Design of 310t/h Circulating Fluidized Bed Boiler Automatic Control System and Application of DCS
Because of its superior performance, circulating fluidized bed (CFB) has been increasingly used in small and medium power plants in China. First of all, the CFB boiler has a high discharge efficiency, a wider range of low-quality coal to be used for combustion, and a better desulfurization effect can be obtained with less limestone in the furnace; secondly, the lowering of the operating temperature reduces the possibility of slagging. It also allows less NOx emissions and produces fewer pollutants; because the fuel stays in the furnace for a longer time, the thermal effect is better; due to the proper relationship between the flue gas velocity and the solid carrying capacity, the CFB boiler can be Operates within a very wide load range. In addition, the ash and slag discharged after combustion can also be comprehensively utilized, which has caused widespread attention and attention at home and abroad. The following describes the automatic control system of the 310t/h circulating fluidized bed boiler of Shanghai Petrochemical Thermal Power Plant.
First, the analysis and application of the main control system I/O of a 300t/h CFB boiler has 1500~1700 points, so it is difficult to realize so many I/O points using the conventional control system. In addition, there are many control functions that require safety and reliability. Practice has shown that only DCS can be used to perform the task. CFB boilers must be ensured to open, stop, and operate safely and economically in the prescribed order, thereby determining the sequential control and automatic adjustment in the DCS.
1.1 Tasks Completed by Sequence Control (1) Starting and Stopping the CFB Boiler
See the startup procedure (Table 1) for the start-up procedure of the CFB boiler. The circuit must not only ensure the opening and stopping according to the steps in Table 1, but also must have a fault in the start-up process to have the function of diagnosis and alarm, so that the operator can quickly eliminate the fault and drive as soon as possible. For example, when starting any auxiliary equipment, the vehicle will not be driven within the specified time and the alarm will be displayed immediately. 
(2) Burner Management System (BMS or FSSS), mainly safety chain system.
During start-up and normal operation, the CFB boiler fails and the main fuel is tripped first. For example, when the temperature of the flue gas reaches 485°C and above, the lowest level of the water level in the drum is immediately realized and the MFT is immediately realized to ensure boiler safety and avoid damage to the equipment.
1.2 Tasks performed by the automatic regulation system The independent areas in which the steam quality index and the economical operation boiler regulation zone can be divided into: steam drum water level adjustment zone, superheated steam temperature adjustment zone, combustion adjustment zone, and salt content adjustment zone. At present, the control of salt content in boiler water is mostly based on manual periodic discharge and continuous discharge. For this reason, the CFB boiler regulates only the drum level, superheated steam temperature and fuel area. The CFB boiler control includes the following control function groups: boiler master control, fuel quantity control, air flow control, bed temperature control, limestone flow control, furnace pressure control, start burner control, feedwater control, main steam temperature control, start burner control , Feedwater control, main steam temperature control, deaerator water level, pressure control, etc. Figure 1 outlines the overall design of the automatic adjustment system for the entire unit. The following describes the main control system on the boiler side. 
1.2.1 Main Control System of Boiler The main control system of the boiler is a certain value control system to control the main steam main pipe pressure pm as the target. The main pressure of the boiler tube is used as the main signal, and the energy balance signal P1*Ps/PT+k*d(P1*Ps/PT)/dt is adopted as the feedforward signal of the boiler control loop. P1 is the turbine adjustment. The steam pressure after the speed, PT is the pressure before the machine, the ratio P1/PT is proportional to the opening of the turbine control valve, and overcomes the non-linearity of the valve opening signal caused by the hysteresis and insensitivity of the door adjustment. Impact. Under the rated control parameters, the steam consumption of the turbine is proportional to the opening of the valve. For the steam extraction unit, whether the change of the steam power or the change of the electric power, is reflected in the change of the turbine control gate. The change in the pre-machine pressure setpoint Ps reflects the change in boiler energy demand. The k*d(P1*Ps/PT)/dt signal represents the fuel demand of the energy storage element (drum). Therefore, the P1*Ps/PT+k*d(P1*Ps/PT)/dt signal can reflect the demand of the steam consumption of the steam turbine for the fuel. It also reflects the change of the boiler parameters for the fuel and also meets the boiler energy storage component. Demand for fuel. The feedforward and feedback compound control method constructed in this way can respond to the pressure demand most quickly and meet the steam boiler demand for boilers.
