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

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