2 Dynamic Modelling of Reactive Fluidized Bed Systems Using …
47
Here, A r is the cross-sectional area and h max the total reactor height. The fluidized
bed reactor model can have two exits: A top exit and an exit over an overflow
weir/standpipe. With all inlet and outlet streams, a new reactor inventory is calculated
for each time step:
dm r
dt
= ˙
m solids,inlet − ˙
m elutriation, f reeboard − ˙
m over f low
(13)
For a CFB riser, ˙
m overflow is set to 0 because all particles are leaving the reactor
together with the gas via elutriation. The number of elutriated particles depends on
the solid concentration at the reactor top and the gas velocity. In a bubbling bed
reactor, most particles are discharged via solid overflow and only little elutriation
takes place. The particle flow over the weir is defined with a correlation by Botsio
and Basu [21]:
˙
m over f low = c v,H b · ρ s ·
2
3
· C · W ·
2g · (H b − H w )
2/3
(14)
The parameter C is the weir coefficient, W is the width of the weir, H w describes the
height of the weir and H b the height of the bubbling bed.
Chemical Reactions
As it is described in the fluid mechanics section, the reactor model is subdivided into
a freeboard zone and a bottom zone with a solid free bubble phase and suspension
phase. Figure 3 (right) illustrates the modeled phases as well as the input and output
streams. Heterogenous gas-solids reactions occur only in the suspension and dilute
freeboard phase. In the model, the solids are assumed to be perfectly mixed in the
whole reactor. The gases rise from the distributor without any back mixing and are
modeled as in a plug flow reactor.
In iG-CLC, the solid fuel is directly fed into the fuel reactor (FR), where it dries,
devolatilization takes place and later the char is gasified. The generated fuel gases
then react with the OC. The reaction mechanism is depicted in Fig. 4 as described
previously by Adanez et al. [9] and Lyngfelt et al. [22].
The modeled devolatilization time is assumed to be immediate and thus independent from the atmosphere as well as other gas concentrations. The devolatilization
of the fuel occurs in the dense bed zone of the fuel reactor. An equal distribution of
the volatile gases through the whole bed height is assumed [23]. This gas mixture
consists of H 2 , CO, CO 2 , CH 4 , H 2 O and N 2 including the volatiles, and the fuel
moisture.
Via the same inlet stream, solid char can be added to the system. The char and
the oxygen carrier are assumed to be perfectly mixed. Further, the modeled fuel is
assumed to be ash and tar free. The watergas-shift reaction and other homogenous gas
reactions are not regarded in the model. Furthermore, higher hydrocarbons, nitrogen
47
Here, A r is the cross-sectional area and h max the total reactor height. The fluidized
bed reactor model can have two exits: A top exit and an exit over an overflow
weir/standpipe. With all inlet and outlet streams, a new reactor inventory is calculated
for each time step:
dm r
dt
= ˙
m solids,inlet − ˙
m elutriation, f reeboard − ˙
m over f low
(13)
For a CFB riser, ˙
m overflow is set to 0 because all particles are leaving the reactor
together with the gas via elutriation. The number of elutriated particles depends on
the solid concentration at the reactor top and the gas velocity. In a bubbling bed
reactor, most particles are discharged via solid overflow and only little elutriation
takes place. The particle flow over the weir is defined with a correlation by Botsio
and Basu [21]:
˙
m over f low = c v,H b · ρ s ·
2
3
· C · W ·
2g · (H b − H w )
2/3
(14)
The parameter C is the weir coefficient, W is the width of the weir, H w describes the
height of the weir and H b the height of the bubbling bed.
Chemical Reactions
As it is described in the fluid mechanics section, the reactor model is subdivided into
a freeboard zone and a bottom zone with a solid free bubble phase and suspension
phase. Figure 3 (right) illustrates the modeled phases as well as the input and output
streams. Heterogenous gas-solids reactions occur only in the suspension and dilute
freeboard phase. In the model, the solids are assumed to be perfectly mixed in the
whole reactor. The gases rise from the distributor without any back mixing and are
modeled as in a plug flow reactor.
In iG-CLC, the solid fuel is directly fed into the fuel reactor (FR), where it dries,
devolatilization takes place and later the char is gasified. The generated fuel gases
then react with the OC. The reaction mechanism is depicted in Fig. 4 as described
previously by Adanez et al. [9] and Lyngfelt et al. [22].
The modeled devolatilization time is assumed to be immediate and thus independent from the atmosphere as well as other gas concentrations. The devolatilization
of the fuel occurs in the dense bed zone of the fuel reactor. An equal distribution of
the volatile gases through the whole bed height is assumed [23]. This gas mixture
consists of H 2 , CO, CO 2 , CH 4 , H 2 O and N 2 including the volatiles, and the fuel
moisture.
Via the same inlet stream, solid char can be added to the system. The char and
the oxygen carrier are assumed to be perfectly mixed. Further, the modeled fuel is
assumed to be ash and tar free. The watergas-shift reaction and other homogenous gas
reactions are not regarded in the model. Furthermore, higher hydrocarbons, nitrogen