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The automatic control system for circulating fluidized bed boilers includes seven aspects of control: drum water level regulation, main steam temperature regulation, furnace negative pressure regulation, bed temperature regulation, bed layer thickness regulation, main steam pressure regulation, and air supply regulation. This article shares the methods for designing and implementing such a control system for circulating fluidized bed boilers, using SAMA diagrams and specific examples. Flow meter: yunrun.com.cn/product/999.html. During the operation of a 75t/h circulating fluidized bed boiler in a certain company, it is necessary to keep the following parameters within specified ranges: ① Drum water level – Stability tolerance: ±5mm. ② Main steam temperature – Stability tolerance: ±5°C. ③ Furnace negative pressure – Stability tolerance: ±5Pa. ④ Bed temperature – Stability tolerance: ±4°C. ⑤ Pressure difference across the material layer – Stability tolerance: ±0.1kPa. ⑥ Main steam pressure – Stability tolerance: ±0.05MPa. ⑦ Oxygen content in flue gas – Stability tolerance: ±3%. There are many factors that affect the stability of these parameters, and they also influence each other. http://yunrun.com.cn/upload/201804/17/201804171742585498.png From the perspective of automatic control, a circulating fluidized bed is a control system characterized by multiple interrelated variables, non-linearity, time variability, and distributed parameters. In order to enable automatic adjustment of these parameters, through research and experimentation conducted by Changhui Instrument Manufacturing Co., Ltd., its control can be roughly divided into two parts: 1. Steam and water system. The steam and water systems of fluidized bed boilers share many similarities with those of other types of boilers; therefore, the typical control schemes used for layer-fired and suspended-furnace boilers can be applied. 2. Combustion system: The combustion system of fluidized bed boilers has its own characteristics and advantages, and the various parameters are interrelated with significant delays. A control scheme can be established after system identification using fuzzy control theory. Automatic control design scheme for 75t/h circulating fluidized bed boilers yunrun.com.cn/tech/1924.html The automatic control of circulating fluidized bed boilers is designed using SAMA diagrams, which helps people understand the control principles and specific implementation methods clearly. If you are not familiar with the SAMA legend, please refer to our website’s technical article titled “Applications of SAMA Diagrams and SAMA Legends in Automatic Control Design”. 1. Drum level control system: The drum level is an important parameter for ensuring the safe operation of the boiler, and its control system utilizes a single-stage three-element control system with temperature and pressure compensation. The SAMA diagram for control is shown in Figure 1: http://yunrun.com.cn/upload/201804/17/201804171355570318.png. Figure 1 depicts the SAMA diagram of the drum water level control system in a circulating fluidized bed boiler. 2. Main steam temperature control: The system employs cascade control, with the main steam temperature serving as the input for the primary controller (a single-loop regulator). The temperature at the exit of the desuperheater is used as an intermediate parameter, which is fed into the secondary controller as feedback for this secondary regulator (an external setpoint regulator). The primary controller operates in a feedback manner, while the secondary controller operates in a feedforward manner; by adjusting the amount of water used for desuperheating, the required main steam temperature can be achieved. The control SAMA graph is shown in Figure 2. http://yunrun.com.cn/upload/201804/17/201804171409313236.png Figure 2: SAMA diagram of the main steam temperature control system in the circulating fluidized bed boiler. Figure 3: Furnace negative pressure control system. The control channels in question exhibit good dynamic characteristics; a furnace negative pressure transmitter with a precision of 0.075% and high stability is used to measure the furnace pressure, and a single-loop control system is employed for regulation. However, due to the relatively fast ascent speed of the perturbed channel, a feedforward-feedback composite system with an air supply volume feedforward signal is adopted. To prevent large fluctuations in negative pressure from posing a safety risk, a limiting element must be incorporated into the circuit to ensure that the draft dampers vary within a certain range of openings. The control SAMA plot is shown in Figure 3. http://yunrun.com.cn/upload/201804/17/201804171419068337.png Figure 3: SAMA diagram of the negative pressure control system for the