Performance prediction of a large-scale circulating fluidized bed boiler by heat exchangers block simulation

Abstract

A simplified mathematical model to predict the performance of a circulating fluidized bed boiler, which was previously developed along with a pilot-scale boiler, was extended to accommodate the configuration of a 300MWe class boiler. The model is a combination of zero-dimensional energy and mass balance model of heat exchanger blocks, across which the streams of air and combustion gas, water and steam, and the circulating solids interact. The system model describes the boiler as a system of heat exchanger blocks through which the solid-gas flow interacts with the water-steam flow and heat transfer rates are determined by defining the heat transfer coefficients and representative temperature differences. The current paper discusses the validity of the approach which has been extended from the previous case. Albeit the significant difference in degree of complexity as well as physical size of the two boilers considered, boilers were represented as a system of a finite number of heat exchanger blocks, while configuration of heat exchangers is incorporated. Heat transfer relations were carefully evaluated by checking if the operating regimes of the flow and heat transfer were adequately represented. Special considerations were given to the significant heat transfer surfaces, which were installed as a form of wing walls, where water-steam inside the tubes could be either superheated steam or evaporating water. Results of the performance prediction of the boiler were cross checked against the measurement data from the operating plants. Calculated results of gas and steam temperatures of each heat exchanger block were compared with measurements over the wide range of operating conditions, whose major indicator was boiler load, which is related to fuel combustion rate and thereby the generated steam flow rate. The model prediction has also shown its additional advantages in providing the property data at the points where measurement probes were not installed or the properties which were difficult or inherently impossible to measure. The paper also discussed the validity of heat transfer relations and sensitivity of their parameters.