Selection of refractory materials for glass melting furnace regenerator

01. Temperature and environmental conditions of regenerator

When the total height H of the checker and the upper and lower temperatures t1 and t0 in the regenerator structure and operating parameters are determined, the flue gas temperature ti at any level can be estimated according to the following formula, which can be used as one of the bases for selecting refractory materials. Therefore, the selection of refractory materials for the regenerator should meet the following use conditions:

(1) Temperature cycle changes;

(2) Oxidation/reduction effect;

(3) Solid fly erosion;

(4) The effect of volatile fly and condensate.

For the checker refractory material, it is also necessary to have a good heat exchange value to meet the needs of the checker thermal efficiency.

02. Reasonable selection of refractory materials

1. Checker upper layer

The temperature drop per meter in the regenerator is generally 80-100℃, and the highest temperature at the top of the grid body reaches 1380-1400℃. In the upper layer of the lattice body at a temperature above 1300℃, it is advisable to use directly bonded high-purity magnesium bricks. This brick is fired at high temperature (1780-1800℃) with high-purity fused sand. The content of CaO, SiO2 and Fe2O3 is low, and the periclase is directly bonded. It is difficult for the gas phase and liquid phase to enter the brick. The brick body has strong corrosion resistance and can reduce the capping phenomenon of surface bonding powder.

Since the SiO2 in the flying material will gradually enter the cracks of the brick body and change the CaO/SiO2 ratio of the matrix part, low-melting phase diopside CMS2, magnesium scapolite C2MS2, forsterite M2S and magnesium rhodonite C3MS2 will be formed, resulting in a large volume effect. The periclase crystals can also gradually grow under the action of alkali vapor, causing the brick body to crack, break and peel off, shortening the service life of the brick body.

In addition, if the concentration of V2O5 in heavy oil is high, it will react with CaO in magnesium bricks as follows:

In a non-weakly reducing atmosphere, calcium vanadate is in liquid phase, which penetrates into the brick to promote the growth of periclase crystals and also causes deformation of the brick.

At present, the proportion of ultrafine powder with quartz sand particles less than 0.1mm in the glass batch of domestic float glass melting furnaces is large. Many manufacturers do not use granular soda ash (heavy alkali), and the particle size is also fine. Therefore, in the upper layer of the regenerator grid body at the position of the 1# and 2# small furnaces near the feeding port.

2. Middle layer of the checker

The temperature of the middle layer of the checker is about 800-1100℃, and magnesium-chromium, forsterite and magnesium-aluminum refractory materials can be selected. Magnesium-aluminum materials have strong resistance to sulfate erosion, but are expensive. This type of refractory material has not yet been widely used in China. The use temperature of forsterite bricks should not exceed 1050℃, and they should be used in the low temperature zone of the middle layer.

There is a phenomenon of repeated liquefaction and solidification of sulfate in the middle layer of the checker. This is caused by the residual V2O5 carbon chain cracking catalyst in the heavy oil, which turns SO2 in the flue gas into SO3 and gradually corrodes the grid body refractory material. Its solidification expansion can cause corresponding stress embrittlement damage to the brick structure. Its chemical reaction formula is as follows:

Above 1000℃, sulfate will react with MgSO4 to form NaxMg(yS2O2)2, and the intensity of the reaction will increase with the increase of the Na2O/SO3 ratio. In order to improve the corrosion resistance of magnesia-chrome bricks, the Cr2O3 content should be increased as much as possible, and the direct bonding degree of the mineral phase should be increased so that the chromium spinel wraps the periclase particles, which can extend its service life.

3. The lower layer of the checker and other parts

The temperature of the lower layer of the checker is below 800℃, and the chemical corrosion is weak, but the total weight of a regenerator grid body is at least 40-50t, and the unit load of the lower layer of the grid body is as high as 8-10t/m2. In addition, there is a need to use the flame method to burn and clean the grid body. Therefore, it is advisable to use high-quality low-porosity clay bricks with strong creep resistance and good thermal shock resistance. In order to prevent the contact reaction between alkaline bricks and clay bricks, high-alumina bricks can be used as a transition layer between the middle and lower layers of the grid body.

Other parts of the regenerator include the crown, side walls and grate crowns, where the erosion of refractory materials is relatively weak. The crown of the general regenerator uses high-quality silica bricks, the side walls are divided into three parts, and the regenerator wall in the upper space of the checker uses high-quality silica bricks. The target wall can also use direct bonding magnesia-chrome bricks. From the part above the grate bar to the top surface of the grid body, the better solution is to use the same material as the checker in the same height section, which can extend the service life of the wall. Another solution is to use alkaline bricks or directly bonded magnesia-chrome bricks that are one level lower than the corresponding lattice material in the upper section, directly bonded magnesia-chrome bricks in the middle section, and low-porosity clay bricks in the lower section. First-level clay bricks can be used below the grate bar, and low-porosity clay bricks are generally used for the grate bar arch. It can also be made of fused AZS bricks with clay bricks arches.

03. Structural form of checker

In glass melting furnaces, the regenerator checker is usually arranged in Siemens and basketry styles with straight bricks. However, the lattice holes are often blocked. When the blockage is serious, measures such as hot repair and replacement of lattice bricks are taken. The hot repair conditions are very bad and the labor intensity is extremely high. The octagonal cylindrical bricks are used to replace the original straight bricks. The lattice body is chimney-shaped and not easy to be blocked. No hot repair is required during the entire kiln period. Just check regularly. If there is a small amount of blockage, the flame method is used to burn and clean the lower part of the checker from bottom to top.

One of the important energy-saving technologies for large glass melting furnaces is to promote the use of cylindrical lattice bricks. Octagonal cylindrical checker bricks retain the physical and chemical properties of the original straight bricks, and are easy to lay. The bricks are aligned up and down with basically no free hanging parts. The structure is stable, the heating area per unit volume of the checker is high, and the service life is long, which is increasingly valued. The wall thickness of the cylindrical brick can be reduced to 40mm, which not only reduces the weight of the unit checker body, but also increases the thermal conductivity. The cost of the cylindrical checker body is about 15% higher than that of the basket checker body and about 15% lower than that of the cross checker body. However, in terms of energy saving, the difference between the cylindrical checker body and the cross checker body is not much. The heat consumption of the basket checker body increases by 1% to 2% per year, and the heat consumption of the cylindrical checker body increases by about 0.5% per year. A lot of energy is saved due to the slowdown of "aging".

In the design of the heat storage chamber structure, special attention should be paid to the connection method of the cylindrical checker bricks and the grate bar arch. The Siemens arrangement of straight bricks should be used for transition between the cylindrical checker bricks and the grate bar arch, with a height of about 1m. In this way, the lattice holes can be smoothly connected up and down, and the uniformity of the gas entering the cylindrical lattice body can be improved, giving full play to the advantages of the cylindrical checker bricks and improving the thermal efficiency of the glass melting furnace.

04. Conclusion

At present, the regenerator of the domestic glass melting furnace has gradually changed from the traditional ascending path structure to a box-shaped partitioned or connected structure. Further strengthening the research on the rational selection of refractory materials for the regenerator, the use of partitioned configurations, and the development of new varieties can meet the requirements of improving the efficiency and mission life of the regenerator. It is of great significance for the production of high-quality glass in domestic glass melting furnaces and the early realization of the development goals of low energy consumption, high thermal efficiency, large tonnage scale and long kiln life.