Circulating Fluidized Bed Boiler Combustion Control and Adjustment

Circulating fluidized bed boiler is a modern new energy efficient, low pollution and clean combustion equipment, is one of the important ways to solve the coal-burning pollution, it also has the advantages of wide fuel adaptability, good performance of load adjustment etc.. In recent years, large capacity circulating fluidized bed boiler has been widely used in China, circulating fluidized bed boiler 's operation process is very different from pulverized coal boiler and grate furnace, but in actual operation, many operator tend to use the original operation ways of pulverized coal boiler and experiences to operating circulating fluidized bed boiler, while the results often lead to economic reduction, even an accident. After consulting a large amount of relevant information and experience of long-term boiler design, and the site operation data’s summary, analysis the combustion characteristics and heat transfer mechanism of circulating fluidized bed boiler, The control and adjustment of circulating fluidized bed boiler combustion are briefly described, hoping to provide some references for boiler operators.

1, The overall structure of CFB boiler

The main include circulating fluidized bed boiler combustion system, material circulation system and flue tail of three parts. The system includes several parts of the combustion chamber, air distributor, combustion chamber, furnace, coal and limestone supply system; material circulation system including cyclone separator and J valve back feeding system of two parts; the tail flue arrangement of superheater, economizer, air preheater heating surface.

2, The combustion characteristics and heat transfer mechanism of circulating fluidized bed boiler

The main feature of circulating fluidized bed boiler is that the particles are sent back to the bed combustion by the appropriate gas-solid separation device and feed back device after leaving the furnace outlet. Fuel from furnace front coal feeding system into the furnace, with one wind and two wind; Fuel from furnace front coal feeding system into the furnace, with one wind and two wind; one wind as a burning fluidized bed wind and fluidizing agent of bed by the air distributor into the combustion chamber; the two wind along the height of the furnace is divided into two layers arranged to ensure enough fuel to the combustion space and combustion in the combustion chamber; the material in the flow of the wind speed under certain severe disturbance, some solid particles leaving the combustion chamber in high speed carry under the airflow enters into the furnace, the larger particles due to gravity along the inner wall of a hearth under flow, gas-solid two-phase flow in the furnace to form smaller; the particles fly out of furnace with flue gas into the cyclone are separated along the lower part of the particle separator material returning device back to the combustion chamber combustion cycle, after separation of the flue gas by heating in tail flue of surface absorption After leaving the boiler. Because of the separation device of circulating fluidized bed boiler with high efficiency, the particles are separated by the return feeder and is sent back into the chamber, make the boiler furnace ash concentration is high enough, strengthen heat transfer, therefore the circulating fluidized bed boiler not only radiation heat transfer, but also has the convection and heat conduction and heat transfer, greatly improved the heat transfer coefficient, ensure the boiler to reach the rated output.

3. Circulating fluidized bed boiler's main parameters control and adjustment.

During the combustion operation of circulating fluidized bed boiler, the most important indexes are bed temperature, air flow, fuel particle size and material layer thickness.

3.1 Bed temperature

Maintain the normal bed temperature is the key to stable operation of circulating fluidized bed boiler. In order to ensure good combustion and heat transfer, the bed temperature is generally controlled at 850~950 degrees centigrade and runs stably. In the process of operation, we should strengthen the monitoring of the bed temperature, the temperature is too high, easy to make coking in the fluidized bed cause shutdown accident, and affect the effect of nitrogen removal; the temperature is too low, prone to low temperature coking and fire extinguishing. The main factors affecting the bed temperature are the load, the amount of coal, the amount of feed back, the amount of air and the ratio of the one or two air, specific aspects are as follows:

1, During the operation, when the coal change, the heat change will change the bed heat balance, thus affecting the combustion, heat transfer and load, will also affect the emissions caused by bed temperature fluctuations, the calorific value is high, the bed temperature is higher.

2, Uneven coal supply, sometimes more, sometimes less, will make the bed temperature rise and fall, especially sometimes careless operation or short time coal will make bed temperature drop sharply.

3, After the load change, the wind coal ratio is not adjusted in time, such as load increase, coal quantity, air flow did not increase, the bed temperature will drop, otherwise, the bed temperature will rise.

4, During the operation, the coal particle size control is not strict or the coal quality is too bad, the slag discharge is not timely, the fluidization quality at the bottom of fluidization layer will deteriorate, and the increase of the material layer resistance will reduce the air volume and the imbalance of the coal to air ratio, resulting in the gradual decline of the bed temperature.

5, Improper adjustment of air coal ratio, excessive coal supply, excessive air flow, coal can not be well burned in the furnace, make the bed temperature is reduced. If the operators mistakenly believe that the amount of coal is not enough, continue to increase the amount of coal, will cause a serious imbalance in the ratio of coal to air, the bed temperature dropped sharply. If the air quantity is too large, it will make the gas particle away heat increases, also can make the bed temperature drop.

