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Slide 2
Source: José D. Figueroa, National Energy Technology Laboratory (NETL), USDOE
Chemical Looping Technology Improvements through PI
Current project objectives are to:1. What are the efficiencies for CLC technology integrated within
generating facilities
2. Hydrodynamics - Identify gas-solid handling systems to improveintegration of key solids handling
3. To investigate reactor choice used for oxide particles fluidisation
4. Methods for intensifying reduction and oxidation reactions
(Fluidised Beds might not be the best technology to carry out
solid-gas reactions)
Slide 3
Chemical Looping Combustion
Reactions:
- Fuel reactor
(2n + m)MyOx + CnH2m → (2n + m)MyOx−1 + mH2O + nCO2
- Air reactor
MyOx−1 + 1/2 O2(air) → MyOx + (air : N2 + unreacted O2)
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Aspen Plus
Adequate contact time between fuel and air and solid oxygen carrier to achieve
maximum conversion
Size of the two reactors
Adequate solid inventory
Adequate molar flowrate ratio between air and fuel
Issues considered:
Aim:
To develop a fluidised bed model for CLC inspired by a model proposed for FCC
(to replace the Gibbs reactor model, largely used in the literature of CLC)
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Aspen Plus Implementation
The red-ox reaction of NiO/Ni supported by bentonite is investigated.
The fuel reactant is pure methane and the oxidising agent is air.
Within the fuel reactor the specific reduction reaction is:
CH4 + 4NiO CO2 + 2H2O + 4Ni
While within the riser the specific oxidation reaction is:
O2 + 2Ni 2NiO
The un-reacted core model was applied and the controlling step for both oxidation
and reduction reactions is the kinetic step
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Aspen Plus implementationFuel reactor: bubbling regime
Two phase theory: bubble phase with low content of solids and emulsion phase characterised by
perfect mixing of gas and solids
Reactor axially divided into several sections consisting a PFR representing the gas flow through
the bubbles and a CSTR representing the gas flow through the emulsion
Gas mass transfer between bubble and emulsion phase occurs at the exit of the each stages between
the outlet streams
External calculator block in Excel defines the operating conditions of the feed
External calculator and transfer blocks to implement the mass transfer terms
External FORTRAN subroutines used to implement the reactions8
Aspen Plus implementationAir reactor: fast fluidisation regime
Axial distribution of solid particles
Reactor split into a lower and an upper region, the dense and lean phase respectively
One CSTR models the dense phase
Three CSTRs model the upper region characterised by three different mean void fraction
External calculator block in Excel is used to define the operating conditions of the feed
External FORTRAN subroutines are used to implement the reactions
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Results: Bubbling Bed Model
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Results: Bubbling bed model
Conversion increases with the number of stages
The comparison with data from the literature shows that 5 stages give
the conversion observed
Gibbs reactor does not consider gas by-pass in the bubble phase: this
explains the higher conversion
A sensitivity analysis was carried out to find the minimum solid
inventory to achieve more than 90% in methane conversion; this was
found t be 92.5%.
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Results: Riser Model
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Results: Riser model
The set of parameters to achieve a conversion of solid that allows the
circulations of solid particles between air and fuel reactor in steady state
condition was found:
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Uo [m/sec] DR [m] HR [m] Lm [m] Ks [m/sec] Fair/FCH4
1.79 0.8 3.5 0.25 4.41E-04 1.25
Conclusions
The model takes into account hydrodynamics and kinetics
The main process variables can be estimated (e.g. diameter and height of
the two beds, solid inventory, molar flowrate ratios)
The model shows higher accuracy than a Gibbs reactor (it considers the
gas bypass through the bubble phase)
The system can be implemented into the full power plant system to
calculate the process thermal efficiency and attempt economic analysis
and LCA.
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CDF Studies
CFD studies on the Bubbling bed (Fuel) reactor show the issues in creating a workable bed condition
Slide 15
Increasing particle size (decreasing IPF)
2 s
27.2 s 36.8 s
Exp.
Group A/Bdp=125 micron
17.0 s 39.9 s
Group Bdp=350 micron
2 s20.5 s 32.0 s
Group Ddp=800 micron
2 s
Simulation Exp.Exp. Simulation Simulation
Process Intensification
Gas-Solid fluidised beds remain an attractive method for use in chemical and processing industries. However, there are several intrinsic weaknesses and issues including:
Bubbling – bubble formation cause some gas to bypass fluidised particles, resulting in lower gas-solids contact
Elutriation – eventuating in loss of reactants, occasional pollutants (hence need for cyclone). This is exacerbated in larger scale CFB’s operated in turbulent regime to maintain throughput
Scale-up - does not come easily
Large scale - reactor with tall cyclone leads to low mobility and compactness leading towards higher capital and running costs
Umf – the need to match Umf values for the oxidiser and reducer to ensure effective fluidisation and throughput
Slide 16
Future work
Integration of Aspen Plus with CFD modelling to study the gas bypass and the
whole efficiency of the process
• CLC Process Intensification
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