Application and Construction of SiC Refractory Materials in Dry Coal Powder Water-cooled Wall Gasification Furnaces

1.1 Fire-resistant insulation structure: Dry coal powder water-cooled wall gasification furnaces are generally divided into upper and lower sections, with the upper section serving as the gasification chamber and the lower section as the quenching chamber. The gasification chamber is the core component of the entire gasifier. Coal powder and oxygen react in the gasification chamber at a temperature of about 1600°C. The gasification chamber is mainly composed of two parts: the internal components and the shell, which primarily include a membrane-type water wall, an annular space, and a vertical cylindrical shell. The internal components of the gasification chamber consist of: the water wall of the gasification chamber, the top cooler, the slag outlet cooler, the burner cooler, etc., along with the corresponding header tubes and connection pipes. The straight-section of the water wall in the gasification chamber features a tube bundle structure, while the upper and lower conical sections have a coiled tube structure. A thin layer of SiC refractory material is applied to the fire-facing side of the water wall tubes to protect them from the erosion caused by the high-temperature syngas flow and to prevent tube damage. The water wall is cooled and protected using cooling water under certain pressure. The internal components of the gasification chamber adopt a fire-resistant and insulating structure of \"water wall + refractory lining\", which allows the gasification furnace to function independently in the face of high pressure and high temperature. This arrangement enables the furnace shell to withstand high pressure while the water wall handles high temperatures, thus facilitating long-term operation of the water-wall gasification furnace. The main features of the refractory and thermal insulation structure for dry coal powder water-cooled wall gasifiers are as follows. (1) A stable solid slag layer can be formed outside the water wall, enabling \"slag to resist slag\" and protecting against the erosion and wear caused by gases and slag, thus creating a self-repairing refractory material structure that increases the service life of the material. (2) The water wall reduces the erosion rate of refractory materials, ensuring the operating temperature of SiC materials

From the above fire-resistant and thermal-insulating structures, it can be seen that the main functions of applying fire-resistant materials are as follows. (1) Prevent wear and corrosion of the membrane wall. (2) Prevent the membrane wall from experiencing excessive thermal shock. (3) Control the heat absorption of the water wall to maintain the temperature field in the gasification chamber of the gasifier at the desired level. 1.2 Selection of refractory materials 1.2.1 Operating environment The main factors considered in selecting refractory materials for gasification furnaces are the operating environment and the material properties. The impact of the usage environment generally takes the following aspects into consideration. (1) Maximum operating temperature and maximum operating pressure of the gasifier. (2) Medium atmosphere in the gasification chamber. (3) Slag discharge method of the gasifier grate. (4) Situation of particles carried by the scouring airflow. (5) Gasifier type and refractory lining methods, etc. The refractory materials selected must have a maximum allowable operating temperature that is higher than the highest operating (design) temperature of the gasification furnace, as well as sufficient fire resistance to withstand the high thermal loads within the furnace without becoming softened. At the same time, it should be noted that higher operating pressures accelerate the wear and erosion of refractory materials by water vapor and H2 in the syngas. 1.2.2 Material properties: The properties of SiC refractory materials generally depend on the bonding condition between the SiC particles, and there are many different types. The main performance indicators of SiC refractory materials are shown in Table 2.

The ramming material is prepared by mixing refractory aggregates with a certain gradation, powder materials, binders, admixtures, along with water or other liquids. It is generally used in areas where the amount of material used is small, or where the wear-resistant layer is thin (usually not exceeding 18δ60mm), as well as in areas with many pins such as corners. Hammering and vibration pouring are commonly employed for its application. Generally, the ramming material is a plasticizable substance, and the binder is phosphate; there is no need for high-temperature drying to remove moisture – only natural curing along with exposure to light from floodlights at a certain temperature is sufficient to enhance its strength. The lining made of rammed material has a low moisture content and is tightly compacted; its performance is superior to that of refractory castables made from the same material. The disadvantages are slow construction speed and high labor intensity, and it tends to be replaced by dry vibrated materials and high-quality refractory castables. The casting material has high fluidity and a wide range of applications; it is generally used in areas where a thick wear-resistant layer is required. It can be poured into machines, and vibration rods can be used to vibrate the material. Its strength is relatively high. Since cement and water are used as binders, it is necessary to dry the material by heating, and this process requires a strict heating curve as well as a considerable amount of time. Based on porosity, castables are divided into dense refractory castables and thermal insulation refractory materials with a porosity of 45% or higher; based on the binder used, they are classified into hydraulic-bonded, chemically bonded, and coagulated-bonded refractory castables. Laying SiC refractory material on the fire-facing side of the water wall in the gasification chamber serves both to reduce heat loss and to form a slag layer; this allows full utilization of the slag layer’s insulating properties, enabling the slag to protect the furnace walls and prevent the gasification furnace lining from being eroded. To ensure effective slag formation, in addition to stably controlling the temperature of the gasifier furnace and the steam-water system during operation, it is also necessary to select appropriate refractory materials based on the solubility of various oxides in coal slag at high temperatures.

