Requirements for Refractory Castables in Various Parts of CFB Circulating Fluidized Bed Boilers

Circulating fluidized bed boilers are currently the most widely used and relatively environmentally friendly waste heat boilers in China. They are primarily used in the power industry, petrochemical industry, waste incineration industry, and other sectors. While their applications vary, their structure is largely the same. During boiler operation, high-temperature gases and solid particles in the gases cause severe wear on the refractory linings in various areas, including the furnace and circulation system. Therefore, the wear protection and thermal insulation requirements of these linings are critical to boiler operation. Taking power plant boilers as an example, the operating environment of various boiler components and the requirements for the refractory castable lining are as follows.

Lower Furnace

The dense phase area in the lower furnace is a highly wear-prone area in circulating fluidized bed boilers. The material within this area is coarse, highly concentrated, and fluidized at high speeds. The combustion atmosphere is somewhat corrosive. The wear-resistant refractory castable used in this area must be wear-resistant, have good thermal conductivity, and exhibit excellent resistance to thermal cracking and thermal shock. Steel fiber wear-resistant refractory castables are recommended.

Separator Inlet Flue Duct

The separator inlet flue duct experiences a change in airflow direction, resulting in fine ash particles and a high concentration. The airflow velocity is generally 4.5-6.0 m/s, and the temperature is 850-950°C. The combustion atmosphere is oxidizing, resulting in severe wear and erosion. The castable must exhibit excellent wear resistance, thermal cracking, and thermal shock resistance. Steel fiber wear-resistant refractory castables are recommended.

Cyclone Separator

The high airflow velocity within the cyclone separator causes centrifugal force, causing coarse particles in the airflow to collide with the wall and separate. This results in severe wear at the separator inlet and on the inner wall of the separator. The operating temperature within the separator is less than 1000°C, and the refractory and wear-resistant layer is relatively thick. The atmosphere is oxidizing. The castable must be lightweight, dense, wear-resistant, and have excellent thermal insulation (for adiabatic separators), as well as good resistance to thermal cracking and thermal shock. High-strength, wear-resistant, and refractory castables or wear-resistant, refractory plastics are recommended.

Return Material Transfer Unit

The return material transfer unit transports ash separated by the separator into the furnace for recirculating combustion. The return material transfer unit features a fluidized bed on one side of the furnace and a moving bed on the other. It is a fluidized, sealed ash conveying device. The fluidization velocity is less than 1 m/s, and the material inside is fine and highly concentrated. The operating temperature is approximately 900°C. Compared to other parts, the return material transfer unit's operating environment is not particularly adverse, but construction is challenging. Since combustion does not occur within the return material transfer unit, the castable must possess excellent thermal insulation, thermal cracking, and thermal shock resistance. High-strength, wear-resistant, and refractory castables are recommended.

Platenose Heating Surfaces

Power plant boiler furnaces feature platenose heating surfaces (water-cooled platen and superheater). The lower elbow of the platen is subject to wear and tear from ash particles. This requires the castable to exhibit excellent wear resistance and thermal conductivity. High-strength, wear-resistant, and refractory castables are recommended.

Precautions for High-Strength Wear-Resistant Castable Construction

The quality of high-strength wear-resistant castable construction directly determines its wear resistance and service life. Key considerations lie in controlling the water-cement ratio, ensuring density, and maintaining proper maintenance to prevent cracking and shedding caused by improper construction. Specific precautions can be divided into four key stages:

Refractory Castables in Various Parts of CFB Circulating Fluidized Bed Boilers

1. Pre-construction Preparation: Clean the base surface to remove oil, dust, and loose debris. Sandblast and roughen as necessary. Install anchors according to design requirements, ensuring uniform spacing (usually 150-200mm). Secure with welds and apply anti-rust paint.

2. Mixing: Use a forced mixer. First, add the dry castable and mix for 2-3 minutes. Then, strictly control the amount of water added according to the instructions (generally a water-cement ratio of 0.10-0.14). Mix until a uniform, lump-free paste forms. The mixing time should be minimal (5-8 minutes total). Avoid initial setting.

