Which Is Better For The Lining Of Waste Incineration Waste Heat Boilers, Hanging Bricks Or Castables? See The Comparison Results Of A Certain Factory.

 Domestic waste sorting is one of the core initiatives for the country to promote the construction of ecological civilization. With the gradual advancement of domestic waste sorting in China, the calorific value of waste has significantly and gradually increased. This is because after sorting, the proportion of kitchen and fruit waste in the incoming waste to the plant decreases, the proportion of paper and rubber waste increases, and the production rate of leachate decreases. After the implementation of the domestic waste sorting policy in Shanghai in July 2019, Jia Yue et al. 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 13,160 kJ/kg. Xu Zhenwei et al. 4 Taking a certain domestic waste incineration plant in Shanghai as an example, studied the comparison of calorific value distribution before and after domestic waste sorting in Shanghai. Before sorting, the calorific value of the waste entering the incineration plant was 6,208 - 7,165 kJ/kg, with an average of 6,634 kJ/kg; after sorting, the calorific value of the waste entering the waste incineration plant was 7,490 - 7,997 kJ/kg, with an average of 7,749 kJ/kg.

 The calorific value of the waste increased by 1,115 kJ/kg, a year - on - year increase of 17%. "Clean, low - carbon, safe and efficient" is the path for the high - quality development of the waste incineration industry. In terms of the lining materials of waste heat boilers, Figure 1 respectively lists the arrangement methods of refractory materials in the first flue of typical waste incineration waste heat boilers in China and abroad (Japan, the United States, the European Union). It can be clearly seen that in China, the castable method is mainly used, while abroad, the hanging brick method is usually mainly used. This is because before sorting, the content of combustible components in the waste per unit mass is low, and the calorific value of the waste entering the furnace is low. The refractory material in the castable method can fully meet the environmental protection requirements (850 °C, 2s), that is, the temperature in the travel of 2s for the flue gas to stay is higher than 850 °C. The calorific value of the waste entering the furnace abroad is high (usually higher than 8,372 kJ/kg), and the refractory material in the first flue is mainly in the hanging brick method, which can make the heat of the flue gas be efficiently absorbed by the working medium in the tube, so as to design an economically appropriate radiant heating surface.

Figure 1: Lining refractory materials inside the waste heat boiler - Layout modes of typical waste incinerators at home and abroad

If the heat in the high-temperature flue gas cannot be efficiently absorbed by the working medium inside the tubes, resulting in overheating inside the furnace, it will cause serious failure modes such as degradation and cracking of the castable. At this time, the corrosive substances in the flue gas will erode the water-cooled walls, causing high-temperature corrosion phenomena, thus bringing challenges to the long-term stable operation of the waste heat boiler of the waste incinerator.

To adapt to the increase in the calorific value of waste, the material selection of the castable in the first flue of the waste heat boiler has currently changed from high-alumina with a low thermal conductivity (～2W/(m·K)) to silicon carbide (～6W/(m·K)). At the same time, the components of corrosive substances in the waste incineration boiler are complex, and the neutral silicon carbide (SiC) material has outstanding resistance to the erosion of acid and alkali components. Therefore, regardless of the castable or hanging brick method, the refractory material in the first flue of the waste heat boiler is mainly based on SiC.

Table 1 lists the performance comparison and analysis of the refractory materials of the two methods. It can be seen that the hanging brick is superior to the castable in terms of porosity, mechanical properties, and thermal conductivity.

Table 1 Comparison of the Properties of Refractory Materials by the Two Methods

To analyze the application feasibility of the hanging brick method for the radiant heating surface of domestic waste incineration waste heat boilers, this paper conducts a comparative analysis of the castable method and the hanging brick method for the refractory materials in the first flue of waste incineration waste heat boilers from aspects such as material inspection, heat exchange effect, and economic benefits. It is expected to provide certain reference for the optimized design of the heating surface of waste incineration boilers.

1.1 Oxidation Mechanism of Materials

In the flue gas from domestic waste incineration in China, the water vapor content is high. Especially when direct water spraying is used for cooling, the water vapor content is in the range of 20% - 30%, which has a strong oxidative damage effect on silicon carbide refractory materials.

The refractory material of the castable method is mainly composed of silicon carbide (SiC) and silicon dioxide (SiO₂), with an SiC content of about 65%. It is formed by casting in the furnace, and needs to be strictly dried and solidified in accordance with the heating curve.

The refractory material of the hanging brick method is a prefabricated brick in the factory, dried and formed in a pure nitrogen atmosphere at a temperature of about 1450 °C. It is mainly composed of silicon carbide (SiC) and silicon nitride (Si₃N₄, Si₂N₂O), with an SiC content of about 75%. Each brick has a size of about 260mm × 260mm and a weight of about 6.6kg.

