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Performance of a Bench Scale Fast Fluidized Bed Carbonator Sharat Kumar Pathi, Weigang Lin, Jytte Boll Illerup, Kim Dam-Johansen, and Klaus Hjuler Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/ef500572b • Publication Date (Web): 01 Jul 2014 Downloaded from http://pubs.acs.org on July 5, 2014
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Performance of a Bench Scale Fast Fluidized Bed Carbonator 1
Sharat K. Pathi 1
1
1
Jytte B. Illerup*
Kim Dam-Johansen 2
1
Weigang Lin
Klaus Hjuler
Department of Chemical and Biochemical Engineering, Technical University of Denmark, Building 229, DK-2800 Kgs. Lyngby, Denmark 2
FLSmidth A/S, DK-2500 Valby, Denmark
*Corresponding author: Jytte Boll Illerup; e-mail:-
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ABSTRACT The carbonate looping process is a promising technology for CO2 capture from flue gas. In this process the CO2 capture efficiency depends on the performance of a carbonator that may be operated as a Circulating Fluidized Bed (CFB). In this paper, the carbonator performance is investigated by applying a new experimental method with accurate control of the particle re-circulation rate. The experimental results show that the inlet calcium to carbon molar ratio is the main factor on the CO2 capture efficiency in the carbonator, i.e. increasing the inlet Ca/C from 4 to 13 results in increasing the CO2 capture efficiency from 40 to 85% with limestone having a maximum CO2 capture capacity of only 11.5%. Furthermore, a reactor model for carbonator is developed based on the Kunii-Levenspiel’s model. A key parameter in the model is the particle distribution along the height of the reactor, which is estimated from experiments under stable operating conditions with constant bed inventory, reactor temperature and exit CO2 concentration. The validated CFB carbonator model was used to simulate different operating conditions relevant for CO2 capture from a power plant and for a cement plant. The results show that particle re-circulation rates of 2-5 kg/(m2s) or ratio of bed inventory to re-circulation rates of 70-176 s are sufficient for attaining 90% CO2 capture efficiency depending on the inlet Ca to C ratio.
KEYWORDS: CCS, CO2 Capture, CFB carbonator, Carbonate Looping, Cement plant, Power plant
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Nomenclature a [1 / m ] :
Solids distribution decay constant
At [m2 ] :
Cross sectional area of reactor
CCO2 ,in [kmol / m3 ] :
CO2 inlet concentration
CCO2 ,eq [kmol / m3 ] :
CO2 equilibrium concentration
CCO2 ,exit [kmol / m3 ] : CO2 exit gas concentration CCO2 ,d [kmol / m3 ] :
CO2 concentration in the dense bed
CCO2 ,l [kmol / m3 ] :
CO2 concentration in the freeboard
d p [mm] :
Particle size
D [m ] :
Reactor diameter 0.06 m
Ecarb,s [−] :
CO2 capture efficiency from solid conversion
Ecarb,g [−] :
CO2 capture efficiency from gas conversion
FCaO,in [kmol / s] :
Flow rate of CaO into carbonator
FCaO,out [kmol / s] :
Flow rate of CaO out of carbonator
FR [kmol / s] :
Flow rate of Ca into carbonator
Gs [kg / m2 ⋅ s] :
Particle re-circulation rate
hl [ m ] :
Location in the free board region
H d [m] :
Height of dense bed
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H l [m ] :
Height of free board region
H t [m] :
Reactor height
k [ −] :
Sorbent decay constant
kcw [1 / s] :
Gas transfer co-efficient
k g [m / s ] :
Gas transfer constant
ks [m4 / kmol ⋅ s] :
Surface rate constant
kr [m3 / kmol ⋅ s] :
Effective rate constant
K r [1 / s ] :
Reactor rate constant
M i [kg / kmol ] :
Molar mass of component ‘i’
N [ −] :
Cycle number
R [ kJ / kmol ⋅ K ] :
Gas constant
So [m2 / m3 ] :
Initial specific surface area
Save [m2 / m3 ] :
Average specific surface area
t[ s ] :
Time
T [K ] :
Temperature
uo [ m / s ] :
Gas velocity
xCaO [−] :
Fraction of CaO in limestone
xCO2 ,in [vol.%] :
CO2 concentration in the inlet gas
xCO2 ,outlet [vol.%] :
CO2 concentration in the exit gas
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X or X carb [ −] :
Average bed conversion
X cal [ − ] :
Fraction of CaCO3 in calcined limestone
X r [−] :
Residual CO2 capture capacity
X max [−] :
Maximum CO2 capture capacity
W [ kg ] :
Bed inventory
δ cd [ − ] :
Bubble or core region in dense bed
ε mf [−] :
Void fraction under minimum fluidization (0.4)
ε s [−] :
Solids fraction
ε sd [− ] :
Solids fraction in the dense bed
ε s*[−] :
Solids fraction under saturating gas carrying
ε sl [−] :
Solids fraction in the freeboard
ε se [ − ] :
Solids fraction at the reactor exit
ρ [kg / m3 ] :
calcined limestone density
η[−] :
Contact efficiency between gas and particles
ηd [ − ] :
Contact efficiency in the dense bed
ηl [− ] :
Contact efficiency in the freeboard
