Ⅰ. Introduction
The circulating fluidized bed combustion technology of coal is a new type of coal-burning technology developed in the past two decades. It is a major innovation of the traditional grate furnace and pulverized coal furnace. Due to the high heat capacity, strong mixing and low-temperature combustion characteristics of the materials in the furnace, it can effectively burn lignite, bituminous coal and anthracite, as well as inferior fuels such as gasification furnace slag and petroleum coke. In the same boiler, multiple types of coal can be burned at the same time, and the fuel adaptability is good. Circulating fluidized bed combustion can be carried out by adding limestone to desulfurize the flue gas in the furnace, providing a way for the rational use of high-sulfur coal. However, the general tail desulfurization technology is costly and difficult to promote and apply in small and medium-sized boilers in China. Circulating fluidized bed boilers are spread all over the world. After years of development, many furnace types with different structures and forms have been formed. In my country, the circulating fluidized bed combustion technology has developed rapidly, and various forms of CFB have continued to appear and gradually developed towards large-scale. The advantages of CFB have been recognized by the majority of users. From the perspective of clean coal combustion, it is also an important starting point for my country to solve environmental protection problems in the near future.
Ⅱ. Wear Mechanism
CFB is different from traditional pulverized coal boilers. The bed material in the furnace rises along the furnace under the flue gas, enters the separator through the upper outlet of the furnace, and separates the gas and solid phases in the separator. The separated flue gas enters the flue at the tail of the boiler, and the separated solid particles return to the lower part of the furnace through the return valve. During the operation of CFB, the solid bed material containing fuel ash, limestone and its reaction products is in a continuous high-temperature circulation flow in the closed circulation loop of furnace-separator-return valve-furnace, and performs high-efficiency combustion and desulfurization reactions at 850~900℃ in the furnace. In addition to the external circulation of the bed material in this loop, the bed material continuously circulates internally in the furnace under the action of gravity. Therefore, wear and tear will inevitably occur in the corresponding parts of the circulation loop. Wear not only affects the safe operation of the boiler, but also limits the realization of some advantages of this type of boiler; the direct harm caused by wear to the boiler is that the wall thickness of the metal pipes on the heating surface that bears the internal pressure is reduced until the tube bursts and the boiler is shut down. Wear increases the operating and maintenance costs of the boiler, reduces the utilization rate of the unit, and causes huge losses to users.
Wear is the impact of coal or ash particles on the surface of the heating surface tube at a certain angle (0~90°), causing erosion and metal loss on the tube surface. Erosion wear is mainly caused by impact and cutting, and cutting is the most important factor. Solid particles act as tiny cutting tools to slide and cut grooves on the relatively soft metal surface. Wear is a very complex failure process, which is not only affected by mechanical reasons, but also closely related to various factors such as materials, environment, and medium. The wear of CFB heating surface tubes is affected by the concentration of coal particles and ash particles, particle characteristics, and flow channel geometry. In areas with high solid particle concentrations, wear mainly depends on the convection movement of solid particles and flue gas flow and heating surface tubes. Loss is closely related to flue gas flow rate. The speed of solid particles is the main factor affecting wear. Therefore, serious damage usually occurs in the area where the particle flow rate changes suddenly. The heating surface damage of CFB mainly occurs in the lower part of the combustion chamber, around the upper outlet of the furnace, and the lower part of the screen-type heating surface arranged in the combustion chamber.
At present, the anti-wear measures of CFB mostly adopt the laying of thick non-metallic refractory bricks, castables or plastics. According to the relevant information available, most of the accidental shutdowns of CFBs invested in foreign countries and some domestic CFBs are due to the erosion and wear of materials, which causes the refractory materials to fail and fall off, block the slag discharge port and material circulation circuit, and cause the heating surface to be worn and cause pipe bursts.
Ⅲ. Causes of refractory failure
At present, people still lack experience in CFB, especially in the selection of materials for large CFB, there are still many problems to be solved, among which the wear resistance of refractory materials is a major problem. Due to the different comprehensive physical and chemical properties of various refractory materials, it is very important to reasonably select refractory materials according to the use parts, purposes and boiler characteristics. According to the current relevant information and usage, the failure of refractory materials for CFB is caused by many factors.
Scouring and wear are problems that all kinds of CFB refractory materials must face. Due to the characteristics of the boiler itself, the refractory materials are subject to wear and tear from the solid bed materials of fuel, fuel ash, limestone and their reaction products every day during operation. Different types of coal have different chemical compositions, volatile matters and ash contents, and the scouring speeds on refractory materials are also different, as well as the wear on refractory materials in the hot state. In addition, the design structure and flow rate of the furnace are directly related to scouring. These problems require actual testing and demonstration to minimize scouring and wear. Special parts of the boiler structure often cause premature failure of refractory materials, especially the refractory materials at the upper outlet of the furnace and the upper part of the separator, which often collapse under the combined effects of thermal cycles and mechanical vibrations. These must be comprehensively considered and solved in terms of design, material selection and installation.
