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Calcination in an electrically heated bubbling fluidized bed applied in calcium looping

Samani, Nastaran Ahmadpour
Master thesis
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URI
https://hdl.handle.net/11250/2724254
Date
2020
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  • Master i teknologi [169]
Abstract
Switching fossil fuels to green electricity as the energy source to decarbonate the raw meal

in the calciner can eliminate the CO2 emissions produced through fuel combustion and

also provide a basis for simple capture of the CO2 generated through calcination, as CO2

is the only gaseous product exiting from the electrified calciner. For this reason, an

electrically-heated fluidized bed reactor was designed as a calciner and its applicability

and cost estimation were carried out.

A mass and energy balance for steady-state conditions was conducted, so that relevant

temperature, flow rates, and duties in the electrically-heated FB reactor and heat

exchanger have been calculated by MATLAB code.

The key parameters of FB reactor such as minimum fluidization velocity, minimum

bubbling velocity, terminal settling velocity, and the reaction time based on the particle

size distribution were calculated. The fluidizability of the fine limestone particles was

tested by a cold-bed BFB unit and it revealed that owing to the fine particle sizes of the

raw meal, there are strong cohesive forces between the particles. Hence, a conventional

bubbling fluidized bed is difficult to fluidize Geldart C particles. The identical system was

simulated by Barracuda® and the results of the simulations had a good consistency with

the experiments.

A binary-particle fluidization system, mixing fine powders with the coarse particles, was

proposed to enhance the flowability of fine particles. The fluidized bed calciner process

was designed as a semi-batch process operating in two modes; the calcination mode (with

a low gas velocity) and the entrainment mode (with a higher velocity). After the raw meal

particles have been calcined, they have to be separated from the coarse, inert particles.

This can be done by increasing the velocity of the CO2 used for fluidization to a value

sufficiently high to entrain the raw meal particles, but still sufficiently low that the coarse,

inert particles are not entrained. The inert particles may provide a homogeneous

distribution of the fine particles and help to fluidize them. The aggregation and clustering

of the fine particles will decrease due to collisions with inert coarse particles. The inert

particles will also provide a thermal energy reservoir through their heat capacity and

thereby contribute to a very stable bed temperature, which is advantageous in the control

of the process.

The operational conditions at 1173 K, such as the particle size distribution of the inert

particles and the fluidization gas velocity were calculated by the Barracuda simulations.

The inert particles with the diameter range of 550-800 µm and the velocities in the

calcination and entrainment modes equal to 0.18 m/s and 3 m/s appeared as suitable for

the calciner operation. The simulations showed that at the velocity of 0.18 m/s, 7.6% of

fine particles may be entrained. However, by comparing the CO2 residence time with the

reaction time of particles, it was concluded that all fine powders were calcined before

leaving the bed
Publisher
University of South-Eastern Norway
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