Application of silicon carbide ramming material in the water wall of coal chemical gasification furnaces

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Application of silicon carbide ramming material in the water wall of coal chemical gasification furnaces Abstract； The article introduces the types and characteristics of the water-cooled wall in gasifiers ； The effects of coal ash melting point, viscosity, furnace temperature, ash content, and refractory ramming material on slag attachment to the water wall were analyzed. The introduction of Cr2O3 improves the resistance of silicon carbide ramming mix to slag erosion on water-cooled walls. The addition of CHC-3 desiccant reduced the drying time and lowered the drying temperature. The ramming material of the SiC-Al2O3-Cr2O3 system helps with slag accumulation on the water wall, achieving an effect of protecting the slag with more slag. Coal chemical engineering is an important method for the clean and efficient utilization of coal; it converts carbon-containing raw materials into CO and H2 through high-temperature, high-pressure chemical processes, which are then used in areas such as ammonia synthesis, methanol production, coal-to-hydrogen production, and olefin-based power generation. The core of the gasification process is the gasification unit. A vaporization furnace for chemical reactions under high temperature and pressure. There are over 10 types of gasification furnaces, which can be divided into two categories based on the form of the raw material ； Powder coal gasifiers and water-coal slurry gasifiers. Based on the structure of the gasifier furnace lining, it can be divided into the refractory brick structure represented by Texaco, and the water wall structure represented by Shell. Shell, Texaco 1. Characteristics of the water wall in the gasifier ； Pulverized coal and oxygen react in the combustion chamber of the gasifier at a temperature of around 1400°C to 1600°C, under an operating pressure of about 4.0 MPa. The water wall of the gasifier is divided into three parts: the main water wall, the upper coil, and the slag outlet coil. Water wall lining. The water wall and quench chamber are made of heat-resistant low-alloy steel. On the fire-facing side of the water wall, heat-resistant steel studs and a 20-mm protective layer of silicon carbide ramming material are welded. Thanks to the protection provided by silicon carbide and the subsequently formed solid slag layer, the temperature of the water in the water wall tubes remains around 270°C. There are 2 arrangements for the water tubes in the water wall ； tubular ； Such as Shell Sehll four-nozzle design, with the syngas swirling upward as it exits the furnace ； ; Coiled-type space furnace with a single nozzle, with syngas exiting downward. The water wall adopts a fully sealed vertical tube (coiled tube) water wall structure; its wall temperature is about 110°C lower than that of the gasifier, whose furnace wall is made of refractory bricks. The water wall converts the heat dissipation from the original refractory brick furnace wall into steam, resulting in a higher efficiency for the gasifier. The wall temperature will not be higher than that of the water-cooled wall, so there is no issue of overheating of the furnace wall. Compared with the furnace chambers of gasification furnaces using refractory bricks, water-cooled wall gasification furnaces require less investment, have a longer service life, lower maintenance costs, and only need about 5 days for drying – offering significant economic benefits. In recent years, a large number of domestically produced water-cooled wall space furnaces have been put into operation, including the second-generation Tsinghua furnace and the SE-2 Dongfang furnace. The schematic diagram of the water-cooled wall gasifier is as follows ； The HTL space furnace features a coiled membrane-type water wall structure; the SE-Eastern furnace uses a vertical tube membrane-type water wall structure. 2. Factors affecting slag accumulation on the water wall: During normal coal-firing operation of the gasification furnace, coal powder, oxygen, and steam are injected into the combustion chamber at high speeds through nozzles. These jets create suction effects and high turbulence, thereby facilitating thorough gasification reactions. The molten liquid slag formed by instantaneous combustion is thrown onto the surface of the water wall by the swirl generated by the burner, and it solidifies under the cooling effect of the circulating water in the water wall, adhering to the surface of the refractory material to form an initial slag layer. Under the combined effect of the furnace temperature and the cooling water, the surface of the slag layer continuously solidifies and thickens or melts and thins; as the initial thickness of the slag layer increases, the thermal resistance rises gradually, and the cooling effect of the water-cooled walls decreases progressively. After the slag layer inside the gasifier reaches dynamic equilibrium, the slag layer on the water wall can be divided into a flowing layer, a transition layer, and a fixed layer. The fluidized layer and transition layer are continuously renewed as the gasification reaction proceeds. The thickness of the slag layer is directly related to the safe operation of the gasifier. The melting point of coal ash, its viscosity, the furnace temperature, the oxygen-to-coal ratio, and the properties of the refractory material used in filling have a significant impact on slag formation. 