1. Basic introduction
Refractory materials are key components of water-coal slurry pressurized gasifiers, and their operating performance is one of the key factors for the long-term, safe and economical operation of gasifiers. This paper analyzes the problems of refractory materials in the operation of gasifiers, aiming to find the key factors affecting the service life, and to make improvements in coal quality, operation, brick shape, etc., to solve the bottleneck problem that restricts the efficient and stable operation of the system. A company uses GE water-coal slurry pressurized gasification technology, with an operating mode of two starts and one standby, a designed operating pressure of 6.5MPa, and a designed coal processing capacity of each gasifier of 1500t/d. The refractory lining of the gasifier uses domestic materials and is still a traditional three-layer structure. Since the start of operation, the service life of the fire-facing surface of the refractory lining of the gasifier has been short, mainly manifested in the partial detachment of the cone bottom bricks and the serious erosion of the cylinder bricks. These phenomena are analyzed and solutions are introduced.
2. Analysis of damage mechanism of refractory lining
During the use of refractory materials, due to the corrosion and erosion of high temperature or temperature drastic change, atmosphere change and slag, the damage forms of refractory materials are complex and the damage mechanisms are diverse. In summary, the damage forms of refractory materials are mainly mechanical damage and chemical erosion.
2.1 Mechanical damage
The mechanical damage of refractory materials mainly includes damage caused by thermal spalling, structural spalling, high temperature thermal fatigue and mechanical impact. In the use of gasifier refractory materials, “thermal shock” phenomenon is inevitable, which is one of the main causes of mechanical damage to refractory materials.
The so-called thermal shock refers to the impact of refractory materials on them when their operating temperature changes significantly. GE has a definition of thermal cycle, that is, when the temperature changes by 100℃ within 1h, it is called a thermal cycle, which can also be classified as thermal shock. Gasifiers are prone to thermal shock during the process of furnace drying, feeding and parking, especially during feeding and parking. During the furnace drying process, due to the negative pressure and improper control of furnace drying fuel, the furnace temperature may change greatly in a short period of time, which is mainly manifested in the increase of the temperature difference between the upper and lower combustion chambers of the gasifier and the excessive temperature of the local area. At this time, the overall thermal expansion will be uneven, which may cause local cracks or bricks to be crushed. During the feeding process, since both coal slurry and oxygen are low-temperature media, they enter the furnace within 1 minute. At this time, the furnace temperature is generally around 1000℃. From the change of furnace temperature observed on the DCS, it can be found that at the moment of feeding, the temperature in the furnace first dropped and then rose. The drop is because the cold medium will absorb a lot of heat when entering the high-temperature environment, and the rise is because when the temperature of coal slurry and oxygen reaches a certain value, they will undergo a violent oxidation reaction and burn. Therefore, each feeding is equivalent to two “thermal shocks” to the refractory materials of the gasifier. During the parking process, in order to ensure the cleanliness of the oxygen pipeline and coal slurry pipeline, the two main material pipelines must be purged with nitrogen. Each time the system is stopped, the nitrogen entering the combustion chamber of the gasifier drops from 13MPa to about 11.3MPa. About 1000m3 of nitrogen enters, which will also cause the temperature of the refractory to drop suddenly, so each shutdown process is equivalent to a “thermal shock”. From the operation process, we can draw the following conclusion: the more times the gasifier is started and stopped, the higher the probability of mechanical damage to the refractory.
In the early stage of system operation, due to the lack of understanding of such large furnaces and the unstable operation of the furnace, the gasifier was started and stopped frequently, and sometimes a low-temperature joint operation was performed, which was one of the reasons for the short life of the refractory.
2.2 Chemical erosion
The refractory used in the water-coal slurry pressurized gasifier is composed of a variety of high-melting-point compounds, the main component of which is Cr₂O₃. At present, the Cr₂O₃ content of the refractory used on the fire surface can be as high as 90%, and the rest are Al₂O₃, ZrO₂ and other components. Among these compounds, some have polymorphism, that is, the same compound has multiple crystal structures (crystal forms). When conditions change, it will transform from one crystal form to another. If a compound has several crystal forms, only the one with the lowest free energy can exist stably under a certain temperature and pressure. For example, a compound has two crystal forms, crystal form I and crystal form II. When the temperature is lower than a certain value, only crystal form I can exist stably; when the temperature rises to a certain value, a transition between crystal form I and crystal form II occurs. When the temperature is higher than this temperature, only crystal form II can exist stably. For the refractory materials of the water-coal slurry pressurized gasifier, when other factors are not considered and only the temperature is considered, the temperature requirement is not more than 1500℃. Since crystal form transformation is often accompanied by changes in volume (or density) and other properties, when crystal form transformation occurs in the refractory material, cracking, loosening or pulverization may occur, thereby affecting its overall service life.
