[b]1. Introduction[/b] With the continuous development of control technology, the control systems of most coal-fired power plants have achieved automated control. Newly constructed units all adopt advanced control technologies, and the control systems of older units have also been upgraded, with most units already upgraded. The control systems of coal-fired power plants generally adopt the currently popular distributed control system (DCS) system to ensure the safe and stable operation of the power generation system. Currently, due to increased environmental awareness and the needs of human survival and development, both domestically and internationally, there is a strong focus on the development of environmental protection technologies and the implementation of environmental protection projects. Newly built large-scale coal-fired units are all designed with desulfurization and denitrification systems. Older units lacked desulfurization (SO2, SO3) and denitrification (NO, NO2, etc.) devices, and future CO2 emission reduction control is also required. According to national environmental protection requirements, existing generating units must be subject to pollutant emission control to meet national environmental protection standards. When upgrading boilers in thermal power plants by adding desulfurization and denitrification devices, the control system must be designed according to the specific circumstances of the power plant and the specific unit. Now, with the continuous development of control technology, several major control systems have emerged in the control field. Therefore, choosing the right control system for the retrofit design is a crucial issue for designers. It's essential to ensure system safety and stability while also considering practical realities and cost-effectiveness. This article briefly introduces the current status of on-site desulfurization and denitrification retrofit units, outlines the characteristics of several control systems, and provides some references for control system design based on different units and actual site conditions. 2. Current Status of Retrofit Units in Thermal Power Plants Currently, the processes for adding desulfurization and denitrification systems to retrofit units are mostly simplified, especially for units of 200MW and below, primarily considering factors such as cost and system footprint. Currently, the desulfurization technology used in retrofit units generally employs simplified wet desulfurization, dry, and semi-dry desulfurization technologies. Denitrification technologies include SCR, SNCR, and combustion adjustment methods. In retrofit units, the main approach is to reduce nitrogen oxide formation through low-NOx burners and secondary air adjustments. From the perspective of current domestic desulfurization and denitrification technologies, the process systems are relatively simple, with fewer control loops. Based on the current design of retrofit units, the requirements for the control system can be broadly categorized into the following two situations. One approach is to integrate the desulfurization and denitrification control systems into the main control system of the corresponding power plant units, achieving centralized control with the original main system. Another approach is to design the desulfurization and denitrification control systems independently, keeping their operation outside the main system. This ensures that the main system's normal operation is not affected during system shutdowns and maintenance. Based on these different approaches, the control system can have different design principles and control implementation methods. In the first approach, a DCS system with the same equipment as the main system can be selected for convenient communication. Considering price and other factors, PLC and FCS bus technology can also be used to connect with the main system through other communication methods. When designing this scheme, the compatibility of the main system must be considered, along with the design margin and backup space of the original system. More importantly, the compatibility of the communication protocols between the original main system and the newly designed system must be ensured. In the second approach, the control system design is much more flexible than the first approach. The selected control system is independent of the original main control system. This independent control system can be designed as any of the three technologies—DCS, PLC, or FCS—according to user requirements, or it can be designed as a comprehensive control system integrating all three technologies. The selection is mainly based on the following factors: the system's operating mode; the capital investment in the control system; the distance between the field and the control room; the number of analog control loops in the system; the number of switching quantities in the system; and the level of intelligence of the primary instruments in the field. Each of the three control systems has its own advantages and disadvantages, and if conditions permit, they can also be designed as a comprehensive control system. They are all widely used in many power plants. [b]3. PLC Control System[/b] Programmable Logic Controller (PLC), although still called PLC, no longer corresponds to its original practical meaning and is not the simple programmable controller it originally was. The original definition of a PLC was a digital control dedicated electronic computer that used programmable memory to store instructions, execute functions such as logic, sequence, timing, counting, and calculation, and control various machines or work programs through analog and digital input and output components. After