Five adjustments for the operation of circulating fluidized bed boilers

Five adjustments for the operation of circulating fluidized bed boilers

Five adjustments for the operation of circulating fluidized bed boilers

Five adjustments for the operation of circulating fluidized bed boilers

Circulating fluidized bed boilers are not only structurally different from conventional coal-fired boilers, but also have their own characteristics in terms of combustion mode and regulation methods. In addition to steam temperature, steam pressure, and furnace negative pressure, the main parameters for normal operation and adjustment of circulating fluidized bed boilers should also focus on monitoring bed temperature, bed pressure, furnace pressure difference, cyclone separator ash temperature, cyclone separator material layer height, slag cooler working status, air distribution plate pressure, slag temperature, slag discharge temperature, etc.

Firstly, bed temperature control

Bed temperature is one of the main parameters that need to be closely monitored in circulating fluidized bed boilers. The high or low bed temperature directly determines the thermal load and combustion efficiency of the entire boiler, which is determined by the characteristic of circulating fluidized bed boilers (power controlled combustion). According to the different types of coal used, the control range of bed temperature is generally around 850-950 ℃. For coal with high volatile matter, it can be appropriately reduced, while for coal with low volatile matter, it may need to be above 900 ℃. But it should not be too high or too low, as being too low may cause incompletenessThe combustion loss increases, the desulfurization effect decreases, the heat transfer coefficient decreases, and in severe cases, a large number of unburned coal particles accumulate in the tail flue for secondary combustion, or the combustion fraction in the dense phase zone is insufficient, resulting in a higher bed temperature and a lower main steam temperature; If the bed temperature is too high, it may cause coking inside the bed, damage the air cap, and force the boiler to shut down. Generally, it should be ensured that the temperature in the dense phase zone is not higher than the deformation temperature of the ash by 100-150 ℃ or more.

The main means of adjusting bed temperature is to adjust the coal feeding rate and the ratio of primary and secondary air flow. If the excess air volume is maintained within an appropriate range, increasing or decreasing the coal feed rate will cause the bed temperature to rise or fall. But at this time, attention should be paid to the size of coal particles. If the particles are too small, once the coal enters the furnace, it will be blown by a primary wind to the dilute phase zone and burned on the heating surface of the dilute phase zone or horizontal flue, without causing a significant increase in bed temperature. When the coal particle size is too large, operators often use a larger operating air volume to maintain the fluidized state of the material layer. Otherwise, bed material stratification, local or overall overheating and coking of the bed layer may occur, which will delay the combustion time, lower the bed temperature, and increase the temperature in the upper part of the furnace after a period of time. When the air volume increases, the heat in the bed will be blown away to the upper part of the furnace, and the temperature of the bed will actually decrease. Conversely, the temperature of the bed will increase. Of course, once the primary air volume stabilizes, it is generally not necessary to adjust it frequently, otherwise it will damage the fluidized state of the bed. Therefore, many circulating fluidized bed boilers use a primary air volume less than a certain value as a condition for the main fuel cut-off (MFT) action. However, adjusting the air volume once within a small range is still an effective means of adjusting the bed temperature. The secondary air can regulate the oxygen content, but it is not as obvious as in coal powder furnaces. Sometimes, increasing the secondary air strengthens the disturbance effect on the upper part of the furnace, resulting in a temporary decrease in bed temperature. However, after a period of time, due to the increase in oxygen content, the overall bed temperature will show an upward trend. In circulating fluidized bed boilers with medium temperature separators, bed temperature is often controlled by changing the amount of return material. In the circulating flow boiler of the high-temperature separator, the effect is not significant because the ash temperature of the return feeder is not significantly different from the bed temperature. If a large amount of material is suddenly returned, it will cause a large number of burning coal particles to be buried by the bed material before they can be burned out, and the bed temperature will drop significantly. Adding limestone can also cause a decrease in bed temperature, as limestone absorbs some heat during calcination. The thickness of the bed layer can also have a significant impact on the regulation of bed temperature. When the bed thickness is very low, the heat storage capacity is insufficient, the bed temperature decreases, and at the same time, the furnace outlet temperature also increases. This is because the combustion fraction in the dense phase zone decreases and the heat release from combustion in the dilute phase zone increases. A low bed thickness can also cause uneven temperature throughout the entire bed, with high bed temperatures in areas with high coal addition and low bed temperatures in areas with low coal addition, which can easily lead to local coking. And the average bed temperature level is low, so the load cannot be increased. When the moisture content of coal increases, it will lower the overall temperature level of the bed.

