Coordinated control of circulating fluidized bed

Foreword: At present, the state has adopted relatively strict restrictive measures for units with high energy consumption and high pollution. Measures such as rectification, technological transformation, and demolition have been adopted for unqualified generators. Therefore, energy-saving and environmental-friendly boilers for new furnaces have been strongly promoted. The large-scale circulating fluidized bed, especially the 400t/h furnace, has been greatly developed. Although the furnace type is relatively mature, according to the characteristics of the circulating fluidized bed, CCS coordination and AGC control are also difficult, but they need to be optimized. The control strategy is also not difficult. According to the Pansan Power Plant Project of Huainan Mining Group, the actual commissioning for more than a year, AGC put into operation to achieve excellent results, in the same industry in a leading level of technology, for the purpose of technology sharing, specially prepared this article during the Dragon Boat Festival holiday, and readers share it.

Keywords: CFB, coordination control, AGC
I. Overview
The circulating fluidized bed boiler (Circulating Fludized Bed Boiler, referred to as CFB boiler hereinafter) as a kind of coal clean and high-efficiency combustion technology has entered the commercial market of coal-fired boilers since the early 1980s, and has occupied quite a bit in small and medium-sized boilers. Share. And as the technology matures, it gradually develops toward greater capacity. It is developed from the bubbling bed boiling furnace and the circulating fluidized bed process of the chemical industry. It has outstanding advantages such as wide fuel adaptability, high combustion efficiency, low nitrogen oxide emission, low-cost limestone furnace desulfurization, and large load regulation ratio. Circulating fluidized bed realizes the strict pollution emission index at low cost, burns inferior fuel at the same time, and has comprehensive advantages in terms of load adaptability and comprehensive utilization of ash residues, etc., which provides an effective way for energy-saving and environmental protection transformation of pulverized coal furnaces. And get a lot of applications.
As we all know, the large-lag combustion phenomenon of CFB boiler is a non-linear, multi-variable, multi-variable coupling control object. Its automatic control system needs to complete more complex control tasks than pulverized coal boilers, and it is also not desirable for traditional control methods. Newer control concepts are needed.
Second, the main control range
The following figure shows the schematic diagram of the main components of the circulating fluidized bed. It is easy to see that the circulating fluidized bed is divided into the following sections: induced wind, primary air, secondary air, flow, wind, return, slag discharge, coal feeding, and hearth , Superheaters, economizers, air preheaters, steam drums, water cooling walls and other major systems.
Figure I. Circulating fluidized bed boiler
From the process flow we can see that the main automatic control of the conventional control subsystem is:
Furnace pressure regulation;
One air volume adjustment;
Secondary air volume regulation;
Fluidized air control system;
Bed level control adjustment;
Coal combustion control regulation;
Drum water level control regulation;
Steam temperature control adjustment;
Limestone desulfurization control regulation;
And other auxiliary automatic adjustment systems, such as fuel pressure, soot steam pressure.
The control from the subsystem alone cannot meet the coordinated control of the entire unit system, and it cannot satisfy the automatic generation control of the power grid. Therefore, the following coordinated control system must also be available:
DEH and CCS coordinate control interface system;
Load control center
Load regulation system;
Control loop protection system;
Auxiliary interlocking action protection system, mainly for RB, fast load rejection;
Machine and furnace coordination protection system;
A frequency-modulated load compensation control system;
Third, the design of the overall design plan
Considering the entire unit as a whole, it is not difficult to analyze. The above components are mainly composed of LMCC (Load Command Control Center), combustion coordination, various control subsystems, and parameter coupling. However, LMCC and coordinated combustion are the entire control system. The brain, which receives AGC instructions, directs and controls, and manages the coordinated operation of various subsystems. If only relying on superior instructions, the entire control will increase the deviation, so it must be in the entire coordination process must have a feedback system of the actual load, through a variety of test conditions, research, qualitative analysis of expert analysis control system, it For this unit alone, there may be some reference for other units. Copying and copying may cause deterioration of the entire system. Therefore, we call it an expert control system and only serve the control of the unit.
Fourth, the specific function description
The following figure shows the actual LMCC Coordinated Control Management Center.
Coordinated control of circulating fluidized bed
The main control screen is mainly divided into the following sections for monitoring and control:
