Is the incinerator made of castables or hanging bricks?

Is it better to use hanging bricks or castables for the lining of the waste heat furnace of waste incineration?

The classification of domestic waste is one of the core measures of the country to promote the construction of ecological civilization. With the gradual advancement of domestic waste classification, the calorific value of waste has increased significantly. This is because after classification, the proportion of kitchen fruits in the incoming waste has decreased, the proportion of paper and rubber has increased, and the leachate yield has decreased. After the implementation of the domestic waste classification policy in Shanghai in July 2019, Jia Yue and other studies showed that compared with mixed disposal, the density of dry waste decreased by about 36.4%, the moisture content decreased by about 36%, and the low calorific value of dry waste suddenly increased to 13160kJ/kg. Xu Zhenwei et al.4 took a domestic waste incineration plant in Shanghai as an example to study the comparison of calorific value distribution before and after the classification of domestic waste in Shanghai. Before classification, the calorific value of garbage entering the incineration plant was 6208~7165kJ/kg, with an average of 6634kJ/kg; after classification, the calorific value of garbage entering the incineration plant was 7490~7997kJ/kg, with an average of 7749kJ/kg. The calorific value of garbage increased by 1115kJ/kg, an increase of 17% year-on-year.

"Clean, low-carbon, safe and efficient" is the path to high-quality development of the waste incineration industry. In terms of waste heat boiler lining materials, Figure 1 lists the layout methods of refractory materials for flue ducts of typical waste heat boilers in China and abroad (Japan, the United States, and the European Union). It can be clearly seen that the domestic method is mainly castable, while the foreign method is usually mainly brick hanging. This is because the combustible content per unit mass of garbage before classification is low, and the calorific value of the garbage entering the furnace is low. The castable refractory materials can fully meet the environmental protection requirements (850℃, 2s), that is, the temperature of the flue gas staying for 2s is higher than 850℃. The calorific value of the garbage entering the furnace abroad is high (usually higher than 8372kJ/kg), and the flue refractory materials are mainly in the form of hanging bricks, which can make the flue gas heat efficiently absorbed by the working fluid in the tube, thereby designing an economically suitable radiation heating surface.

If the heat in the high-temperature flue gas cannot be efficiently absorbed by the working fluid in the tube, it will lead to overheating in the furnace, causing serious failure forms such as castable degradation and cracking. At this time, the corrosive substances in the flue gas will corrode the water-cooled wall and produce high-temperature corrosion, which will bring challenges to the long-term stable operation of the waste heat boiler of the waste incinerator. In order to adapt to the increase in the calorific value of garbage, the current choice of castable materials for the flue of the waste heat boiler has changed from high aluminum with low thermal conductivity (~2W/(m·K)) to silicon carbide (~6W/(m·K)). At the same time, the composition of corrosive substances in the waste incineration boiler is complex, and the neutral silicon carbide (SiC) material has outstanding resistance to acid and alkali corrosion. Therefore, regardless of the castable or brick hanging method, the flue of the waste heat boiler is a refractory material with SiC as the main body. Table 1 lists the performance comparison analysis of the two methods of refractory materials. It can be seen that the hanging bricks are superior to castables in terms of porosity, mechanical properties, and thermal conductivity.

In order to analyze the feasibility of applying the brick hanging method to the radiation heating surface of domestic waste incineration waste heat boilers, this paper compares and analyzes the refractory castables method and brick hanging method of the flue refractory material of the waste incineration waste heat boiler from the aspects of material inspection, heat exchange effect, economic benefit, etc., in order to provide a certain reference for the optimization design of the heating surface of the waste incineration boiler.

Material inspection

Material oxidation mechanism

The water vapor content in the flue gas of domestic waste incineration is high, especially when direct water spraying is used for cooling, the water vapor content is between 20% and 30%, which has a strong oxidation damage effect on silicon carbide refractory materials. The main components of castable refractory materials are silicon carbide (SiC) and silicon oxide (SiO₂), and the SiC content is about 65%. They are cast in the furnace and must be dried and solidified strictly according to the heating curve. The brick-hanging refractory materials are prefabricated bricks in the factory, which are dried and formed in a pure nitrogen atmosphere at a temperature of about 1450℃. The main components are silicon carbide (SiC) and silicon nitride (Si₃N₄, Si₂N₂O), and the SiC content is about 75%. Each brick is about 260mm×260mm in size and weighs about 6.6kg.

