Analysis and suggestions on unstable circulating r

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Cause analysis and suggestions of unstable circulating return material of CFB boiler

this paper introduces the cause analysis of the unstable circulating return material of CFB boiler fly ash circulating system in the ignition process, which affects the boiler operation, so as to find suitable methods and ways for the stable operation of the boiler fly ash circulating system

1 system introduction

our thermal power plant has three 220t/h circulating fluidized bed boilers, which were constructed in 2002 and put into operation in 2003. The boiler index parameters and performance are summarized as follows:

the 220t/h circulating fluidized bed boiler is a high-temperature and high-pressure parameter (9.81MPa, 540 ℃), single drum, natural circulation steam boiler, which adopts the circulating fluidized bed combustion mode, and the material separation adopts high-temperature adiabatic cyclone separation and balanced ventilation

the boiler is mainly composed of four parts: combustion chamber, high-temperature cyclone separator, self balancing U-shaped sealing return valve and tail convection flue. The combustion chamber is located in the front of the boiler, and membrane water walls are arranged around and on the ceiling to ensure the air tightness of the furnace. The bottom is a slightly inclined water-cooled air distributor, which is equipped with a large-diameter bell jar hood. The upper part of the furnace is vertically arranged with four water-cooled panels and four secondary superheaters to improve radiant heat transfer. There are two high-temperature insulated cyclone separators arranged in parallel behind the combustion chamber, with diameters of φ 5160mm, lined with wear-resistant insulation material. The sealing return valve is located at the lower part of the cyclone separator and is connected with the combustion chamber and separator. The return material adopts the self balancing method, and the fluidizing seal air is supplied separately by the high-pressure fan. The combustion chamber, cyclone separator and sealed return valve form the material particle circulation circuit, and the coal and limestone complete the combustion and desulfurization reaction in the combustion chamber. The tail convection flue is located at the rear of the boiler. The four sides and ceiling of the upper part of the flue are composed of wrapped wall superheaters, which are successively arranged with three-stage superheaters and one-stage superheaters along the flue gas flow. The lower flue is successively arranged with economizers and horizontal air preheaters, and the primary and secondary air are arranged separately

the boiler adopts a single-stage evaporation system, and the downcomer adopts a combination of centralized and decentralized water supply. The superheated steam temperature is regulated by two-stage feedwater spray desuperheating. The boiler is arranged in the de interval of the main power house, and the boiler body adopts the tight closed mode. The 8m operation floor is designed as a fully closed one, with a small room on the top of the boiler. The boiler frame adopts all steel welded structure and is designed according to 7-degree seismic crack. The boiler adopts the fixed method of supporting and hanging. Except that the cyclone separator and air preheater are supporting structures, the rest are suspension structures. In order to prevent the damage of water-cooled wall and furnace wall caused by deflagration in the boiler, the boiler is equipped with rigid beams. The boiler sets the center line of the furnace, the center line of the cyclone separator and the center line of the flue as the expansion center, and expands freely with the expansion center as the origin. Nonmetallic expansion joints are set at the connections of the separator, the furnace, the return valve and the tail flue to solve the problem of thermal displacement sealing and ensure the tightness of the boiler

the boiler combustion system adopts four point coal feeding on the front wall. The coal in the coal hopper in front of the boiler is sent to the scraper coal feeder in front of the boiler through the weighing belt coal feeder, and enters the furnace through the coal chute. In order to prevent the positive pressure flue gas from escaping into the coal feeding system, the positive pressure seal of secondary air is introduced into the coal feeding system

the limestone system adopts positive pressure pneumatic conveying, and the limestone in the daily warehouse is sent by the limestone fan through the air supply pipeline to the secondary air pipe into the furnace and the inclined leg of the return valve

the boiler slag is discharged from two slag discharge holes on the bed surface. The high-temperature ash is cooled by two drum slag coolers, and then the low-temperature ash is sent to the slag removal system

the boiler air distribution is equipped with primary air fan, secondary air fan, high-pressure fan, limestone fan and induced draft fan. The balanced ventilation method is adopted, and the pressure balance point is located at the furnace outlet

the boiler ignition system adopts the combined starting mode of bed and bed. Four starting burners are arranged on the bed and two hot flue gas generators are arranged under the bed, which has the advantages of high thermal efficiency, uniform heating, fast starting speed and high ignition reliability

