Under the combustion condition of CFB boiler, the wear of solid particles to the heating surface is always the key problem that restricts the economic operation and further development of CFB boiler. There are different air flows, jets and bubbles in different directions, speeds, angles and concentrations in CFB boilers. They act on the inner surface in various forms, in addition to the existence of corrosive gases and other media, to form a complex wear process. The main way to maintain a boiler is to place wear resistant Refractory in certain areas to protect the heating surface. Under reducing conditions, the Refractory is more wear resistant than steel, and the wear resistant material is a low cost barrier that prevents wear and protects the boiler from safe and reliable operation.
The CFB boiler runs at high temperature, and the temperature changes frequently in the furnace, which results in the circulating thermal shock. At the same time, there are a large number of high-speed moving high-temperature solid particles in the furnace, which continuously scour the heating surface. Therefore, it is necessary to lay wear-resistant Refractory to protect the furnace. This paper introduces only the wear-resistant Refractory to the fire side (flue gas side) .
1.1 types of Refractory
Brief introduction of wear resistant Refractory for cfb boilers
Wear-resistant Refractory can be divided into shaped materials and non-shaped materials according to their state of supply, according to their function can be divided into three categories: wear-resistant Refractory (including brick, castable, plastic and mortar) ; Refractory (including brick, castable and mortar) ; Refractory insulating material (incl. Bricks, castable and mortar)。
A shaped material
1) wear Refractory (high density Refractory) : silica-alumina brick (silica brick, refractory brick, high alumina brick) , zircon-silica brick, non-oxide brick (carbon brick, silica-carbon brick) , magnesia-calcium chrome brick and fused magnesia products (Magnesia brick, magnesia brick, chrome brick, dolomite brick) .
2) heat preservation material: fire-resistant heat preservation brick, heat preservation brick, heat preservation block, ceramic fiber.
B Amorphous material
The Amorphous materials include castable materials, pressing materials, plastic materials, patching materials, gunning materials, casting materials, vibrating materials, mortar coating materials, etc. . In form, it can be divided into powder, mud and agglomerate.
1. 2 properties of wear resistant Refractory
The wear resistant Refractory is a special product which is not easy to damage and deform at high temperature. In order to prevent the smoke and fly ash from wearing out, it is important to select and install the wear resistant materials correctly, it can guarantee the long-term performance of the system and reduce the times of shedding and maintenance of wear-resistant materials.
The chemical composition of wear resistant Refractory is mainly composed of aluminum and silicon compounds, the total content is 80% ~ 95% .
In CFB boilers, to withstand the effects of the environment, the wear Refractory must have a certain degree of fire resistance, compressive strength, flexural strength, thermal shock resistance and a small enough linear rate of change. The physical and chemical indexes of wear-resistant Refractory mainly consist of the following aspects:
A fire-resistant degree
Refractoriness is the ability of a Refractory to resist melting at high temperatures without external forces. Refractoriness is usually also expressed as the maximum service temperature at which the line change rate is less than 1. 5% temperature.
Table 1 refractoriness of common refractory materials and refractory products
B volume density
Bulk density, also known as bulk density, is the mass of the wear resistant Refractory per unit volume, which reflects the density of the Refractory in KGM3.
C Coefficient of thermal conductivity
Thermal conductivity is the amount of heat per unit area of vertical wear Refractory per unit time, in terms of W/(m. k) , per unit temperature gradient. The thermal conductivity of the Refractory is not only related to its use, but also an important factor that directly affects the thermal shock stability of the products.
D Thermal Shock Stability
Thermal shock stability is the ability of wear-resistant refractory products to withstand rapid changes in temperature without being destroyed, also known as thermal shock resistance, resistance to rapid temperature change, resistance to rapid cold and rapid heat. The use of Refractory is often affected by rapid changes in ambient temperature, resulting in cracks, spalling and even collapse of the material. The factors affecting thermal shock stability include thermal expansion rate, thermal conductivity, microstructure, product shape and particle composition, etc.
E Rate of change of e line
The linear rate of change is the ratio of the length of the wear resistant Refractory to its original length under the change of unit temperature, expressed as a percentage, also known as the linear expansion coefficient. Refractory is one of the basis of structural design and expansion joint design, line change rate is too large, wear-resistant Refractory easy to peel。
α = ( L2 - L1) /L1
In the formula: L1 is the length of the specimen at room temperature, mm; L2 is the length of the specimen when heated to test temperature t, mm.
