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Causes and countermeasures of wear on heating surface of CFB boiler

Words:[Big][Medium][Small] Mobile Page Two-Dimensional Code 2018-06-12     

Abstract: the wear mechanism of heating surface of CFB boiler was discussed.The causes of wear of water wall and rear heating surface of circulating fluidized bed boiler are analyzed, and the solutions are put forward.

Key words: circulating fluidized bed boiler;Heating surface.Furnace water wall;Tail heating surface;Wear and tear

Genesis and Countermeasures for Abrasion of Heating Surface in CFB Boiler

WANG Xiaoxia, CHEN Zhiwei, GAO Shaojuan

The Abstract:The mechanism of abrasion of heating surface in circulating fluidized bed (CFB) boiler is discussed. The genesis for abrasion of furnace water wall and rear heating surface of CFB boiler is analyzed,And the solution seheme is put forward.

Key words: circulating tluidized bed (CFB) boiler;Heating surface.The furnace water wall;Rear heating surface.abrasion

The wear of the heating surface of CFB boiler seriously affects the safe operation of the boiler, and the direct harm is to make the wall thickness of the pressure metal tube thin until the tube burst and stop the boiler.Wear and tear causes boiler maintenance cost increase, unit utilization rate decrease, bring economic loss to the enterprise.Therefore, solving the wear problem of CFB boiler has important practical significance.The causes and countermeasures of wear on the heating surface of circulating fluidized bed are discussed in this paper.

1 wear mechanism

The bed material in the circulating fluidized bed furnace rises along the furnace chamber under the flue gas, enters the separator through the upper exit of the furnace chamber, and carries out gas-solid two-phase separation in the separator. The separated flue gas enters the flue passage at the end of the boiler through the upper exit of the separator, and the separated solid bed material returns to the lower part of the furnace through the return valve.In the operation of circulating fluidized bed boiler, the solid bed material containing fuel, fly ash, limestone and its reactant flows continuously in the closed circulation loop of furnace, separator, return material valve and furnace, and carries out efficient combustion and desulfurization reaction in the furnace.In addition to circulating in this closed loop, the solid bed material also continuously carries on the internal circulation.Therefore, severe wear on the contact surface is bound to occur in the closed loop [1].

The heating surface of CFB boiler mainly includes water cooling wall in furnace, heating surface inside furnace, heating surface in tail (economizer, convection superheater, air preheater), external heat exchanger and so on.Wear on the heating surface is caused by scour, shock and microvibration wear.Erosion wear refers to the wear caused by the erosion of flue gas and solid materials on the heating surface when the flow direction of flue gas and solid materials is parallel to the heating surface (or tube bundle).Impact wear refers to the wear caused by impact and collision of flue gas and solid materials on the heating surface (or tube bundle) when the flow direction of flue gas and solid materials is at a certain Angle.Micro-vibration wear refers to the phenomenon of heat transfer tube wall loss caused by vertical movement between heat transfer tube and supporting member under the condition of heat transfer.

Wear and tear of furnace water wall

The typical "annular core" flow of CFB boiler is as follows: the bed material in the central area of the furnace is mainly upwelling flow, while the water wall around the boiler is mainly downwelling and adherent flow.There is a large amount of solid bed material circulating in the furnace. When the solid bed material falls along the water wall, the solid bed material flowing down the fin groove is much more than the solid bed material flowing down the arc surface outside the water wall.If the fin is blocked by wear-resistant pouring material, welding tumor and other obstacles, the solid bed material will be rapidly separated to both sides of the obstacle, forming the erosion and wear of the water wall.Or bounce onto the water wall tube, forming erosion wear or impact wear on the water wall.Finally led to the leakage of the water wall tube burst.The common wear parts and solutions of furnace water wall are discussed below.

(1) part 1

Wear of the water-wall at the junction of wear-resistant castable and water-wall in the dense phase area of the furnace.This part is one of the most important wear parts in CFB boiler.Due to a bulge at the end of the wear-resistant castable material in the dense phase area and the transition between the pipe walls in the thin phase area, eddy currents are generated in the adherent flow of the solid bed material, which forms a rapid wear on this part.

The circulating fluidized bed boiler used in the early stage has been worn seriously in this area.At present, "let tube" technology has been adopted by many circulating fluidized bed boiler manufacturers, and has achieved remarkable results.Let the pipe, that is, bend the water-wall pipe to the outside of the furnace. During the ramming construction of wear-resistant pouring material, beat the wear-resistant pouring material to the innermost side of the water-wall pipe to prevent the water-wall wear.The structure of water wall pipe before and after modification is shown in figure 1.

 

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(2) part 2

Wear of water wall in furnace corner area.In some circulating fluidized bed boilers that have been running, it is found that the water wall wear in the corner area of the furnace is quite serious. The reason is that the solid bed material flowing down the inner wall in the corner area has a relatively high concentration, and the flow state is also vulnerable to damage.The improvement is to smooth the corner area of the furnace.

(3) part 3

Wear of irregular tube walls, including through the wall, furnace wall opening at the elbow, weld line on the wall, in addition to some measuring elements in the furnace, such as thermocouple, etc.Operation experience shows that even small irregular geometry can cause severe local wear.Therefore, the following improvement measures are proposed: for the parts that need to be ramming wear-resistant pouring material, construction quality should be strictly guaranteed, wear-resistant pouring material should be solid and smooth and complete, especially the corner parts should not appear uneven and convex.Water wall between the tube, fin between the butt weld to the fire surface should be smoothed.