2. Laying of refractory materials: To ensure the optimal performance of the refractory lining, it is necessary to strictly control the construction techniques and the quality of shaping of the refractory materials. The installation of refractory materials is generally carried out by the supplier of those materials. Before installation, a detailed construction plan and quality control measures must be established, and a qualified and experienced construction team should be selected. 2.1 Pre-construction preparations (1) Inspection of anchorages: The material, dimensions, spacing of the anchorages, as well as the welding techniques, must meet the requirements specified in the design drawings. All welded pins, claws, or other components shall undergo 100% visual inspection as well as a hammer test to check the quality of the welds; any loose or damaged fasteners shall be rewelded. (2) Equipment cleaning: The substrate and working area must be thoroughly cleaned before pouring to prevent impurities from entering the casting material. Mixers, conveyors, transport vehicles, etc. should also be thoroughly cleaned before use, with the equipment being cleaned once per shift. The mixer that has been used to mix plasticizable materials must be thoroughly cleaned before it can be used for mixing castables.

2.2 Control during construction process (1) Water quality and quantity: The water quality required for mixing refractory materials is clean potable water with a pH value within the range of 6.0 to 8.0; saltwater and other types of water containing suspended particles must not be used. For refractory materials that require water for mixing, the amount of water to be added must be strictly in accordance with the process requirements. (2) Temperature control: Generally, the base temperature for applying refractory materials should not be lower than 5°C or higher than 35°C; for applying mortar, the base temperature should be kept between 5°C and 30°C. Refractory materials must not be frozen before natural curing, nor must they become damp before furnace drying. (3) Molding The molding process is a very crucial step; the quality of the mold directly affects the quality of the casting. Template control refers to controlling its stability and the accuracy of its dimensions. After pouring is complete, the mold must not be moved or removed for at least 12 hours; before heating the furnace, it is necessary to thoroughly check that the mold has been completely removed. (4) Mixing and stirring: During mixing, dry mixing should be carried out first; after the dry mixture is well blended, water should be added gradually while stirring. The specific mixing time shall be in accordance with the written instructions provided by the manufacturer; it is strictly prohibited to stir the casting material for an extended period of time. (5) Vibration pouring: During the pouring process, it is necessary to constantly check the stability of the mold. After pouring is complete, gently tap the sides of the mold with a vibration rod to remove any bubbles. The component casting must be carried out in one continuous operation without interruption; after completion, any further work such as finishing or leveling is strictly prohibited. (6) The end position of the water wall lining material shall be compacted tightly using silicon carbide plastic; there shall be a smooth transition at the upper chamfer without any pits, and a serrated yet smooth transition shall be formed at the fins, with a neat finish. (7) Oil paper should be placed between the castable and the refractory fiber blanket, or a waterproofing agent should be applied to prevent water penetration. (8) During construction, expansion joints should be installed scientifically, in accordance with the design requirements and the actual conditions on site, to prevent through-cracks from forming in the cast material during operation. (9) At the locations where temperature and pressure measurement points are installed, during construction it is necessary to prevent the sleeves of these measurement points from loosening due to vibration of the pouring material, which could cause deviations in elevation and angle.

2.3 Maintenance and Repair Before starting the furnace heating, thermosetting, water-bonded, or chemically bonded refractory materials that have been installed recently must undergo natural curing for at least 3 days. During natural curing, the temperature of the refractory material layer should be maintained above 10°C; chemically bonded refractory materials must not get wet, and their surfaces should not be covered; thermosetting refractory materials do not require special curing, but they must not get wet or frozen before being fired. After completion of construction, a visual inspection of the lining surface must be carried out; the surface of the lining working layer should be smooth and even, without any crushing, peeling, bubbling, or through-cracks. All identified defects should be repaired promptly before baking the furnace. 3. Conclusion The dry coal powder water-cooled wall gasifier technology represents one of the more advanced modern clean coal technologies in the world today; however, its practical application still faces certain limitations. Regarding the selection and design of refractory materials, the following suggestions are offered: (1) Prefer domestic products for refractory materials. The dry coal powder water-cooled wall gasifiers are mainly of the Shell and GSP types, which are imported technologies. Gasification technology is protected by patents, and domestic authorized manufacturers usually only carry out adaptation designs, with generally no say in the selection of refractory materials. Technology patent holders often choose foreign products, which are costly and have long delivery times. SiC materials are already in widespread use in domestic power plant boilers; technology patent holders and authorized manufacturers should actively engage in discussions and exchanges, conduct a thorough study of the existing refractory materials in China, analyze their thermal conductivity, mechanical properties, etc., and select high-quality refractory materials suitable for dry coal powder water-cooled wall gasification furnaces. Based on digestion, absorption, and re-innovation, in collaboration with refractory material manufacturers, efforts are made to develop new types of insulation materials in order to reduce the difficulties associated with project implementation. (2) Develop construction procedures for refractory materials: Standards should be established for the design of key areas such as expansion joints and monitoring holes, as well as for requirements regarding water addition and mixing time. Since refractory insulation structures are considered non-standard equipment, there are no design specifications for such devices either domestically or internationally, nor is there any mature experience available for reference. Generally, the construction methods for refractory materials are developed by the manufacturers, but since each manufacturer has different technical capabilities and varying levels of understanding of the on-site conditions, coupled with differences in the skills of various construction teams, the quality of the linings in different projects varies greatly. Therefore, technology patent holders should actively communicate with licensed manufacturers and capable refractory material producers to develop construction procedures specific to different coal types and furnace designs, thereby facilitating project implementation and management.