3. Casting Process: Use an inserted vibrator (30-50mm diameter) to vibrate the lining layer by layer, with each layer no thicker than 300mm. Vibrate until the slurry is smooth and free of bubbles, avoiding missed or over-vibration. Casting should be continuous, with the interval between each layer no longer than the initial setting time to prevent cold joints.

4. Curing and Baking: Cover with plastic sheeting or wet burlap for 12 hours after casting, maintaining an ambient temperature of 5-35°C for at least 7 days. After curing, bake slowly according to a heating curve to remove moisture and avoid cracking caused by rapid heating. Baking time varies depending on the thickness of the lining and generally takes 3-7 days.

Unshaped Refractory Lining Materials for Cupola Linings

Different from the traditional cupola ladle lining, the two types of ladles built with refractory ramming material or refractory castable material can greatly increase the working life of the ladle lining and reduce the number of spare ladles. What are the specific material choices for the two new types of ladle linings?

High-Alumina Refractory Ramming Materials

Refractory Ramming Material Lining

Ramming material is a general term for plastic refractory materials composed of refractory aggregate, binder, and moisture. The mineral composition of refractory aggregate coated with ramming material is generally bauxite, corundum, quartz sand, and quartz powder. The chemical composition is mainly composed of alumina (Al2O3) and silicon oxide (SiO2) two parts. Due to the different mineral compositions and particle sizes, various suppliers have developed a variety of packaging ramming material specifications. At present, the chemical composition of the lining ramming material, among which the higher the alumina content, the higher the use temperature, the greater the drying density, the stronger the thermal conductivity of the lining, and the longer the life of the lining.

Bauxite, corundum, quartz sand, and quartz powder in the lining ramming material are materials with high thermal conductivity. In addition, due to the size grading of refractory materials, the compaction density of ramming materials is larger. Therefore, the thermal conductivity of the lining ramming material is higher. If the ramming material is used completely to build the lining, the heat conduction of the lining causes the temperature of the iron liquid to drop more. Therefore, before using the lining ramming material to build the lining, a heat insulation layer of a certain thickness should be designed inside the cladding.

The ramming die must be prepared in advance to build the packing lining with ramming material. The ramming die is specially designed according to the ladle structure and lining to be located, and either has a certain inclination or has the performance of split disassembly. When building the lining, first use the ramming material layer by layer, ramming the bottom to a certain thickness. Then, the ramming mold is placed in the shell, and the ramming material is filled layer by layer to the mouth of the bag. Finally, pull out or remove the ramming die, and brush the surface of the lining with graphite powder coating.

The construction quality of ramming material lining has a decisive influence on the life of the lining, and the difference in construction quality may make the same material have different life. When building the lining with ramming material, it should be rammed tightly. The ramming compactness must conform to the drying compactness range of the lining material. The drying and roasting temperature and time of the packaging should also meet the technical requirements of the relevant products.

SiC Refractory Castable Lining

Refractory Castable Lining

Refractory castable is a class of amorphous refractory materials with cement as binder. The mineral composition of the refractory aggregate is the same as that of the refractory aggregate used for lining ramming. Calcium oxide (CaO) is an essential component of cement as the binder lining cement castable is high alumina cement. Therefore, the calcium oxide content can indirectly represent the cement content in the castable. Cement-lined castable can be divided into general cement castable, low cement castable and ultra-low cement castable. The lower the cement content, the higher the refractoriness of the material. Chemical composition range of coated low cement and ultra low cement castables. Among them, the higher the Al2O3 and the lower the CaO, the higher the refractoriness and the longer the life of the lining.

Before building the lining with refractory castable, it is necessary to make the casting mold with thin steel plate in advance, and the casting mold needs a certain taper to facilitate smooth pulling out after pouring. A certain thickness of heat insulation layer is set in the interior of the cladding to improve the heat preservation property of the ladle before constructing the cladding with cement castable. During construction, the bottom is covered with castable to the required thickness. After vibrating with a vibrator commonly used in concrete construction, the casting mold is fixed in the cladding. The castable is then filled layer by layer between the insulation layer and the casting mold. At the same time, vibrator is used to vibrate to the envelope, and the casting mold is pulled out in time after the initial solidification of the castable. Finally, the coating surface is coated with graphite powder.