If the slag penetrates into the pores inside the refractory material, it will not only promote the melting and erosion of the refractory material, but also be an important reason for the chemical corrosion of the material, resulting in structural peeling and accelerating its damage. Because once the slag penetrates into the pores inside the refractory material, it will immediately react with it, causing the working surface to deteriorate, and as a result, the penetrated area will become very loose under high temperature conditions. Some people have done relevant experiments, and when the temperature reaches 1300℃, the penetration depth of the slag into the interior of the refractory brick can reach 30mm. For the water-coal slurry pressurized gasification technology itself, within the existing range of coal types, the operating temperature of most coal types cannot reach below 1300℃. From this, we can draw the following conclusion: In the normal production process, the erosion of slag on refractory materials is almost inevitable. In order to extend the service life of refractory materials, after a large number of industrial exploration tests, we found that Cr₂O₃ reacts with Fe₂O₃, Al₂O₃, FeO, and MgO in the slag to form a composite spinel (Mg, Fe)O(Al, Cr, Fe)₂O₃, which forms a dense protective layer on the surface of the refractory material and can prevent further erosion of the slag. The most effective way to prevent slag from penetrating into the interior of the refractory material is to increase the viscosity of the slag, which can be achieved by controlling the viscosity-temperature characteristics of the slag.
The viscosity-temperature characteristics of the company’s coal are poor, as shown in Table 1.
From the data in the table, it can be seen that the operating furnace temperature to achieve the critical viscosity of 25.42Pa·s has reached 1283℃, and when the furnace temperature is slightly reduced, its viscosity will increase sharply. In order to ensure smooth slag discharge, the operating furnace temperature must be increased. At this time, it is extremely difficult to form a protective layer. The refractory material is greatly damaged by long-term exposure to high temperatures! Therefore, high-temperature operation and failure to form a protective layer are another major reason for the short life of refractory bricks.
3. How to extend the service life of refractory lining
The service life of refractory lining is directly related to the operating cost and economic benefits of the entire plant. For this reason, we have classified and summarized the problems that have occurred, aiming to continuously optimize various indicators during operation.
3.1 Stable operating conditions
(1) Strictly follow the furnace drying curve to prevent the displacement of refractory bricks due to rapid changes in furnace temperature. In particular, the first maintenance of cold new bricks must be carried out strictly according to the time. The constant temperature time of each stage can only be extended, not shortened.
(2) During normal operation, the melting point of raw coal and coal slurry ash in the furnace should be analyzed every day. According to the analysis data, control the appropriate oxygen-coal ratio and control the furnace temperature. It is necessary to prevent heat accumulation caused by slag mouth blockage and direct damage to furnace bricks caused by excessive temperature.
(3) Switch the gasifier regularly during operation. The gasifier load should not be too high to ensure good burner atomization and reduce the chance of accidental jump. After the burner has been running for more than 30 days, it is no longer necessary to operate the joint operation. When the joint operation is carried out, try to ensure that the furnace temperature is above 900℃ to avoid vibration caused by low temperature feeding.
(4) Measure refractory bricks regularly. Adjust the center ratio and the position of the reaction center area in a timely manner according to the measurement data to keep the refractory bricks evenly used and prevent the “barrel” effect.
3.2 Optimizing the viscosity-temperature characteristics of ash
Due to the poor viscosity-temperature characteristics of coal ash, the operation flexibility is small and it is difficult to form a protective layer. Through analysis, the alkalinity in the coal ash composition is improper. Here we use (CaO+Fe₂O₃+MgO)/(SiO₂+Al₂O₃) to measure, and this ratio is generally between 0.1 and 1. When the ratio is about 0.5, the alkali-acid ratio is moderate, and the test coal sample has this ratio of 1.268, so some acidic substances should be introduced into the coal.
After the test, sand from the local desert was added to the test coal sample to adjust the alkali-acid ratio. Through the test, it can be seen that when sand equivalent to 40% of the coal ash is added to the test coal sample, the viscosity-temperature characteristics of the coal are improved, and the ash melting point is also reduced. The viscosity-temperature characteristics are shown in Figure 1.
4. Overall evaluation of operation effect
(1) With the enhancement of the ability to master the system and the continuous optimization of operation, the number of system start-up and shutdown times has decreased, and the number of joint operations has also decreased accordingly, reducing the number of “thermal shock” to the refractory bricks. Regular inspection of refractory bricks and replacement of process burners ensure that the system is within the controllable range. From the inspection of several furnaces, there are almost no large cracks in the refractory lining and the overall thermal expansion is good.
(2) By optimizing the viscosity-temperature characteristics of ash, the temperature operation elasticity of the gasifier is increased, and a protective layer is gradually formed on the surface of the refractory material, which is expected to increase the service life of the bricks.