The adjustment and control of bed temperature mainly adjusts the amount of coal according to the change of load and coal quality, and maintains the suitable ratio of air to coal and the thickness of material layer, make the bed temperature is kept within the optimum range. In operation when the bed temperature pressure changes, timely change trend according to corresponding adjustment to the amount of coal and air. In operation when the bed temperature pressure changes, timely change trend according to corresponding adjustment to the amount of coal and air. To adjust and control the bed temperature should be especially careful operation because of the reaction temperature of the thermocouple always lags behind the actual temperature, so can not wait until the table indicating the bed temperature has exceeded the normal range and then to adjust, then even if the full stop to the coal or coal added to the maximum, the bed temperature is likely to continue to rise or fall, there are the danger caused by coking and flameout. When the bed temperature fluctuations are not great, to fine tune several times. When the bed temperature changes greatly, and make substantial adjustments is wrong. In any case, to adjust the amount of coal and air flow, it is necessary to ensure the good fluidizing quality of the bottom material, so as to prevent coking and flameout.

3.2 Air volume

The effect of one wind is to keep the bed material in a good boiling condition and to supply the oxygen needed for fuel combustion. The main function of the two air is to increase the disturbance of the flue gas, strengthen the mixing of gas and solid, reduce the thermal deviation in the furnace, increase the smoke temperature at the furnace outlet, and also provide the oxygen needed for combustion. The adjustment principle of air volume is that the two air is adjusted accordingly under the premise that the air flow satisfies the fluidization.One air flow is directly related to the quality of Fluidization. Circulating fluidized bed boiler to cold test before operation, and make the thickness in different material layer (layer differential pressure) under the critical flow volume curve, at run time as the lower limit of air volume adjustment, if less than this value, the material layer may flow is not good for a long time, there will be coking. The two air volume adjustment is mainly based on the flue gas oxygen content in the number of general control in 3 ~ 5%, such as high oxygen, that air volume is too large, will increase the boiler heat loss of flue gas Q2, at the same time, gas flow rate is large, the abrasion of heating surface; if too small will cause combustion incompletely, increase chemical incomplete combustion loss Q3 and mechanical loss of incomplete combustion q4. If the total air flow is not enough in operation, the air volume should be increased gradually to meet the combustion requirements, and the ratio of the one or two air flow should be adjusted constantly so that the boiler can achieve the best economic operation index. It should be noted that the input of the two air must be based on the continuous increase of the load and furnace temperature, and gradually slow, and avoid rapid and massive investment. Just because of the boiler operation, the heat intensity is low, the combustion system is still not stable enough, if a large amount of fast input two wind temperature is low, temperature will cause large fluctuations, bring many difficulties to the operation adjustment, if the control is not good will cause flameout.

3.3 Fuel particle size

The coal used for circulating fluidized bed boiler is a wide screening material in our country. It is generally required to be 0-8mm. The size of fuel particle will cause the change of the air supply rate, combustion fraction and fly ash concentration, thus affecting the change of the steam temperature. Such as coal particle size greater than 8-10mm, if maintained in the design capacity of running may make the coarse particle deposition caused accidents (this is one of the main reasons for the long-term stability of fluidized bed boiler in China can not run) in order that the coarse particles flow, must increase the air volume, resulting in particle elutriation rate increase, dense phase combustion fraction within the area reduced, dilute phase region combustion share increased, while increasing the air volume and the superheater area smoke increases, the temperature rise, serious when still can make some fine coal in the superheater zone caused by combustion, steam temperature gauge. The cause of the coal particle size incompatibility is strictly controlled by the following aspects, so as to ensure the safe and economical operation of the boiler. (1) the coal making system is not suitable, and the raw coal is not broken before screening, resulting in too much fine coal content. (2) the sieve runs abnormally. After a period of operation, especially when the coal is wet, the sieve hole is partly blocked, which makes the particle size of coal finer and finer. (3) no iron absorption device or is not running properly on the coal handling system, the iron nails, etc. in a fluidized bed. (4) the operation of the crusher is not normal, the crushing effect is not good, and the broken coal will not be screened, which will cause large particles of coal to enter the bed. (5) the screen is broken, the sieve pores become larger, resulting in a large number of coarse coal into bed.

3.4 Thickness of material layer

The thickness of the material layer is obtained by monitoring the differential pressure of the material layer. The pressure difference between the measured air chamber and the upper interface of the combustor is usually used as the monitoring value of the differential pressure of the material layer. One layer thickness control reasonable, directly affect the wind chamber pressure and fan power consumption, material layer is too thick will make the air flow is reduced, may cause flow caused by bad furnace coking and fire; the material layer is too thin, although the air volume adjustment range, but the operation is not stable, low load wind turbine throttle loss big. The general low load with small amount of thin layer with high load operation, large volume, thick material layer operation. Ensure the material layer thickness and the fluidization quality can be controlled by the bottom slag, when slag discharge，should be based on the set upper and lower coal types.