2.1 Melting point: Under high temperature and pressure, part of the coal powder turns into CO and H2, while another part forms ash and becomes slag. The melting point and viscosity of coal ash mainly depend on its composition. Coal ash consists of a mixture of silicates, sulfates, and various metal oxides; the main components include SiO2, Al2O3, Fe2O3, CaO, MgO, K2O, Na2O, TiO2, SO3, etc. Typical values are as follows. Project: Al2O3, SiO2, Fe2O3, CaO, MgO, K2O, Na2O; Composition %: 13–21, 30–45, 7–11, 3–15, 2–4, 1–3, 2–9; Eutectic temperature °C: K2O-Na2O, 800–1000. SiO2-Fe2O3-CaO-Al2O3, 1200 SiO2-CaO, 1544. SiO2-Fe2O3-CaO, 1204. The higher the contents of CaO-SiO2-Al2O3, 1553 SiO2, and Al2O3, the higher the ash fusion point. The higher the contents of Fe2O3, CaO, MgO, K2O, and Na2O, the lower the ash melting point and the lower the viscosity. It is not conducive to slag formation. The ash fusion temperature range for coal types should be 1150–1350°C. 2.2 Operating temperature ； The operating temperature of the gasifier combustion chamber has a direct effect on the oxygen-coal ratio at the burners, which in turn affects the heat load on the water-cooled walls, carbon conversion rate, cold gas efficiency, etc. As the oxygen-to-coal ratio increases, the temperature rise in the high-temperature zone of the gasifier’s water wall, at temperatures above 1550°C, is significantly greater than that in the low-temperature zone; this leads to **increased damage to the pins and SiC ramming materials on the water wall in the high-temperature zone**. Therefore, the control of the oxygen-coal ratio in the main burner is crucial for the integrity of the water wall. ②The oxygen-coal ratio is low, the gasification reaction temperature is low, resulting in incomplete combustion. Unreacted fine coal powder and fly ash, along with the gas, cause increased erosion and abration of the water wall lining and studs, leading to a decrease in the thermal conductivity of the water wall and even its damage. 2.3 Ash content: When the ash content in coal powder is too low, there is significant heat loss in the gasification furnace, which hinders the protection of the furnace walls against slag formation; in some cases, no slag can even form, thereby affecting the service life of the gasification furnace. When the ash content is too high, a portion of the carbon surface is covered by ash, reducing the contact area between the gasifying agent and the carbon and thus lowering the gasification efficiency. This also leads to faster erosion of the furnace’s internal components by the slag. The ash content in coal is optimal when it falls within the range of 12% to 25%. 2.4 Reaction between the refractory material of the water wall and slag, and its effect on slag adhesion to the water wall. At the beginning of furnace operation, slag adheres directly to the surface of the refractory lining of the water wall. When the device is operating normally, influenced by factors such as furnace temperature and coal quality, the viscosity of the slag decreases. The water cooling walls of the gasification furnace become thinner, and the liquid molten slag does not adhere tightly to these walls. As a result, the ash and slag erode the refractory materials on the surface of the water cooling walls; this erosion also affects the studs and silicon carbide packing materials located on those walls. As the temperature rises, the solubility of various oxides in the slag increases; at the same temperature, the solubility of Cr2O3 in coal slag is the lowest. Thereby ensuring the integrity of the material ； The slag has poor wettability for Cr203 material, thereby slowing down Cr203. Dissolution in slag ； After the Cr203 material dissolves into the slag, it increases the viscosity of the slag; it can even form a uniform protective layer on the surface of the material, blocking its pores and preventing the slag from penetrating. Sulfur in coal is a harmful substance; gases such as H2S formed at high temperatures can reduce the viscosity of slag. Fe and SO2 form an eutectic point at 1170°C, which severely erodes the heat-resistant steel studs and water wall