Process and Reactor Level Simulations of Coal-Direct Chemical-looping Combustion - Novel Combustion Concepts for Sustainable Energy Development - page 353

on the flue stream concentration of CO 2 and overall energy output; bituminous coal

and anthracitic coal show signi

fl

cantly improved CLOU performance over lignite
coal. The similarity in the CLOU performance of bituminous coal and anthracitic
coal can be explained by the fact that they have similar carbon content, which
indicates that char gasi

cation is no longer a performance factor of high relevance
in CLOU since the presence of oxygen enables the solid gas combustion to take

-

place without gasi cation.

To account for changes in the hydrodynamic characteristics, detailed multiphase

fl flow simulations are conducted using the CFD/DEM coupled approach in ANSYS

Fluent. The initial reactor-level simulation shows excellent agreement with the
experimental results obtained by the research group at Darmstadt University of
Technology. Based on the insights from reactor-level modeling, simulations for a
complete three-dimensional CD-CLC con

guration are performed to determine the

cold '

results that the continuous formation of new bubbles and solid recirculation from
the loop seal to the fuel reactor

fl flow performance of the con guration. It is observed from the simulations

— two key factors important for successful CD-CLC

operation — are not satisfactorily achieved for the con guration used in the exper-

iment at TU-Darmstadt. Subsequently, a new con guration was designed to redress

these issues, i.e., to enhance the particle recirculation in the entire system and to
increase the possibility of continuous bubble formation by eliminating the dead
zone in the spouted bed and the resulting pressure imbalance. The cold flow sim-
ulations results for the modi cant

improvements on the performance can be achieved with the introduction of proper
design modi

ed CD-CLC con guration con rmed that signi

cations.
Finally, the CFD/DEM cold fl flow model for the modi ed CD-CLC con guration

was expanded to consider a chemically reacting flow. The oxygen carrier selected

was 60 wt% Fe 2 O 3 supported by 40 wt% MgAl 2 O 4 due to its favorable properties
for CD-CLC operation. The system behavior was investigated in the presence of
chemical reactions between the Fe 2 O 3 and gaseous CH 4 . It was found that the initial
bubble formation and continuous recirculation of solid particles from the loop seal
to the fuel reactor were adequate and that the chemical reactions were successfully
incorporated. One shortcoming observed in the simulation of

fl

the complete

CD-CLC system with reactive fl flow was that subsequent bubbles formed in the fuel

reactor lacked suf cient kinetic energy required to reach the top of the reactor and

into the cyclone. By investigating the pressures in the system, it was found that the
high-velocity inlet jet formed a straight pathway that bypassed much of the particle
bed after the

first bubble collapsed, which in turn prevented the critical pressure
buildup required for subsequent bubbles to develop. Based on this insight, a few
alternatives are proposed to circumvent this issue, which will be investigated in the
future work. More importantly, the system performance of the modi ed CD-CLC

con guration demonstrates the necessity of developing a credible CFD/DEM model

with high fidelity for future optimization designs of industrial-scale CD-CLC

systems.