Numerical simulation of a high temperature CO2 capture

fluidized bed

Carlos Herce 1, Antonio Calabrò

1, Stefano Stendardo

1,2

1.ENEA (Italian National Agency for New Technologies, Energy and Sustainable Economic

Development), Rome, ITALY - carlos.herce@enea.it

2. Joint Research Centre of the European Commission, Le Petten, THE NETHERLANDS.

1. Introduction

Carbon capture and storage technologies are the most hopeful solutions to reduce the CO2

emissions on fossil fuel power plants. Fig. 1 shows an adaptation of the ZECOMIX project

[1], divided in four sections: coal hydrogasification island, steam methane reforming with

water shift unit, carbon capture unit and a power plant based on an hydrogen-fuelled internal

combustion turbine. The most notable aspect of the whole process is the way by which carbon

dioxide is separate from the syngas in the aforementioned carbon capture unit, called

carbonator.

A good looking route seems to be the high temperature Chemical Looping Carbon Capture

(CLCC). In this approach, a porous solid sorbent captures the carbon dioxide by means of a

chemi-sorption process. Once the solid sorbent reaches at its ultimate conversion, it is sent

back to the regeneration step and a high-concentrated CO2 stream is released and sent to final

disposal. Particularly, calcined dolomite is selected as CO2-acceptor due its excellent

experimental performance. This work present a numerical analysis of the operation of the CO2

capture reactor.

2. Computational model

In order to simulate this system some assumptions were done:

• It includes two phases: solid and gas.

• The particles are spherical and uniform in size.

• The particles are assumed inelastic, smooth and mono-dispersed spheres.

• The system is isothermal.

• No gas phase turbulence [2]

Fig. 1. Plant layout

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Processes and Technologies for a Sustainable Energy

2.1. Gas-solid hydrodynamics

The simulation of the fluidized bed was carried out using a commercial CFD code, FLUENT

6.3. This code allows, using multifluid Eulerian model, to solve the mass and momentum

conservation equations for each phase [3]. A Maxwellian velocity distribution is used to the

particles. The kinetic theory of granular flows was used for closure of the solid stress terms,

whilst a drag function [4] serves to calculate the momentum exchange coefficients.

Other constitutive equations are needed to close the granular-phase momentum equation. The

energy dissipation due to the collision of inelastic particles was calculated with a particle

temperature [5] model, including an algebraic formulation of granular temperature [4].

Moreover, the solids shear stress model included the collisional [4], the kinetic [6] and the

frictional [7] parts. 2.2. CO2 capture kinetic model

High temperature CO2 capture with calcium based sorbents is been studied for years [8-11].

Dolomite is a mineral formed of calcium and magnesium carbonates. Such a material presents

a very good performance in the CO2 capture if it undergoes a calcination process and

carbonates are decomposed into their respective oxides. Initially, indeed, dolomite is calcined,

and the carbonates become oxides, releasing CO2 contained in natural way. Calcium oxide

(CaO), not magnesium oxide (MgO), reacts with the CO2 to obtain calcium carbonate

(CaCO3). This reactions can be done in a loop capturing and releasing CO2 in function of the

operation (carbonation and calcinations, respectively). This numerical study is centered in the

carbonation reaction, and in the first cycle, in which the conversion of CaO to CaCO3 is the

maximum. In order to model the kinetics of the carbonation a grain model was selected [12,

13] is:

molkJHsCaCOCOsCaO /178),()( 0

29832 −=∆↔+ (1)

( )

⋅+−

−−−+

−−=

33

,

3/2

,0

1

111

21

)(1

ZXX

XX

D

k

CCXk

dt

dX

aOC

PL

eqAACaO

δ

σ

(2)

3. Simulation method

A two dimensional rectangular and symmetrical section formed by 3000 cells (presented in

Fig. 2), instead of cylindrical domain, is assumed in the simulation for simplicity [14].

An Eulerian-Eulerian description was adopted to resolve the coupling of gas and particulate

phase, and it was modeled by solving a Unsteady-state Reynolds Averaged Navier-Stokes

equations (URANS) along the computational domain. At the walls, no slip conditions were

selected for both phases.

A time step of 0.0001 s with 50 iterations by time step was chosen to achieve to convergence

[15]. The second order QUICK scheme was used to evaluate the convective terms due gives a

better resolution on bubbles because of presents less numerical diffusion than first order

schemes. The pressure-velocity coupling is resolved by means of the phase-coupled SIMPLE

(PC-SIMPLE) algorithm [16]. The residual tolerances were set of 10-3

for each scaled

residual.

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Ischia, June, 27-30 - 2010

Table 1. Simulation Parameters.

Description Value Comment

Particle density (kg/m3) 1550 Calcined Dolomite

Particle density (kg/m3) 2200 Carbonated Dolomite

Particle diameter (m) 1.95 e-4

Uniform

Initial solids packing 0.6 Fixed Value

Maximum solids packing 0.63 Fixed Value

Superficial gas velocity (m/s) 0.2 ~ 10 Vmf

Temperature (°C) 600 Fixed Value

Inlet boundary conditions Velocity Uniform

Outlet boundary conditions Pressure Outlet

Static bed height (m) 0.5 Fixed Value

Bed height (m) 4 Fixed Value

Bed width (m) 1 Fixed Value

Time steps (s) 0.0001 Specified

Convergence criteria 10-3

Specified

The bed was assumed to be at minimum fluidization condition at the beginning of the simulation.