tubes. The thickness of the slag layer decreases, and the slag on the water wall comes into direct contact with the studs and refractory materials on the water wall, resulting in severe erosion. Thereby shortening its service life. The volume erosion rates of various oxides and SiC in cinder are as follows ； Volume erosion rate %: Cao, SiO2 – 100, 90; SiC, Al2O3 – 75, 60; 3, 45; Cr2O3 – 30, 15, 0. Temperatures: 1350℃, 1400, 1450, 1500, 1550, 1600, 1650, 1700℃ for 3 hours. It can be seen from the graph that Cao, SiO2, as well as SiC, Al2O3, and Cr2O3, have poor resistance to slag erosion. Cr2O3 has the lowest solubility and poor wettability in coal slag, thereby delaying its dissolution in the coal slag. When Cr203 material is dissolved in slag, it increases the viscosity of the slag; it can even form a uniform protective layer on the surface of the material, blocking its pores and preventing the slag from penetrating. Cr203 itself possesses excellent resistance to slag erosion. The addition of Cr2O3 to the ramming material for water wall refractories significantly improved resistance to slag erosion. 3 Determination of the refractory lining for ramming on the water wall: To prevent the water wall lining from being corroded and damaged by coal slag, to increase heat output, and to ensure smooth flow of slag on its surface, a wear-resistant lining with a thickness of 14 mm to 25 mm is rammed onto the surface of the water wall. In order to remove the heat from within the furnace through the tube banks, it is necessary for the wear-resistant lining to have high thermal conductivity as well as good resistance to slag erosion in a reducing atmosphere. With the assistance of relevant experts from Wuhan University of Science and Technology and Sinopec Anqing Branch, and by incorporating advanced foreign technologies, a silicon carbide ramming compound based on the SiC-Al2O3-Cr2O3 system for use in the furnace of gasifiers has been successfully developed. The compounds formed through covalent bonding in silicon carbide exhibit characteristics such as chemical stability, high-temperature resistance, wear resistance, erosion resistance, and corrosion resistance. Silicon carbide also has a very high thermal conductivity—64.4 W/(m·K) at 500°C—enabling it to rapidly transfer heat from the slag layer on the water-cooled walls to the walls themselves. The main crystalline phase of Al2O3 at high temperatures is α-Al2O3 (corundum). Density 3.9 g/cm3, refractoriness 2250°C. Mohs hardness 9.0. High wear resistance and corrosion resistance. Cr2O3 has an α-Al2O3 crystal structure, a melting point of 2435°C, and good stability against acids and bases; it can form solid solutions with oxides such as Al2O3 and SiO2. Increasing the viscosity of the slag effectively prevents its penetration. The silicon carbide ramming material for gasifiers is prepared under specific conditions from high-purity, large-grain silicon carbide, corundum, ultra-fine chromium corundum powder, an aluminum-chromium phosphate binder, and additives. It belongs to a hard ramming material that can cure in air. Its strength arises from the dehydration and concentration of the colloidal binder at temperatures ranging from 35°C to 240°C, followed by polymerization reactions, as well as chemical bonding with silicon carbide, corundum, and fine powders. Refractory materials such as aluminum chromium phosphate, high-temperature oxide micropowders, corundum, chromia corundum, and nanoscale Cr2O3 undergo ceramic bonding at temperatures between 1100°C and 1450°C, resulting in the formation of polycrystalline composite phases. At high temperatures, nanoscale chromium oxide and aluminum oxide powders form a solid solution. It enhances the erosion resistance and wear resistance of the lining. In refractory linings for water-wall applications, the content of SiO2 should be reduced. Whether in its free or bound form, SiO2 in refractory materials can react with hydrogen gas inside the gasifier, producing gaseous products that escape from the refractory material. This process destroys the original structure of the refractory material, reducing its strength and accelerating its degradation. The chemical reaction equation is: SiO2(s) + H2(g) → H2O(g) + SiO(g). 4. Product properties 4.1 The properties of GSJ-75 and imported materials are shown in Table 1 ； (Applicable to furnace linings) Table 1 Items: GSJ-75, SiC75 P (imported). Bulk density (g/cm³): at 110°C for 24 hours – 2.60, 2.55; at 1100°C for 3 hours – 2.54, ----. Compressive strength (MPa): at 110°C for 24 hours – 85, ----; at 1100°C for 3 hours – 82, 54.4. Flexural strength (MPa): at 110°C for 24 hours – 15.6, ----; at 1100°C for 3 hours – 15.0, ----. Linear change after firing (%): at 1100°C for 3 hours – -0.2, -0.2. Thermal conductivity (W/m·K): at an average temperature of 1000°C – 6.502; at an average temperature of 500°C – 3.85. Wear resistance (cc): at 1100°C – 5.1, 6.5. Chemical composition (%): Al₂O₃ + Cr₂O₃ – 21.31, ≤ 26; Fe₂O₃ – 0.30, ≤ 0.2; SiC – 74.22, ≥ 70. The properties of GSJ-85 and imported materials are shown in Table 2 ； (Used for wall boxes and fluid grid linings) Table 2: Product GSJ-85 RGB F85LC (imported). Volume density: 2.65 g/cm3 at 110°C for 24 hours; 2.6 g/cm3. 