The changing characteristics of the hanging brick material and the castable material during the oxidation erosion process. Since the density of the castable material is lower than that of the hanging brick, and the volume expansion of silicon carbide during oxidation is obvious, the castable will eventually crack and accelerate the degradation of the material. For hanging bricks, antioxidant additives are added during the brick making process in the factory, and the brick burning atmosphere is regulated at the same time. Therefore, the hanging brick material has a stable volume and is not easy to crack/peel off, so the antioxidant capacity is much higher than that of the castable. At the same time, the higher the density of the material, the stronger the antioxidant degree.

Formula (1) to (4) list the composition changes of the castable and hanging brick materials under the high temperature and high water vapor oxidizing atmosphere in the furnace. It is generally believed that when the temperature is greater than 500℃, silicon carbide and silicon nitride will first be oxidized into silicon oxide grains 4, and at the same time, the generated silicon oxide will be further oxidized by water vapor into gaseous hydroxide.

Based on GB/T2997-2015, the volume density and apparent porosity of the samples before and after oxidation were tested, and the volume change rate and mass change rate of the samples before and after oxidation were calculated. The results show that the volume change rate of silicon nitride after oxidation is lower than that of silicon carbide, and the hanging brick material of silicon carbide combined with silicon nitride shows good oxidation resistance.

Material oxidation test

In order to adapt to the high water vapor content in the flue gas of domestic waste incineration, the material of the hanging bricks has been improved. According to the ASTMC863 (83/88/2000) test method for evaluating the high-temperature oxidation resistance of silicon carbide refractory materials, the hanging brick material and the castable material are placed in a constant temperature resistance furnace, the temperature is controlled at 900-1000℃, and the measured water vapor flow rate is 32kg/m³ per hour, and the test time is 500h.

The mass and volume change rates of the hanging brick material and the castable material after oxidation show that the weight gain of the hanging brick material gradually slows down, which means that the material forms an anti-oxidation protective layer. At the same time, the volume change rate of the hanging brick material is less than 0.8%, which meets the factory acceptance requirements. This is because the oxidation products fill the pores in the hanging bricks, forming a protective layer to prevent further oxidation of the substrate. By comparison, it can be clearly seen that the castable material has obvious weight gain and volume expansion, and is prone to cracking and falling off, which is consistent with the above-mentioned material oxidation mechanism.

2.1 Comparison of heat transfer coefficients

The calculation object is a domestic waste incinerator with a single-line processing scale of 750t/d and a designed low calorific value of 8372kJ/kg. Its main steam parameters are 6.4MPa/450℃, the water-cooled wall tube specification is φ60mm×5mm, and the tube spacing is 85mm. The above two methods are used to lay the refractory materials. As shown in Figure 4, the thickness of the refractory materials laid on the tube surface in the two methods is the same, both of which are 40mm. In addition, during the actual operation of the boiler, a layer of ash will adhere to the surface of the refractory materials. The total heat transfer coefficient of the heating surface can be calculated by heat flow balance

In the formula: K is the total heat transfer coefficient of the heating surface, W/(m²·K); h₁ is the surface heat transfer coefficient of the flue gas side to the refractory material, W/(m²·K); h₂ is the surface heat transfer coefficient of the working fluid in the tube to the inner wall of the tube, W/(m²·K); δ; are the thickness of the tube wall, refractory material and coking layer, m; λ; are the thermal conductivity of the tube wall, refractory material and ash layer, W/(m·K).

Assuming that the ash thickness of the castable and the hanging brick surface is 5mm, the total heat transfer coefficient of the heating surface is 84W/(m²·K) and 109W/(m²·K) respectively. Considering the strong tendency of the castable surface to accumulate ash, assuming that the surface ash thickness is 15mm, the total heat transfer coefficient of the heating surface is 65W/(m²·K). Therefore, the total heat transfer coefficient of the heating surface using the hanging brick method is 1.3 to 1.67 times that of the castable.