φCO ,in [ NL / min] :
Inlet CO2 flow rate
φCO ,out [ NL / min] :
Outlet CO2 flow rate
φt ,in [ NL / min] :
Fluidizing gas flow rate
CaO
2
2
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1. Introduction One of the emerging technologies for CO2 capture from flue gas is the Carbonate Looping Process (CLP)1. The calcium based sorbent material is continuously circulated between two reactors, one for capturing CO2 from flue gas and the other for regeneration of the sorbent along with releasing CO2 gas 2-12
. The CLP is considered to have a large potential as an alternative to wet chemical processes
(amines), especially in terms of a lower energy penalty and corrosive material3. Other advantages of CLP are that abundantly available natural limestone can be used as sorbent, and that high-temperature streams are available for energy efficient co-generation. So this process is of special interest for combining CO2 capture simultaneously for power and cement production 13-15. Different aspects of the carbonate looping process has been investigated, such as particle scale studies12,
16, 17
, reactor scale studies18-21, process integration studies1,
6, 7, 14, 22
and techno-economic
evaluations23, 24. Using a thermo-gravimetric analyzer (TGA), the conversion of CaO to CaCO3 has been studied as function of the carbonation temperature, the inlet CO2 concentration, the calcination conditions, the type of limestone and the looping cycle number.12,
25-30
. However, these results are
difficult to apply directly to scaling up because the flow conditions in industrial scale reactors are completely different from the ideal conditions tested in TGA. A number of experimental facilities have been established recently in scales ranging from 10 kWth to 1.7 MWth as summarized in Table 1 (single fluidized beds) and Table 2 (dual fluidized beds). The reactor level studies have shown that the performance of the carbonator determines the overall efficiency of the carbonate looping process. The two main factors are 1) the maximum CO2 capture capacity of the calcined limestone, i.e. a particle structure related parameter31 and 2) the operating conditions of the carbonator. The particle scale studies were useful in determining the reaction rate which can be used in reactor models for simulating
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the influence of operating parameters. A brief literature review on the experimental work with single and dual fluidized bed reactors, as well as, the carbonator models is summarized as follows.
1.1 Single Fluidized Bed Results The setup with a single fluidized bed is normally used for cycling experiments. Table 1 summarizes the experimental conditions applied to investigate the carbonate looping process in single fluidized bed reactors along with reactor dimensions and scales. The general trend of the CO2 capture capacity as a function of cycle number is similar to TGA results. The results of cycle experiments in large pilot scale facility operated as a bubbling fluidized bed reactor by Galloy et al.20 indicated the reliability of the carbonate looping process for industrial applications. However, for industrial applications fast fluidized bed reactors with particle recirculation are probably more relevant for continuous operation with a lower footprint, and a more efficient mixing of particulates. Table 1: Cyclic experiments in a single fluidized bed reactor
INCAR KIER CANMET TUD
Hbed (HT)
D
Scale
dp
Calcination
Carbonation
[m]
[m]
[kWth]
[mm]
1 (5)
0.1
75
0.65-1.68
850°C, Air
650°C,15%CO2
0.2 (1.2)
0.1
n/a
0.35-0.6
850°C, Air
700°C,16%CO2
1 (5)
0.1
75
0.65-1.67
850°C,Air
700°C,15%CO2
1.2 (8.7)
0.6
470
0.43*
800°C,Air
700°C,15%CO2
Ref.
Abanades et al.18 Ryu et al.32 Salvador et al.33 Galloy et al. 20
*:Mean particle size; H is height of dense bed and H is total height of reactor. bed T
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1.2 Dual Fluidized Bed Results The dual fluidized bed systems are used for investigating the carbonate looping process in continuous operation. Table 2 summarizes the dual fluidized bed facilities in open literature. The results in these studies show that the most important parameter in the process is the particle recirculation rate which was controlled by different methods. Some of these studies were carried out for parametric investigations while others for demonstrating the industrial applicability of the carbonate looping process. Table 2: Looping experiments in dual fluidized bed reactors Scale Particle recirculation rate
Hbed (HT)
D
Ref.
[kWth]
[kg/m2.s]
[m]
[m]
CANMET
75
n/a
1 (5)
0.1
Lu et al.19, 34
OSU
120
n/a
n/a
n/a
Wang et al.35
Tsinghua
10’s
0.8
0.3 (1)
0.15
Fan et al.36
IFK
10’s
