A Brief Discussion on the Automatic Thermal Combustion Control of Industrial Steam Boilers

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Abstract: The production of industrial boilers is a complex control and regulation process involving multiple inputs and outputs, multiple loops, and nonlinear interconnections. Current control theories struggle to address the control challenges associated with this process, and its mathematical models remain semi-empirical. This paper discusses some aspects of automatic control in thermal combustion. 　　 　　1. Thermal regulation The boiler’s thermal system consists of components such as the feedwater system, steam system, flue gas system, and air system. The thermal combustion control system for industrial boilers essentially provides automatic control over the operation of these systems. Its main control circuits include automatic control of the boiler drum water level and an automatic combustion adjustment system for the boiler. 　　1.1 Automatic regulation of boiler drum water level 　The water level in the boiler drum is one of the key indicators for normal operation, and it represents a very important parameter for regulation. Since the drum water level plays a crucial role in boiler operation, boiler automation always begins with automatic feedwater control. 　　Boilers with a capacity greater than 4 t/h must be equipped with an automatic feedwater control device. The task of automatic feedwater regulation is to ensure that the feedwater volume keeps up with the boiler’s evaporation rate and maintains the water level in the drum within the limits permitted by the process. The automatic control of the boiler drum water level is designed based on the dynamic characteristics of the drum water level. There are many factors that cause changes in water level, but the main ones are the step changes in water supply and evaporation rates. The regulator adjusts itself based on the water level signal, as well as the deviation signals related to steam flow and water supply flow. To ensure the safe operation of the boiler, the feedwater automatic control system should employ instruments and control systems with high reliability. 　　For steam boilers with a capacity of over 20 t/h, an automatic feedwater control system for the boiler drum using a three-pulse principle is generally employed. The three impulse boiler drum feedwater automatic control system is a control system that uses the drum water level as the main control signal, steam flow rate as the feedforward signal for the controller, and feedwater flow rate as the feedback signal for the controller. By using the steam flow signal for feedforward control of the feedwater flow to counteract external disturbances, and by utilizing the feedwater flow signal as a feedback signal to counteract internal disturbances, the amplitude of fluctuations in feedwater regulation quality is **improved**. 　　In boilers equipped with a three-pulse feedwater automatic control device, regulation signals for both steam flow and feedwater flow are utilized during operation, enabling the control system to respond promptly and with strong resistance to interference. When there is a sudden change in the steam load, the steam flow signal causes the feedwater control valve to act in the correct direction right away; that is, if the steam flow increases, the feedwater control valve opens further. It counteracts the reverse action caused by false water levels, reducing the amplitude of fluctuations in the feedwater flow. If the feedwater flow decreases, the regulator immediately opens the feedwater valve further in response to the signal of this decrease, thereby maintaining a constant feedwater flow. 　　1.2 Automatic regulation of boiler combustion In boilers of larger capacity, an automatic regulation system for boiler combustion can be installed, depending on the requirements for energy savings and automation, as well as the level of maintenance and available investment. 　　The basic task of the automatic control system for the boiler combustion system is to adjust the heat generated by fuel combustion to meet the requirements of the steam load, while also ensuring efficient combustion and the safe operation of the boiler. There are three aspects that need to be adjusted, namely maintaining the pressure in the steam main constant ； Maintain the economic efficiency of boiler combustion ； Maintain the negative pressure in the furnace chamber within a certain range. These three adjustment tasks are interrelated, and they can be accomplished by adjusting the fuel volume, air supply volume, and exhaust air volume. For the automatic regulation of the combustion process under stable load conditions, it is necessary to keep the combustion volume, air supply volume, and exhaust air volume constant, while promptly compensating for internal disturbances within the system. These internal disturbances include changes in the quality of the fuel, as well as variations in the fuel volume, air supply volume, and exhaust air volume caused by changes in the power grid frequency. In the presence of external disturbances that cause changes in load, the fuel volume, air supply volume, and exhaust air volume should be adjusted proportionally, so as to meet the load requirements while keeping the three controlled parameters—steam pressure, negative pressure on the furnace walls, and combustion efficiency—within acceptable ranges. 　　 　　