Modeling a Circulating Fluidized Bed
Andrew Griesmer | February 25, 2014
A circulating fluidized bed (CFB) is used to create a homogeneous mixture of gas (usually air) and solid particles to increase the efficiency of the combustion process in boilers. A better understanding of this process will help engineers to optimize their design parameters based on their individual needs. The Circulating Fluidized Bed model in COMSOL does just this, simulating a CFB with a given set of parameters that are easily interchangeable, depending on the needs of the user.
Why Use a Circulating Fluidized Bed?
A regular boiler uses solid particles for the combustion process, producing residues that need to be scrubbed away on a regular basis. In a circulating fluidized bed (CFB), air is sent upwards through the bed, lifting the particles into the air. When they reach the top and exit the bed, they are sent back into an inlet towards the bottom of the bed, and so on.
A model of the circulating fluidized bed system.
Since CFB’s produce a fluid-like fuel to burn (particles suspended in air), the increased surface area of the individual particles leads to higher efficiency, lower operating temperatures (~1500°F vs. 2200°F), fewer emissions, and increased fuel options.
Simulating Flow for Better Understanding
Circulating fluidized beds are already being used in industry today, but to keep increasing overall efficiency, we need to better understand the process of fluidizing the beds. This will help to answer questions such as “What should the fluid inlet velocity be?”, “What width and height dimensions should my fluidized bed have?”, and “How will changing the particle density effect my overall output?”.
Instead of answering these questions with experimental testing, you can use simulation tools. The Circulating Fluidized Bed model, created with COMSOL Multiphysics and the CFD Module, uses the Euler-Euler Model, Laminar Flow Interface to model the different phases of the process.
First, there is the dispersion phase, which is shown in the image below. The darker areas indicate a higher ratio of particles-to-air. As you can see, after only five seconds the particles are much more dispersed, though none have reached the top of the bed.
Snapshots of the fluidized bed showing the start of dispersion phase. The bed width has been scaled by a factor of ten.
After about ten seconds, the system reaches a steady state, which is known as the continuous phase. At this point, the fluid/particle mixture has reached its maximum average volume fraction. The volume fraction is the volume of air divided by the total volume of the system (air plus particles).
At time zero, the packed bed consists of 50% particles and 50% air, or a volume factor of 0.5. As seen below, once the system reaches the continuous phase, the volume fraction ranges between 0.86-0.935, depending on the location in the bed.
The volume fraction along the width of the bed, at three different vertical positions, during continuous phase.