reduces its toughness; the oxide layer tends to crack under impact, which further accelerates material wear. The degree of wear is highly dependent on the impact angle; at an impact angle of 90°, there is no chipping wear, only fatigue wear, which is very mild. The wear is most severe when the impact angle is between 20° and 50°. Therefore, the angle between the insertion direction of the protective tube and the flow velocity direction should avoid this range of angles. 　　Upon close inspection and analysis of the failure points in the thermocouple protection sleeve, there are roughly two areas that are prone to failure. One is the end of the thermocouple protection sleeve, where the material is subject to severe wear due to the combined effects of direct exposure to flue gas and materials, as well as swirling currents and vortices. Another area is the junction between the thermocouple protection sleeve and the furnace wall; wear in this area is mainly caused by abrasive erosion resulting from the eddies of flue gas and material at that location. The above analysis indicates that high-temperature erosion wear failure is the main cause of failure for the special thermocouple protective sleeves used in circulating fluidized bed boilers, and this is an issue that should be given priority attention in the selection of materials and manufacturing process for such protective sleeves. 3 Material selection and development of thermocouple protective sleeves Thermocouples are equipped with protective sleeves for two main purposes: one is to protect them from mechanical damage, and the other is to insert a shielding sleeve between the thermocouple and its surroundings, thereby allowing the thermocouple to remain in an environment as close as possible to its optimal conditions. Therefore, selecting an appropriate thermocouple protection tube based on the specific operating environment is quite important for extending the service life of the thermoelectrode and improving the accuracy of temperature measurement. 3.1 Principles for material selection and advantages and disadvantages of common materials Based on the environment in which the thermocouple protection tube operates within a circulating fluidized bed boiler, the following points should be considered when selecting materials: (1) The material should have good airtightness (i.e., low porosity) ； 　　(2) It should have sufficient strength and stability, as well as good resistance to thermal shock ； 　　(3) The material should have good red hardness; it is necessary that the material be wear-resistant ； 　　(4) It has good resistance to chemical corrosion, as coal-fired media contain corrosive gases such as SO2, SO3, Na2O, and K2O ； 　　(5) Good thermal conductivity. The materials used for thermocouple protective sleeves, which are currently in industrial use, can be classified into three main categories based on their composition: metallic materials, non-metallic materials, and composite materials consisting of both metallic and non-metallic elements. 　　　　1. Common materials for metal protective tubes include stainless steel and superalloys. The advantages of these materials are their high mechanical strength, good corrosion resistance, and excellent thermal conductivity; however, they have poor high-temperature strength, which results in low wear resistance at high temperatures. Stainless steel protective tubes can generally be used for 7 to 8 days, while other common materials can be used for 1 to 2 months. In recent years, there has been no ideal solution domestically or internationally; as a result, the only option available has been to thicken the walls of the protective tubes. The wall thickness of some thermocouple protective sleeves has reached over 10 millimeters. Therefore, its performance in circulating fluidized bed boilers is not very satisfactory, and the cost-performance ratio is poor. 　　　　2. Common materials for non-metallic protective tubes include quartz, high-temperature ceramics, magnesium oxide, aluminum oxide, etc. Compared to metal materials, the advantages of these materials lie in their good wear resistance at high temperatures and their excellent corrosion resistance. However, they also have significant drawbacks: firstly, their thermal conductivity is low and their sensitivity to temperature changes is high, which prevents the tube walls from being made very thick. Moreover, thinner tube walls pose a range of problems in terms of application. Ceramic materials are generally brittle and have poor thermal shock resistance; they are prone to cracking due to impacts and thermal shocks during use, especially in circulating fluidized bed boilers where intense erosion can lead to premature failure. 　　　　