Dynamic simulation of the circulating fluidized bed loop performance under the various operating conditions

Abstract

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.

Dynamic simulation study of the steam temperature in a ultra-supercritical circulating fluidized bed boiler system

“…Accordingly, the heat transfer rates of the major sections of the boiler (Q·OTU,Q·Platen,Q·CYC,Q·EHEX,Q·HEX1,…,Q·HEX5) were determined.

Figure 1.Schematic configuration of a USC-CFB boiler model, represented as an integrated system of discretized heat exchangers: the solid–gas blocks (FUR: core-annulus model, 21 CYC, LS, EHEX: lumped model, 21 back pass: discretized model 14 ) of the solid flow and the flue gas flow interact via heat transfer (Q·OTU,Q·Platen,Q·CYC,Q·EHEX,Q·HEX1,…,Q·HEX5) with the corresponding blocks (OTU water wall, Platen HEX, HEXs 1-5, CYC, FSH, FRH: discretized model 22 ) of the steam flow. COD represents the cross over duct.

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“…The fluid property values for the steam were calculated at the prevailing pressure and temperature. A detailed description of the solid–gas flow modeling including the conservation laws and the correlations for the transport phenomena can be found in paper, 21 and that of the supercritical steam flow modeling can be found in paper. 22…”

“…Circulating solids served as the energy distribution mechanism of the CFB loop. 21 Therefore, in order to evaluate the effect of the circulating solids on the steam temperature, the solid flow rate in the solid return section was adjusted by bypassing the inlet solid flow rate into the EHEX. Since the main steam temperature was simulated through interactions involving the solid–gas flow of the FUR, circulating solids of the solid return section, and the flue gas flow of the BP, this simulation tool enables the study of the relationship between the steam temperatures and operating parameters based on a model of the dominant physical processes.…”

“…Accordingly, it was proven that the solid circulation rate affects the temperatures of each component in the CFB loop and determines the heat transfer rate in the solid return section. 21 Therefore, to predict the behavior of the steam temperature in the USC-CFB boiler system, the flow rate and state of the circulating solids should be properly addressed in the modeling. In general, the solid circulation rate (m·cir) is determined by the airflow rates in the FUR during operation and cannot be treated as a manipulated variable.…”

Simulation of a CFB Boiler Integrated With a Thermal Energy Storage System During Transient Operation

“…The coal and air inflow rates were changed and the transient response of the bed temperature was estimated; good agreement was found between the simulations and experimental data from the same boiler regarding the aforementioned transient variables as well as critical steady-state parameters during 30 and 50% load operation. Finally, Kim et al (2016Kim et al ( , 2019 extended the model of Kim et al (2014Kim et al ( , 2015 to perform dynamic simulations as well, by calculating the incremental difference from the previous time step, and progressing for the next time step. Two dynamic simulations were performed for a 340 MWe CFB boiler and one with 1500 t/h steam production capacity.…”