2 ash circulation system problem

on December 16, 2006, during the boiler ignition process, the boiler returned material fluctuation. At that time, the temperature of the bed temperature point in the middle of the boiler furnace was too high, the boiler outlet temperature and the inlet and outlet temperature of the cyclone separator were low, the ash circulating return temperature was low, the ash pressure was low, and there were fluctuations from time to time

3 problem analysis

3.1 CFB boiler material analysis

cfb boiler material distribution can be divided into four forms: fly ash, bottom slag, internal circulation and external circulation materials. Among them, the realization of internal circulation material and external circulation material balance is the key to the operation of CFB boiler. If the return material is unstable or suddenly stops working, it will cause insufficient circulating material in the furnace, and the air temperature and air pressure bed temperature are difficult to control, endangering the normal operation. In addition to the pouring material falling off and blocking, the most important reason for the instability of external circulation return is that the insufficient amount of circulating ash in the boiler leads to a short circuit of flue gas, which makes it difficult to establish a continuous and stable external circulation material balance. If we don't pay attention to it and take measures to adjust it in time, the consequences will be very serious

3.2 analysis of boiler ignition and startup process

the reason for insufficient circulating ash during boiler ignition and startup is very simple, because when we start the boiler before ignition and startup, the furnace charge is certain, 700mm thick, and no coal is added 5 hours before the boiler temperature rises, so there is no medium into the boiler to supplement the circulating ash that is continuously taken away with the flue gas, At this time, most of the furnace burden left in the combustion chamber is large particles (which is also the main reason for the bed temperature fluctuation this time). In addition, our primary air volume control at the initial stage of ignition is not accurate enough, so that a large amount of fly ash is further taken away with the flue gas. Where is the circulating ash taken away by the flue gas? They all accumulate in the return valve, because the return valve is empty at the initial stage of ignition. Only when the return valve reaches a certain thickness of material level, can a good cycle be established. Why? Let's first understand the function and operation characteristics of the return valve, return riser and high-pressure fan

3.2.1 u return valve

is actually a small fluidized bed, and the return air enters the valve through the fluidization hood from the lower two small air chambers, and the high-pressure air passes through during operation, Fig. 1; A1 and A2 enter the air chamber and fluidize the materials in the U-shaped valve through the air distributor and hood. The U-shaped valve is a self balancing valve, which can automatically adjust the outflow and inflow. The function of the valve itself to adjust the flow is weak. It also has one of the most important functions: to seal the return material

3.2.2 return riser

the function of the riser is to transport materials, seal the system, generate a certain pressure head to avoid the anti cross flow of furnace flue gas, and cooperate with the return valve and high-pressure fan to enable the continuous and stable transmission of materials from low pressure to high pressure (furnace)

3.2.3 high pressure fan

high pressure fan is a kind of equipment with high pressure head and low air volume. It has the characteristics of high pressure head, increased resistance and increased fan head to overcome the high pressure in the furnace hall and realize the continuous and stable transportation of materials. Low air volume is to avoid coking of high-temperature materials in the return valve

3.3 cause analysis of unstable return material

3.3.1 establishment of return material cycle

one of the most important functions is "return material sealing". High pressure 5 in the furnace will return and cause flue gas short circuit, thus hindering the normal circulation of fly ash. Section B1 is a return riser. When the external circulating ash enters the self weight b1b2 of the material in the riser, the circulating ash can enter the U-shaped valve. However, at this time, the circulating ash entering the U-shaped valve is small, it is difficult to be transported back to the furnace chamber, and can only be accumulated in the U-shaped valve. At this time, several or more cycles mentioned above will be formed, so that a certain amount of circulating ash can be accumulated in the U-shaped valve. Finally, only when, b1=b, 2 or When B1 ≥ B2, a good and stable external circulation material balance can be established