3 tail heating surface wear

In the circulating fluidized bed boiler, although the gas-solid two-phase separation of flue gas is carried out by high efficiency separator, the concentration of fly ash is still very high because of the high concentration of solid bed material in the furnace and the failure of separator to catch it.The flue gas flows downward in the tail flue, and the higher fly ash concentration and the higher flue gas velocity often lead to serious wear on the heating surface and flue of the tail of economizer.

The factors affecting the wear of the heating surface of the tail include the velocity of flue gas, the concentration and average particle size of fly ash, the physical and chemical properties of fly ash, the properties of the pipe materials, the temperature of the pipe wall, the Angle of flue gas scour the tube bundles, and the arrangement of the tube bundles.

The main parts of wear occurred in: convection superheater near the flue gas corridor on both sides of the wall 1 ~ 5 rows of pipe, at the bottom of the horizontal flue pipe elbow and a few pipe and elbow protruding outside the alignment of the tube bundles;The second and third rows of the snaking pipe of the economizer along the flue gas flow direction and the elbows on both sides of the wall and the pipe through the wall;Air preheater flue gas inlet.The main measures to prevent and reduce the wear on the heating surface of the tail are:

Reduce the concentration of fly ash in flue gas.The greater the concentration of fly ash, the more serious the wear on the heating surface of the tail will be [2].The fly ash concentration depends on the ash content in the fuel. The coal quality of power boilers in China is relatively poor, and the ash content of the fuel is relatively high, which makes it difficult to reduce the fly ash concentration. Therefore, it should be required not to mix with other impurities in the process of coal mining and transportation.

Improve separation efficiency of separator.Separator is one of the key devices to ensure the reliable circulation of solid bed material in circulating fluidized bed boiler. It separates more than 95% solid bed material carried by the flue gas at the outlet of the furnace, and then returns it to the furnace through the return valve for circulating combustion.Therefore, the concentration of fly ash in flue gas can be reduced by optimizing the separator structure and improving the separation efficiency.

(2) use the appropriate smoke velocity.The velocity of flue gas has great influence on wear, so the velocity of flue gas should not be too high.In the design of circulating fluidized bed structure, the section area of tail flue passage can be appropriately increased to reduce the gas velocity.In operation, the velocity of flue gas can be reduced by reducing the primary air volume and wind speed.

(3) as far as possible to make the smoke velocity field uniform, avoid the formation of smoke corridor.The horizontal pitch of heating surface should be arranged evenly, and the space between heating surface and furnace wall should not be too large.

(4) on the flow of superheater, economizer wear prone to add wear protective tiles, tube type air preheater with wear casing.

4 macro solutions

In view of the wear problem of the heating surface of CFB boiler, the above anti-wear measures focus on the improvement of boiler structure.Macroscopically, there are many factors affecting the wear of the heating surface of CFB boiler, including fuel characteristics, bed characteristics, circulation mode, operating parameters, heating surface structure and layout, etc.Therefore, the following macro solutions to reduce wear and tear are proposed.

(1) reduce the proper amount of air and wind speed.On the premise of stable bed pressure and temperature, the primary air volume and wind speed should be reduced as much as possible.The velocity of flue gas decreases when the primary air volume and wind speed decrease.Although the velocity of flue gas and the velocity of solid bed are different, the velocity of solid bed depends on the velocity of flue gas.Therefore, the lower the flue gas velocity is, the lower the solid bed material velocity is.

(2) furnace design section area should not be small.Under the condition that the total air volume is constant, the smaller the furnace section area is, the bigger the fluidization velocity and furnace velocity will be, and the faster the wear will be.

(3) the use of low wear fuel, bedding.In terms of wear, soft coal (lignite) will reduce the wear of heating surface compared with hard coal (anthracite).From the point of view of design, a certain furnace type corresponds to a certain type of coal, so the boiler operation should ensure that the coal quality and particle size of furnace coal in the design range, and the use of limestone quality and particle size should be strictly controlled.Also pay special attention to remove raw coal stone, iron, etc., eliminate potential wear.

(4) adopt advanced combustion technology.Tsinghua university and so on proposed the low pressure drop circulating fluidized bed boiler combustion technology.Under the guidance of steady-state design theory, the effective bed stock of fast bed is reduced appropriately.With the large particle burnout as the limiting condition, the quantity of invalid bed which constituted the bubbling bed at the bottom of the circulating bed was reduced appropriately.Avoid unnecessary fan energy consumption and heating surface wear caused by excess bed stock.

(5) supersonic arc spraying overcomes the defects of traditional flame spraying on pipe base material damage and easy peeling of coating, plays a good role in reducing the wear of boiler pipe, and can bring obvious economic benefits [3].

5 conclusion

In this way, the circulating fluidized bed boiler can operate safely and effectively, give full play to the advantages of circulating fluidized bed technology, reduce the maintenance cost of the boiler, improve the utilization rate, and promote the faster and better development of the enterprise.

References:

Li shuo. Practical manual of design, commissioning, operation and maintenance of CFB boiler [M]. Changchun: jilin science and technology press, 2004:1070-1100.

Che defu, zhuang zhengning, li jun, et al. Boiler [M]. Xi 'an: xi 'an jiaotong university press, 2004:384-385.

Zhang zhang, zhou huiren, Chen shifeng, et al. Anti-wear spraying for the tube of circulating fluidized bed boiler [J]. Gas & heat, 2004,24 (11) : 641-643.



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