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1. Introduction The circulating fluidized combustion technology for coal is a new type of coal-fired technology that began to develop in the 1960s. It has gained increasing widespread use due to its advantages such as wide fuel adaptability, high combustion efficiency, low nitrogen oxide emissions, inexpensive SO2 control, large load regulation range, and fast load adjustment. Especially in the past decade, in order to effectively protect the environment, circulating fluidized bed boilers have seen rapid development, progressing toward higher parameters and larger scale. Currently, circulating fluidized bed boilers with 300MW units have been put into operation in China. Currently, an increasing number of medium and large-scale circulating fluidized bed boilers are in operation in China, and these power plants generally use DCS control systems for the operation control of their units. The use of DCS control systems in coal-fired boilers has a well-established track record, with high levels of automation and safety. For domestic circulating fluidized bed boilers, the current DCS control systems essentially use the DCS control logic designed for coal-fired boilers, with only minor modifications made ； Furthermore, given the current status of power plant construction in China, most units are put into operation under tight deadlines, leaving almost no time for researchers working on control systems. However, the combustion mechanism of circulating fluidized bed boilers is highly complex. The design of such boilers is still at the empirical design stage, with strong coupling between the various variables in the system, as well as significant non-linearity. The thermal automatic control of circulating fluidized bed boilers, particularly issues related to combustion automation, has become a major obstacle to their further widespread use. The automation of the operation of circulating fluidized bed boilers is one of the key factors for making them practical in real applications. 2. Current Situation During the commissioning phase of the unit, people generally considered that it was sufficient for the control system to ensure the proper start-up and shutdown of the boiler; as for optimizing the control system, improving its logic, enabling automatic operations and optimizing them, as well as setting up boiler protection mechanisms, these aspects were handled by making simple adjustments within the control concepts applicable to coal-fired boilers. However, circulating fluidized bed boilers and pulverized coal boilers differ significantly in terms of combustion mechanism, which determines that their control logic and concepts should also be quite different. Therefore, applying the control concepts of coal powder boilers often does not suit circulating fluidized bed boilers. This is also why, at present, many circulating fluidized bed boilers cannot be automated, and many safety protections cannot be activated; as a result, there is a high need for operators to manage these boilers, the workload is heavy, efficiency is low, and the operation of the boilers is extremely unstable. This poses a challenge for our manufacturing plants, power plants, and research personnel: how to make the DCS control system more suitable for circulating fluidized bed boilers. 3. Characteristics of automatic control in the combustion process of circulating fluidized bed boilers Circulating fluidized bed boilers differ from coal-fired boilers in that they have numerous control loops and a relatively complex system. The control system 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 ； Circulating ash control. The control characteristics for drum water level and superheated steam temperature are the same as those in conventional pulverized coal 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 generated by the burning of coal fed into the boiler to meet the requirements of the boiler’s steam load, while also ensuring safe and economical operation of the boiler. The tasks of the combustion control system can be summarized as follows: 1) Maintaining stable main 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 boiler’s evaporation rate ； 2) Ensure the economic efficiency of the boiler’s combustion process. While changing the fuel amount, adjust the air supply volume accordingly to match it, thereby ensuring the economic efficiency of boiler combustion ； 3) The intake air volume and the exhaust air volume are coordinated with each other to keep the furnace pressure within a normal range, ensuring the safe operation of the furnace ； 4) 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 during boiler operation as well as on the amount of NOx generated. Under normal circumstances, a bed temperature of around 860°C is the optimal temperature for desulfurization in the furnace, with lower levels of NOx production as well. An excessively low bed temperature not only reduces the efficiency of the boiler but also leads to unstable operation and an increased risk of flameout ； Excessively high bed temperature reduces desulfurization efficiency and increases NOx emissions; it also tends to cause coking of the furnace bed material, preventing fluidized combustion 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 ； 5) The control of the bed height 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 fully fluidized state ； If the layer of material is too thin, it not only fails to meet the load requirements but also allows the primary air to pass through the layer and extinguish the fire in the furnace ； 6) Circulating ash control directly affects the boiler’s circulation ratio, and it also has a certain impact on the bed temperature. 4. Implementation of automatic control for the combustion process in circulating fluidized bed boilers Circulating fluidized bed boilers are typical multi-variable controlled systems; however, due to insufficient research on such systems, as well as a lack of experience and in-depth understanding, traditional methods must still be used when designing, analyzing, and studying their control systems. Currently, the combustion control system design for circulating fluidized bed boilers still relies on conventional PID control, and it typically consists of several interrelated control units such as fuel control, total air volume control, primary air control, secondary air control, combustion chamber negative pressure control, bed temperature control, bed height control, and circulating ash 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. Regarding the control of combustion in circulating fluidized bed boilers, from a macro perspective, no matter how you attempt to control it, it is essential first to maintain stability in the bed temperature. The main factors that affect the bed temperature can be seen in the diagram below; the relationships among the various control variables are quite complex. As can be seen from the diagram, the various parameters are interrelated. To achieve automated control, simple PID control is far from sufficient; therefore, advanced control concepts such as fuzzy control must be incorporated into the control system of circulating fluidized bed boilers. This is a challenge for us who are engaged in the research of fluidized bed boilers. Practice has shown that fuzzy control can effectively control time-varying, nonlinear, and complex controlled systems. 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 a circulating fluidized bed boiler, which is a nonlinear and complex system, satisfactory results can be achieved. 5. Optimization methods for automatic control of the combustion process in circulating fluidized bed boilers The combustion control in circulating fluidized bed boilers is intended to ensure the safe operation of the boiler, maintain stable bed temperature, and enable flexible participation in the coordinated control of the power unit. To achieve these objectives, it is necessary to understand several relationships, namely: the relationship between coal consumption and load ； Relationship between coal consumption and bed temperature ； Relationship between load and bed temperature ； The relationship between the heat absorption of the furnace heating surface and bed temperature as well as coal quantity. Furthermore, we must pay attention to the thermal energy storage capacity of circulating fluidized bed boilers. By understanding these, it becomes easy to determine the elements we need to control, and thus we can use fuzzy control in combination with DCS functions to achieve our control objectives. In fact, on a macro level, as long as we understand the heat storage capacity of circulating fluidized bed boilers, we can control them effectively. This is what distinguishes the control of circulating fluidized bed boilers from that of coal-fired boilers. Unfortunately, it is very difficult to determine exactly how much heat storage capacity a running fluidized bed boiler has. To effectively control the boiler, we can use fuzzy control to solve this problem. We have the control system mimic human experiential thinking, then use theoretical calculations for correction, and finally achieve our goal through DCS. For example: in coordinated control, it is necessary to reduce the load. If done manually, adjustments would be made based on experience – less coal and less air would be used. Considering the boiler’s capacity to store energy, operators would first reduce the amount of coal used significantly; once the load has been reduced, they would then increase the coal supply to match the current load level. During this process, parameters such as the boiler’s bed temperature, primary air volume, secondary air volume, oxygen level, bed height, and circulating ash amount all change to varying degrees, and these also need to be adjusted. By understanding this process and the relationships mentioned above, we can achieve automatic control of the control system through it. Other situations such as load increase, fault conditions, and various unforeseen disturbances (such as changes in the medium) can also be handled using the same method. 6. Conclusion At present, there are still many imperfections in the design and implementation of automatic control systems for the combustion process in domestically produced circulating fluidized bed boilers. Considering the characteristics of the combustion operation in such boilers, it is essential to optimize and modify these automatic control systems in order to ensure the safe and efficient operation of the units.