Rotary Kiln Incinerator for Hazardous Waste Treatment

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The diverse types, complex compositions, and regional variations of hazardous waste pose a significant challenge, and improper handling can lead to a range of environmental and societal issues. High-temperature incineration has proven to be an effective method for the safe disposal and volume reduction of hazardous waste. Among these methods, the rotary kiln incineration process stands out as an efficient treatment option due to its capability to handle various types of hazardous waste and accommodate large processing capacities.

In China, since the 1990s, there has been a gradual adoption of rotary kiln incineration systems for hazardous waste treatment. However, in recent years, incidents of kiln stoppages due to the deterioration of refractory materials have become more prevalent, significantly impacting the stable operation of hazardous waste incineration production lines. This situation has prompted an urgent need for the optimization of refractory lining design and efficient operation in rotary kiln hazardous waste incinerators.

The current practice of co-processing hazardous waste in cement kilns primarily involves solid waste, waste liquids, and the use of RDF (Refuse-Derived Fuel) as a substitute fuel. This co-processing occurs in various sections of the cement kiln, including the preheater, decomposing furnace, post-kiln inlet, and kiln hood.

This underscores the importance of addressing the specific challenges associated with refractory lining in rotary kiln incinerators, ensuring their resilience to withstand harsh conditions, and optimizing their design for efficient and stable operation in hazardous waste treatment processes.

Rotary Kiln Incinerator Configuration

The rotary kiln incinerator is a specialized facility for the combustion of hazardous waste, consisting of the rotary kiln, secondary combustion chamber, waste heat boiler, quench tower, acid removal, and dust removal systems. Various types of pre-treated hazardous waste enter the combustion system through different feeding methods. Under the combined action of gravity and the continuous rotation of the rotary kiln, the materials come into full contact with the combustion-supporting air, completing the processes of drying, gasification, and combustion. Finally, the remaining portion is quenched and cooled into ash residue.

Depending on the direction of gas and solid flow or the position of the burner in the rotary kiln, rotary kiln incinerators can be classified into two forms: countercurrent and concurrent. In the concurrent design, the burner heat source is at the feeding end of the rotary kiln, and the solid materials and flue gas move in the same direction. Conversely, in the countercurrent design, the burner heat source is at the discharge end, and the solid materials and flue gas move in opposite directions. Currently, hazardous waste incineration systems mainly adopt the design of concurrent rotary kilns, while countercurrent rotary kilns are more suitable for waste with high moisture content or low calorific value.

Rotary kiln incinerators exhibit broad applicability, capable of simultaneously treating solid, liquid, and gaseous hazardous waste. In the context of increasingly stringent environmental requirements, the advantages of specialized rotary kiln hazardous waste incinerators are evident.

Refractory Lining Design for Rotary Kiln Hazardous Waste Incinerator

Rotary Kiln Body

The most critical component of a rotary kiln incinerator is the rotary kiln body. As it is a dynamically operating device, the design of the refractory lining structure requires high standards. Firstly, it is essential to ensure the stability of the overall lining under dynamic high-temperature conditions, preventing issues such as brick dislodgment and spalling. For hazardous waste rotary kiln systems, considering the relatively slow rotation speed, generally below 1 revolution per minute, different structures such as single-layer bricks, double-layer bricks, composite bricks, and integral casting can be selected based on factory design and energy-saving requirements. The total thickness of the refractory material in the rotary kiln is generally in the range of 250 to 300 mm. The material of the working layer refractory brick can be chosen based on the processing temperature and the type of hazardous waste, including options such as high-alumina, corundum-mullite, chrome-zircon corundum refractory materials, etc.

When using a single-layer structure with a thickness of 230 to 300 mm, as the thermal conductivity of refractory materials is mostly above 1.7 to 2.0, the outer wall temperature of the high-temperature zone can reach 350°C or more in the later stage. However, the overall structure remains stable, making it suitable for outdoor projects with rotary kilns.

For a double-layer structure using insulating bricks and refractory bricks, the low thermal conductivity of insulating bricks can effectively reduce the kiln shell temperature, allowing the outer wall temperature to be controlled at around 220°C or even lower, providing good energy-saving effects. However, this method requires higher construction standards and is suitable for indoor projects. It is important to note that the insulating brick strength generally needs to be greater than 20 MPa, and the insulation brick thickness should not be less than 50 mm.

To achieve both stability similar to a single-layer brick and reduced outer wall temperature, a composite brick design can be used. In this case, the working face consists of corundum bricks and chrome corundum bricks, while the back face is made of hollow alumina balls, high-alumina, or clay materials, with an insulation layer thickness of approximately 50 to 70 mm. However, this type of product has lower production efficiency and may experience higher rejection rates due to differences in thermal expansion coefficients and sintering shrinkage rates between the working layer and the insulation layer.

To address these challenges, a grooved composite brick design, as shown in Figure 4, can be employed. In this design, the working layer material remains unchanged, and the tail is grooved, filled with insulating materials such as nanometer boards. This grooved composite brick not only facilitates production and construction but also achieves a reduction in the outer wall temperature of the cylinder by approximately 30 to 50°C.

Additionally, some rotary kilns use lightweight castables or fiberboards as insulation materials, with a dual-layer casting structure where the working layer is made of heavy castables (anchored with metal studs). This design allows for effective control of the outer wall temperature of the rotary kiln. The structure uses metal components to connect the refractory materials with the kiln shell, eliminating issues related to refractory brick detachment. However, the key factors in this design are the welding quality of the metal components and the proper drying and removal of moisture from the castable material.

Secondary Combustion Chamber

The primary function of the secondary combustion chamber is to perform secondary combustion treatment on the flue gas, burning combustible components, fly ash particles, and decomposing substances like dioxins. Physical and chemical reactions occur simultaneously, resulting in intense reactions. The secondary combustion chamber generally includes refractory materials, insulation materials, and insulation materials. Refractory materials refer to the working layer of heavy castables, plastics, or refractory bricks. Insulation materials include insulation castables and insulation bricks, while insulation materials refer to calcium silicate boards, ceramic fiber boards, and nano boards.

Currently, the design of the secondary combustion chamber lining mainly includes three approaches, as shown in the diagram below:

design of the secondary combustion chamber lining

Scheme one includes refractory materials, insulation materials, and insulation materials. In this three-layer structural design, taking a total thickness of 450mm as an example, the outer wall temperature is around 80-90℃.

Scheme two includes refractory materials and insulation materials. This structure has a higher outer wall temperature, and with a total thickness of 305mm as an example, the temperature reaches 150-180℃.

Scheme three includes refractory materials and insulation materials. This scheme falls between the first two, with a total thickness of 270mm as an example, and the outer wall temperature is approximately 110-140℃.

Generally, the temperature in the secondary combustion chamber is around 1100-1200℃, and the local temperature of the burner can even exceed 1300℃. Therefore, selecting materials like corundum mullite for the working layer can meet the usage requirements. For the burner area, it is recommended to use materials like corundum or chromium corundum. In comparison, scheme one with a three-layer structural design offers good stability, reducing the risk of gas leaks or flameouts that could lead to high-temperature corrosion or overheating deformation of the kiln, requiring shutdowns for maintenance.

Other Sections

In the heat recovery boiler section, the ash hopper, collection box, top seal, and the portion of the flue gas outlet require the use of some abrasion-resistant castables. The ash hopper often employs a double-layer structure with insulating materials and abrasion-resistant castables, with a total thickness generally ranging from 200 to 250mm.

For the quenching tower, acid-resistant castables or acid-resistant mortar are used. In the region approximately 2 meters downward from the top, where temperatures are higher, a 25mm insulation material can be applied as a thermal insulation layer. The total thickness of the lining is typically 100mm.

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