Using low cement or ultra-low cement castable to build the lining is short in construction time, less in physical exertion, and easy to ensure the construction quality. At the same time, the lining can be repaired after partial damage. Therefore, the lining life is long and the consumption of tons of liquid iron refractory is low.

Unshaped Refractory Lining Materials for Cupola

Selection of Amorphous Refractory Castable for Cupola Lining

Cupola is subjected to high temperature and chemical erosion during melting. The melting time of cupola is 6~10 hours, the furnace lining erosion is large, and the furnace condition is unstable.

The corrosive atmosphere of furnace lining has a high requirement on the selection of refractory castable. It is better to use refractory castable with 70%~80% aluminum content in the feeding area and flow channel. Because of the high aluminum content, the high temperature strength and wear resistance and thermal shock performance are good, and the resistance to alternating hot and cold shock is strong.

The main parts of the cupola are hearth, bridge and slag separator. These parts have slag contact, chemical erosion is serious. Refractory castables with high aluminum content, due to the purity of the substrate itself, less impurities, strong resistance to iron wash, and very good oxidation resistance.

When selecting castable material for cupola lining, the slag composition should be considered, because there is a certain proportion of limestone in the burned material. Limestone has an alkaline composition, so the refractory castable used should have the ability to resist alkali erosion. That is, there should be a silicon carbide component in the castable, silicon carbide wear resistance and slag erosion resistance.

If it is alkaline erosion, magnesium or spinel are good choices. If spinel and silicon carbide are added to the castable, then this composite material is the most ideal material. However, silicon carbide and spinel should be added in different proportions according to the different slag composition.

If the top of the furnace lining can also be selected plastic, so easy to construction. Plastic should also consider the composition of ceramic combination, it is best to use phosphoric acid combination of plastic, do not use water glass combination. Because phosphate bonded plastic, strength and high temperature resistance are good. If castable is selected, plastic is still used for daily repair, which can shorten the maintenance time.

It should also be noted that in the furnace production process if the coke does this cupola melting heat source. It is necessary to control the quality of coke to stabilize the use of furnace lining.

In the final analysis, the refractory castable selected for the lining is still a refractory castable with high aluminum content. Because the pure castable has good wear resistance, the castable with silicon carbide or spinel composition has better corrosion resistance.

Rotary Kiln Refractory Castable Lining for Oxidation and Roasting of Chromium and Nickel-iron Ore

Chromium, nickel-iron ore oxidation roasting material for rotary kiln. A rotary kiln is a special kiln working under dynamic conditions. It is mainly used for calcination and roasting of cement clinker, petroleum coke, magnesia dolomite, limestone, iron oxide magnetization, nickel-iron laterite ore, and non-ferrous metal sulfide ore. Due to the particularity of the working conditions, there are higher requirements for the refractory lining to a certain extent, and the traditional refractory bricks are severely challenged in this type of kiln. According to the working characteristics of rotary kiln, Rongsheng Refractory Co., Ltd. ( http://www.aluminabricks.com/ ) has developed special refractory castables for the rotary kiln series.

Rotary Kiln Anti-Skinning Castable

Refractory Castable for Rotary Kiln

In addition to the common feature that refractory castables for rotary kilns can be used directly without firing, the production process is relatively simple among many types of unshaped refractory materials, and no special equipment is required. Low cement castable for nickel-iron rotary kiln. Many manufacturers' new dry-process cement rotary kiln production lines have improved their ignition and feeding production conditions, and their operating rates have increased year by year. However, there are still some problems with the use of refractory castables. Through comparative analysis, Rongsheng refractory manufacturers have made a summary of the use of refractory castables in the lining of rotary kilns.

1. Rotary kiln refractory castable configuration scheme

High-temperature alkali-resistant castables for preheaters, high-aluminum high-strength low-cement refractory castables for kiln tail smoke chambers and calciners. Corundum high-strength low-cement refractory castables for grate coolers and kiln mouths. Steel fiber high-aluminum refractory castables are used for the kiln hood, and high-aluminum low-cement refractory castables are used for the tertiary air duct. Special castable for corundum coal injection pipe for coal injection pipe.