Generally speaking, bed temperature is accurately monitored through thermocouples arranged in the dense phase zone and various parts of the furnace, and the position of the bed temperature measuring point has a significant impact on the bed temperature value. Because the surface temperature of the material layer inside the bed is the highest, while the temperature at the bottom is lowerTherefore, the bed temperature measuring point must be arranged in a suitable position. Thermocouples for temperature measurement are arranged at the top, middle, and bottom heights of the dense phase zone. During ignition, due to the effect of the hot flue gas generated by the under bed igniter, the upper temperature cannot represent the bed material temperature, and the temperature in the middle and lower parts should be used as the standard. When there is no external heat source, the temperature difference between the upper and lower parts of the dense phase zone is less than or equal to 50-80 ℃. When the thermometer is abnormal, the temperature can be determined by the color of the bed material through the observation hole and temporary observation hole. Based on experience, when the color of the bed material turns dark red, the bed temperature is approximately 500 ℃; When the color of the bed material is red or bright red, the bed temperature is about 800-900 ℃; When the color of the bed material is shiny and white, the bed temperature may exceed 1000 ℃.

When the bed temperature fluctuates, it is necessary to first confirm whether the coal feeding amount is uniform, and then check the amount of coal feeding. Excessive or insufficient coal supply, as well as excessive or insufficient air flow, can lead to combustion deterioration and a decrease in bed temperature. When adjusting the bed temperature during normal operation, it is necessary to maintain uniform coal feeding and air flow, following the principles of "adding air first and then adding coal" and "reducing coal first and then reducing air". The adjustment range should be as small as possible, and attention should be paid to controlling the advance time according to the trend of bed temperature changes.

Secondly, bed pressure control

Bed pressure, also known as bed layer pressure drop, refers to the static pressure at the air distribution plate and the pressure difference at the boundary between the dense phase zone and the dilute phase zone. The pressure drop of the air distribution plate generally accounts for 20% to 25% of the total pressure drop in the furnace, and in rare cases, it can be appropriately increased or decreased to ensure the requirements of fluidization quality. Under the premise of a constant fluidization air volume, it directly reflects the height of the bed layer. Maintaining relatively stable bed pressure and furnace pressure is essential for boiler operation and crucial for ensuring normal operation. If the bed pressure is too low, the combustion in the furnace will become suspended combustion, and the bed temperature will rapidly increase with the increase of coal addition, but the load cannot be carried, and the temperature difference of the entire bed is very large, which is prone to local coking; If the bed pressure is too high, more fluidization air is needed, otherwise it will also cause the bed material to not be fluidized and also lead to local coking. On the other hand, the pressure of the water-cooled air chamber will increase with the increase of bed pressure, and the pressure borne by the primary air system will increase, which can easily damage the fan and the pipeline of the air system. Moreover, in practice, it has been shown that when the bed pressure is too high, that is, when the bed thickness is too high, it can also hinder the normal return of the material feeder. The bed material falling into the water-cooled air chamber hinders the smooth flow of the primary air system, thereby affecting the total amount of primary fluidized air.

The main means of controlling bed pressure during normal operation is to adjust the amount of slag discharge. The slag discharge method of this boiler adopts bottom discharge. In the case of continuous slag discharge, the slag discharge speed is determined by the coal feeding rate, fuel ash content, and bottom slag fraction, and is coordinated with the working conditions of the slag discharge equipment or the slag cooler itself. When regularly discharging slag, the upper limit of bed pressure or control point pressure is generally set as the standard for starting to discharge bottom slag. Set the lower limit of bed pressure or control point pressure as the standard for stopping slag discharge. Continuous slag discharge is also achieved by adjusting the speed of the slag cooler to control the bed pressure within a certain range.

When discharging slag, the amount of slag is controlled by adjusting the speed of the slag cooler drum. For drum type slag coolers, it is crucial to control the stability of the slag discharge volume, otherwise it may cause the slag discharge system to overheat, deform or burn out. If it is intermittent slag discharge, it may cause ash agglomeration and block the slag cooler. For bottom slag discharge, some large or high-density wear-resistant materials, insulation materials, or debris, coke blocks, etc. may be discharged. When these blocks are too large, they may block the slag discharge pipe or slag cooler, causing poor slag discharge.

Thirdly, steam temperature adjustment

Generally speaking, the main steam temperature of a circulating fluidized bed boiler increases with the increase of bed temperature and decreases with the decrease of bed temperature. Due to the large heat storage capacity of circulating fluidized bed bed materials, the bed temperature does not change significantly when the load changes significantly, so the steam temperature of circulating fluidized bed boilers is relatively easy to control. When the load increases, the bed temperature tends to rise and the steam temperature also rises; When the load decreases, the bed temperature tends to decrease, and the steam temperature also decreases accordingly. Of course, this is not absolute, it is related to the structural characteristics and capacity of the unit: if the boiler superheater is mainly a convection superheater, when the load increases, the particle concentration increases, the heat absorption of the convection heating surface increases, and the steam temperature of the superheater rises; But the heat absorption of the radiant superheater is only proportional to the temperature level, as long as the temperature of the suspended space in the upper part of the furnace does not rise, its steam temperature will not rise. Changing the ratio of primary and secondary air can also alter the combustion ratio between the dense and dilute phases in the furnace, thereby changing the bed temperature to achieve the goal of regulating steam temperature.