Refractory materials expand or contract with the rise and fall of temperature. If this expansion and contraction is constrained, stress will be generated inside the material. Refractory materials are inhomogeneous and brittle materials. Compared with metal products, refractory materials have lower thermal conductivity and elasticity, lower tensile strength, poor resistance to thermal stress damage, and lower thermal shock resistance. Under the action of thermal shock cycles, refractory materials are prone to cracking and spalling first, and eventually overall damage. This is another important reason for the premature failure of CFB refractory materials. The factors that affect the thermal shock stability of refractory materials are relatively complex, and the influence of various factors must be considered comprehensively.
The physical and chemical properties of refractory materials are very important. Generally speaking, the compressive strength, flexural strength, wear resistance, thermal shock stability and reburning line change of refractory materials are the main assessment indicators. At the same time, the high-temperature compressive strength index should also be considered. In particular, some castables cannot reach the sintering temperature and have low strength when used at the combustion temperature of CFB. For example, castables with high-alumina cement have a peak strength after drying at room temperature and 110°C due to the increase in the amount of calcium brought in, but the strength decreases with the increase in temperature, and the strength is lowest at 700~1100°C. There are also many refractory binders that must be sintered at a temperature above 1200°C to have a certain strength. When used below 1200°C, the refractory material cannot reach the sintering temperature and the strength is very low, so the effect of using it on CFB is not ideal.
Unreasonable construction, installation and furnace drying are also one of the main reasons for refractory damage. Most manufacturers have no experience in construction and installation, and cannot strictly supervise the construction in accordance with the design requirements and the refractory construction requirements provided by the material manufacturer. Some installation companies also have no experience in installing CFB refractory materials, and they carry out construction according to the masonry methods and experience of industrial boilers, resulting in large installation quality defects. In addition, some projects are put into operation ahead of schedule in order to shorten the installation period. Because the boiler requirements are not met, the moisture contained in the materials is not completely converted into water vapor and escapes. After the furnace is ignited and operated, the water vapor pressure in the refractory exceeds the tensile strength of the material, causing lining delamination and collapse, resulting in large-scale collapse of the furnace wall shortly after the furnace is ignited.
Ⅳ. Current status of refractories for CFB
At present, the refractory materials used in domestic CFB can be generally divided into three categories according to their functions: bricks, castables, plastics and mortars of refractory materials; bricks, castables and mortars of refractory materials; bricks, castables and mortars of refractory insulation materials. The types of wear-resistant refractory materials commonly used are: phosphate bricks and castables: sillimanite bricks and castables; silicon carbide bricks and castables; corundum bricks and castables; wear-resistant refractory bricks and castables; the highest-grade ones are silicon carbide combined with silicon carbide products. Judging from the results of the existing CFB use, the use of these materials is not very ideal.
Phosphate bricks are unfired bricks that have been heat-treated at low temperature (500℃) and are usually used in the range of 1200~1600℃. They have been used in cement kilns for many years. Most of the design materials of early CFBs use phosphate bricks and phosphate castables. At that time, the national building materials industry formulated a standard for aluminum phosphate wear-resistant bricks, while other industries did not formulate standards for wear-resistant materials. Especially for refractory materials for CFB, few people know and study them. Since CFB operates in the range of 850~900℃, at this temperature, the physical properties of the refractory material are unstable and the wear resistance cannot be fully exerted. The physical and chemical properties of phosphate castables are the same as those of bricks, except that its construction is more complicated and is restricted by the environment. Although phosphate materials have their shortcomings when used on CFB, their prices are acceptable to users.
Sillimanite is a high-quality refractory material. It is usually added to refractory materials to increase the load softening temperature by 100~150℃. The temperature at which refractory materials change is 1450~1600℃. The molding and sintering temperature of sillimanite bricks reaches this temperature. Therefore, sillimanite bricks are an ideal wear-resistant refractory material for use on CFB: However, sillimanite castables cannot fully exert their wear resistance because CFB combustion cannot reach this temperature range. In addition, the price of sillimanite materials is relatively high, which increases the cost of users. These have affected the promotion and application of sillimanite castables on CFB.
Silicon carbide products have good resistance and good thermal shock stability when used in a high-temperature non-oxidizing atmosphere. When sintered at a certain temperature, a glaze protective layer can be formed on the surface. The main reason is that there is a small amount of oxidizing atmosphere in the combustion of CFB. Relevant literature points out that the United States also strictly prohibits the use of silicon carbide refractory products in CFB, and the price is relatively high.
