Abstract: This paper analyzes the characteristics of the automatic control system of circulating fluidized bed boiler, elaborates on the concept, architecture and characteristics of fieldbus, and illustrates the specific development and application methods of fieldbus with examples.
Keywords: Circulating fluidized bed boiler; Fieldbus; Distributed architecture; Communication protocol; Profibus industrial Ethernet
1. Characteristics of Circulating Fluidized Bed (CFB) Boiler Control Systems
Circulating fluidized bed (CFB) boiler combustion technology is a coal-fired technology that has developed over the past 20 years. It boasts significant advantages such as wide fuel adaptability, high combustion efficiency, low nitrogen oxide emissions, large load regulation ratio, and rapid load adjustment. China's national industrial policy strongly supports the development of this technology and its products. Since its emergence, CFB boilers have been widely used worldwide, and large-capacity CFB power plant boilers have been accepted by the power generation industry. The world's largest capacity 250MW CFB boiler was put into operation in 1997, and several large-capacity CFB boilers of 200-250MW have also been put into operation. my country has focused on the research and development of medium-sized CFB boilers, which are now fully commercialized. Currently, my country has approximately 2,000 CFB boilers with a capacity of 35t/h or less in operation or on order, which have begun to enter the power market, and research and development of large-scale CFB boilers has also commenced.
The control system of a circulating fluidized bed (CFB) boiler includes both typical thermal process control (such as boiler drum water level control and steam temperature control) and program control (interlocking protection system). Field equipment includes thermal instruments such as temperature, pressure, flow, and level transmitters and regulating valves, as well as analytical instruments such as speed-regulating motors and zirconia generators, and mechanical quantity measurement and control equipment. The working environment is relatively harsh, mainly due to temperature, dust, vibration, and electromagnetic interference, requiring the equipment to have strong environmental tolerance.
From an operational perspective, boiler equipment operates 24/7, placing high demands on the reliability of control equipment. Circulating fluidized bed boilers have unique characteristics, requiring strict control over parameters such as air distribution and furnace temperature. From an economic perspective, efficient operation necessitates a high-level automated control system. However, market competition has shrunk profit margins, forcing small and medium-sized enterprises, in particular, to downgrade their automation equipment, often limiting their automation levels to basic detection and necessary stand-alone control.
2. Features and advantages of fieldbus
2.1 Generation of Fieldbus
Looking at the history of control system development, it is not difficult to find that the introduction of each new generation of control systems is a solution to the shortcomings of the older generation of control systems. Ultimately, driven by the two major external factors of user demand and market competition, they occupy a dominant position in the market. The emergence of fieldbus and fieldbus control systems is no exception.
2.1.1 Analog Instrument Control System
Analog instrument control systems dominated in the 1960s and 70s. Their significant drawbacks were low accuracy and susceptibility to interference.
2.1.2 Centralized digital control system.
Centralized digital control systems dominated in the 1970s and 80s. They used microcontrollers, PLCs, SLCs, or microcomputers as controllers, and transmitted digital signals internally. This overcame the low accuracy of analog signals in analog instrument control systems and improved the system's anti-interference capability.
The advantages of a centralized digital automation information control system are that it is easy to perform control calculations and judgments based on the overall situation, and the selection of control methods and control timing can be uniformly scheduled and arranged. The disadvantage is that it places high demands on the controller itself, which must have sufficient processing power and extremely high reliability. When the system tasks increase, the efficiency and reliability of the controller will drop sharply.
2.1.3 Distributed Control System (DCS)
Distributed control systems (DCS) dominated the market in the 1980s and 1990s. Their core concept is centralized management and decentralized control, separating management from control. A host computer is used for centralized monitoring and management, while several slave computers are distributed across the field to achieve distributed control. These host and slave computers are interconnected via a control network to facilitate information exchange. Therefore, this distributed control system architecture effectively overcomes the shortcomings of centralized digital control systems, which require high processing power and reliability from the controllers.
In distributed control systems (DCS), the realization of distributed control concepts benefits from the development and application of network technology. Unfortunately, different DCS manufacturers, aiming for monopolistic practices, employ their own proprietary and closed control communication networks. This makes interconnection and information sharing difficult between DCS systems from different manufacturers, and between DCS and upper-level Intranet or Internet information networks. Therefore, from this perspective, a DCS is essentially a closed, proprietary, and non-interoperable distributed control system, and DCS is also expensive. Under these circumstances, users urgently demand openness and cost reduction in network control systems.
2.1.4 Fieldbus Control System (FCS)
FCS emerged in response to these trends. It uses the open and interoperable fieldbus network to interconnect various field controllers and instruments, forming a fieldbus control system. Simultaneously, control functions are completely decentralized to the field, reducing installation and maintenance costs. Therefore, FCS is essentially an open, interoperable, and thoroughly distributed control system, and it is expected to become the mainstream control system product of the 21st century.
