Sangmin Choi

A dynamic simulation of a modern ultra-supercritical circulating fluidized bed (USC-CFB) boiler system was performed using a physics-based model, where the boiler system was composed of an integrated system of heat exchanger (HEX) blocks. In each of the discretized elements of the HEX blocks, supercritical steam flow interacted with the solid–gas flow via heat transfer, and conservation laws, and physical phenomena of the CFB, and the supercritical steam flow was modeled to obtain the flow properties of each material side. By utilizing this model, the dynamic behavior of the main steam temperature, which was selected as the representative performance parameter for achieving safe and fast load changes, was simulated in response to changes applied to the main operating parameters such as the feedwater and fuel flow rates. The dynamic characteristics of the USC-CFB boiler were demonstrated by presenting the temperature responses of the steam and the circulating solid–gas after a step change in the feedwater and fuel flow rates. In the case of a load change, the dynamic response of the steam temperature was quantitatively presented by showing the transient overshoot and undershoot behavior, depending on the selection of the ramp speed when changing the feedwater and fuel flow rates. The steam temperature was also shown to be controlled by manipulating the inlet solid mass flow rate into the final superheater section of the external HEX. The comprehensive set of plant performance data that was generated from the model simulation can be utilized in setting up the operation strategies and/or in determining the control parameters for achieving stable steam temperature behavior during a load change.

In a circulating fluidized bed boiler, the large thermal mass and flow characteristics of the solids strongly affect the transient response of the circulating fluidized bed loop temperature, which determines the heat transfer rate to steam flow. Therefore, it is essential to interpret the dynamic response of the solid behavior in the circulating fluidized bed loop for the stable and efficient operation of the circulating fluidized bed boiler. In this study, the dynamic simulation of the solid behavior along with the flue gas flow in a circulating fluidized bed loop was performed by applying the core-annulus approach for the solid-gas flow inside the furnace and selected models for other physical phenomena of the fluidized bed. The circulating fluidized bed loop of a commercial boiler was selected as the target system. Especially, the model simulates the characteristics of the solid behavior, such as the local solid mass distribution, and the solid flow inside the furnace and the circulating solid according to the various operating conditions. These aspects are difficult to measure and quantify in a real power plant. In this paper, the simulated furnace temperature behavior as the representative performance parameter of the circulating fluidized bed loop was discussed along with the qualitative operation experiences reported in the literature. The operating conditions include the feed rate of fuel and air, the particle size, the solid inventory and the solid circulation rate. Furnace temperature behavior was reproduced through the simulation for each operating case in the literature and was analyzed with the solid behavior along with the combustion rate and heat transfer rate of the circulating fluidized bed loop. The simulation enables quantitative evaluation of the effect of the solid behavior on the temperatures of the furnace and return part in the various operating conditions.

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 300 MWe 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.