The superheater uses a hybrid cooler to regulate the steam temperature, and this can also be used to eliminate the temperature difference between the two side tube walls. The superheater adopts a two-stage desuperheater, with the first stage being coarse adjustment, arranged on the outlet of the low-temperature superheater and the inlet pipeline of the screen superheater; The second level is arranged for fine adjustment, located on the connecting pipeline between the low-temperature section of the high-temperature superheater and the high-temperature section of the high-temperature superheater.

Fourth: Load Adjustment

One of the significant advantages of circulating fluidized bed boilers is their good load regulation performance. The key to normal operation is to establish a stable material circulation. A large amount of circulating material plays a role in transferring mass and heat, carrying a large amount of heat to the entire furnace, thereby reducing the temperature gradient above and below the furnace and increasing the range of load regulation.

Circulating fluidized bed boilers should focus on achieving two balances during load adjustment, namely material balance and heat balance. Material balance refers to the balance between the coal, limestone, and other materials entering the furnace and the slag, fly ash, and circulating materials coming back from the feeder. Heat balance refers to the sum of the heat generated by the fuel entering the furnace, the heat carried by the circulating material, and the unburned portion of the materialThe heat generated by the combustion of coal particles is equal to the heat absorbed by the water-cooled wall tubes, circulating materials, and flue gas formed by the blowing of primary air. Among these three parts of heat, the heat carried away by the smoke formed by the heating of the first wind is the largest; The heat carried away by the circulating ash is secondary; The water walls around absorb the least amount of heat. If the combustion fraction in the dense phase zone is determined, the heat carried away by the primary air and the heat absorbed by the water-cooled walls around the dense phase zone are also determined for a given bed temperature. The heat balance determined to achieve this bed temperature is the heat carried away by the circulating ash.

When the external load increases, the total heat absorption required by the boiler increases. If the combustion is not adjusted, the steam temperature and pressure will decrease accordingly. In order to maintain the stability of steam temperature and pressure, operators will increase the amount of coal and primary and secondary air flow, strengthen combustion, improve bed temperature levels, and correspondingly increase the amount of circulating ash. The separation efficiency of the cyclone separator is greatly improved. For the evaporation surface, due to the increased bed temperature and combustion in the dilute phase zone, the heat absorption of the evaporation surface increases; For the screen superheater, due to the enhanced combustion in the upper part of the furnace, its temperature level also increases to a certain extent, resulting in an increase in heat absorption; For the convective heating surface arranged in the tail flue, as the smoke velocity increases, the heat absorption also increases. In this way, the heat absorption capacity of the entire boiler heating surface increases compared to before, which promotes the steam temperature and pressure to return to normal values. At this point, the boiler evaporation capacity adapts to the increased demand of the entire unit's power generation load, achieving a new balance. When the external load decreases, the particle concentration in the furnace and the combustion fraction in the upper part of the furnace both decrease and approach the operating conditions of the bubbling bed. The decrease in particle concentration in the bed further reduces the heat flux density of the water-cooled wall, thereby affecting heat transfer. The separation efficiency of the cyclone separator decreases with the decreasing surface of the inlet particle concentration. The decrease in separation efficiency, in turn, makes it difficult to maintain suspended particle concentration and circulation rate, resulting in a decrease in overall furnace suction. However, the combustion fraction in the dense phase zone increases due to the decrease in circulation rate, which to some extent slows down the decrease in bed temperature. The other processes are opposite to when the load increases.

When various parameters change, they will have a certain impact on the operation of circulating fluidized bed boilers. among which

When the calorific value of coal affects the load, but the calorific value of coal changes, the change in bed temperature will also affect the load. The higher the calorific value, the higher the theoretical combustion temperature. Under the premise of constant combustion fraction in the dense phase zone, the higher the bed temperature, steam temperature and pressure will increase, and the load will increase.

2. The impact of changes in coal particle size on load. The larger the coal particle size, the less particles escape from the bed material, and the less heat is carried to the rear heating surface. As a result, the boiler cannot maintain normal material return, and the amount of heat that should be carried by the circulating material decreases. In this way, both material balance and heat balance are disrupted and correspondingly weakened, resulting in a decrease in boiler load.