2.58 g/cm3 at 1100°C for 3 hours. Compressive strength: 95 MPa at 110°C for 24 hours; ——. 97 MPa at 1100°C for 3 hours; ≥80 MPa. Flexural strength: 17.1 MPa at 110°C for 24 hours; ——. 17.6 MPa at 1100°C for 3 hours. Percentage change in linear dimension after firing: -0.3% at 1100°C for 3 hours; -0.2%. Thermal conductivity: 6.731 W/m·K at an average temperature of 1000°C; 3.85 W/m·K at an average temperature of 500°C. Wear resistance: 4.3 cc at 1100°C; ——. Chemical composition: Al2O3+Cr2O3: 10.1%; Fe2O3: 0.1%, ≤0.1%. SiC: 85%, ≥84%. 4.3. Performance of the highly wear-resistant linings CAJ-3.1RC and AA-22S is shown in Table 3 ； (Applicable to gas transmission pipelines) Table 3: Specifications for CAJ-3.1 RC AA-22S. Volume density: g/cm3 – 3100 at 110°C for 24 hours; ----; 3050 at 1100°C for 3 hours; 2500 at 400°C. Compressive strength: MPa – 90.0 at 110°C for 24 hours; ------; 113.0 at 1100°C for 3 hours; ≥60 at 800°C. Flexural strength: MPa – 115.0 at 110°C for 24 hours; ------; 12.0 at 1100°C for 3 hours; ------. Linear change rate after firing: % – 11.0 at 1100°C for 3 hours; ------. Thermal conductivity: W/m·K – -0.20 at an average temperature of 1000°C. Wear resistance: cc – 2.5 at 1100°C; <6.5. Chemical composition: % Al2O3 – 84.1; Fe2O3 – 0.1; Cr2O3 – 10.5. 5. Usage methods: 5.1 Set up scaffolding inside the furnace and provide sufficient lighting. Remove the old lining that needs to be repaired from the water-cooled walls, clean away any loose cast material on the furnace walls and pipe walls, and use a vacuum cleaner to remove any loose particles and dust. Then, weld the fixing nails in accordance with the design requirements. (See the cleaned water wall in the figure on the right.) 5.2 The construction temperature for the refractory ramming material used in the gasification furnace should be between +5°C and +30°C. When the ambient temperature is below +5°C, the construction area must be preheated to the required temperature using appropriate heating methods. If the temperature at the construction site exceeds +30°C, cooling measures must be taken to meet the construction requirements. 5.3 During construction, the ratio of the unfired refractory materials must not be altered arbitrarily; no water or other materials should be added to the mixed and cleaned water wall refractory ramming material without permission. Refractory ramming material is mixed using a special mixer, and the mixer as well as the feeding hoppers and weighing containers must all be cleaned thoroughly. Refractory lining materials are installed by manual tamping; the mixed refractory mass must be applied within 30 minutes, and refractory materials that have begun to set cannot be used. The water used for mixing refractory materials must meet the **\"drinking water\" standards** and be kept within the range of +5°C to +25°C. The tools used for construction and the containers for holding refractory materials should be clean and ready for use. 5.4 Silicon carbide ramming mixtures and chromia-alumina ramming mixtures are prepared from powder materials, curing agents, and high-temperature binders. Each bag of the mixing material (powder) weighs 25 kg, while the curing agent weighs 0.75 kg. High-temperature colloid: 30 kg per barrel. 5.5 The mixing ratio for silicon carbide ramming material is as follows: the powder form of this material has its plasticity adjusted using high-temperature colloids, and no water may be added to the mixture. 5.6 A HOBART-type high-power mixer is used for construction. The Type B impellers and the mixing tank body shall be made of stainless steel, and the clearance between the impellers and each side of the mixer shall not exceed 0.25 inch (6 mm). If the gap is too large, or if the amount of refractory material in the mixer is greater than or less than 1/3 of its capacity, the mixing efficiency and the quality of the material will decline. Increase the stirring speed to between approximately 175 and 210 revolutions per minute; either too fast or too slow a stirring speed will affect the stirring time. 5.7 Depending on the capacity of the mixer and the amount used each time, the silicon carbide ramming material powder is first added to the mixer; after mixing for 1 minute, four-fifths of the high-temperature colloid is added, and after mixing for 3 to 2.4 minutes, the curing agent is added and mixing continues for another 2 to 3 minutes, after which the remaining high-temperature colloid is added. Until well mixed. 