3. Composite material protective sleeves: Among those that have seen rapid development in recent years are cerametallic composite materials and special composite coating materials for protective sleeves. Cerammetal composites are multi-component composites composed of a metal matrix, ceramic components, and lubricants. Thermocouple protective sleeves made from such materials possess advantages such as good thermal conductivity, strong resistance to thermal shock, heat resistance, and wear resistance. Special composite coating protective sleeves currently mostly involve applying hard and wear-resistant composite materials such as cermets to the surface of a metal substrate using methods like laser cladding. The aforementioned composite protective sleeve has the advantage of combining the characteristics of two different materials, thereby offering complementary strengths; however, its drawback is that the complex manufacturing process results from the special bonding mechanism between these two materials. 3.2 Development of wear-resistant materials and their application in thermocouple protection sleeves After more than 30 years of research and application, the development and use of wear-resistant materials abroad have reached a stable state; developed countries already have their own series of products and relevant standards. Among non-metallic materials, ceramics exhibit excellent wear resistance. However, these materials are quite brittle, lack sufficient thermal shock resistance, and have relatively poor thermal conductivity; moreover, to reduce thermal sensitivity, the wall thickness of such tubes cannot be made too thick. Therefore, thermocouple protective sleeves made from such materials do not perform well in circulating fluidized bed systems. Currently, the research trends in the field of ceramic materials, both domestically and internationally, tend to focus on developing metal-ceramic composites, namely cermets, or using surface coating technologies to apply them as coating materials. 　　The development of metal wear-resistant materials has gone through several stages, from high-manganese steel, ordinary white cast iron, nickel-hard cast iron to high-chromium cast iron. It has currently developed into three categories: wear-resistant cast iron, wear-resistant steel, and wear-resistant alloys. Although wear-resistant steels and wear-resistant alloys each have their own advantages, they are far inferior to white cast iron in terms of cost and wear resistance; as a result, the latter is more widely used. In particular, alloyed white cast iron has a service life that is several times, or even dozens of times, longer than that of ordinary white cast iron. Its chromium-based white cast iron can see its overall performance greatly improved by adjusting the chromium and carbon contents. 　　The high-chromium cast iron often mentioned is actually high-chromium-molybdenum white cast iron, which can be classified into two types based on carbon content: Cr15 and Cr20. The former is used to withstand normal impact wear, while the latter is suitable for applications with high impact wear as well as for wear-resistant components with greater thickness. High-chromium cast iron with a chromium content of 30% is used for workpieces that require heat resistance, wear resistance, or corrosion resistance. High-chromium cast iron has been increasingly used as an anti-abrasive wear material at room temperature. However, under high-temperature wear conditions, the service environment for this material is more severe than that under room-temperature wear; during its service life, the material suffers damage not only due to abrasive wear but also from the oxidative corrosion caused by high-temperature gases. Materials previously used in high-temperature wear conditions (such as CrMnN steel) focused solely on their oxidation and corrosion resistance, requiring a very low carbon content in the material; as a result, its wear resistance was compromised. Therefore, to study high-temperature wear-resistant materials, a comprehensive consideration of these materials is necessary in order to properly design and select the appropriate materials to use. During the solidification of high-chromium cast iron, (Cr, Fe)7C3-type carbides are formed. These carbides have a high hardness of 1500–1800 HV, which is much higher than that of (Cr, Fe)3C-type carbides (840–1100 HV). This is the main reason why high-chromium cast iron possesses high wear resistance. The eutectic structure of high-chromium cast iron is opposite to that of ledeburite in ordinary cast irons: the eutectic carbides are discontinuous phases existing as dispersed particles within the austenite, whereas the eutectic austenite is a continuous phase. This special structure is equivalent to embedding high-hardness particles within a certain matrix, **which reduces the brittleness of the high-hardness phases; as a result, high-chromium cast iron possesses good toughness. In high-chromium cast iron, part of the chromium goes into the carbides while another part goes into the matrix, thereby enhancing the oxidation resistance of the matrix. Based on the microstructural characteristics of high-chromium cast iron, it is entirely feasible to use it as a material for heat electrode protective sleeves intended for wear resistance. Further research and experimentation are still needed to determine how to adjust its composition in order to achieve the best levels of wear and corrosion resistance. 