3. The impact of coal moisture content on load. When the moisture content increases, the bed temperature will decrease due to the increased latent heat of vaporization absorbed by the steam. However, moisture can simultaneously promote volatilization analysis and coke combustion. After deducting the smoke exhaust loss caused by adding moisture, the overall trend is a decrease in bed temperature and load.

In general, the load of circulating fluidized bed boilers changes in the same direction as the airflow, wind speed, and material concentration, and automatically increases or decreases with the increase or decrease of load, demonstrating good automatic adaptability.

Fifth: Adjustment of the return feeder

Due to the use of non mechanical J valves in the return devices of most units, the sealing between the furnace and the return feeder is achieved by the material difference between the discharge leg and the feeding leg. How to maintain the level difference within a certain range and ensure the continuous and stable return of materials to the furnace is crucial for controlling bed temperature and pressure.

The return air is provided by a high-pressure fan, and pressure measuring points and ash temperature measuring points are arranged inside the J valve return feeder. Among them, there are pressure measuring points on the discharge leg to indirectly monitor the material level height. Because the entire return feeder system has negative pressure above the material level, if any point has a positive pressure, it means that the position where this point is located is the material level height. During normal operation, it is necessary to ensure that the pressure value at the top point is at least negative, otherwise the material level height cannot be monitored, which may cause blockage of the J valve return feeder and prevent material return.

At the beginning of ignition, a certain amount of bed material should be added to the J valve feeder in advance, otherwise a short circuit of flue gas will form between the furnace and the J valve feeder, and there will be no material circulation. During normal operation, the values of each pressure measuring point should fluctuate within a certain range, and the temperature measuring point values should not differ from the bed temperature values by more than 30-50 ℃. During the commissioning process of the unit, the air pressure of the loose air and return air has already been determined. Therefore, during normal operation, the air pressure of the loose air and return air should not be adjusted as much as possible. If the air pressure is too low, it is easy to cause blockage. Loose air should be used to clear the pipeline to ensure smooth operation; If the wind pressure is too high, it can easily affect the circulation ratio. In short, both of these situations will result in a decrease in the performance of the cyclone separator, and the output will also decrease accordingly.

If the loose air and return air are not properly matched, it will also hinder the full fluidization of bed materials, causing the wear-resistant lining of the return feeder to fall off, and even exacerbating the degree of redness of the return feeder. Generally, there is no need to adjust the loose air and return air during normal operation.

The negative pressure at the inlet of the two central cylinders should be the same. If the deviation is too large, it indicates that there is a tendency for blockage in one of the cyclone separators, and its fluidization air pressure should be adjusted in a timely manner to clear it as soon as possible. In general, the bottom of the J valve return feeder is equipped with an emergency discharge pipe, which can be opened in case of emergency to ensure that the bed material drops to the normal level as soon as possible.

New wear-resistant thermocouples for cement kilns and circulating vulcanization beds. This instrument uses special heat-resistant and wear-resistant alloy materials as the temperature measuring outer protective tube and wear-resistant head, and is equipped with an armored core body. It not only has high corrosion resistance to fly ash particle erosion, but also provides good protection for the inner core body under high temperature conditions. Using flange or threaded connection, it can measure temperature between 0-1000 ℃ for a long time and 1100 ℃ for short-term use.

Details

New wear-resistant thermocouples for cement kilns and circulating vulcanization beds. This instrument uses special heat-resistant and wear-resistant alloy materials as the temperature measuring outer protective tube and wear-resistant head, and is equipped with an armored core body. It not only has high corrosion resistance to fly ash particle erosion, but also provides good protection for the inner core body under high temperature conditions. Using flange or threaded connection, it can measure temperature between 0-1000 ℃ for a long time and 1100 ℃ for short-term use. The service life of wear-resistant thermocouples in circulating vulcanization beds can reach 8-12 months. The service life of wear-resistant thermocouples in cement kilns can reach 3-6 months.
High temperature wear-resistant thermocouple protection tube material is a high-tech material with a long-term use temperature of up to 1000-1150 ℃ and a short-term use temperature of 1200-1250 ℃.

The main products are:

Instrument series:

WR and WZ series temperature measuring instruments, pressure, liquid level, temperature and humidity sensors, various digital instruments, and various intelligent instruments. Temperature and pressure transmitters, heating elements, pressure gauges, bimetallic electronic components, electronic modules, and complete sets of instrument accessories. Cable series: various power cables, control cables, intrinsic safety cables, silicone rubber cables, computer cables, shielded cables, fire-resistant and high-temperature resistant cables, high, medium, and low-temperature heat tracing cables, and various compensation wires.

Model and Specifications

The material of the protective tube is1Cr18Ni9TiOther materials will be ordered according to the agreement