5.8 The mixed silicon carbide ramming material should be carefully placed, in blocks, at the appropriate locations on the surface where the silicon carbide ramming material lining for the water wall is to be installed. Each layer added should not be thicker than 20 mm; it should be compacted using a rubber mallet, with the heads of the mallets overlapping by 2/3. The compaction process should be repeated 2–4 times to ensure full density and the absence of any gaps. It is particularly important to ensure that the ramming material fills the area around the pipe fittings completely. Knead until compact. The silicon carbide refractory lining 5.9 of the water wall after construction aims to minimize the amount of high-temperature colloid and curing agent used, while still ensuring proper application of the silicon carbide ramming material. The optimal amount of curing agent to add is around 2%. 5.10 The construction of the fire-resistant rammed material lining shall result in a smooth surface free of voids, peeling, or inclusions. Individual cracks smaller than 2 mm can be filled and repaired, while those larger than 2 mm must be addressed in accordance with relevant repair standards. 5.11 The silicon carbide ramming material after construction shall be cured naturally in a dry and well-ventilated environment at temperatures above 10°C for 2–5 days. If the humidity is too high, an air dryer can be used to ventilate the silicon carbide ramming material lining. 6. Furnace Drying System 6.1 The silicon carbide ramming material and chromia-alumina ramming material used in gasifiers are air-hardening ramming materials, which are two-component composite ramming materials. Dry powders, colloids, setting agents, special moisture-evacuating agents, etc., are mixed in a forced-action mixer to form an putty-like paste with high plasticity. This clay material contains free water, crystalline water, and volatiles generated by condensation and polymerization reactions of colloids at certain temperatures, which cause thermal stress within the lining. To allow physical water, crystalline water, and volatile substances to evaporate slowly, it prevents rapid evaporation of moisture in the hot lining, which could otherwise generate high stresses and damage the lining structure. It also enables the refractory lining material to expand evenly, slowly, and fully when heated, so that its properties reach those required for use; this prevents heat stress concentration from causing damage to the lining, ensuring that no cracks or peeling occur in the refractory lining during the operation of the gasification furnace. 6.2 To accelerate the evaporation of gases such as water, a special dehumidifying agent was added to the compacted material; it decomposes at temperatures between 50°C and 100°C to form nanoscale capillaries, which facilitates the rapid removal of water and volatile substances from the colloid in the compacted material. The furnace drying time has been reduced from 84 hours to 64 hours. By reducing the maximum temperature of the furnace during drying from 450°C to 230°C, superheated steam in the furnace walls’ pipes can be used as a heat source for drying, eliminating the need for a dedicated drying machine. It not only saves maintenance costs but also shortens the maintenance cycle. For use in gasification furnaces, the ramming materials GSJ-75, GSJ-85, and CAJ-3.1 are suitable for the following furnace heating curves. 500℃, 400, 300; constant temperature at 230℃: 200; constant temperature at 100℃: 100; 0, 6, 12, 18, 24, 30, 36, 42, 48, 54, 60, 66, 72, 78 hours. 4. Performance in practice: Since its introduction to the market in 2008, the silicon carbide ramming material for water-cooled wall gasifiers has been widely used in the gasifier furnaces and gas transmission pipelines of various units, including Sinopec Anqing Branch’s fertilizer plant, Henan Longyu Coal Chemical Co., Ltd., Henan Zhongyuan Dahuagroup Co., Ltd., Xinxiang Zhongxin Chemical Co., Ltd., Tianjin Huaneng Power Plant, and Luoyang Yonglong Energy Chemical Co., Ltd. Under normal operating conditions, by adjusting different coal qualities and coal-to-oxygen ratios, it is possible to effectively control the melting point and viscosity of coal ash as well as the temperature in the gasifier furnace, thereby ensuring the optimal thickness and enabling the use of slag to protect against further damage. The SiC-Al2O3-Cr2O3 ramming material can form a solid solution at high temperatures, preventing the penetration of coal slag into the lining made of this ramming material. This ensures that the service life of the lining yields satisfactory results. This product has been granted a **patent (patent number ZL 20111 0256439.5)**. Slag deposition on the water wall of Shell’s gasifier after 10 months of use