more than 30 years of development, PLC has become very mature and complete, and analog closed-loop control functions have been developed. For a long time, PLC has been widely used in the field of automation control in various industries, providing very reliable control applications for various automated equipment. The main reason is that it can provide safe, reliable, and relatively complete solutions for automation control applications, suitable for the current automation needs of industrial enterprises. Current PLCs not only have early logic operation functions but have also evolved towards integrated control. New PLCs are constantly improving their PID closed-loop control functions, and various other functions are also being continuously improved. PLCs have been widely used in the field of continuous process control, and the development trend based on continuous process control technology is further growing. Communication is a key technology for the widespread application of PLCs, and this technology has been expanded in the PLC field. Similar to the system, decentralized processing of PLCs has become possible, making them easier to manage and better integrated. The price of PLC systems is also gradually decreasing. Since the price of the smallest PLC system module unit is only around a thousand yuan, or even lower, most users no longer repair damaged modules but directly replace them with new ones, as repairing such faulty modules may cost the same or even more. Now, some small and even ultra-small PLC systems have provided industrial users with analog I/O, PID control loops, communication interfaces, and even fieldbuses for connection to enterprise network systems. A PLC system with 14 I/O channels and 4 PID control loops costs only around 1,000 yuan, making it ideal for small-system control applications. Some PLC suppliers leverage the strong application market to develop small PLC products, and many industrial users even consider them everyday necessities in the low-end application market. While PLC hardware continues to evolve, PLC programming software is also advancing. This is because the entire system must not only fully consider the performance of the hardware but also take corresponding measures to provide industrial users with usable configuration software tools, fault diagnosis technology, network communication capabilities, and the applicability of additional software packages beyond basic automation hardware. Currently, not only are PLC manufacturers constantly striving to develop programming software suitable for PLC systems, but other software developers are also continuously releasing matching configuration software, and each software is suitable for multiple brands of PLC products, greatly facilitating user programming. A fundamental characteristic of PLCs is their flexibility in system configuration. In some small systems, where the number of control and monitoring points is limited, small PLC control systems can be used to monitor and control the process. Small systems use a single PC as the master station and multiple PLCs of the same model as slave stations to monitor and control the system. With the development of PLC network technology, large-scale control systems composed of multiple PLCs are increasingly being adopted, and their functions are becoming increasingly similar to DCS systems. A system can consist of multiple PLCs connected via a network, with each PLC serving as a low-level data acquisition unit and enabling local control. Larger control systems can use one PLC as the master station and multiple PLCs of the same model as slave stations, forming a PLC network to achieve comprehensive monitoring and control functions for more complex systems. A PLC can also exist as a subsystem within a DCS system. [b]4. DCS Control System[/b] Distributed Control System (DCS) means that control hazards are distributed and centrally displayed. A DCS is a monitoring technology integrating 4C (Communication, Computer, Control, CRT) technologies. Its characteristic is a large-scale, top-down tree-like topology system, with communication being its key technology. A DCS (Distributed Control System) consists of four parts: I/O boards, controllers, operator stations, and a communication network. The technical levels of I/O boards and controllers from various international DCS manufacturers are relatively similar. The main differences lie in the number and combination of algorithms within the controllers, and the presence or absence of intelligent features in some I/O boards. However, the controller must complete a cycle of reading all I/O data within one second. Operator stations vary considerably, primarily in whether they use PCs or minicomputers, UNIX or NT operating systems, and dedicated or general-purpose monitoring software. Good coordination between the operating system and monitoring software can reduce system crashes. The biggest difference lies in the communication network, with polling being the worst and exception reporting the best, resulting in a significant speed difference. The controller I/O components connect to the production process, the operator stations connect to people, and the communication network links these two parts into a system. Therefore, the operator station is a crucial component of the DCS. The engineer station configures the control and operator stations, while the historical data station records historical production data. Data in a DCS system primarily originates from field signals and various variables, manifested in the control station as corresponding measured values ​​(PV), setpoints (SV), operational output values ​​(MV), and loop statuses. This data is collected into the corresponding memory of the DCS control station, forming real-time data. Other configuration information related to the workstation, such as range, engineering units, loop connection information, and sequence control information, is also stored in the control station, but must simultaneously be stored in the operator station or engineer station, and there must be a mapping relationship. Small and medium-sized DCS control stations, limited to controlling 16-32 loops, are more readily accepted due to their decentralized nature. Currently, the market share of small DCS is gradually sharing with PLCs and FCSs; in the future, small DCS may first integrate with these two systems, and "soft DCS" technology will initially develop in small DCS. The control station is the foundation of the entire DCS; its reliability and safety are paramount. System crashes and control failures are absolutely unacceptable. Furthermore, redundancy, power-off protection, anti-interference, and explosion-proof system configurations must all be highly effective and reliable to meet user requirements. Years of practical experience have proven that the vast majority of manufacturers' DCS control stations are capable of meeting user requirements. DCS systems are relatively more expensive than PLC systems, and this should be considered comprehensively from the perspectives of performance-price ratio, product lifecycle, user selection based on the actual automation requirements of production facilities, engineering costs, and maintenance costs. More importantly, PLC systems and industrial PC systems (IPCs) are currently known for their low prices, so DCS manufacturers face a challenging situation. While DCS can survive for the next five years, they should focus on reducing costs, minimizing maintenance expenses, developing remote diagnostics and maintenance, and improving service systems. They should also adjust their current direct sales model to allow more competitors to participate in DCS applications. The initial purpose of DCS development was primarily to solve the control algorithms of control loops within the system. In the mid-1970s, the process industry developed rapidly. However, due to the increasing size of equipment, high requirements for process continuity, the growing number of process parameters to be controlled, and stringent conditions, such as the need for centralized display and operation, the already widespread electric unit combination instruments could not fully meet the requirements. Under these circumstances, after market research, industry manufacturers determined that the DCS products they developed should be based on analog feedback control, supplemented by sequential control of switching quantities and batch control of analog and switching quantities. Therefore, the advantage of DCS today is still the realization of analog control, and analog control and its algorithms are the core technologies of DCS. From the current DCS perspective, a single controller can complete the calculation of dozens of loops and the acquisition of hundreds of points, plus a suitable amount of logic operations. In field use, the effect is quite good. This raises the issue of controller upgrades. Sometimes the distance between the controller and the sensing element is still relatively far, which promotes the development of fieldbus. [b]5. FCS Control System[/b] Fieldbus control technology (FIELDBUS CONTROL SYSTEM) is a new type of control system that has been developed in recent years. The core of the FCS system is the bus protocol, i.e., the bus standard. Once the bus protocol of a particular type of bus is determined, the related key technologies and equipment are also determined. In terms of the basic principles of their bus protocols, all types of buses are the same, based on solving bidirectional serial digital communication transmission. However, due to various reasons, there are significant differences in the bus protocols of different types of buses. To ensure that fieldbuses meet interoperability requirements and become truly open systems, the IEC international standard explicitly stipulates that the user layer of the fieldbus communication protocol model has device description functions. Digital communication replaces 4-20mA analog signals: In traditional technology, the connection between field-level devices and controllers is one-to-one (one I/O point to one measurement and control point of the device), the so-called I/O wiring method, transmitting 4-20mA (transmitting analog information) or 24VDC (transmitting digital information) signals. Using fieldbus technology, a single communication cable can connect the controller to field devices (intelligent devices with communication interfaces), using digital communication to complete the communication and control requirements of the underlying devices. One of the main technical characteristics of fieldbus is the requirement for intelligent field devices; that is, the application of fieldbus technology requires that field devices (sensors, drivers, actuators, etc.) be intelligent (programmable or parameterizable) devices with serial communication interfaces. Therefore, fieldbus technology is based on the development of large-scale integrated circuits of computers. The second characteristic is that it integrates remote control, parameterization and fault diagnosis of field devices; fieldbus uses computer digital communication technology to connect intelligent field devices. Therefore, the controller can obtain a lot of rich information from the field devices, realize the transmission of equipment status, fault and parameter information, and complete the remote control, parameterization and fault diagnosis of equipment. The