The above parameters is crucial to the stable and safe operation of circulating fluidized bed boiler combustion. In the operation according to the burning time and medium load, strictly control the material layer pressure and bed temperature, by continuously adjusting the quantity of coal, air volume, the boiler operation to achieve the best effect, to maximize the high efficiency and energy conservation of circulating fluidized bed boiler advantage.

Here is our company in recent years boiler field operation case analysis:

Case one:

According to the fax of the user, it is found that after the operation of a SHX20-1.25 CFB boiler for a period of time, the output of the boiler has been reduced, and some problems have been found through the operation analysis:

1, Boiler 's burning coal is powdered anthracite, separator and return feeder load increase caused by blockage due to the combustion characteristics of anthracite after, will appear over temperature and even coking phenomenon, long-term operation and not timely clean-up of Coke will be piled up, finally blocked.

2, found in the separator into elliptical cylinder masonry, have a great relationship with the separation efficiency of the cyclone separator cyclone shape, reduce the efficiency of the cyclone separator, easy to destroy the normal boiler ash circulation, the boiler feed back system does not work properly, causing the boiler burning difficult, is not conducive to the normal operation of the boiler, and the combustion efficiency and reduce the operating efficiency of the boiler. The separation efficiency is reduced and the circulation is reduced, and the boiler output is affected. At the same time, the furnace outlet temperature is low, which indicates that the circulation is less.

Terms of settlement：

1, Adjust the fuel particle size, particle size should be controlled between 0 ~ 8 anthracite mm, 0 ~ 2 mm at the same time, should guarantee the size of not more than 20%, to reduce the operating load of the return feeder.

2, Repair separator, ensure the size of the separator, smooth inner wall, improve the collection effect and circulation.

3, Adjust the original refeeder air distribution mode, increase the return feeder channel, from the current 200 to Phi Phi 250, while exports to demand the return of a fan material should ensure material returning wind, wind pressure, so as to avoid and return feeder jam over temperature of pulverized coal combustion.

4, Improve the temperature of the hearth can solve the difficult problem of burning anthracite, but lower than the coal ash melting point of 200 DEG C can be, but it should not exceed 1000 degrees for a long time, should increase the boiler output by improving circulation boiler.

Case two:

According to the user's reflection, a F25-2.5 boiler supplied by our company can not reach the rated load, and the right return feeder is not easy to return and block the ash. According to the analysis of the on-site inspection and the operation records of Party A, it is found that:

1, the coal supply of the boiler is too large, the raw coal is not broken, directly into the boiler, the fluidized bed slag particles mostly 20-40mm. According to the boiler operation records, in September before the operation, a fan current between 130A-150A, and the operation record after October, a fan current maintained at more than 180A, and even up to 200A; and the boiler load, material pressure differential little change. It can be explained that the larger the coal particle, the higher the resistance of the material layer, and the influence of the primary air flow rate. When the primary air volume is too small, the boiler is limited when the load is raised, and because of the increase of the number of deslagging, the combustion of the fuel is not complete, which causes the heat loss to be aggravated, and the thermal efficiency of the boiler is reduced. At the same time due to coal particles is too large, easy to aggravate wear cap, refractory belt, heating surface and other parts, so the service life of the boiler is affected, affect the production safety of boiler.

2, According to the scene also found that users adjust the dust way, from the original water film precipitator instead of bag dust collector, and the induced draft fan did not do the corresponding adjustment. Boiler fan before the elbow increased from the original water film dust remover is directly connected with the fan inlet by dust collector through 3 at 90 degrees bend into the fan, causing local resistance is too large; and the bag dust collector itself compared to the resistance of water film dust collector for the resistance is too large, so that the boiler the fan working load change. Because of the draught system of boiler is a whole system, this change caused by the boiler formation pressure head can not overcome the wind resistance of the system fan at high load, resulting in the form of negative pressure, so that the boiler can not improve the load.

3, The boiler is not equipped with deaerator system, boiler feed water temperature is the backwater of the external system, the water temperature is about 80 degrees. The first water temperature does not reach the boiler design of water temperature is 105 degrees centigrade, cause the boiler actual load than the rated load reduction; secondly because water is not easy to cause the boiler deaerator system to form oxygen corrosion, seriously affect the service life of the boiler, endanger the safe operation of boiler.

4, The oven right back feeder material returning poor ash blocking. After the boiler operation log view found in previous two refeeder temperature difference is small, the temperature change is consistent, that the return feeder structure is no problem, the work is completely normal, the right back feeder material returning poor analysis, probably in the boiler, due to revert over temperature or other reasons, resulting in the upper back device and separator taper parts or other debris blocking suspended coke, thereby affecting the normal work of the return feeder.

So users cannot reach the rated load of boiler, the fuel particles and users through the large and dust and flue resistance increases and other reasons have a great relationship, and to remind the user to increase the boiler deaerator, so as not to appear more security hidden trouble.