A Mathematical Model of a Drum-Type Boiler with Thermal Stress Calculation in the Circulating Fluidized Bed Combustor

A mathematical model has been developed to predict the dynamic behavior of a natural circulating drum-type boiler system. The basic conservation rules of mass, momentum, and energy were applied to the system. The six state variables of the model include the total volume of water, steam pressure, steam quality at the top of the riser, drum water level, downcomer-riser loop flow, and the volumetric fraction of steam in the water-steam mixture under the drum water level. This model can simulate the dynamic drum water level directly based on the mass balance of the drum while the other models in the literature computed it by off-line with the aid of empirical equations and assumptions. A sub-model was applied to calculate the temperature and thermal stress distribution both in the metal layers and in the insulation layers of the drum shell. The sub-model was derived from a finite differences method based on the discretization of the Fourier’s heat conduction equation. The simulation results with the dynamic model showed a good agreement with the data in the literature.