3.3.2 self balance of return valve

according to the above analysis and the characteristics of U-shaped valve, let's try to analyze again: when there is a certain thickness of material layer in B2 section in the figure in the U-shaped valve, under the action of high-pressure fan, the material level expansion is fluidized, (it will never be blown out. We have done experiments when shutting down the boiler. After two high-pressure fans are started and operated at the same time for a long time, there is still a circulation volume with the thickness of section B2 in the U-shaped valve). If we compare the original ash storage in section B2 of the U-shaped valve to the principle of "expansion and fluidization of water", that is, the "connector". Then, when the circulating ash separated by the separator is returned to the furnace by the return riser B1 ≥ B2, it continuously enters the U-shaped valve (i.e. the "connector" principle), so as to establish a good external circulation circuit and realize its original design concept, "the U-shaped valve is a self balancing valve, which automatically regulates the outflow and inflow"

3.3.3 unstable phenomenon of return material

this reminds us that several times of unstable return material that have occurred during normal operation mainly occur in the rapid and too fast load reduction of the boiler. The reasons are the self weight of the material in the riser when the external circulation ash quantity suddenly decreases, the resistance of the high-pressure fan increases when b1b2, the pressure head increases, and a large number of materials in B1 section return to the furnace, so that the pressure in B1 section is small and unstable return material in B2 section, forming a vicious cycle. Only when, b1=b2 or, B1 ≥ B2, can a good and stable external circulation material balance be established. During the adjustment, there should be no sudden noise. It may take several hours of adjustment to stabilize the external circulation material balance

3.3.4 impact of unstable return circulation

through the low inlet temperature and slow temperature rise of the cyclone separator during ignition startup, it can also be proved that the circulating ash separated by the separator is retained in the U-shaped valve with a sharp year-on-year decline of 33.4% and cannot be sent back to the furnace combustion chamber to participate in the ash circulation. If there is circulating ash to participate in the ash circulation, the inlet temperature of the cyclone separator is bound to increase with the increase of the bed temperature, Because it is the circulating ash that brings the temperature to the inlet of the cyclone separator, and in the early stage of our ignition startup, the deviation between the inlet of the separator and the bed temperature is great when we do not put coal in, which is not conducive to the control of the temperature rise of the separator, and is bound to cause the cracking and falling off of the separator

3.3.5 situation of boiler circulating return system in other power plants

according to the experience of boiler ignition and startup process in other power plants, it can also be proved that the instability of return material during ignition and startup is related to the emptying of return valve without a certain thickness of material level. Take the ignition starting process of Jilin Dongguan power plant, Liaoning Dalian Xianghai power plant and Liaoning Panjin power plant as an example. Jilin Dongguan power plant has never seen the phenomenon of return material fluctuation during the ignition and start-up process and normal operation. Generally, during the minor repair of the boiler, they never clean up the ash in the U-shaped return valve, and all of them are reserved for use during the ignition and start-up again. Therefore, the instability of return material has never occurred, and Panjin power plant is also in this way. In Dalian Xianghai power plant, the ignition bed material is relatively fine, generally less than 5mm, and the boiler has to be recharged twice according to the changes of the bed pressure gauge during the ignition start-up process to supplement the shortage of circulating ash and the lack of bed material. Dalian Xianghai power plant has designed its own bed material refilling port, which can be recharged during the ignition start-up of the boiler, which is very convenient, so during the ignition start-up process, There has never been any return material fluctuation

4 suggestions on measures to avoid unstable return material

4.1 retention of circulating ash

if there is no falling off of separator, return riser, U-shaped valve and inner pouring material, the ash in the U-shaped valve will not be removed and will be left

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