Process and Reactor Level Simulations of Coal-Direct Chemical-looping Combustion - Novel Combustion Concepts for Sustainable Energy Development - page 339

fuel reactor at Darmstadt University of Technology (TU-Darmstadt) (Alobaid et al.
2013 ). For the cold flow simulations, only the hydrodynamics and particle motion

are considered, without chemical kinetics and heat transfer. The TU-Darmstadt
experiments were performed with a spouted

fl

fluidized bed similar to that in the test
rig used by Link ( 1975 ). It is a Plexiglas model 100 cm in height, 15 cm in width, and
2 cm in depth. The apparatus was loaded with 36,500 glass beads with an average
diameter of 2.5 mm and a density of 2,500 kg/m 3 . A high-speed air jet was supplied
through a centrally placed nozzle with dimension of 1 cm (width) by 2 cm (depth),
while low-speed background air

fl

flow was introduced through the two side panels.
The glass beads used in the experiment fall into the category of Group D particles
based on their size and density according to Geldart

fl

' s powder classi cation ( 1973 ).

Such particles are generally very hard to get fluidized because once the bubbles are

formed, they tend to rise slower than the rest of the gas percolating through the
emulsion and coalesce rapidly as a result. According to Link ( 1975 ), the background
fl

fl

uidize the bed so that prominent gas bubbles can form
when high-velocity central jet is supplied. The top of the domain in the apparatus
was open to the atmosphere. A high-speed camera was used to record the distribution
of the particles with a 1-ms resolution in time. The static pressure was monitored at
four different height locations of z = 2, 12, 22, and 40 cm by pressure sensors. The
experiment was initially loaded with 36,500 glass beads with an average diameter of
2.5 mm and a density of 2,500 kg/m 3 . The TU-Darmstadt experimental apparatus
and the corresponding computational mesh are shown in Fig. 9 .
The computational domain shown in Fig. 9 consists of 11,000 hexahedral cells.
The multiphase simulation is set up with the continuum

flow is necessary to pre- fl

fluid (air) as the primary
phase and the discrete particulate solid (glass beads) as the secondary phase. The
interaction between particles and

fl

fl fluid, that is the two-way exchange of momentum,

is de Brien drag law ( 1989 ). In the simulation, a total of

36,500 particles are released with a small offset in z-direction and start to experience
free fall due to gravity. When the aggregate kinetic energy of the particles becomes
suf

ned by the Syamlal-O '

ciently small, the bed is considered settled and the simulation time is set as
zero. Such an initialization is important since it ensures the random packing of the
particles in the static bed. For the continuous gas phase, the velocity inlet boundary
condition is applied at the bottom panels of the domain, with 22.69 m/s gas velocity
at the center panel and 0.85 m/s gas velocity at the side panels corresponding to the
experimental conditions (Alobaid et al. 2013 ). Pressure outlet boundary condition
with zero gauge pressure is applied to the top surface of the domain. Standard non-
slip boundary condition is applied to the lateral walls. For the discrete phase, all
walls are treated as re

cient of restitution of 0.97. To
ensure numerical accuracy, second-order discretization in both space and time is
applied. Due to large value of the particle spring constant, the particle-tracking time
step is set suf

fl ecting surfaces with coef

ciently small to capture the key particle collisions. The key numerical
parameters are summarized in Table 8 .