2 Applications of Microcomputers in the Automatic Control of Thermal Combustion in Boilers 　　2.1 Automatic Regulation by Microcomputers in Boilers 　　2.1.1 Automatic Regulation of the Water Level in the Drum 　　The water level in the drum of industrial boilers is a crucial factor for proper operation; an excessively high water level can affect the separation of steam from water within the drum, leading to steam containing liquid droplets. On the other hand, an excessively low water level can disrupt the natural circulation of steam and water in the boiler. If not adjusted in time, all the water in the drum may vaporize, which could result in damage to the boiler or even an explosion. The water level in the boiler drum is influenced not only by the balance between the water supply volume (inflow) and the evaporation volume (outflow), but also by factors such as the steam-water circulation system, changes in the volume of steam and water, variations in fuel quantity, changes in drum pressure, and disturbances in the water supply and steam volumes – all of these can affect the water level. In manual mode, in addition to using DDZ-Ⅲ type instruments for three-shot automatic water level control, a microcomputer-based automatic control system is also employed for three-shot automatic drum water level control. The design concept of the three-impulse microcomputer control system is to analyze the dynamic characteristics affecting the water level control object. Given that the drum water level as a control object has certain delays and inertial properties, it is not possible for this object to immediately change in the opposite direction when there are changes in steam flow or feedwater volume. Especially when steam flow changes, the volume of steam and water does not change in the opposite direction, resulting in a phenomenon known as \"false water level\". On the other hand, it was analyzed that the control system using DDZ-Ⅲ type instruments fails to achieve satisfactory results in dealing with the phenomenon of \"false water level.\" By utilizing the computational capabilities of microcomputers, adopting the PID control principle as the basis for control, along with a program for detecting \"false water levels\" and the use of appropriate delay elements for pressure and differential pressure compensation, it is possible to effectively address this critical issue of water level control. 　　2.1.2 Automatic control of the boiler combustion system The boiler combustion system is primarily composed of six parameters: drum pressure, steam flow rate, air supply volume, coal feed rate, furnace negative pressure, and flue gas oxygen content. The purpose of regulation is to ensure that the heat generated by combustion meets the requirements of the steam load; this requires that (1) the steam pressure in the main pipes be maintained constant ； (2) Maintain economic combustion in the boiler ； (3) Maintain the negative pressure in the furnace chamber within a certain range. 　　To accomplish the above tasks, the microcomputer control system of the boiler adopts a comprehensive automatic control scheme, namely: (1) adjusting the frequency of the blower’s variable-frequency controller based on the outlet pressure in order to change the blower volume, and making fine adjustments using heat signals derived from the drum pressure and steam flow; to ensure accurate calculation of the outlet steam, temperature and pressure compensation is employed ； (2) The grate speed is adjusted based on the optimal wind-coal ratio; in other words, the ratio of the amount of coal burned to the amount of air supplied is adjusted, thereby changing the heat output of the boiler and consequently its evaporation capacity. A residual oxygen signal is used for correction in order to maintain the boiler’s main pipe pressure at a constant level. Additionally, the frequency of the exhaust fan’s variable frequency controller is adjusted according to the negative pressure in the furnace, thus changing the amount of exhaust air. Differential signals are used for predictive control, meaning that when the load is stable (i.e., when the steam flow remains constant), the amounts of fuel, supply air, and exhaust air should all remain unchanged ； When the load changes (due to external disturbances), the fuel volume, air supply volume, and exhaust air volume are adjusted proportionally. Based on the above requirements, the microcomputer control scheme for the combustion system can be determined, a system block diagram can be drawn, and the microcomputer programming can be utilized to the fullest extent possible to replace instrument units (such as function generators, adders, differentiators, limiters, filters, etc.) and thus perform rapid calculations of various parameters. The above explains the principles of the microcomputer-based water level and combustion control systems. After being processed by the microcomputer, the output signals are sent via D/A conversion devices to the feedwater valve, the frequency control units for the blowers and exhaust fans, as well as actuators such as the grate and coal feeder. The computer is used to directly adjust the valve positions, air volume, and rotation speed in a step-by-step manner, thereby enabling the system to select the optimal parameters for optimal regulation. 　　