Solids Flow in Large-Scale Circulating Fluidized Bed Furnaces

Doktorsavhandling, 2021

The flow pattern of solids established in large-scale circulating fluidized bed (CFB) furnaces is of great importance for the performance of commercial CFB boilers, as it governs the heat transfer and mixing of the fuel and any other reactive solids. The solids flow pattern in the riser is crucial for the design and scaling up of large-scale CFB technologies for the thermochemical conversion of solids. The aim of this work is to acquire new knowledge and understanding of the solids flow patterns in CFBs that are representative of large-scale furnaces. The goal is to improve the reliability of predictive modeling tools and, thereby expand the development of new and existing CFB technologies within the energy field. The solids flow of a CFB furnace is characterized by a bottom region with a high concentration of solids, a splash zone with strong solids back-mixing, and a transport zone that covers the major height of the furnace and has a lower level of solids back-mixing, from the bottom and upwards.

This thesis uses experimental campaigns and various modeling tools to elucidate the CFB solids flow. The experimental work is carried out in two cold units: a pseudo-2-dimensional unit that allows visual observation of the flow; and a 3-dimensional unit that can be operated under fluid-dynamical scaling, which has been shown to reflect accurately the solids flow in an existing reference >200-MWth CFB boiler. Furthermore, the data derived from the different sizes and operational ranges of these experimental units are linked to previous measurements of large-scale CFB combustion. Examinations of the solids back-mixing phenomena are supported by different modeling tools, including Direct Number Simulations, semi-empirical modeling through the Finite Volume Method, and Monte Carlo modeling.

The results of this work show that: (i) the presence/absence of a dense bottom bed affects the extent of solids entrainment from the bottom region; (ii) a fluid-dynamical region similar to the splash zone is established even in the absence of a dense bottom bed; (iii) the rate of solids back-mixing in the splash zone can be predicted from modeling of the gravity-driven ballistic trajectories; (iv) the solids back-mixing in the transport zone is governed by the transfer of solids through the core-wall layer boundary, which is driven by turbophoresis (i.e., the migration of particles in the direction of increasing particle concentration), and for which a Sherwood number-based expression is proposed that improves on the former empirical expressions; and (v) the solids back-flow effect at the riser exit cannot generally be neglected when predicting the in-furnace back-flow, and is substantial at gas velocities that are typical for commercial CFB boilers. Validated expressions are proposed for the decay coefficients of the splash and transport zones and the solids entrainment from the bottom region. Taking together this collected knowledge, this thesis improves the reliability of semi-empirical modeling tools for the prediction of the solids flow patterns in large-scale CFB furnaces for a wide range of operational conditions.