Research On Superheated Steam Temperature Control

The superheated steam temperature is a very important parameter, related to the save operation of the unit. The proportion of wind power station that has priority for power generation in the power grid is rising and the wind power (or other new developed power source) is not stable due to the weather. So the thermal power station is responsible to regulate the peak frequently to fit consumers need. Thus it would causes the parameters of thermal power station vary, especially the steam temperature varies fast in large range and that is not allowed. It is a tough task to control the steam temperature well. Because the super-heater is a facility that parameter distributed, nonlinear, have large inertia and time delay, time varying system. A lot of factors would influence the main steam temperature. The cascade PID controller performance is not efficient enough for superheated steam temperature control in CFB boiler. So the influencing factors were discussed in this thesis. A useable simple model of superheated steam temperature was built based on enthalpy. And a reasonable controlling method was designed to improve the adjusting ability to load change.The objective of this thesis is to improve the control performance for superheated steam temperature. So the influencing factor of the main steam temperature was analyzed in many aspects. A simple model that suitable for engineering was built based on thermodynamics and heat transfer theory, and also based on assuming the super-heater is a lumped parameter system,.After the model was built, two control methods suitable to the model were designed. The one was a SMITH-PID controller which the SMITH is main controller and the PID is auxiliary. Other was enthalpy calculated PID (EC-PID) controller, which EC is the main controller and the PID is auxiliary. The main controller section is responsible to calculate the increase in enthalpy of the steam that flue threw the super-heater and obtain the value of the inlet steam temperature of the super-heater. The value would be the set point of the PID controller.The configuration logic that used in the experiment was built on a virtual distributed processing unit (VDPU) of the DCS system. The VDPU is running on one of the engineer station. The result of the calculation is transferred to modulating control system (MCS) that responsible to control the steam temperature. The experiment shows that the both methods can control the temperature effectively. The temperature was varying in the range allowed by design of the boiler and the performance was better than traditional cascade PID controller. The experiment also shows that SMITH-PID controller’s performance is more effective than the EC-PID controller, it have shorter regulating time, small scale of propagation and can absorb strong disturbance. So the SMITH-PID controller is the advised choice for further improvement that was done in this thesis.The design of super-heated steam temperature control method in this thesis can adjust both process of changing load and stable running of the unit. The control efficiency is very nice. It have higher accuracy in temperature and easy for maintain. The research could be guidance to stable running of steam temperature in CFB, engineering practice and could be accordance in theory.