2. Shedding phenomenon of refractory castable

In many cases, due to the unreasonable design of the configuration scheme, there have been many accidents of pouring material falling off the rotary kiln just over a year after it was put into operation, and the kiln shutdown failure rate is very high. The main reason is that the junction between the calciner and the tertiary air duct has been burned through many times, the shrinkage of the calciner has been worn out, and the tertiary air duct has fallen off. The head of the coal injection pipe burst and fell off three times, the low wall of the grate cooler was worn out, the kiln head cover was burned three times, and the arch beam of the kiln tail smoke chamber collapsed.

Magnesia Refractory Castable for Rotary Kiln

3. Analysis of the reasons for castable shedding

The change of fuel, especially after the use of anthracite, the coal ash content suddenly settled in a large amount, resulting in the slope of the kiln tail smoke chamber and the frequent shrinkage of the calciner. It needs to be cleaned manually with iron tools, which will inevitably cause mechanical damage to the refractory material. At the same time, the saturation ratio of the raw material entering the kiln fluctuates greatly, and chemical erosion and the infiltration of liquid alkali salts deteriorate the structure of the castable. Reduce thermal fatigue resistance, thermal shock stability, alkali corrosion resistance, and resistance to changes in oxidation and reducing atmospheres. Especially when used in kilns that start and stop frequently, the service life will be greatly shortened. Coupled with clinker dust erosion, erosion, and kiln temperature expansion due to cold and heat, many castables such as the kiln hood, tertiary air duct, and grate cooler have fallen off and failed.

Traditional cement refractory castables can obtain sufficient room temperature strength due to the high amount of cement. However, the crystal transformation of cement at medium temperature will significantly reduce the strength, which leads to the reduction of high-temperature strength and erosion resistance of the material.

Rapid cooling and rapid heating aggravate the frequent accidents of refractory materials. A large number of documents point out that the alkali expansion coefficient is large, and huge thermal stress is generated during the heating process. Therefore, when the kiln is baked, the temperature should be raised slowly, so that the expansion of the kiln system can supplement the expansion of the refractory material to play a compensating role. This is the key to how to use good refractory materials. However, it is difficult to accept a long kiln drying time in actual production, and when a problem is found, the high-temperature fan is directly turned on to quickly cool the kiln temperature. In order to grab the output after the overhaul, it is required to raise the temperature quickly, which paves the way for the damage of the refractory material.

4. Solutions to castable shedding

Different grades of refractory castables are selected for different parts of the pre-calciner kiln system. The shrinkage of the calciner is made of anti-skinning refractory castable, and the kiln hood is made of steel fiber high-aluminum refractory castable. The low wall of the grate cooler is easy to wear and tear, so mullite high-strength wear-resistant castable is used. Corundum castables are easy to burst and fall off when used on coal injection pipes, so steel fiber high-aluminum high-strength wear-resistant refractory castables are used instead, and high-aluminum refractory castables are used for quick repairs during emergency repairs.

Since the castables currently used are basically not crusted, it not only reduces the work intensity of the inspectors but also improves the running time of the rotary kiln stop material cleaning and decomposition furnace shrinkage crusting, which greatly enhances production efficiency.

Rongsheng refractory manufacturer, Rongsheng refractory manufacturer, is a refractory manufacturer with rich production and sales experience. In the design and solution of rotary kiln refractory lining materials, we also have many customer cases. Please contact us to design the configuration of refractory materials for the insulation layer of the rotary kiln for free. Our new type of insulation material products are of superior quality and can effectively reduce the temperature of the outer wall of the rotary kiln.

Refractory Anchors the Skeleton of Refractory Castable Lining

The Refractory Anchor is a non-metal or metal component installed on the furnace shell or steel structure supporting the furnace lining and is embedded in the lining body for anchoring and connection. It is an important part of modern unshaped refractory linings and a key technical measure to improve the service life of linings. Especially it is used as the skeleton structure of refractory castable lining.

