2 Dynamic Modelling of Reactive Fluidized Bed Systems Using …
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of bound oxygen directly, which then reacts with a solid fuel. This reaction route
is called Chemical Looping with Oxygen Uncoupling (CLOU). This work focuses
entirely on iG-CLC.
The simulation of the CLC process was conducted with a variety of methods
and approaches. One typical method is Computational Fluid Dynamics (CFD). In
CFD a high amount of equations is solved on a dense numerical grid which can be
two or three dimensional. In contrast to this, flowsheet simulation describes macroscopic effects in the individual units with empirical models. In comparison to CFD,
the operation of a whole process system can be quickly simulated regarding long
term effects. Flowsheet modelling approaches for CLC were conducted by various
research groups mainly with Aspen Plus [1–4] but also with MatLab/Simulink [5, 6]
and IPSEpro [7]. A summary of the mentioned works can be found in Haus et al. [8].
Other mathematical modeling on macro-scale are summarized by Adanez et al. [9].
In all the mentioned research, only steady state operations were simulated. Often,
the modeling of CLC concentrates on the FR since it is the most crucial part in the
process. The reactions in the AR are mostly disregarded, due to a fast and complete
re-oxidation of the OC. Also, the conversion of carbon, which slipped from the FR to
the AR can be considered as complete. A fluid mechanical macroscale model for
a fluidized bed reactor for flowsheet simulation was implemented into the SolidSim
environment by Puettmann et al. [10]. The basics of this approach were used in the
ASPEN Plus 8.4 package of ASPEN Tech. Prior research at TUHH has shown, that
this model cannot be used directly for CLC processes.
The CLC process is characterized by the complex transient behavior between the
process units due to the big holdup of the reactive units. This is disregarded in most
CLC modeling approaches since most works are focused on the FR. Often artificial
in- and output streams are used for every unit to mimic the effect of solid circulation.
These model systems cannot predict the dynamic process behavior, since e.g. the
solid circulation rate is not calculated but assumed as input value.
In the presented work, an entire CLC system, consisting of fluidized bed reactors,
loops seals and a cyclone was implemented into DYSSOL. The process network is
described with empirical and semi-empirical correlations. With the models, longterm behavior as well as process variations, such as start-up, shut-down and fuel load
changes, can be simulated. Due to the complexity of the whole system with numerous
gas solid reactions, the modeled fluidized bed reactor unit was advanced step-by-step.
First, the focus was on the fluid dynamics of the interconnected system [11]. In a second step, the dynamic reactions of the OC with gaseous fuels were implemented [8].
Afterwards, the volatile gasification was added into the model. In this way, the combustion of biomass was simulated, by neglecting the char content [12]. In a last step,
the focus is on the conversion of high-carbon fuels. Additionally, to the OC, char was
introduced as a second reactive solid to the system. All simulations were compared
to experiments on a 25 kWth CLC pilot plant with the respective fuel. The measured
gas concentrations in the pilot plant’s off-gas were used to fit the kinetic data to the
simulations. In the following, the experimental facility and flowsheet are explained
and afterwards all modeled units are described in detail.
41
of bound oxygen directly, which then reacts with a solid fuel. This reaction route
is called Chemical Looping with Oxygen Uncoupling (CLOU). This work focuses
entirely on iG-CLC.
The simulation of the CLC process was conducted with a variety of methods
and approaches. One typical method is Computational Fluid Dynamics (CFD). In
CFD a high amount of equations is solved on a dense numerical grid which can be
two or three dimensional. In contrast to this, flowsheet simulation describes macroscopic effects in the individual units with empirical models. In comparison to CFD,
the operation of a whole process system can be quickly simulated regarding long
term effects. Flowsheet modelling approaches for CLC were conducted by various
research groups mainly with Aspen Plus [1–4] but also with MatLab/Simulink [5, 6]
and IPSEpro [7]. A summary of the mentioned works can be found in Haus et al. [8].
Other mathematical modeling on macro-scale are summarized by Adanez et al. [9].
In all the mentioned research, only steady state operations were simulated. Often,
the modeling of CLC concentrates on the FR since it is the most crucial part in the
process. The reactions in the AR are mostly disregarded, due to a fast and complete
re-oxidation of the OC. Also, the conversion of carbon, which slipped from the FR to
the AR can be considered as complete. A fluid mechanical macroscale model for
a fluidized bed reactor for flowsheet simulation was implemented into the SolidSim
environment by Puettmann et al. [10]. The basics of this approach were used in the
ASPEN Plus 8.4 package of ASPEN Tech. Prior research at TUHH has shown, that
this model cannot be used directly for CLC processes.
The CLC process is characterized by the complex transient behavior between the
process units due to the big holdup of the reactive units. This is disregarded in most
CLC modeling approaches since most works are focused on the FR. Often artificial
in- and output streams are used for every unit to mimic the effect of solid circulation.
These model systems cannot predict the dynamic process behavior, since e.g. the
solid circulation rate is not calculated but assumed as input value.
In the presented work, an entire CLC system, consisting of fluidized bed reactors,
loops seals and a cyclone was implemented into DYSSOL. The process network is
described with empirical and semi-empirical correlations. With the models, longterm behavior as well as process variations, such as start-up, shut-down and fuel load
changes, can be simulated. Due to the complexity of the whole system with numerous
gas solid reactions, the modeled fluidized bed reactor unit was advanced step-by-step.
First, the focus was on the fluid dynamics of the interconnected system [11]. In a second step, the dynamic reactions of the OC with gaseous fuels were implemented [8].
Afterwards, the volatile gasification was added into the model. In this way, the combustion of biomass was simulated, by neglecting the char content [12]. In a last step,
the focus is on the conversion of high-carbon fuels. Additionally, to the OC, char was
introduced as a second reactive solid to the system. All simulations were compared
to experiments on a 25 kWth CLC pilot plant with the respective fuel. The measured
gas concentrations in the pilot plant’s off-gas were used to fit the kinetic data to the
simulations. In the following, the experimental facility and flowsheet are explained
and afterwards all modeled units are described in detail.