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 be
