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Abstract: The problems and challenges existing in the control systems of circulating fluidized bed boilers are analyzed, their characteristics are discussed and summarized, and finally, the development trends and research directions for the control system strategies of circulating fluidized bed boilers are explored and predicted. Keywords: circulating fluidized bed ； Mathematical model ； Control law ； Fuzzy control in circulating fluidized bed combustion of coal is a new coal-fired technology that began to develop in the 1960s. It has seen increasing use due to its advantages such as wide fuel adaptability, high combustion efficiency, low nitrogen oxide emissions, large load regulation range, and fast load adjustment. This is especially true for environmental-friendly power plant boilers, which have developed rapidly over the past decade to help protect the environment. However, the combustion mechanism of this combustion method is highly complex. The design of circulating fluidized bed boilers is still at the empirical design stage; the coupling between various variables in the system is strong, and there is significant non-linearity. Problems related to the thermal automatic control of circulating fluidized bed boilers have become the main obstacle to their widespread adoption, and the automation of their operation is one of the key factors for making them practical. 1 Existing problems: Implementing automatic control for circulating fluidized bed boilers is not an easy task, and there are many issues that need to be explored and studied. These are summarized as follows: a) Circulating fluidized bed boilers are control systems with distributed parameters, nonlinear behavior, time variability, and close coupling among multiple variables; therefore, their automatic control systems must carry out more complex control tasks compared to those of ordinary boilers ； b) The basis for employing modern control theory is the need for a relatively accurate mathematical model to describe the characteristics of the controlled system. However, due to the complexity of the combustion characteristics of circulating fluidized bed boilers, it is very difficult to establish such a mathematical model ； c) Due to the complexity and uniqueness of combustion in circulating fluidized bed boilers, it is very difficult to achieve automatic control of them. Conventional control methods that are effective for ordinary boilers and other process control systems are no longer sufficient to ensure the fulfillment of all the control requirements for circulating fluidized bed boilers. Given the aforementioned problems and their causes, it is of great practical significance to develop a control scheme suitable for this type of furnace. 2 Control System Design and Characteristics Circulating fluidized bed boilers differ from coal-fired and oil-fired boilers in that they have numerous control loops, resulting in a relatively complex system. The design of such control systems generally includes the following main loops: drum water level control ； Superheated steam temperature control ； Fuel control ； Control of air volume and flue gas oxygen content ； Furnace negative pressure control ； Bed temperature control ； Bed height control ； Control of secondary return material. The control characteristics for drum water level and superheated steam temperature are the same as those of conventional pulverized coal boilers and oil-fired boilers, so they will not be analyzed here; only the features of the control systems related to the combustion in circulating fluidized bed boilers will be discussed. The basic task of automatic control in the combustion process of circulating fluidized bed boilers is to adjust the heat supplied by fuel combustion to meet the requirements of the boiler’s steam load, while also ensuring the safe and economical operation of the boiler. The tasks of the combustion control system can be summarized as follows: a) Maintaining stable steam pressure. Changes in steam pressure indicate that the amount of steam produced by the boiler does not match the steam demand of the load; it is therefore necessary to adjust the fuel supply accordingly in order to change the amount of steam generated by the boiler ； b) Ensure the economic efficiency of the boiler combustion process. While changing the fuel quantity, adjust the air supply volume accordingly to match it, thereby ensuring the economic efficiency of boiler combustion ； c) The intake air volume and the exhaust air volume are coordinated with each other to keep the furnace pressure within normal limits, ensuring the safe operation of the boiler ； d) The bed temperature is an important parameter that directly affects whether the boiler can operate safely and continuously; it also has a direct impact on the desulfurization efficiency and NO2 levels during boiler operation. The amount generated. Generally, a bed temperature of 856°C is the optimal temperature for desulfurization in the furnace, with lower levels of NO2 production as well. An excessively low bed temperature not only reduces the efficiency of the boiler but also leads to unstable operation and a higher risk of flameout ； Excessively high bed temperature reduces desulfurization efficiency and increases NO2 production; it also tends to cause coking in the furnace bed, preventing sulfurized combustion from occurring and leading to shutdown of the furnace. It can be seen that the bed temperature is an extremely important parameter in the operation of circulating fluidized bed boilers ； e) Bed height control is also closely related to the safe and continuous operation of the boiler; if the bed is too thick, it will suppress the flow of primary air, preventing the furnace material from reaching a state of complete sulfidation ； If the layer of material is too thin, it not only fails to meet the load requirements but also allows the primary air to penetrate through the layer and extinguish the flame in the furnace ； f) The control of secondary return material will directly affect the boiler’s circulation ratio, as well as having a certain impact on the bed temperature. A circulating fluidized bed boiler is a typical multivariable controlled object; however, since research on its system has only just begun and there is still a lack of experience and in-depth understanding, traditional methods are still used in the design, analysis, and study of its control systems. Currently, the combustion control system design for circulating fluidized bed boilers still relies on conventional PID control, and it typically consists of 8 interrelated control units: fuel control, total air volume control, primary air control, secondary air control, combustion chamber negative pressure control, bed temperature control, bed height control, and secondary return material control. In other words, the controlled object is artificially divided into many single-variable systems for control. Although this control method is simple and easy to implement, and local analysis is reasonable, there are many problems when considering the whole system; it poses significant limitations to further improving the level of automatic control, and in some cases it even fails to ensure the normal operation of the equipment. 