2.2 Boiler Microcomputer Monitoring System 　　1) It is capable of monitoring and sampling various on-site signals such as the water level in the steam drum, feedwater flow rate, feedwater pressure, water temperature at the inlet and outlet of the economizer, flue gas temperature at the inlet of the economizer, flue gas temperature at the outlets of both economizers, air temperature at the outlets of both air preheaters, flue gas temperature at the outlets of both air preheaters, flue gas temperature at the dust collector outlet, pressure at the dust collector outlet, air volume for forced draft and induced draft ventilation, flue gas pressure at the inlet of the economizer, flue gas pressure at the outlet of the economizer, air pressure at the outlet of the air preheaters, steam flow rate, steam pressure, steam temperature, furnace temperature, negative pressure in the furnace, coal feeding rate, and oxygen content. It also features a color closed-circuit monitoring system for the water level in the steam drum ； 2) Inspection and sampling of analog tracking signals such as water supply valve position, frequencies of various fans, and grate rotation speed ； 3) Inspection, sampling, and display of digital signals such as various valve positions, the start/stop status of the draft fans and grate motors, as well as the manual/automatic mode of various control loops ； 4) Sampling of frequency conversion signals for blowers and induced draft fans. 　　2.3 Scheme for the Automatic Thermal Combustion Control System of Boilers The automatic control system for boilers uses a PLC as the main control element, enabling rapid and accurate control of the boiler’s operation process. This approach helps to save labor and resources, improve the thermal efficiency of the boiler, and reduce energy consumption. The computer serves only as a monitoring and display device and is not involved in the control process. The automatic control system for boilers is based on the hardware architecture of the SPLC-9000 system, which features a multi-layer network structure consisting of three layers in total. 　　Measurement and control layer ； It is composed of high-performance PLC programmable controllers, which are equipped with CPU modules, I/O modules, and communication modules, and allow for flexible expansion. Programming in a PLC can be done using ladder logic, enabling one PLC to achieve automatic control of multiple boilers. Through the remote expansion solution, the SPLC-9000 system can also support centralized control of multiple control rooms that are located at great distances. 　　Operation layer ； An IPC industrial computer is used as the field workstation, with the main field workstation and the slave field workstation installed in separate control rooms, providing functions for screen display and data management. Given that PLCs offer high reliability but at a higher cost, while ordinary interface boards have lower reliability and are less expensive, in order to improve the performance-to-cost ratio of the monitoring system, the SPLC-9000 system provides both a programmable controller signal connection method and an interface board signal connection method. The output control of the system is handled independently by the PLC; the workstation is responsible only for providing a interface for users to modify parameters, and it transmits the control parameters set by the users to the PLC in order to adjust the control performance. 　　Management ； A regular computer is used as the management machine; managers in a dispatch room located away from the control room can view the images displayed on the field workstations through this management machine, retrieve the report data stored on those workstations, and carry out data analysis tasks. It is possible to accurately monitor the operating status of the boiler in real time, enabling rational scheduling. 　　The collection, processing, and control of important data are all carried out by the PLC. Since the PLC is designed specifically for industrial environments and is a true industrial computer, its reliability is **higher than that of ordinary interface boards and modules. The modular structure of the PLC and its convenient expansion features enable flexible system configuration. The host computer serves only for monitoring and management; damage to the interface board or the workstation does not affect the system’s automatic control. The software used in the field monitoring slave station and the field monitoring master station is the same; the only difference lies in the data configuration, and data from all boilers can be accessed from either station. 　　3.4 Effect Analysis Due to the high thermal inertia of the boiler system and the sharp fluctuations in load, operators rely on sensory parameters to control the boiler’s operation, which introduces a high degree of randomness and makes it difficult to maintain optimal operating conditions for the boiler. However, with the implementation of an automatic thermal combustion control system, the thermal efficiency of the boiler improves; coal burns more completely, pollutant emissions are kept within acceptable levels, and energy utilization increases. This reduces energy waste resulting from manual control methods, saves labor and resources, lowers operating costs, and enhances the reliability of the boiler’s operation.