main advantages of fieldbus-based automated monitoring system are: (1) Fieldbus-based automated monitoring system enhances the field-level information integration capability. Fieldbus can obtain a lot of rich information from field devices, which can better meet the information integration requirements of factory automation and CIMS system. Fieldbus is a digital communication network. It does not simply replace 4-20mA signal, but can also realize the transmission of equipment status, fault and parameter information. In addition to completing remote control, the system can also complete remote parameterization work. (2) Open, interoperable, interchangeable and integrable. Products from different manufacturers are interoperable and interchangeable as long as they use the same bus standard, so the equipment has good integrability. The system is open, allowing other manufacturers to integrate their own specialized control technologies, such as control algorithms, process flow, and formulas, into the general system. Therefore, there will be many monitoring systems on the market that are tailored to industry characteristics. (3) The system has high reliability and good maintainability. The fieldbus-based automated monitoring system uses a bus connection method to replace one-to-one I/O wiring. For large-scale I/O systems, this reduces unreliable factors caused by connection points. At the same time, the system has online fault diagnosis, alarm, and recording functions for field-level devices. It can complete the parameterization work such as remote parameter setting and modification of field devices, which also enhances the maintainability of the system. (4) Cost saving: For large-scale distributed I/O systems, it saves a lot of cable, I/O module and cable laying engineering costs, reducing system and engineering costs. Digital intelligent field devices are the hardware support of the FCS system. The reason is simple: the FCS system executes a two-way digital communication fieldbus signal system between automatic control devices and field devices. If the field devices do not follow a unified bus protocol, i.e., the relevant communication protocol, and do not have digital communication capabilities, then the so-called two-way digital communication is just empty talk, and it cannot be called a fieldbus control system. Furthermore, a major characteristic of fieldbus is the addition of field-level control functions. If the field devices are not multifunctional and intelligent products, then the characteristics of the fieldbus control system do not exist, and the so-called advantages of simplified systems, convenient design, and easy maintenance are also meaningless. For a control system, whether using DCS, PLC, or fieldbus, the amount of information the system needs to process is at least the same. In fact, after adopting fieldbus, more information can be obtained from the field. The amount of information in a fieldbus system does not decrease, and may even increase, while the number of cables transmitting information is greatly reduced. This is a major advantage of the FCS system. [b]6. PCBCS Control System[/b] In addition to the three major control systems mentioned above, a control system called PCBCS has recently emerged. This system will only be briefly introduced here, without comparison to the three systems mentioned above. PCBCS (Printed Circuit Board Control System) combines ruggedized PC hardware with control software to implement control functions typically performed by dedicated PLCs and DCSs, or in other words, "encapsulates" the PLC's control functions within software, running in a PC environment. A PCBCS control system mainly consists of three parts: a PC; I/O components and their connectors; and operating system software and application software. The PC integrates the operator station and control station of traditional PLC and DCS control systems, possessing multiple functions including control, communication, and operation display. Rapidly developing computer technology allows the PC to provide a truly open platform, integrating all system functions onto this unified and open platform. This reduces installation space, saves cabling, simplifies complex communication connections, and allows access to important production information via the Internet or corporate intranet, enabling production process optimization. From the above three components, it can be seen that the openness of the PCBCS system is comprehensive. Therefore, its current and future development speed will be very fast. It will never be like the PLC and DCS control systems of the past, which suffered from long-term development lag due to their closed and specialized nature. Instead, it will keep pace with the development of computer technology, communication technology, I/O component manufacturing technology, fieldbus technology and software technology and improve rapidly. [b]7. System Comparison[/b] (1) The DCS system is a large system with strong closed-loop control function. The PLC system is suitable for medium and small systems with strong logic control function. The FCS is suitable for various control systems, but it must have digital intelligent field devices as a prerequisite to show its intelligent advantages. (2) The DCS system has a large initial investment, while the PLC system has a relatively small investment. The FCS system requires a high degree of intelligence of primary instruments, and the investment in primary instruments is larger. (3) The DCS system is a closed system, and the products of different companies are basically incompatible, making subsequent expansion difficult. The PLC system can expand the same type of PLC unit through the network, and