2.2 Fieldbus and Fieldbus Control System
2.2.1 Concept of Fieldbus
Fieldbus is a real-time control communication network that interconnects the lowest-level field controllers and intelligent field instruments in automation, following all or part of the communication protocols of the ISO OSI Open Systems Interconnection reference model. FCS, on the other hand, is a real-time network control system that uses an open fieldbus control communication network to interconnect the lowest-level field controllers and intelligent field instruments in automation.
2.2.2 Differences between Fieldbus and Local Area Network
(1) In terms of function, fieldbus connects the lowest-level field controllers and field intelligent instruments in automation. Small batches of data, such as detection, status, and control information, are transmitted over the network cable. The transmission rate is low, but real-time performance is high. In short, fieldbus is a real-time control network. Local area networks (LANs) are used to connect computers within a local area. Large batches of digital information, such as text, sound, and images, are transmitted over the network cable. The transmission rate is high, but real-time performance is not required. In this sense, LANs are high-speed information networks.
(2) In terms of implementation method, fieldbus can use various communication media, such as twisted pair, power line, optical fiber, wireless, infrared, etc., and has a low implementation cost. Local area network requires dedicated cables, such as coaxial cable, optical fiber, etc., which has a high implementation cost.
2.3 Fieldbus Control System Architecture
The lowest level Intranet control network, or FCS, distributes controller nodes to the field, forming a thoroughly distributed control architecture. The network topology is arbitrary, including bus, star, and ring topologies. Communication media is unrestricted, utilizing various methods such as twisted-pair cabling, power lines, wireless, and infrared. The Intranet control network formed by FCS easily interconnects with enterprise intranets and the Internet, creating a complete three-tier enterprise network architecture.
2.4 Characteristics of Fieldbus Control Systems
2.4.1 Openness and interoperability.
Openness means that FCS will break the monopoly of large DCS manufacturers, bringing equal opportunities for competition to small and medium-sized enterprises. Interoperability enables the "plug-and-play" function of control products, thus giving users more choices for industrial control products from different manufacturers.
2.4.2 Thorough Dispersion
Thorough decentralization means that the system has high reliability and flexibility, is easy to reorganize and expand, and is easy to maintain.
2.5 Low cost
To assess the overall cost of a control system, one should consider not only its initial construction cost but also the total investment throughout its entire lifecycle, from installation and commissioning to operation and maintenance. Compared to DCS, FCS's open architecture and OEM technology significantly shorten development cycles and reduce development costs. Its thoroughly distributed structure transforms one-to-one analog signal transmission into one-to-N digital signal transmission, saving substantial costs associated with analog signal transmission, including A/D and D/A converters, wiring, and maintenance. Therefore, overall, the cost of FCS is significantly lower than that of DCS.
3. Current Status of Fieldbus Development
The fieldbus industry is developing rapidly and is currently in a stage of fierce competition among numerous players. More than 40 fieldbuses have been developed to date, such as Interbus, Bitbus, DeviceNet, MODbus, Arcnet, P-Net, FIP, and ISP. Among them, five are considered the most influential: FF, Profibus, HART, CAN, and Lon-Works.
(1) FF (Foundation Fieldbus) was launched by the American Instruments Association (ISA) in 1994. Representative companies include Honeywell and Fish-er-Rosemount. It is mainly used in instruments for petrochemical and continuous industrial process control. FF's distinctive feature is that its communication protocol adds a user layer on top of the ISO OSI physical layer, data link layer, and application layer, achieving interoperability through an object dictionary (OD) and a device description language (DDL). Currently, FF-based fieldbus products include pressure and temperature transmitters manufactured by Smar, the PressLogix system launched by Honeywell & R.Kwell, and PlantWeb launched by Fish-er-Rosemount.
(2) Profibus (Process Field Bus) was launched by Siemens in Germany in 1987 and is mainly used in PLCs. There are three types of products: FMS for communication between master stations; DP for communication between slave stations in the manufacturing industry; and PA for communication between slave stations in the process industry. Since the development of Profibus fieldbus products started as early as ten years ago, due to the limitations of computer network technology at that time, most of them were based on IT network standards. As the application fields continued to expand and user requirements became higher, fieldbus products could only be partially modified and supplemented on the original IT protocol framework, which led to the addition of many conversion units (such as various couplers) in the control system. This has brought certain limitations to the further development of the product.
(3) HART (Highway Addressable Remote Transducer): Introduced by Rosemount in 1989, primarily used in smart transmitters. As a transitional standard to ART, it transmits digital signals by superimposing sine waves of different frequencies (2200Hz for 0', 1200Hz for 1) onto a 4-20mA power signal line, thus ensuring compatibility between digital and traditional analog systems. Its lifespan is expected to be within the next 20 years.