Figure 2 shows the change characteristics of the hanging brick material and the castable material during the oxidation and erosion process. Because the density of the castable material is lower than that of the hanging brick, and the volume expansion during the oxidation of silicon carbide is obvious, it eventually leads to the cracking of the castable and accelerates the material degradation. For hanging bricks, anti-oxidation additives are added during the brick-making process in the factory, and the brick-firing atmosphere is adjusted at the same time. Therefore, the volume of the hanging brick material is stable, not easy to crack/spall, so its anti-oxidation ability is much higher than that of the castable. At the same time, the higher the density of the material, the stronger the anti-oxidation degree.

Figure 2 Oxidation Processes of Hanging Brick Material and Castable Material

Equations (1) to (4) list the composition changes of the castable and hanging brick materials in the high-temperature and high-water-vapor oxidizing atmosphere inside the furnace. It is generally believed that when the temperature is higher than 500 °C, silicon carbide and silicon nitride will first be oxidized into silicon oxide crystals ⁴. Meanwhile, the generated silicon oxide will be further oxidized by water vapor into gaseous hydroxides.

Xiang Yubo et al. based on GB/T2997-2015, detected the bulk density and apparent porosity of the samples before and after oxidation, and calculated the volume change rate and mass change rate of the samples before and after the oxidation treatment. 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 with silicon carbide combined with silicon nitride shows good oxidation resistance.

1.2 Material Oxidation Test

To adapt to the characteristic of high water vapor content in the flue gas from domestic waste incineration in China, the material of the hanging brick has been improved. According to the test method for evaluating the high-temperature oxidation resistance of silicon carbide refractory materials in ASTM C863 (83/88/2000), the hanging brick material and the castable material were placed in a constant-temperature resistance furnace, the temperature was controlled at 900 - 1000 °C, and a metered water vapor flow rate of 32 kg/m³ per hour was introduced, with a test duration of 500 h.

Figure 3 shows the mass and volume change rates of the hanging brick material and the castable material after oxidation. It can be seen that the mass gain of the hanging brick material gradually slows down, which means that the material has formed an oxidation-resistant protective layer. At the same time, the volume change rate of the hanging brick material is less than 0.8%, meeting the factory acceptance requirements. This is because the oxidation products fill the pores in the hanging brick, forming a protective layer that prevents further oxidation of the base material.

In comparison, it can be clearly seen that the mass gain and volume expansion of the castable material are obvious, and it is prone to cracking and spalling, which is consistent with the above-mentioned material oxidation mechanism.

Figure 3 Mass and Volume Change Rates of Hanging Brick Material and Castable Material After Oxidation

2.1 Comparison of Heat Transfer Coefficients

Taking a domestic waste incinerator with a single-line processing capacity of 750 t/d and a designed low calorific value of 8372 kJ/kg as the calculation object, its main steam parameters are 6.4 MPa/450 °C, and the water wall tube specification is φ60 mm × 5 mm with a tube spacing of 85 mm. The above two methods are used to lay the refractory materials. As shown in Figure 4, the refractory materials of the two methods are laid on the surface of the tubes with the same thickness, both being 40 mm. In addition, during the actual operation of the boiler, an ash layer will also adhere to the surface of the refractory material. The total heat transfer coefficient of the heating surface can be calculated by the heat flow balance, as shown in Equation (5).

K is the total heat transfer coefficient of the heating surface, W/(m2⋅K);

h1​ is the surface heat transfer coefficient from the flue gas side to the refractory material, W/(m2⋅K);

h2​ is the surface heat transfer coefficient from the working medium inside the tube to the inner wall of the tube, W/(m2⋅K);

δi​ are the thicknesses of the tube wall, refractory material, and coke layer, respectively, m;

λi​ are the thermal conductivities of the tube wall, refractory material, and ash layer, respectively, W/(m⋅K).

Assuming the ash layer thicknesses on both the castable and hanging brick surfaces are 5 mm, the total heat transfer coefficients of the heating surface are 84 W/(m2⋅K) and 109 W/(m2⋅K), respectively. Considering the stronger ash deposition tendency on the castable surface, assuming its ash layer thickness is 15 mm, the total heat transfer coefficient of the heating surface is 65 W/(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 method.