Analysis of P1*Ps/PT+k*d(P1*Ps/PT)/dt and main steam flow as feedforward can be seen as the main difference: if main steam flow is used as feedforward signal, when the combustion rate changes When the internal disturbance is caused, the feedforward of the main steam flow will form a positive action, which will intensify the spontaneous disturbance of the unit. The boiler control system that uses the P1*Ps/PT+k*d(P1*Ps/PT)/dt signal as the feedforward signal only responds quickly to the combustion when the load is disturbed, and acts on the air volume and fuel volume in advance. Control unit to overcome the lag and inertia of the controlled object. However, P1*Ps/PT+k*d(P1*Ps/PT)/dt does not change when fuel is disturbed, which is very important for stable combustion.
1.2.2 Total air volume adjustment The total air volume adjustment is used as the main control regulator for air volume to meet the control of the total air volume and oxygen volume, and the air volume is issued to the boiler combustion secondary air and fluidized primary air.
The total air volume adjustment receives the boiler master control command signal and sends the air volume command signal to the air volume master controller via the wind coal cross interlocking loop, so as to ensure that the dynamic load is added first, and the load is reduced first.
1.2.3 Fuel Regulation The main factors influencing the pressure of the main pipe are the change of the turbine control door and the change of the fuel quantity. Using P1*Ps/PT+k*d(P1*Ps/PT)/dt signal can overcome the impact of changes in the turbine control valve on the boiler pressure.
The effect of fuel on pressure depends on two things: the amount of fuel that is disturbed, and the change in the fuel's calorific value.
(1) Disturbance of fuel volume The system control scheme uses two control strategies to overcome fuel disturbances: 1 Fast fuel response loop. By using the direct measurement of the fuel in the circulating furnace, the fuel side disturbance due to the unstable operation of the fuel feeder can be quickly eliminated. 2 system uses multiple output control loops. The multi-output control loop consists of a multi-output controller and the device's manual operator. For multi-actuator control systems, the use of multiple output systems has the following advantages:
· Hand automatically switch the balance, no disturbance;
· Manual feeding or automatic injection of any feeder does not affect the change of fuel instructions;
· Multi-output control loop gain automatic correction;
· Feeder instructions are automatically adjusted according to the feeder's automatic number of running feeders, ensuring a certain loop control gain.
Bias control of each operating loop. The offset of the feeder command can be set under automatic conditions to change the load distribution of each feeder without affecting the fuel command.
(2) Changes in fuel calorific value In order to overcome the fuel disturbance caused by changes in the fuel calorific value, a control loop for automatically compensating the fuel calorific value is designed in the system. In the automatic compensation loop, Honeywell's steam flow rate and feedwater temperature are used to correct the function curve of the boiler fuel, and the influence of the fuel change on the control parameters of the system is overcome as quickly as possible.
In the scheme, a control system using P1*Ps/PT+k*d(P1*Ps/PT)/dt as boiler command signal and P1+d(Pd/dt) as feedback signal is also designed, where pd is the pressure of the steam drum . analyse as below:
(1) P1*Ps/PT+k*d(P1*Ps/PT)/dt represents the demand for steam from the steam turbine, while P1+d(Pd/dt) not only reflects the change in fuel volume, but also Reflects changes in the fuel quality, the "caloric value."