(4) CAN (Controller Area Network) was introduced by Bosch in Germany in 1993 and is used in automotive monitoring, switch control, manufacturing, etc. The media access method is a non-destructive bit arbitration method, suitable for small networks with high real-time requirements, and the development tools are inexpensive. Motorola, Intel, and Philips all produce independent CAN chips and 80C51 chips with CAN interfaces. CAN bus products include AB's DeviceNet and Advantech's ADAM data acquisition products.
(5) LonWorks (Local Operating System) was launched by Echelon Corporation in 1991 and is mainly used in building automation, industrial automation, and the power industry. LonTalk uses a full 7-layer protocol, P-PCSMA (Predictive P-Persistent Carrier Sense Multiplexing) for media access, network logical addressing, and a priority mechanism to ensure real-time communication. Its security mechanism uses a verification method, thus enabling the construction of large-scale network control systems. Echelon's Neuron chip is essentially a network microcontroller. Its powerful network communication processing capabilities, combined with object-oriented network communication, greatly reduce the time and cost developers need to spend on constructing application network communication, allowing them to focus on the application layer they are good at for developing control strategies. Therefore, many experts in the industry believe that the LonWorks bus is a promising fieldbus. LonWorks-based bus products include Action's Flexnet & Flexlink, etc., and the 435T/h-400” circulating fluidized bed boiler fieldbus control system.
4 Overview
4.1.1 Functions of the Control System for a 35t/h Circulating Fluidized Bed Boiler
• Collect and display the boiler's operating parameters, such as steam pressure, air pressure, and furnace temperature.
• Perform manual control to complete tasks such as starting and stopping the furnace;
The Development of Fieldbus and Its Application in Circulating Fluidized Bed Boiler Control Systems
• After the boiler is in normal operation, automatic control is implemented to ensure stable and efficient operation of the boiler;
• It has boiler interlock protection function;
• Store one year's worth of historical data:
4.1.2 Overall Design Concept
Hardware: The system includes one engineer/operator station combo unit (ES/OS), one dedicated operator station (OS), one printer, two redundant ABB AC8OOF controllers, and three PROFIBUS buses. See Figure 1.
PBI bus: Configured with 1 remote $800 I/O, operating speed 12MB/s
PB2 bus: connects 26 DP instruments;
PB3 bus: connects 26 DP instruments;
4.1.3 Control Loop and Measuring Points
(1) Steam drum water level control loop.
(2) Main steam temperature control loop.
(3) Main steam pressure control loop.
(4) Coal spreading air control circuit.
(5) Exhaust fan control circuit.
(6) Slag discharge regulation and control loop.
(7) Return air control circuit.
(8) Ignition operating system.
(9) Interlocking protection system. Measurement points are as follows:
System features:
(1) Since the controller wtOFOBUS bus module supports the DPV1 standard, it has increased the instrument configuration capability compared with the pure DP standard module. We do not need to install dedicated configuration software for each instrument. We only need to import the development of the fieldbus of each instrument and the application parameter file (GSD) in the circulating fluidized bed boiler control system. This allows us to program and configure the parameters of all PA standard instruments under a unified system programming software, which effectively saves software costs and improves the efficiency of engineering configuration.
(2) Open control functions. The ABBAC800F controller uses a RISC processing chip and the program adopts a multi-task operation mode. Each task can be set with its operating cycle and priority. Therefore, the controller can meet the control requirements of complex regulation loops and the control requirements of fast electrical switching quantities. One controller can cover the control requirements of all links in the factory and the system programming language conforms to the IEC61131-3 standard.
(3) System Global Database Technology. The core database technology of "distributed storage and global management". Data is stored on the controller, but it is only a global system variable table in the system programming software. Cross-referencing of variables between process stations does not require writing data exchange programs. Data between operator stations and process stations does not need to be converted and can be directly accessed. Variable modification and inspection are also performed globally.
(4) Open Ethernet technology. The ABBAC800F controller system communication module is a standard TCPdP protocol module, which allows the system to connect to the factory management LAN without the need for additional equipment.
This system fully leverages the technical advantages of the POFIBUS bus, enabling the control level of circulating fluidized bed boilers to reach a new height.
5. Conclusion
Fieldbus technology has reached a mature theoretical stage and is now evolving from high-parameter, large-scale systems to general engineering applications. Its application in circulating fluidized bed boiler control systems can significantly improve equipment operation and management, thereby increasing economic efficiency.
Click to download: The Development of Fieldbus and Its Application in Circulating Fluidized Bed Boiler Control Systems
Edited by: Chen Dong