3.3 Application of new material coating technology in thermocouple protective sleeves The new material coating technology holds extremely broad application prospects in thermocouple protective sleeves. Its feature is the ability to combine two different materials, thereby fully leveraging the properties of the matrix and the coating to effectively meet practical needs. Therefore, research in this area is quite active internationally, and applications aimed at improving the wear resistance of materials include laser-clad cermets and gradient functional ceramic coatings, among others. In general wear-resistant coating technologies, the WC+metal material system is most commonly used. However, due to the high operating temperature of boilers, WC decomposes at such temperatures; therefore, the operating temperature of WC cannot exceed 550°C, and thus WC-based systems cannot be used in boiler wear-resistant coatings. Currently, the coating system of Cr3C2+NiCr is widely used internationally. This system performs well, but its cost is high. It is worth noting that, in practical applications in the field of thermocouples, our research group has developed intermetallic compound-coated thermocouple protective sleeves using new materials and new manufacturing processes. Intermetallic compounds are high-melting-point structural materials that lie between metals and ceramics, thereby endowing this thermocouple protection sleeve with entirely new properties. First, it possesses excellent corrosion resistance and good oxidation resistance, particularly with very strong resistance to hydrogen sulfide ； Secondly, it possesses excellent wear resistance; the coating has the high temperature and high hardness characteristics of intermetallic compounds, which contributes to its wear resistance ； Thirdly, they possess excellent heat resistance. Currently, nickel-based wrought superalloys can be used at temperatures of 950–1000°C, while nickel-based cast superalloys can operate at temperatures of 1050–1100°C, which is around 0.8 times the material’s absolute melting point. If a similar value of 0.8 is applied to Ni-Al alloys, their operating temperature could reach around 1250°C, thereby exceeding the performance of existing superalloys by 150–200°C and reaching a level that is leading in China and advanced on the international scale. Fourthly, ordinary materials can be used in place of high-quality ones; for example, the 1Cr18Ni9Ti material combined with a surface intermetallic compound coating can be used as an alternative to expensive heat-resistant superalloys. This not only reduces costs but also results in performance that **exceeds that of the expensive superalloys ； Fifth, it has a low cost while maintaining good performance. For some thermocouple protective sleeves used in high-temperature and wear-resistant environments, thermocouples manufactured using this intermetallic compound coating technology have achieved satisfactory results when used for temperature measurement at the outlets and in the pipelines of boiling furnaces, circulating fluidized bed boilers, the flue gas chambers at the end of cement rotary kilns, ball mills, fan mills, coal grinders, and other similar equipment. 　　The gradient functional material FGM coating technology is highly suitable for the coating of thermocouple protective sleeves, as the use of coating technology on such sleeves inevitably leads to problems related to mismatched thermal stresses. The development of ultra-high-heat-resistant materials with the ability to mitigate thermal stress not only increases the service life of the surface coatings on protective sleeves, but also allows for effective bonding with intermetallic compounds, resulting in composite coatings with improved performance. Abroad, there have been reports on intermetallic ceramic composite coatings using intermetallic compounds as binders, such as intermetallic compound-bonded TiC. This material not only possesses high hardness but also exhibits excellent corrosion and wear resistance; furthermore, by adjusting the content of the intermetallic compound, its coefficient of thermal expansion can be controlled to be closer to that of steel. Research in this area is also quite active in China. Through several years of research, our team has achieved **significant success** in the field of intermetallic compound + fine ceramic composite coating technologies. Intermetallic compounds possess excellent heat resistance and corrosion resistance, while fine ceramics, in addition to these properties, also have outstanding wear resistance and non-stick characteristics. Thermocouple protective sleeves made from such materials are bound to exhibit ideal wear resistance. They have already been applied in industries such as cement, boilers, and glass, with satisfactory results. 4. Conclusion 　In summary, the use of new materials and new processes is an effective way to improve the wear resistance of thermocouples used in circulating fluidized bed boilers. In particular, the introduction of surface coating technology into the manufacturing of thermocouples has **broadened perspectives and effectively solved the problem of how to comprehensively improve the performance of thermocouple protective sleeves with minimal investment. Modern technology is advancing by leaps and bounds, and in the future, new materials and processes will surely be applied to the field of thermocouples.