Application of Refractory Materials in Fluidized Bed Waste Incineration Boilers

At present, the refractory materials used in fluidized bed incineration boilers can be generally divided into three categories according to their functions: (1) Bricks, castables, plastics and mortars of wear-resistant refractory materials. (2) Bricks, castables and mortars of refractory materials. (3) Bricks, castables and mortars of refractory insulation materials. The commonly used types of wear-resistant refractory materials are: phosphate bricks and castables, sillimanite bricks and castables; silicon carbide bricks and castables; corundum bricks and castables; wear-resistant refractory bricks and castables; and silicon nitride combined with silicon carbide products.

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Application of Refractory Materials in Waste Incinerators

The boiler wall is an important part of the boiler, which plays the role of furnace sealing, insulation, wear resistance, corrosion resistance, and flue formation. The refractory materials used usually require good volume stability, good strength, and good refractory and insulation properties. Since the waste incineration boiler has a large water content and complex composition, the high-temperature flue gas generated by the burning of waste contains a large amount of corrosive gases. The low-melting-point substances in the waste are easy to melt and adhere to the furnace wall. The moving waste or bed material in the fluidized bed furnace at high temperature causes wear and impact on the furnace wall. It is more likely to cause serious damage to the refractory materials and their supporting metal parts.

With the use of efficient power generation and gasification melting technology of waste incineration, the temperature of waste incineration flue gas is higher, the concentration of corrosive gases such as HCl and H2S increases, and the use conditions of refractory materials are more severe. Refractory materials are required to have better thermal shock resistance, acid corrosion resistance and slag erosion resistance. The refractory layer is required to have a longer service life and more economical construction and maintenance costs.

The temperature inside the garbage incinerator generally does not exceed 1400℃. The furnace wall is usually designed with multiple layers of refractory layer, thermal insulation layer and heat insulation layer, so that the temperature of the outer surface of the furnace wall is lower than 70℃, which can improve the surrounding environment of the incinerator and reduce heat dissipation loss. The garbage incineration boiler has a special working environment, and suitable materials should be selected according to the working environment requirements of different parts. The heat insulation layer and the heat insulation layer mainly play a role of insulation, and the heat insulation layer can also play a role of protecting the heat insulation layer. On the basis of considering acid corrosion resistance and appropriate strength, it is mainly selected according to the working temperature and the thermal conductivity required by the design, usually medium and light refractory materials. As the refractory material of the working layer, on the basis of considering the working temperature, it is mainly selected according to the wear resistance, thermal shock resistance, acid corrosion resistance and slag erosion resistance.

Refractory materials can be roughly divided into two categories: shaped materials (refractory bricks) and amorphous materials. Amorphous materials can be divided into two types: plastic and castable. In waste incineration boilers, parts with complex structures and irregular shapes usually use amorphous materials, while shaped materials can be used. Amorphous refractory materials are basically composed of granular aggregates, powdered refractory materials and binders, and can be made of various materials. A small change in each component can obtain material properties with great differences, so the most suitable materials and construction methods can be selected according to the requirements of various parts of the waste incineration boiler. Amorphous refractory materials are most commonly used due to convenient construction, diverse types, and easy maintenance, while shaped materials are usually only used in limited parts such as the lower part of the two side walls of the grate furnace combustion chamber.

The construction methods of amorphous refractory materials mainly include pouring, ramming, spraying, and coating. Usually, they can be subdivided into different materials according to different construction methods. Among them, spray-coated plastics that can be sprayed have the advantages of good thermal shock resistance, slag erosion resistance, and easy construction. They have been widely promoted and used in Japan in recent years. They are used in everything from high-strength refractory materials to low-quality thermal insulation materials.