furnace of a circulating fluidized bed boiler. Figure 4: Bed temperature control system. The bed temperature in a circulating fluidized bed boiler is an important parameter; too high a temperature can lead to coking, while too low a temperature reduces the efficiency of desulfurization and may also cause the boiler to shut down. The main factors affecting the bed temperature are: coal quality, coal particle size, and layer thickness. The layer thickness is also related to the coal feed rate and the return material amount, but the effects of these two on the bed temperature are opposite. To prevent malfunction, it is necessary to include the oxygen content in the flue gas, an important parameter that reflects this effect. The requirements for bed temperature control are met by adjusting the amount of return material (the speed of the Roots blower or the opening degree of the ash return air damper). The control SAMA plot is shown in Figure 4. http://yunrun.com.cn/upload/201804/17/201804171442479110.png Figure 4: SAMA diagram of the bed temperature control system in a circulating fluidized bed boiler. Figure 5: Bed thickness control system. A circulating fluidized bed does not have a distinct bed thickness, but it still has a dense phase region and a dilute phase region; the bed thickness refers to the thickness of the bed when it is at rest in the dense phase region. The layer thickness not only affects the bed temperature but also has a significant impact on the economic operation of the boiler. The actual bed thickness is calculated by measuring the pressure difference between the primary air chamber and the dilute phase zone as well as the volume of primary air, and then it is compared with the specified bed thickness. The speed of the water-cooled spiral slag discharger is controlled through PI regulation in order to maintain the thickness of the material layer. The control SAMA plot is shown in Figure 5. http://yunrun.com.cn/upload/201804/17/201804171650307524.png Figure 5: SAMA diagram of the bed thickness control system for circulating fluidized bed boilers. Figure 6: Main steam pressure control system. The main steam pressure is one of the essential conditions for the safe and efficient operation of turbines and boilers. Due to the large delay in the control pathways, a two-loop system with intermediate parameter integration is required to accelerate the elimination of internal disturbances. The midpoint parameter is a heat signal calculated based on the steam volume and drum pressure, and the fuel amount (coal feeder speed) is adjusted to meet the requirements for pressure control. The control SAMA plot is shown in Figure 6. http://yunrun.com.cn/upload/201804/17/201804171632083039.png Figure 6: SAMA diagram of the main steam pressure control system in a circulating fluidized bed boiler. Figure 7: Air supply control system. The air supply control system allows the amount of air supplied to be adjusted so that it matches the amount of fuel used, thereby ensuring an appropriate excess air coefficient and optimizing combustion efficiency. The oxygen content in the flue gas is used as the control variable, and the requirements for regulation are met by adjusting the amount of primary air and secondary air. To further improve the control quality, a load signal is introduced as a feedforward signal, enabling the air supply system to operate simultaneously with the steam pressure system in response to load disturbances. The control SAMA plot is shown in Figure 7. http://yunrun.com.cn/upload/201804/17/201804171812177730.png Figure 7: SAMA diagram of the air supply control system for the circulating fluidized bed boiler. The circulating fluidized bed boiler used in the automatic control project at Wu County Jiangyuan Thermal Power Co., Ltd. is a medium-temperature and medium-pressure fluidized bed boiler manufactured by Beijing Boiler Factory, with a capacity of 75 t/h. Although both heat signals and bed temperature signals can be used in the main steam pressure control system of circulating fluidized bed boilers to overcome the lag in main steam pressure changes caused by variations in coal feed, the use of bed temperature signals results in a faster response time. However, due to the combined action of these two control systems, bed temperature fluctuations become significant, and coking can occur if proper attention is not paid. By using the heat signal alone, it is possible to overcome time delays and eliminate the coupling between the two control systems, allowing the tuning of each system to be done independently. This is the control requirement designed by Changhui Instrument Manufacturing Co., Ltd.; practical tests have shown that the automatic control system for circulating fluidized bed boilers is stable and reliable, meeting the long-term automatic control needs of the users.