Research On Distributed Parameter Modeling And Simulation For Utility Boiler Based On Combustion Monitoring

Boiler is an important part of the power plant. The boiler security is still a valuable research. The temperature, heat flux and mass velocity heavily affect the boiler safety, which are non-uniformly distributed on the heated surface. Traditional methods such as thermocouple and heat flow meter are pointwise and hardly used for distributed measurements, so the boiler safety can't be effectively evaluated. In this paper, based on the combustion monitoring information such as the 3-D temperature distribution and the emissivity of particle phase, the distributed parameter modeling for power plant boiler is studied, which can comprehensively descript the distributed parameter characteristics. In addition, considering the coupling relationship of the control systems, a new modeling method for drum boiler is developed to reflect the effect of the water level control system and the main steam temperature control system to load-pressure characteristics. The detailed description is as below.In the study, considering the coupling relationship of the control systems, a research on simplified model for drum boiler is performed. Using the system pressure deviation enthalpy, a model is developed, which can reflect the influence of feed water and spray water to the load-pressure characteristics. And a new type feedforword signal in water level control system and main steam temperature contol system is constructed, which can effectively weaken the influenc of other coupling control systems and further improve the control effects.Based on the 3-D combustion monitoring in furnace, a distributed parameter modeling method for evaporation system is developed. The transient, 2-D radiation flux is obtained by using the 3-D temperature distribution and the emissivity of the particle phase inside its furnace. And a factor of equivalent flame emissivity is proposed to reflect the gases radiation and convection to the heat transfer in furnace. The effect of the steam below liquid level to the drum level is considered, and an empirical express is proposed to calculate the coefficient of the equation of the steam mass flow rate at the outlet of drum, which varies with the boiler load.Then we obtain a distributed parameter model for the evaporation system. A simulation is made to validate the model by the comparison of the simulated value with the measured data of the drum pressure, the drum level and the steam mass flow rate at the outlet of drum. The transient, 2-D distribution of thermodynamic parameters, such as the wall temperature, the steam quality and the mass velocity can be predicted by the model, which forms an important basis to effectively evaluate the security and the reliability of the epaporation system.A simulation research on water circulation characteristics is performed by using the distributed parameter model for evaporation system. The results show that the diameter of risers and the thermal load deviation have the greatest influence on the deviation in flow rates. The diameter of the risers has an optimum design value, under the condition of such a diameter, a boiler can adapt to any changes in operation parameters, maintaining a comparatively good water circulation state. Moreover, when the structural parameters of a boiler have been determined, the boiler unit can secure a small deviation in flow rates favorable to water circulation while operating under the condition of a relatively high boiler drum pressure, relatively low feedwater specific enthalpy and moderately high boost-prssure of circulating pumps.A distributed parameter modeling for superheater/reheater system is performed. According to heat sources, the energies transferred in the superheater system are classified. By using the smoke vent plane, the heat transfers occur in an enclosure system, and then a distributed parameter model for the heat transfer of superheater panel is deduced. Based on the research of predecessors about the pressure distribution in headers, the effect of heat load distribution along the width to the pressure distribution in headers and flow distribution of parallel tubes is studied in this paper. Then, a distributed parameter model for superheater and reheater system is obtained, which is validated by the simulation and is useful to reflect the thermodynamic parameter distribution on the same platen or between the platens.A simulation study is made on the wall temperature characteristics of superheater and reheater system by using the distributed parameter model for superheater/reheater system. For the rear platen superheater or platen reheater, under the small gas deviation, there exist two high wall temperature zones for the outer tubes, one is at the horizontal part of U-tube, and the other is at the outlet of U-tube, however, there exists only one high wall temperature zone at the outlet of U-tube for the inner tubes. With the increase of gas deviation, the wall temperature at the tube's outlet decreases, and the wall temperature of horizontal part of U-tube increases remarkably. For the high temperature superheater, the maximum wall temperature is at the tail part of the snake-tube. With the increase of gas deviation, the maximum wall temperature moves toward the bottom. These results are helpful for design and modification of the superheaters and reheaters.