Refractory Material Layout of Incinerator

1. Combustion chamber

The water-cooled wall heating surface arranged in the combustion chamber and at the furnace outlet of the incinerator is radiated by the high-temperature flame in the furnace. Usually, a 50~100mm refractory lining is laid on the flue gas side of the water-cooled wall. Refractory materials usually use SiC materials that are resistant to slag erosion and have good thermal conductivity to prevent slag from adhering to the wall and growing. When the secondary combustion space of the furnace is insufficient, refractory materials are laid in the radiation cooling chamber after the furnace outlet as a secondary combustion zone. As part of the secondary combustion chamber, it can ensure that the flue gas has sufficient residence time in the high-temperature area. For grate furnace incinerators, the laying range also needs to cover the range that the flame may contact. When using high-temperature combustion and low excess air coefficient combustion technology, the laying range of the refractory lining should be appropriately increased along the flue gas flow direction to prevent high-temperature corrosion of the water-cooled wall heating surface.

2. Hearth

Due to the relatively low calorific value of garbage, garbage incinerators usually do not have a heating surface in the furnace part and adopt an insulated steel shell structure in the furnace. The furnace wall plays a role of fire resistance, heat insulation and heat insulation, and the shell plays a sealing role. In developed countries, due to the high calorific value of garbage, it can reach 2500~3500kcal/kg. If domestic garbage is sorted, it can also reach 1800~2500kcal/kg. At this time, it is necessary to set up a radiation heating surface in the furnace to absorb part of the combustion heat and reduce the furnace outlet temperature. The grate furnace usually designs the two side walls or the front and rear arches of the furnace as a water-cooled wall structure. The fluidized bed boiler can usually design the dilute phase zone as a water-cooled wall structure (equivalent to the flue gas radiation cooling chamber). The flue gas side surface of the radiation heating surface in the furnace is laid with a 70~100mm refractory insulation lining, and the outer surface of the water-cooled wall is laid with a thermal insulation layer. Compared with the design of the heat-insulating furnace wall, although the structure is complex, the thickness of the refractory layer is low, which can greatly reduce the weight of the refractory material and reduce the load and cost of the steel structure.

3. Water-cooled wall

The water-cooled wall in the furnace usually adopts a membrane-water-cooled wall structure with good sealing performance. Due to the high temperature in the furnace, it is necessary to use a smaller pitch and lay refractory materials to prevent high-temperature corrosion and wear of the water pipes. Usually, pipes with a diameter of 51mm or 60mm and a pitch of 80~110mm are used. During operation, the water-cooled wall produces a thermal expansion difference with the steel shell part. The connection between the water-cooled wall and the furnace body must not only ensure complete sealing, but also set up a suitable structure to absorb this expansion difference to ensure reliable sealing of the furnace body. In addition, if the refractory material laid on the water-cooled wall in the furnace falls off, a strong heat exchange will occur between the high-temperature flue gas and the water pipe, resulting in overheating and high-temperature corrosion of the water pipe. Therefore, in addition to strengthening the inspection of the laying of refractory materials during daily inspections, the water-cooled wall in the furnace usually adopts a reliable and simple water circulation loop to ensure the safe operation of the water pipe.

The water-cooled walls on both sides of the first channel of the grate furnace waste incineration boiler extend downward to form the walls on both sides of the grate furnace combustion zone. The furnace combustion in this part is the most intense. When the insulated furnace wall structure is adopted, it is easy to melt the low-melting-point materials in the garbage under the action of high temperature to form coke blocks and adhere to the walls on both sides. In severe cases, the furnace will have to be stopped for manual cleaning of the coke. The setting of the water-cooled wall can effectively suppress the coking in this part while reducing the furnace temperature. The water-cooled wall of the front wall of the first channel of the boiler extends downward to form structures such as the grate furnace front arch wall.

At present, there is no unified standard for refractory materials for fluidized bed waste incineration boilers. When selecting refractory materials, the design unit should select refractory materials with a grade superior to that of conventional circulating fluidized bed coal-fired boilers in order to achieve specific needs such as wear resistance and high-temperature resistance. After the user unit purchases the refractory materials, it should promptly urge the construction unit to carry out reasonable storage or timely construction. Avoid improper storage affecting the quality of refractory materials. When refractory materials are installed during masonry, corresponding test blocks should be made to test the strength of the relevant materials, which can be used as the basis for entrusting a third party to inspect if necessary. And supervise the construction and installation of refractory materials.

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