it can also be used as the front-end processing field I/O of the DCS and FCS systems. The FCS system is an open system, and users can choose various devices from different manufacturers and brands to connect to the fieldbus to achieve the best system integration. (4) The information of DCS and PLC systems is all formed by binary or analog signals, which requires D/A and A/D conversion. The FCS system is fully digital and uses digital signal transmission, which eliminates the need for D/A and A/D conversion. It has high integration and high performance, which can improve the accuracy from ±0.5% to ±0.1%. (5) The FCS system can install PID closed-loop control function into transmitters or actuators, shortening the control cycle. Currently, it can be increased from 2 to 5 times per second in DCS to 10 to 20 times per second in FCS, thereby improving the regulation performance. (6) DCS and PLC can control and monitor the entire process and perform self-diagnosis, maintenance and configuration. However, because its I/O signals use traditional analog signals, it cannot perform remote diagnosis, maintenance and configuration of field instruments (including transmitters, actuators, etc.) on the DCS engineer station. FCS adopts fully digital technology, and digital intelligent field devices send multi-variable information, not just single-variable information, and also have the function of detecting information errors. FCS uses a bidirectional digital communication fieldbus signal system. Therefore, it can remotely diagnose, maintain and configure field devices (including transmitters, actuators, etc.). This advantage of FCS is unmatched by DCS and PLC. (7) Due to the field-based information processing of FCS, it can save a considerable number of isolators, terminal cabinets, I/O terminals, I/O cards, I/O files and I/O cabinets compared with DCS and PLC, and also save the space and floor area of ​​I/O devices and device rooms. FCS can reduce a large number of cables and cable trays for laying cables, and also save design, installation and maintenance costs. However, it is based on digital intelligent field devices. Due to years of development and research, the current DCS and the new PLC have complemented each other to form a new system while retaining their original characteristics. The current DCS is not the original DCS, and the same is true for the new PLC, which is not the PLC in the early stage of development. It's inappropriate to say that DCS has replaced PLC or vice versa. Currently, due to the development of network technology, PLC systems can also form large-scale DCS systems. Meanwhile, DCS is also developing smaller DCS systems to meet market needs. FCS evolved from DCS and PLC, retaining the characteristics of DCS, or rather, FCS absorbed years of development, research, and field experience from DCS, including lessons learned. With the development of FCS technology, it is likely to become the mainstream control system in the near future. As various technologies continue to develop, several systems are merging to form more advanced control system networks. In DCS, PLCs can be used as the underlying layer of control to complete basic control tasks. In fact, multiple PLCs can also form a control network, which, from its structure and the perspective of distributing risks, can be called a DCS control system. FCS systems also coexist with other systems; long-distance data acquisition and connection to intelligent local devices using FCS systems will further enhance the entire control system. Meanwhile, PCBCS control systems are quietly emerging and show strong development momentum in terms of performance and technical support. [b]8. Conclusion[/b] Based on the characteristics of the control system and the specific circumstances of the power plant renovation, the design and selection of the control system should consider the following factors when adding desulfurization and denitrification devices to the renovated unit. From a practical perspective, the PLC control system should be the first choice. Its cost is much lower than that of DCS, and the desulfurization and denitrification systems in the renovation typically have around 500 control points, making a PLC system suitable. Furthermore, the system has fewer analog controls and more digital logic controls, so using a PLC system better leverages its strengths. The FCS system is a relatively advanced control system, and with the development and popularization of primary intelligent instruments, FCS will be the mainstream control system in the future. However, due to current site limitations, the advantages of using an FCS system in the renovated unit cannot be fully realized. Of course, if the site requires an advanced control system and most primary instruments are intelligent, then choosing an FCS control system is the most ideal. This system can easily transmit data to the main DCS system via network technology, achieving centralized monitoring. DCS systems can also be used in system retrofitting, as their stability and reliability have been proven over a long period. However, their high cost and limited application of powerful control functions in retrofitting systems lead to resource waste. Currently, small-scale DCS systems are developing rapidly, and besides PLC systems, they can be considered as an alternative. PCBCS systems are still in their early stages, and their reliability requires further verification; however, they may represent the future trend in control systems. Ultimately, the selection and design should be based on the user's specific circumstances, considering various factors to choose a control system suitable for the retrofitted unit and acceptable to the user.