The varieties of corundum products used in CFB include white corundum, high-aluminum corundum (also called sub-white corundum) and brown corundum. The main properties of corundum are high refractoriness, high bulk density and good resistance, but its thermal shock stability is poor, which brings difficulties to the use of CFB. Corundum castables often collapse during use. The reason is that there are many fire pressure and fire lifting phenomena during boiler operation, and the temperature changes frequently in a short period of time, resulting in a shortened service life of the refractory. Another factor is that the boiler is used at a low temperature, the refractory cannot reach the sintering temperature, and the resistance cannot be fully exerted.
The above-mentioned refractory materials for CFB are representative in China. According to the understanding of the use of most users, the user reactions are different. The main reason is that various boiler manufacturers and design units use a wide variety of refractory materials without a unified standard. There are few refractory materials with high wear resistance, which can achieve the same cycle operation and periodic maintenance as the furnace, and can produce good economic value and social benefits. The refractory materials used in the early 35t/h and 75t/h CFBs in China were basically based on the refractory materials used in related industries (such as metallurgy and petrochemicals). However, unlike the high refractoriness and high resistance to slag leaching of refractory materials in the metallurgical and petrochemical industries, CFB requires refractory materials to have high temperature resistance and high thermal shock stability. During construction, refractory materials and thermal insulation sealing materials are required to be layered and bonded together, that is, heavy and light, fixed and unfixed matching construction, and refractory materials with different production processes and construction methods are required to meet the same indicators.
With the introduction of foreign CFB technology, some domestic CFB design and manufacturing manufacturers have formulated technical requirements for refractory materials based on the requirements of foreign companies for CFB refractory materials, supplementing and improving the domestic CFB refractory material specification system. However, at present, domestic boiler manufacturers and design units have their own views, and users have a certain degree of blindness in material selection. In addition, the quality of some manufacturers’ products is not up to standard. After the construction and installation of refractory materials, large-scale collapse or wear occurs within 72 hours of operation. As a result, users have spent a lot of money but cannot meet their needs, causing great economic losses to users; it has also had an adverse impact on the promotion and application of CFB. When some users mention CFB when selecting boilers, they think that the damage problem cannot be solved. CFB refractory materials have been listed as a key scientific and technological research project of the country’s “Ninth Five-Year Plan”. According to the different requirements for refractory materials for different furnace types and different parts, systematic and comprehensive research work has been carried out.
ⅴ. Recommendations on the selection, installation and maintenance of refractory materials
In the absence of a unified standard for CFB refractory materials in China, it is recommended that when design units and manufacturers use refractory materials, they should hire professional technicians and experts with practical experience in CFB refractory materials to evaluate and demonstrate the following contents in order to achieve specific requirements such as wear resistance and high temperature resistance.
Whether the selected refractory materials are specially developed for CFB use: whether the physical and chemical properties of refractory materials can meet the special performance requirements of CFB (the main indicators of refractory materials are compressive strength, flexural strength, performance resistance and thermal shock stability, etc.); whether the physical and chemical properties of refractory materials have been inspected by national authoritative departments, and whether the inspection indicators meet the requirements of the design specifications; the use of refractory materials should be carefully investigated and understood, whether the periodic use performance of refractory materials is recognized by many users; whether the price, service life and economic benefits of refractory materials can achieve the best effect; whether the production capacity, production equipment, inspection methods and quality assurance system of refractory manufacturers can meet the production requirements of refractory materials; whether comprehensive technical descriptions and construction materials of refractory materials can be provided, and whether there are professional technicians to provide on-site construction technical guidance and supervision.
It is recommended that when the user purchases refractory materials, it should promptly entrust a professional inspection agency to inspect; the refractory user unit should use a third party as an inspector to supervise the construction and installation of the refractory materials; the technical level of the installer should be tested in advance, which will greatly reduce future maintenance costs: during construction and installation, in addition to complying with the design requirements, the furnace should be baked according to the heating curve provided by the refractory manufacturer.
It is recommended that users strictly implement regular maintenance, generally a minor repair once a year and a major repair once every three years. Minor repairs are repairs to severely worn areas. The number of shutdowns during operation of the boiler should be minimized as many shutdowns and starts will cause thermal shock cycle stress fatigue in the refractory materials, and the widening and increase of local cracks. If permanent cracks are found during maintenance, they should be repaired in time. Acidic high-temperature adhesives and part of high-aluminum fine powder can be used for repair, or special furnace fillers can be used for repair. Attention should be paid to small cracks. If they are not repaired in time, the cracks will continue to expand. Sometimes ash enters the insulation layer from the cracks. Over time, the gradual increase in the amount of ash in the gaps will cause the furnace wall to protrude and collapse.