Real-time curve part: It can directly and eye-catchingly check the running status of the unit for half an hour;
Important parameters: display real-time data, and use classification, key mode to mark the important parameters of the working conditions;
Data instruction part: Through the input window to set the instruction, rate, offset;
System regulation and control part: Mainly to complete the input of the main system interface before putting into AGC;
LMCC status indication part: mainly shows the coordination status of the unit;
LMCC command control part: According to the unit's operating conditions;
DEH valve flow chart part: mainly check whether the command and feedback are consistent.
The control system has four operating modes:
(1) Coordinated control and operation mode: The steam turbine side pressure regulator is in follow-up, and the turbine power and boiler pressure regulator are all put into automatic;
(2) Furnace control operation mode (BF): automatic pressure regulation of the boiler, manual operation of the turbine (power adjustment);
(3) Machine-to-furnace control operation mode (TF): automatic pressure regulation of the turbine, boiler manual (power regulation) operation mode;
(4) Manual operation of the furnace.
The feed-forward signal in the boiler combustion is especially important. This reflects the key to whether the boiler can quickly increase or decrease the load. This requires the following four signals to be processed as effective feed-forward signals:
(a) Power command signals as feedforward signals;
(b) P1×PS/PT as feedforward signal;
(c) Speed ​​regulator pressure P1 as feedforward signal;
(d) Energy balance between supply and demand*Coefficient of coal combustion efficiency.
In the above, the power command signal is adjusted as the feed-forward signal the fastest, P1×PS/PT is the next, and P1 is the slowest. However, the four signals compensate each other to achieve the purpose of rapid but overshoot.
When the unit unit operation coordinated control mode is in good condition, the coordinated control operation mode can be selected through the LMCC panel. At this time, the pressure regulator on the machine side is in follow-up, and the power regulator on the machine side and the pressure regulator on the furnace side are all put into automatic operation.
The turbine power regulator is put into closed-loop control, and DEH is used as the DCS-controlled actuator. It is controlled by the DCS and controls the opening of the turbine to meet the requirements of external loads.
In the coordinated control mode, the boiler pressure regulator introduces a synthetic signal according to the load command and frequency difference ( δ is a primary frequency inequality factor. After being corrected by the function module as a feedforward signal, the main steam pressure is closed-loop controlled. The role of the module in the feedforward signal is to convert the load command into numerical value with fuel and air flow (especially The primary wind and secondary wind are used as matching supplements to match the appropriate directives, It is actually a differential link so that the feedforward signal has more suitable dynamic characteristics.
When the control system is static, the inlet deviation signal of the fuel regulator is 0. There are:
Energy produced by fuel combustion Energy required by the outside world equal.
which is
In steady state, due to
Since P1≠0
Then there are:
If the AGC button on the LMCC panel is in the input state, ie, the load demand station is in the automatic state, the control system accepts the ADS (Power Dispatch Center) load instruction and participates in the AGC operation of the power grid. The AGC button is not in the input state and the load is set or increased by the operator. Less adjustments.
Furnace machine mode (machine manual (power adjustment), furnace automatic pressure regulation), machine-to-hearth mode (furnace manual (power adjustment), machine automatic pressure regulation) is a coordinated subsystem, so each can complete its own role .
The sliding pressure operation of the unit is an important operating mode for coordinated operation. It means that the steam turbine is approaching fully open, the pressure required by the unit and the actual load command form the best ratio, and it is necessary to ensure that the unit performs its maximum work under the premise of minimum pressure loss. He is a correction curve based on actual operating conditions.
Fifth, summary analysis
After many tests and improvements, especially the experts' analysis of the control subsystem's experience library, organic combination and correction of other subsystem deviations, it can well meet the boiler's combustion stability, and can quickly respond to the grid's load requirements. Solved the problems of AGC's automatic investment and AGC assessment.
about the author:

Tian Guiming was born in 1979 and is currently working in the Control Department of Nanjing Keyuan Automation Group Co., Ltd. He is the deputy manager of the department. After graduating in 2002, the author specializes in automatic control. In 2003, he officially started the design and research of thermal control and control systems for thermal power generation, and achieved good results in many units of more than 100,000 units, especially boilers. Automatic control, coordinated control, AGC, primary frequency modulation have deep theoretical research and practical experience in operation.

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