Coordinated control system (CCS)
The circulation fluidised bed possesses characteristics such as being multivariable, having strong coupling, and large inertia. During load adjustment, it is much slower than the pulverise coal boiler. Therefore, the energy changes in the boiler are unable to catch up with the energy changes of the turbine. This is a major issue with inertia, and very difficult to overcome. HollySys released the CFB unit’s CCS control solution after optimising their control program over the years. They engaged in regular discussions, conducted static and dynamic debugging, and leveraged on their many years of experience in circulation fluidised bed control to cater to the characteristics of the large 300MW generator’s CFB boiler. HollySys’ CCS uses the direct energy balance method (DEB) + rapid energy calibration, effectively reducing overshooting, improving the response of the energy changes of boilers to turbine energy changes, and is successfully applied in many 300MW circulation fluidised bed generators.
The CCS comprises circuits such as the generator load management centre, pressure regulation circuit in front of the machine, boiler master controller, and turbine master controller. It coordinates the operations, speed, and accuracy of the boiler and turbine, steadily responding to the load instructions of the central coordinator or operator, enabling the generator to generate the necessary electricity.
The CCS has functions such as load management centre, primary frequency regulation, runback, and automatic generator control (AGC). It is highly adaptive to generator operations, satisfying operating requirements for continuous, stable, and safe operations. Based on the status of generator processes, it is possible to adopt the direct energy balancing strategies, direct command balancing strategies, and rapid energy calibration strategies, so as to maintain a supply/demand balance in the turbine and boiler, rapidly responding to load command changes of the generator. The CCS load management centre will process load setting commands with speed limiting, amplitude limiting, and the increase/decrease in locking, generate actual load commands, coordinate the regulation of control circuits, and stabilise generator operations. Based on the coordinated control of the turbine and boiler, the AGC sub-system is able to accept load commands sent by the power grid and directly respond to grid load requirements.
The boiler regulation and control system comprises several sub-systems, and the sub-systems are inter-coordinated, ensuring the boiler’s safety and speed, steadily generating the required steam for the unit’s load instructions.
The generally included boiler regulation control functions:
1) Combustion Control
The CFB (circulating fluidized bed)boiler combustion equipment mainly comprises coalsystem, combustion chamber, separator, and return feed system. The combustion process of this is very different from the conventionalpulverised coal fired boiler, due to its outstanding performance in circulation and flow. The burning system is a non-linear system with high inertia and coupling, where its variables affect each other, making the control system very complicated.
Flue gas oxygen content; total amount of fuel; boiler master control instruction; total air flow; primary wind volume bias adjustment; wind – coal cross amplitude limiting; primary wind volume converters; wind – coal conversion; fuel volume regulation ; fuel volume tracking; primary / secondary air volume change regulation ; total air volume conversion; main steam pressure deviation rapid calibration; oxygen to secondary air flow calibration; flue gas oxygen content regulation; stoker load distribution and bias adjustment; primary wind volume instruction; secondary wind volume instruction; coal feed volume instruction; boiler master controller (combustion regulator) control system.
2) Wind Control
The CFB boiler’s wind system is more completed than a general conventional pulverise coal boiler. It mainly comprises primary and secondary wind, wind return feed, and fuel spreader air blowers. Given the different models, there are those with primary/secondary air blowers and high-pressure Roots blower, and there are also those boilers with fuel spreaders that are designed with fuel spreader air blowers. Total boiler wind volume and load instructions are mainly used for the wind volume control where fuel volume is matched with the load to strike a balance, including total wind volume control and primary/secondary wind ratio control.
3)CFB Temperature Control
The CFB temperature control system is unique to the circulating fluidised bed boiler and is also a control system of utmost importance. Once the circulating fluidised bed boiler operates in its best bed temperature region, it is able to obtain better desulfurisation and denitrification effects, and is more economical and safety. The sulfur in the burning fuel such as coal becomes sulfides during the burning process, and the limestone absorbs the sulfides to become drysolidcalcium sulfate, thus reducing sulfate emissions.
4)CFB Pressure Control
The bed pressure control is generally achieved through regulating the slag volume. Currently, the CFB boiler has two types of main slag discharge cooling device. One type is the water-cooled slag cooler drum, and the other is the air-cooled or water-cooled fluidised slag cooler. HollySys performs PI computing on the furnace’s CFB pressure signal and the main steam-flow function signal to generate a slag discharge volume instruction to control the slagging machine speed, and regulate the differential pressure to maintain the set value.
5)Flue Gas SO2 Volume Regulation
The limestone control system measures the SO2obtained to adjust the volume of limestone that has to be added. Once the additional volume of limestone reaches the required level, the speed of transfer to the feeder shall remain constant.
Turbine control generally includes the following functions:
SCS functions: CFB boilers have many controlled objects with complicated control processes, and the manual control is 1-on-1, which makes impossible to adapt to safety and economical operating requirements. The SCS adopts sequence control to realise the orderly control, interlocking, and protection of the start and stop sequences of the sets and all the auxiliary equipment. It is able to improve the work efficiency of operators, prevent accidents from erroneous control, and reduce the labor intensity of the persons on duty.
The functional unit level sequence control and sub-unit level sequence control accepts control instructions from the operator. After the necessary conditional logical computations and issuing of control instructions to the relevant equipment, the current level of control is completed after obtaining the response signal that signifies that the task is completed.
The unit level sequence control system is also called the Automatic Plant Startup system (APS). Once the sequence control system accepts the unit level’s sequence control activation command, the unit will transit from its initial state to the activation of its first load based on the program control step procedures. During the activation process, only a minimal amount of breakpoints have to be set during the activation process, to be verified by the operator before proceeding automatically.
The CFB boiler’s combustion approach is different from the pulverised coal boiler, a large volume of the high-temperature bed material can be used as the constant ignition source, and would not easily combust or explode due to the inappropriate accumulation of explosive mixtures arising from fire extinguishing.
The FSSS, also known as the Furnace Safety Supervisory System, is a necessary monitoring system for large modern thermal power boilers. Regardless of whether the boiler is operating normally, started, or stopped, the system will constantly and closely monitor the large number of parameters and statuses of the combustion system, and constantly conduct logic analysis and computation, and issue action commands where necessary. It can use different types of locking equipment to cause the relevant components of the combustion equipment to complete the necessary operations or deal with averted incidents based on the predefined reasonable procedures, so as to ensure the safety of the boiler combustion system.
The FSSS includes two components: Burner Control System (BCS), and the furnace safety system (FSS).
The FSSS system’s design is in line with the NFPA8502 standards of the National Fire Protection Association, compliant with and implemented in accordance with the relevant national technological standards.
General conditions for CFB boiler master fuel trip:
The general conditions for CFB boiler trip are: