Dynamics of large-scale fluidized bed combustion plants

Licentiate thesis, 2021

Fluidized bed combustion (FBC) plants are widely used in energy systems across the world for the thermochemical conversion of solid fuels, and are especially suitable for low-rank fuels (a category to which renewable solid fuels belong). FBC plants are traditionally operated for base-load electricity production and for heat production, both of which processes are characterized by steady and stable operation. As the share of variable renewable electricity (VRE) sources is expected to increase dramatically, FBC plants will have to adapt their operations to the new flexibility requirements related to the inherent variability of VRE sources. By enhancing their operational and product flexibilities, FBC plants can remain financially attractive and offer services to support the balancing of the grid. As tools for assessing the operational flexibility of thermal power plants, dynamic modeling and simulation are gaining attention from both researchers and plant operators. However, it is a common practice to assume that the dynamics of the gas side are much faster than those of the water-steam side, i.e., not accounting for the in-furnace dynamic mechanisms.

This thesis aims to characterize the dynamic behaviors of commercial-scale FBC plants, accounting for both the gas and water-steam sides of bubbling and circulating fluidized bed (BFB and CFB) units. For this purpose, a dynamic semiempirical model of the gas side of FBC plants is developed and integrated into a process model of the water-steam side. The models are validated against steady-state and transient operational data measured at two commercial-scale industrial units. The model is then used to analyze the inherent dynamics of the gas and water-steam sides, to compare the transient behaviors of BFB and CFB units, and to assess the dynamic performances of FBC plants when operated under different control structures.

The results of the dynamic analysis show that the stabilization times of the temperatures across the furnace differ, largely based on the local heat capacity of the region in the furnace, i.e., the amount of bulk solids. The work includes an assessment of the impact of the characteristic times of the in-furnace mechanisms (i.e., fluid dynamics, fuel conversion and heat transfer) on the computed stabilization times of key in-furnace variables at plant level, and suggests some simple mathematical relationships for predicting these times. When accounting for the water-steam side, the results show that the inherent dynamics of variables such as live steam pressure, flow and power production are in the same order of magnitude as the dynamics of the gas side, particularly for the CFB case. This highlights the importance of accounting for the gas side when attempting to model accurately the dynamics of FBC plants. Furthermore, FBC plants are found to be able to provide fast load changes when operated under control structures that manipulate the live steam valve, although this is found to trigger operational issues, such as pressure overshoots.

The results of this thesis are of particular importance in terms of assessing the transient capabilities of FBC plants to operate in electricity-driven markets where fast operation is required, and they can be used to identify opportunities and challenges. Furthermore, knowledge about the transient operation of large-scale FB reactors will be crucial for the development of FB applications other than combustion, such as polygeneration or thermochemical energy storage.