Solids flows in circulating fluidized beds: explorations of phenomena with applications to boilers

Licentiate thesis, 2020

The circulating fluidized bed (CFB), which is a technology that is commonly used in the heat and power sector, efficiently converts renewable and/or and low-grade solid fuels, such as biomass and biogenic waste fractions. CFB units serve also as a technological platform for carbon capture processes (combustion by oxy-firing or chemical looping), which are foreseen to play a key role in the transition of the energy system towards decreased atmospheric CO2 emissions. However, the development of commercial CFB boilers is limited by key gaps in the knowledge, one of which is the solids flow pattern, which is not sufficiently understood even though it is an important phenomenon that governs both mass and heat transfers in the system.

This thesis aims to elucidate the solids flow patterns in CFB units that are representative of large-scale boilers. More specifically, the objectives are to identify and characterize the interlinked underlying phenomena that govern the solids flow pattern in the riser and the external circulation of solids. The specific phenomena studied here are: depletion of the dense bed; solids entrainment from the bottom region; the back-mixing in the splash and transport zones; and the riser exit backflow. Also examined is how these factors are affected by the unit size. For these purposes we conducted experimental analyses in two units: a pseudo-2-dimensional unit; and a fluid-dynamically down-scaled unit (in which studies were carried out with and without scaled bed material). Validation of the cold-flow scale model with scaled material shows very good similarity between the concentration profiles obtained from the cold-flow scale model and from the large-scale (>200-MWth) CFB reference boiler.

The results show that the presence/absence of a dense bed affects the entrainment of particles from the bottom region into the freeboard. The expansion of the splash zone immediately above the dense bed is affected by the dense bed height due to the modified bubble growth. The solids back-mixing from the core region to the wall layers is mostly affected by the gas velocity and the cross-sectional geometry of the riser. Finally, the external circulation of solids is shown to be non-equal the upwards solids flux at the top of the riser, with the exceptions of cases with very low gas velocities, revealing significant local back-mixing of solids at the furnace exit at nominal loads.