The initial solid packing (εs) was 0.6 and the maximum value was set at 0.63. The particles’ bed

is 0.5 m high. The value of restitution coefficient between particles was 0.9 [17], which present a

realistic behavior of Geldart B particles. Table 2 present lists the main conditions used in the

simulation.

Fig. 2 Schematic of the reactor with CFD boundary conditions (a) and grid (b)

Table 2. Gas phase compositions.

Zone H2 CO2 CH4 CO H2O

SMR Reactor Inlet 12.8 4.8 12.4 8.5 61.5

Carbonation Reactor Inlet 56.6 20.5 0 0 22.9

Outlet 70.3 0 0 0 29.7

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Processes and Technologies for a Sustainable Energy

Fig. 3 Solid volume fraction with time

The carbonation reaction presents a three well-defined zones behavior. During the first stage

the chemical kinetics is the limiting reaction whereas in the third one the diffusion through the

particle is the controlling step and the reaction rate decreases strongly; between these two

periods a transition zone, named breakthrough period, was found where both surface reaction

and diffusional resistance are the controlling mechanisms of the whole CO2 capture process.

Fig. 3 (where blue represents pure gas and red dense gas-solid mixture) shows the distribution

of solid volume fraction along the domain for different times. The bubble formation is well

captured by CFD simulations. Fig.3 also shows a symmetric behavior due to the grid which

simulates half reactor, as well as a chaotic transient generation of bubbles. Moreover, the

mono-size particle approximation ignores the possible particle segregation. Then any

phenomena transporting denser particles to the bottom of the reactor are considered

negligible.

This computational simulation is centered in the first 30 seconds of the operation. In this

operational period diffusion effects are negligible and we can simplify the kinetic scheme,

taking account only the kinetically controlled regime. With this in mind, heterogeneous

carbonation reaction takes place with a reaction rate shown by (3).

( ) )(1 ,

3/2

,0 eqAACaO CCXkdt

dX−−⋅=σ (3)

The CO2 in the gas phase reacts with CaO producing CaCO3, both in the solid phase. The

maximum conversion which dolomite can achieve is function of the composition and also the

cycle of operation. This simulation is centered in the first capture cycle. Hence, both reaction

rate and solid volume fraction are interconnected. In addition, the bubbles produce intense

oscillations in the dense region, which produce the chemical species mole fraction

oscillations. Simulations show a good solid back-mixing ensuring a continuous replacement

of spent solid sorbent (carbonated dolomite) with fresh solid sorbent (calcined dolomite) in

the bottom of the reactor. It worth noting that such solid circulation permits a CO2 uptake at

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Ischia, June, 27-30 - 2010

the reactor inlet. Another important aim of this study is the application of a grain model to a

fluidized environment. In order to evaluate it the solid conversion of calcined dolomite was

studied as a function of time.Fig. 4 shows the conversion of calcined dolomite as a function of

time for different bed heights (0.1, 0.2, 0.3 and 0.5m) (Fig.4).

The trends of conversion are consistent with another carbonation studies [18] for analyzed

operating conditions. The gas and solid phase into the fluidized bed are simulated at 600°C as

well as the gas entering the reactor. Due to the small particle diameter (195 µm) and the

intrinsic bed fluidization we can consider the solid phase isotherm.

0

0,001

0,002

0,003

0,004

0,005

0,006

0,007

0,008

0,1 0,15 0,2 0,25 0,3 0,35 0,4 0,45 0,5

HEIGHT BED Y (m)

CONVERSIO

N (-) ..

t = 1 sec

t = 3.5 sec

t = 8.5 sec

t = 15 sec

t = 20 sec

t = 27 sec

t = 30 sec

Theory

Fig. 4. CaO conversion along the bed height with time.

5. Conclusions

A high temperature chemical looping carbon capture was investigated in a two dimensional,

bubbling fluidized bed reactor

In order to carry out the numerical study chemical kinetics were incorporated into a

commercial CFD code. An Eulerian-Eurelian description was assumed to solve the governing

equations in the continua in transient state.

Under the operational conditions, the numerical simulations shows a good coupling between

bed hydrodynamics and heterogeneous reactions.

Thus, this study could be a first step to the scale-up of high temperature CO2 capture systems

with solids sorbents, and it could also serve to design the experiments. However, the future

efforts will be done to reduce the impact of some hypothesis in the solution (e.g. isothermal

system, monotype particles, little time steps, coupling of different kinetic schemes).

Nomenclature

C molar concentration (kmol/m3)

d diameter (m)

D diffusion coefficient (m2/s)

f volume fraction

H enthalpy (kJ/mol)

k kinetic constant for surface reaction (m/s)

V axial velocity (m/s)

X conversion

Z molar volume ratio VCaCO3 / VCaO

Greek symbols

δ average grain diameter (m)

ε volume fraction

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Processes and Technologies for a Sustainable Energy

ε0 particle porosity

σ grain surface area per unit particle volume (m)

Subscripts and superscripts

A carbon dioxide

eq equilibrium

g gas phase

mf minimum fluidization

p particle

PL product layer

s solid phase

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