F compressive strength and flexural strength at room temperature
Compressive strength, commonly referred to as compressive strength at room temperature, is the ultimate pressure that the wear resistant Refractory can withstand per unit area at room temperature, the compressive strength, which mainly represents the sintering, bonding and structure related properties of Refractory, is a common item in Refractory testing, also known as cold compressive strength. Formula for calculating compressive strength:
CCS = F /A
In the Formula: CCS for compressive strength, UNITS MPA; F for the material can withstand the ultimate pressure; a for the material compression area.
In addition to compressive stresses, wear resistant Refractory are subjected to tensile, flexural and Shear stresses. Flexural strength, commonly referred to as flexural strength at room temperature, refers to the ultimate stress at room temperature when the specimen is broken by bending load, its unit is MPA. The compressive strength and flexural strength depend on the kind and amount of the binder and its admixtures, as well as the purity of the raw materials, the ratio, the quantity of mixing liquid, the construction method and the maintenance method.
G Wear Resistance Index
A 1 pound weight of quartz sand is sprayed at a rate into the wear resistant material, and the amount of wear resistant material is called the wear resistant index, which is measured in GCM2. In general, wear resistance index is an important index to measure the wear resistance of castable and wear resistant brick. Design of wear-resistant material for CFB boiler
2.1 causes of wear resistant material damage
In the CFB boiler, on the one hand, a large number of high-speed moving high-temperature solid particles continuously wash away the wear-resistant materials and cause their destruction, on the other hand, the wear-resistant materials produce cracks and spalling because of the temperature fluctuation, thermal shock and mechanical stress in the furnace, and at the same time, the penetration of Alkali metals also causes the failure and destruction of wear-resistant materials.
2. 2 design of wear resistant materials
In the CFB boiler, the wear area mainly includes the water wall, the heating surface arranged in the furnace, the separation and return system, the tail flue entrance and the slag discharge system. The main areas in which wear resistant materials are used include furnace dense phase zone, furnace underside zone, furnace outlet, separator, stock leg, return valve, separator outlet flue and tail convection flue. In consideration of the above wear characteristics, physical properties of wear resistant materials and construction characteristics, the general use of single-layer or multi-layer wear Refractory. The single-layer laying areas include thin linings such as water-cooled walls, panel heating surfaces, water-cooled partition walls, double-sided water-cooled walls, steam-cooled separators, etc. , the wear-resistant castable material is supported on the flue gas side of the pipe wall by 6 or 10 cylindrical pins (or y type V pins) . The thin lining can withstand the quick thermal shock when the boiler starts and stops, meanwhile, in order to enhance its rigidity and impact ability, it is necessary to add the stainless steel fiber in the castable. The thick lining is divided into two or three layers. The flue gas side is made of wear-resisting firebrick or wear-resisting plastic. Construction and maintenance of wear Refractory
3.1 setting of expansion joints
In the course of construction, the shaped wear-resistant material is unloaded through the supporting plate layer by layer and the load is transferred to the shell steel plate. Construction joints are expansion joints, too large for the area of the castable, should alternate construction, block to block construction joints, construction joints should be staggered layout.
3.2 treatment of metal anchorages
Due to the difference of linear expansion coefficient between metal materials and wear resistant materials after heating, asphalt must be applied to the surface of metal materials (including pins, supporting plates and pipes) before the construction of wear resistant Refractory, there is a certain gap between the castable and the heating surface, and the difference of the heating surface thermal expansion will not destroy the wear-resistant material.
3.3 vibration
If the vibration is not enough or not in place, the density of the wear-resistant castable can not meet the requirement or is not uniform, which can result in the castable not having enough strength and can not play the role of anti-wear, difficult to construct the position of the template should be as small as possible.
3.4 Refractory
Since non-base mineral raw materials contain a certain amount of water, and a certain amount of water is added as a solvent in the process of making castable, the water needs to be drained out through natural drying and heat curing, due to the high density of wear-resistant refractory castable and its crystal water in its molecular structure, so heat curing oven is a process that needs enough time. Although heat curing adopts controllable temperature heat flue gas curing, water should be added to boiler heating surface to protect its safety. The heating curve of heat curing is usually provided by the material supplier or by the approved heating curve of the supplier. After heat curing, all pouring holes opened on the sealing box shall be sealed and welded, and all steam passages opened for heat curing shall be sealed and welded. Conclusion to ensure the safe operation of the CFB boiler, the wear resistant Refractory shall have the following properties: High Ambient and thermal strength, low wear, excellent corrosion resistance, and high temperature volumetric stability. At present, YB/t 4109-2002 standard is mostly applied to wear-resistant refractory castable for circulating fluidized bed boiler. There are three grades of refractory castable, NMJ-1, NMJ-2 and NMJ-3. The corresponding aluminium contents are 60,65 and 70 respectively. But in the actual application process, may need according to the different production condition, the different position to carry on the corresponding adjustment to each physical and chemical index.
