Process and Reactor Level Simulations of Coal-Direct Chemical-looping Combustion - Novel Combustion Concepts for Sustainable Energy Development - page 352

The key shortcoming from the results of the simulation of the complete CD-CLC
system using 60 wt% Fe 2 O 3 particles as the oxygen carrier is that the high jet
velocities required to

fluidize these relatively heavy particles lead to the formation of
an unsteady pathway through the bed after the

fl

first bubble collapses that hinders the
pressure buildup required for subsequent bubbles to reach the top of the reactor and
continuously recirculate. Since this material is already among the lightest Fe-based
oxygen carrier studied by Johansson et al. ( 2004 ) and lighter alternatives likely to be
expensive, one alternate way to mitigate this problem is to reduce the height of the
fuel reactor. The central jet velocity is speci first bubble has enough

kinetic energy to reach the top of the reactor; with a smaller reactor, this velocity can
be reduced which in turn reduces the likelihood of the unsteady pathway to form.
However, this also reduces the residence time of the fuel in the reactor, which can
greatly affect the rate of fuel conversion, particularly when using solid coal that has
to go through a slow gasi

ed such that the

cation process before the reaction with the metal oxide can
occur. Since the unsteady pathway only forms after the first bubble collapses,

another alternative is to use a cyclic flow injection whereby the jet is turned off

intermittently to allow the bed particles to re-settle down into the original packed bed
con

fl

fluidization to the initial bubble formation stage once
the jet is turned back on. Cyclic injections are already used in the laboratory-scale
CLC experiments to switch between N 2 and CH 4 in lieu of separate fuel and air
reactors as considered in the work of Son and Kim ( 2006 ); their operational feasi-
bility on an Industrial scale can also be readily studied.

guration, which resets the fl

5 Conclusions

In this paper, ASPEN Plus is first employed to model and study the CLC and

CLOU processes at the system level. The CLC process model is validated against
previous work and shows good agreement, following which further studies are
conducted to investigate the effect of varying the air

flow rate and oxygen carrier
feeding rate. It is found that the maximization of energy output from CLC can be
accomplished by using the optimum ratio of coal, air, and oxygen carrier in the
system equal to 1:10:70, generating a 48 % net increase in power over the baseline
experimental case of Sahir et al. ( 2012 ). In the CLOU process modeling also using
ASPEN Plus, excellent agreement is obtained between the experimental results and
the simulation for various quantities such as the oxygen carrier conversion kinetics,
fl

fl

ue stream O 2 and CO 2 concentrations, and power output. It is clearly demonstrated
that the ASPEN Plus can provide a creditable process simulation platform for the
study of CLOU process. Scaled-up simulations of CLOU process were also con-
ducted using different types of coal and coal feeding rates. The results show that the
total power output is nearly linear with the increase in coal feeding rate and the
carrier circulation. Such linearity in general will not be expected for actual scale-up,
since the ASPEN Plus system modeling software neglects miscellaneous energy
losses in the system. Furthermore, the coal rank appears to have signi

cant impact