Figure 4 Schematic Diagrams of Two Arrangement Modes of Refractory Materials

2.2 Comparison of Boiler Thermal Calculation Results

For the refractory material in the first flue of the waste heat boiler, the castable method and the hanging brick method are adopted respectively. Taking a domestic waste incinerator with a single-line processing capacity of 750 t/d and a designed low calorific value of 8372 kJ/kg as the calculation object, the boiler thermal calculation results are shown in Table 2. It can be seen that compared with the castable, the calculated flue gas temperature at the outlet of the first flue with the hanging brick method can be reduced by 32 °C.

Meanwhile, according to the calculation model where the temperature is higher than 850 °C within the 2s flue gas residence time in the incinerator [8], it can be calculated that the flue gas residence time with the temperature higher than 850 °C under the boiler maximum continuous rating (BMCR) condition is 3.3s, and the flue gas residence time with the temperature higher than 850 °C under the 70% BMCR condition is 3.1s.

Therefore, from the perspective of thermal calculation, the refractory material with the hanging brick method can not only effectively reduce the flue gas temperature in the furnace but also meet the environmental monitoring requirements.

Table 2 Comparison of Boiler thermal calculation results of refractory materials for the two methods
 

3.1 Process Flow Description

Process flow of refractory materials for the hanging brick method in the first flue: Firing and forming in the processing plant + Welding hooks inside the furnace + Splicing with self-flowing material + Curing by baking the self-flowing material.

Process flow of refractory materials for the castable method: Welding pins on water-cooled wall tubes in the processing plant + On-site preparation of castable and setting up molds for castable + Curing by baking the furnace.

The installation model diagram of the hanging brick method is shown in Figure 5. The metal hooks are welded on the rib plates instead of on the heating surface, ensuring the safety of the heating surface. For the castable method, the fixed pins are welded on the heating surface, and the welding points are stress concentration points with the weakest corrosion resistance. Therefore, the welding points are risk points for tube bursting and require regular maintenance. To prevent the hanging bricks from falling off, each hanging brick is independently supported by 2 metal hooks, and high-temperature resistant fiber paper and self-flowing material are used around to block the corrosion of metal components by the hot gas flow inside the furnace.

Figure 5 Installation Model Diagram of the Hanging Brick Method

To verify the construction quality of the refractory material for the hanging brick method, a hanging brick construction test was carried out. The tube panel material/size, hook material/size, hanging brick material, self-flowing material, welding machine parameters and other auxiliary materials were exactly the same as those at the construction site of the actual project. Figure 6 shows the process of the hanging brick construction test. After pouring the self-flowing material, the liquid level heights of the self-flowing material at different positions are consistent, indicating that the fluidity of the self-flowing material is good.

Figure 6 Construction Test Process of Hanging Bricks

To better detect the bonding strength of the hanging bricks and the filling degree of the poured self-flowing material within the bricks, a destructive test was conducted on the brick surface after hanging the bricks. Figure 7 shows the process of the destructive test. The field test indicates that the refractory material of the hanging brick method is impact-resistant and compressive, with a high-temperature compressive strength reaching up to 147 MPa at room temperature, which can meet the mechanical property requirements in the high-temperature atmosphere of the furnace. Meanwhile, observing the self-flowing material solidified on the back, as shown in Figure 7(b), no bubbles or voids are found, and the surface is smooth. The shape of the hanging opening matches that of the hanging brick, indicating that the self-flowing material is fully filled.

Figure 7 Destructive Test Process of Hanging Brick Materials

This paper systematically compares and studies the castable method and hanging brick method of refractory materials in the first flue of waste incineration waste heat boilers from the aspects of material inspection, heat exchange effect, and economic benefits. Taking a domestic waste incinerator with a single-line processing capacity of 750 t/d and a designed low calorific value of 8372 kJ/kg as an example, the following conclusions are drawn:

(1) With the gradual maturity of domestic waste classification and the increase in waste calorific value, the reasonable arrangement of hanging brick refractory materials in the first flue helps to strengthen the heat absorption capacity of the radiant heating surface of the waste incineration waste heat boiler, providing a basis for the economic arrangement of the heating surface.

(2) To adapt to the characteristic of high water vapor content in the flue gas from domestic waste incineration in China, the material of the hanging brick has been improved. Therefore, compared with the castable, the hanging brick method has the advantages of excellent heat absorption performance and strong oxidation resistance. In terms of economic benefits, the total annual investment in refractory materials is reduced by 100,000 yuan, and the total income is increased by 2.35 million yuan, showing good economic benefits.

(3) The calculated flue gas temperature at the outlet of the first flue using the hanging brick method can be reduced by 32 °C (under BMCR), and the flue gas residence time meets the environmental monitoring requirements of 850 °C for 2 seconds. The application effect combined with the actual furnace in the later stage provides a reference for the optimized design and technical transformation of the heating surface of high-calorific value boilers.