(2) At fuel inlet: P1*Ps/PT+k*d(P1*Ps/PT)/dt=P1+d(Pd/dt)
Steady-state seasonal d(P1*Ps/PT)/dt and dPd/dt are 0, there are:
P1*Ps/PT=P1
So there is a relationship PT = Ps, to ensure the main steam pressure.
1.2.4 Primary air volume adjustment The primary air volume command output by the total air volume regulation is corrected by the bed temperature controller as the set value of the primary air volume regulator. The output of the regulator controls the primary fan inlet guide vane to maintain the required output control of the boiler. Once the air volume. Primary air is mainly used for the bed material in the fluidized furnace.
1.2.5 Secondary Air Volume Adjustment The total air volume of the boiler is adjusted by the correction of the oxygen volume controller and the bed temperature controller as the set value of the regulator, and the output of the regulator is divided into two channels to control the upper secondary air flow. Baffle and lower secondary air flow baffle.
1.2.6 Adjustment of secondary air pressure The adjustment of secondary air is a two-stage control method to control the upper secondary air control baffle and the lower secondary air control baffle to ensure the secondary air flow and to adjust the fan inlet guide vane control twice. Wind pressure.
1.2.7 Oxygen content adjustment Oxygen content adjustment mainly adopts the static balance relation of wind and coal ratio plus the correction of oxygen controller to achieve oxygen content adjustment.
Oxygen content is not a fixed value adjustment over the entire load range. The system introduces a steam flow rate that characterizes the boiler load, and is used as a set value of the regulator after a function calculation and a given correction. The output of the regulator is used to correct the upper secondary air flow rate and the lower secondary air flow rate in two directions.
1.2.8 Bed temperature regulation The task of this control system is to control the bed temperature from 850°C to 900°C to ensure that the fluidized bed stabilizes the fuel and allows the limestone to fully react with the sulphur in the fuel to achieve the best desulfurization effect. The main idea of ​​the system design is that the bed temperature control is based on the statically calibrated primary and secondary air, and the bed temperature controller is dynamically modified. The output of the controller adjusts the ratio of the primary and secondary air.
1.2.9 Primary Air Heater Outlet Air Temperature Adjustment The purpose of this regulation is to prevent primary air temperatures below the dew point and corrode the air preheater pipes. The outlet temperature of the primary air heater is adjusted by changing the opening of the regulating valve into the primary heater auxiliary steam line.
1.2.10 Secondary air heater The purpose of this regulation is to prevent secondary air temperatures below the dew point and to corrode the air preheater pipes. The outlet temperature of the secondary air heater is adjusted by changing the opening of the regulating valve on the secondary steam heater auxiliary steam line.
1.2.11 Adjustment of coal/petroleum coke content The feed rate of the coal/petroleum coke feeder is adjusted according to the fuel command of the boiler master control system so as to achieve the adjustment of the coal/petroleum coke amount.
1.2.12 Limestone Regulation
The main feature of the CFB boiler is that when the temperature of the hearth is controlled between 850°C and 900°C, limestone CaCO3 is added to the combustion chamber. After the sulfur in the coal is burned, SO2 and CaO are generated and reacted to generate calcium sulfate (CaSO4). . The circuit provides enough limestone to maintain SO2 emissions below the environmental requirements.
Compared with coal gangue, petroleum coke has less moisture and ash content, and a higher calorific value. To ensure the adjustability of the bed temperature, limestone investment is even more critical. In this control system, the predetermined calcium/sulfur ratio is obtained by the multiplier to obtain the limestone to fuel ratio. After the SO2 amount is corrected and compensated, this signal serves as a given signal of the limestone regulator. The rotational speed of the limestone rotary feeder is adjusted to achieve limestone feed adjustment.
II. Conclusion The circulating fluidized bed unit of Jinshan Petrochemical Thermal Power Plant had completed the boiler ignition, unit rushing and grid connection at the end of February 2002. According to its operation in the field, the DCS system has successfully completed the system commissioning and commissioning tasks.
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