RS High-Quality Refractory Anchor Bricks

Classification of Refractory Anchors

Anchors are divided into two categories: metal anchors and non-metal anchors, including supports, thrusts, and anchor nails. Hanging bricks are also refractory anchoring bricks(http://www.aluminabricks.com/refractory-bric/anchor-brick/). The technical requirements for anchoring bricks are high strength and good thermal stability. Therefore, three-level clinker is generally used as the raw material and fired at high temperatures. For example, the chemical composition of high alumina anchor bricks in a factory is mainly A1203≥50% and Si02≥46%. Refractoriness is 1750℃, strength is 3.8MPa, 1200℃ linear expansion coefficient is 5.8X10-6K-1, 1450℃ returning linear change +0.1%, load softening starting temperature is 1430℃.

There are various shapes and connection methods of anchor bricks, and their sizes are also different. The most commonly used anchor bricks are 330, 380, 440, and 500mm in length, and the thickness is generally about 100mm. Mainly used for refractory castable lining. The shape without holes is generally used in the refractory plastic lining.

Metal anchors are generally made of ordinary steel, heat-resistant steel is the best, but it increases the cost of thermal equipment. Metal supports and thrusts are generally made of heat-resistant cast steel.

Metal anchors are easily oxidized at high temperatures, so they should be buried in the lining. The distance between the hot end and the working surface of the lining body and the thickness of the dependent body need to be determined according to the operating temperature, generally 20-200mm.

Metal anchors are generally welded to the furnace shell and have various shapes. The most commonly used ones are Y, V, and L shapes. There are also several shapes of support and thrust. The former is used in higher furnace walls and the latter is used in furnace roofs with steep slopes.

The installation holes or necks of the anchor bricks shall not have cracks and the dimensions shall be accurate. The welding between the metal anchor and the furnace shell should be fully welded, and there should be no false welding or missing places, and the size should be accurate. The ceramic anchors and metal anchors of the refractory fiber lining also have various shapes and sizes, and their function is mainly to fix the refractory fiber lining.

Masonry of Refractory Anchor Bricks

How to Build Refractory Anchor Bricks

The anchor brick is a special type of refractory brick that adopts the hanging form to support the structure of the furnace roof and furnace wall. It can also be called hanging bricks and anchors. The anchor bricks are used to prevent the leakage of reactive gas and hot gas in the furnace from damaging the metal hangers and causing the furnace roof to collapse.

Industrial kiln and flue walls are composed of the inner lining, heat insulation layer, and outer wall layer. The refractory bricks for the lining and the fired bricks for the outer wall need to be connected with anchor bricks. The anchor has high tensile and flexural strength, especially the stress generated at the groove is blocked at the rib and cannot be transmitted. Therefore, it is not easy to break when the anchor brick is used in this structure.

Hanging Anchor Bricks

Precautions for refractory anchoring brick masonry

1. When laying bricks, apply mortar carefully from both sides to prevent gas from leaking from the stovetop.

2. The hanging part of the anchor brick should not be damaged.

3. Ensure the size of the joints, accurately determine the position when laying on the top, and let the hanging metal parts play a private role.

4. Expansion joints should be properly left between the furnace body and the anchor bricks.

5. Air hardening mud should be used for laying anchor bricks.

6. The arrangement of anchoring bricks should be determined according to the range and frequency of temperature changes and the size of the direct wall area. Usually, 6 anchor bricks are used per square.

7. When the masonry is close to the anchor brick position, the lower row of bricks should be made in advance to determine the accurate position of the anchor brick. Use a wire brush to thoroughly clean the welding parts of the metal shell, and use welding rods that are compatible with the welding parts to firmly weld the anchor pipe.

8. After the anchoring brick is built, insert the anchoring hook. Fill the airstrike with refractory fiber felt and plug it tightly to form a certain degree of protection for the anchors.

9. The thickness of the anchorage hook has certain requirements. Not too thin, too thin will burn slowly and cause early damage to the lining. The heat-resistant steel material of the anchoring hook must be selected according to the national standard heat-resistant steel to meet the demand.

 Learn more about the refractory anchors, please contact us by email: info@aluminabricks.com.