2.2 Comparison of boiler thermal calculation results

The refractory materials of the flue of the waste heat boiler are castable and brick-hanging respectively. The calculation object is a domestic waste incinerator with a single-line processing scale of 750t/d and a designed low calorific value of 8372kJ/kg. The thermal calculation results of the boiler are shown in Table 2. It can be seen that compared with the castable, the calculated flue temperature at the flue outlet of the brick-hanging method can be reduced by 32℃. At the same time, according to the calculation model [8] that the temperature of the flue gas in the incinerator is higher than 850℃ during the 2s stay of the flue gas in the incinerator, it can be calculated that the residence time of the flue gas with a temperature higher than 850℃ under the boiler maximum continuous output (BMCR) condition is 3.3s, and the residence time of the flue gas with a temperature higher than 850℃ under the 70% BMCR condition is 3.1s. Therefore, from the perspective of thermal calculation, the brick-hanging refractory can not only effectively reduce the flue gas temperature in the furnace, but also meet the environmental monitoring requirements.

Process description

The process of refractory materials in the flue brick hanging method: firing and molding in the processing plant + welding hooks in the furnace + splicing of self-flowing materials + curing in the self-flowing furnace. The process of refractory materials in the casting material method: welding pins on water-cooled wall tubes in the processing plant + on-site mixing of casting materials, casting material molds + curing in the furnace.

The installation model diagram of the brick hanging method is shown in Figure 5. The metal hooks are welded on the ribs instead of the heating surface, which ensures the safety of the heating surface. The fixed pins of the casting material method are welded on the heating surface. The welding point is a stress concentration point with the weakest corrosion resistance. Therefore, the welding point is a risk point for pipe bursting and requires regular maintenance. In order to prevent the hanging bricks from falling off, each hanging brick is independently supported by 2 metal hooks, and the surrounding area is equipped with high-temperature resistant fiber paper and self-flowing materials to block the corrosion of metal parts by the hot air flow in the furnace.

Process description

The process of refractory materials in the flue brick hanging method: firing and molding in the processing plant + welding hooks in the furnace + splicing of self-flowing materials + curing in the self-flowing furnace. The process of refractory materials in the casting method: welding pins on water-cooled wall tubes in the processing plant + on-site mixing of casting materials, casting molds + curing in the furnace.

The installation model diagram of the brick hanging method is shown in Figure 5. The metal hooks are welded on the ribs instead of the heating surface, which ensures the safety of the heating surface. The fixed pins of the casting method are welded on the heating surface. The welding point is a stress concentration point with the weakest corrosion resistance. Therefore, the welding point is a risk point for pipe bursting and requires regular maintenance. In order to prevent the hanging bricks from falling off, each hanging brick is independently supported by 2 metal hooks, and the surrounding area is equipped with high-temperature resistant fiber paper and self-flowing materials to block the corrosion of metal parts by the hot air flow in the furnace.

In order to verify the quality of refractory construction by brick hanging, a brick hanging construction test was carried out, and the material/size of the tube screen, the material/size of the hook, the material of the hanging brick, the self-flowing material, the welding machine parameters and other auxiliary materials were completely consistent with the actual project construction site. Figure 6 shows the brick hanging construction test process. After pouring the self-flowing material, the liquid level of the self-flowing material at different parts is consistent, indicating that the self-flowing material has good fluidity.

In order to better detect the bonding strength of the hanging fire bricks and the fullness of the poured self-flowing material in the bricks, a destructive test was carried out on the brick surface after hanging the bricks. Figure 7 shows the destructive test process. The field test shows that the refractory materials of the hanging brick method are impact-resistant and compressive-resistant. The compressive strength at room temperature is as high as 147MPa, which can meet the mechanical performance requirements under the high temperature atmosphere in the furnace. At the same time, the self-flowing material on the back after solidification is observed, as shown in Figure 7(b). No bubbles or voids are found, and the surface is smooth. The position of the hanging mouth is consistent with the shape of the hanging mouth of the hanging brick, indicating that the self-flowing material is fully filled.