3 Research Directions and Specific Contents Recent research and applications have shown that, due to the complexity of the combustion system in circulating fluidized bed boilers, particularly the close coupling between various control variables, conventional control methods struggle to meet the requirements for the automatic operation of such boilers as well as for further improvements in their automatic control capabilities. Therefore, it is of great significance to apply advanced control theories based on in-depth studies of the combustion systems in circulating fluidized bed boilers, in order to achieve optimal multi-variable control; this represents one of the key research topics in the field of thermal control today. 3.1 Improvement and Quantification of the Mathematical Model: The dynamic multivariable mathematical model of the circulating fluidized bed boiler is improved and quantified, so as to determine the quantitative coupling relationships among various variables of the boiler. This provides a mathematical basis for identifying the control variables, controlled variables, and disturbances in the control system, as well as for establishing the control framework ； To provide reliable data for the implementation of decoupled control ； It provides guidance for setting the control system parameters. 3.2 Study on control laws: By considering the operating mode of the boiler, the control laws for circulating fluidized bed boilers are studied in order to determine the general control strategy. As mentioned earlier, circulating fluidized bed boilers feature wide fuel adaptability, high combustion efficiency, effective desulfurization, low NO2 emissions, a small furnace volume, a large range of load adjustment, and fast load regulation speeds. However, its combustion mechanism is very complex. The heat transfer within the furnace includes four types: heat transfer between gases and solid particles, heat transfer between the bed and the water-cooled walls, heat transfer between the bed and the tubes embedded in the furnace, and heat transfer within the cyclone separator or primary air separator; among these, suspended solids affect heat transfer ； The sulfidation rate affects heat transfer by influencing the density of the suspension; both the vertical length of the heat transfer surface and the bed temperature have a significant impact on heat transfer. The structural uniqueness of circulating fluidized bed boilers, along with the complexity of their combustion mechanisms, makes control more complicated compared to other types of boilers. In addition to the conventional control variables, this type of furnace requires control over the bed temperature and bed height, among which the bed temperature is the most important parameter in the control of circulating fluidized bed boilers. In the control of circulating fluidized bed boilers, since changes in primary air have a greater impact on the bed temperature than changes in coal feed rate, primary air is used as the control variable for regulating the bed temperature, while the coal feed rate is adjusted to meet the load requirements. However, changing the coal feed rate will directly affect the furnace temperature; therefore, the coal feed rate and primary air are two control variables that are closely coupled. In the combustion control system of circulating fluidized bed boilers, how to coordinate these two variables to achieve automatic control. It is the focus of control scheme research. Adjusting the boiler load must be achieved by adjusting the heat transfer mechanism within the furnace, that is, by adjusting the amount of primary air in order to change the solid concentration distribution inside the furnace. At the same time, as the main control parameter for bed temperature, the primary air must also take into account bed temperature regulation. Since the gas and solid mixture in a circulating fluidized bed boiler is relatively uniform, resulting in a good temperature field distribution, the coal fed into the boiler can burn rapidly; therefore, the delay compared to other types of boilers is smaller. As a result, it is more reasonable to regulate the thermal balance by adjusting the coal supply. 3.3 Control Scheme Based on the above two research findings, a conventional control scheme and advanced control theories are integrated to develop a comprehensive combustion control scheme for circulating fluidized bed boilers. In recent years, fuzzy control technology has made significant progress, with many successful applications emerging. Practice has shown that fuzzy control can effectively manage time-varying, nonlinear, and complex controlled systems. However, the establishment of fuzzy control rules is constrained by the experience and knowledge level of experts, which in turn affects the effectiveness of fuzzy control. Moreover, fuzzy control cannot eliminate steady-state errors. Additionally, PID control is widely used in industrial settings and holds a dominant position; as a result, it is neither possible nor necessary for fuzzy control to completely replace PID control. To this end, fuzzy control theory is applied to improve the conventional PID control, and it is integrated with fuzzy control to form a \"comprehensive control scheme\". When combined with various feedforward control schemes and applied to the combustion system of cyclic fluidized bed boilers, which is a nonlinear and complex system, satisfactory results can be achieved. 4 Conclusions The objective of the research on the combustion control system for circulating fluidized bed boilers is to refine and quantify their dynamic multivariable mathematical models based on numerous field tests, determine the quantitative coupling relationships between various variables, and study the control principles for such boilers in relation to their operating modes. Building on existing feedback and feedforward PID control structures, fuzzy control theory is applied to develop a comprehensive combustion control scheme for circulating fluidized bed boilers, which is then refined through practical application.