Investigation On Dynamic Characteristics Of Circulating Fluidized Bed Boiler

Circulating fluidized bed boiler (CFBB) becomes more and more attractive to industries due to its advantages mainly of wide fuel adaptability, high combustion efficiency and low pollutants emissions. It is of great practical significance to analyse and investigate its dynamic characteristics for improving the design and optimizing parameters of the boiler. Modeling so as to the simulation of CFBB becomes indispensable as utilizing mathematical model can easily study the dynamic characteristics of boiler without or fewer experiments, which are normally costly.Focus on the industrial 220t·h^-1 CFB boiler, models for normal operation condition and process startup were developed and simulations under these circumstances were carried out in this work.In detail, model for normal operation condition was composed of a combustion system and a steam-water system, in which dense and dilute regions were considered in the combustion system and the dense region was regarded as a stirred tank reactor. Equations describing oxygen concentration, total solid particles in the bed, unburned carbon and total energy were established using the lumped-parameter method. While the gas and solid particles in the dilute region were regard as at the tubular flow condition, and equations for oxygen concentration, total solid particles in the bed, unburned carbon and total energy were obtained based on distributed parameter solution. As the models were built up, the step changes and the real-time simulation for the combustion system were able to be implemented. The results for step changes simulation with amount of coal, primary and secondary air quantity as input variables could reflect well the dynamic response of system. And the real-time simulation is in a good agreement with the industrial data.The steam-water system includes an evaporation region and a super-heater. In this work, the evaporation region was divided into two sections: drum and riser-tube, and downcomer. For each section, the lumped-parameter method is applied, and the dynamic equations about drum pressure and water level were established by mass and energy balance. The model of the super-heater is established directly by the lumped-parameter method, and the dynamic equations of the superheated steam pressure and temperature can be gained through the mass and energy balance of working fluid within the super-heater. Using these models, the step changes simulation for the steam-water system was performed, and the results of simulation reflect the dynamic response of system fairly well.Bridging the combustion and the steam-water system of the CFBB via the heat exchange formulas of the water-cooled wall and super-heater, the comprehensive dynamic mathematic model was finally buildup for the whole CFBB system, by which dynamic behaviours for CFBB operations can be simulated.The model of startup process was composed of two phases: the heating by the gas from preheat chamber for solid particles before adding coal; and the reaction after coal was fed. The model for the former is obtained based on the energy balance of the whole bed, while for the latter the model consists of dynamic equations of oxygen concentration, total bed material in the bed, unbumed carbon mass and total energy. The curves for temperature increase in the whole startup process can be simulated by solving these two models cooperatively. The model system were validated by comparing the prediction with actual performances of the CFBB. Finally, the validated model system was used to simulate the curves for temperature increase of CFBB in different operating conditions, and the results also indicated the model is right.