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Calcination rate of limestone under regenerator conditions of
Ca-L process
T. Shimizu, S. Furukawa, H.-J. Kim, L.Y. Li Niigata University, Japan
Background
CO2 capture and storage(CCS) Removal of CO2 from the flue gas followed by geostorage
Global warming caused by the increase in CO2 concentration in the atmosphere
CO2 capture processes • Flue gas wet scrubbing
Heat demand for regeneration of amine solution is large.
• Oxyfuel combustion Power consumption to separate oxygen
from nitrogen is large. • Chemical Looping Combustion (CLC) Existing power plant cannot be used. • Calcium Looping CO2 capture (CaL) Existing power plant is used by adding
ASU and CaL.
Calcium-Looping (CaL) process CaL process consists of a carbonator (CO2 absorber) and a CaO regenerator. In the regenerator, CaCO3 is decomposed by heat.
Carbonator Regenerator
CaCO3
CO2, H2O
CaO
Fuel O2CO2Flue gas
CO2-free gas
(CO2 10 - 15%)
CaO+CO2 CaCO3→
→CaCO3 CaO+CO2
Design of regenerator For the design of regenerator (calciner), i.e., determination of solid residence time, CaCO3 decomposition rate is necessary.
Temperature: about 950℃ CO2 concentration: nearly 100%
High heat transfer rate Small particles: < 1 mm
Requirements Measurement of CaCO3 decomposition rate
Objective of this work
TGA
Measurement of CaCO3 decomposition rate
Change in reactant (e.g. mass change)
• Accurate measurement of mass change
• Slow heat transfer from heater to CaCO3
• Difficult to attain 950℃, CO2100% condition
Principles of the rate measurement
Principle of rate measurement
Change in product (e.g. CO2 formation rate) ◆Dilution of produced gas by diluent followed by measurement of CO2 concentration
◆Direct measurement of CO2 flow rate
CO2 formation
Diluent feed
CO2 feed
Time
Principles of CO2 formation rate measurement
CO2 concentration measurement by NDIR
IR Det.
In Out It takes relatively long time to replace gas in the absorption cell. Slow response
Principles of CO2 formation rate measurement
Thermal mass-flow sensor Heater
Temp. Difference
For the case of constant gas composition, good linearity and relatively fast response
Flow
Comparison of response Concentration
Flow rate Mass-flow sensor The response of mass flow sensor was fast (90% response in 5 s).
mass flow sensorCO2 analyzer
NDIR analyzer
Experimental
Experimental apparatus
CO2
TC
⊿PLimestone
HeaterQS
Mass flow sensor
Filter
CO2
TC
⊿PLimestone
HeaterQS
Mass flow sensor
Filter
Quartz fluidized bed I.D.:26 mm Bed height:80 mm BM: Quartz sand
Mass flow sensor
A batch of CaCO3 (1 g) was injected to the bed fluidized by CO2. The flow rate of CO2 was measured at the exit.
Experimental conditions
BM: Quartz sand, 96 μm and 148 μm CO2 flow rate :0.24 NL/min 0.54 NL/min (about 5Umf at 1223 K) Limestone size:350~420 μm
CaCO3 MgCO3 SiO2 Al2O3 Fe2O3
96.9 1.4 0.6 0.8 0.3
Analysis of Chichibu limestone (wt.%)
Bed temp (initial):1223±5K 1243±5K 1263±5K
Gas:CO2 100%
Results and discussion
Bed temperature after limestone feed
1210
1220
1230
1240
1250
1260
1270
1280
0 10 20 30 40 50 60Time after charging limestone [s]
Tem
pera
ture
[K]
Initial temperature 1223KInitial temperature 1243KInitial temperature 1263K
Constant temperature was attained except for initial 10 s after injection.
Increment of CO2 flow rate
0.000
0.004
0.008
0.012
0 50 100 150 200Time after charging limestone [s]
Prod
uced
CO 2
flow
[L/s] Average temperature 1216K
Average temperature 1233KAverage temperature 1251K
Time scale of CO2 formation was sufficiently longer than the time scale of sensor’s response.
Final conversion of CaCO3 to CO2
Final conversion of CaCO3 was higher than 93%. Total amount of produced CO2 was calculated by integrating formation profile.
Final conversion QS average size
96μm 148μm Average temperature
[K] Final conversion [-] [-]
1216 1233 1251
0.930 0.988 0.987
0.945 0.964 0.972
Change in conversion (X) with time
0.0
0.2
0.4
0.6
0.8
1.0
0 50 100 150Time [s]
Con
vers
ion
ratio
X [-
]
QS:average particlediameter of 96μm
QS:average particlediameter of 148μm
0.0
0.2
0.4
0.6
0.8
1.0
0 50 100 150Time [s]
Conv
ersio
n ra
tio X
[-]
QS:average particlediameter of 96μmQS:average particlediameter of 148μm
0.0
0.2
0.4
0.6
0.8
1.0
0 50 100 150 200 250Time [s]
Conv
ersio
n ra
tio X
[-]
QS:average particlediameter of 96μmQS:average particlediameter of 148μm
Only minor influence of bed material size on rate of conversion change was observed.
T=1216K
T=1233K T=1251K
Apparent reaction order
A straight line relationship between ln(1-X) and time:
dX/dt=k(1-X)
-1.6
-1.4
-1.2
-1.0
-0.8
-0.6
-0.4
-0.2
0.0
0 30 60 90 120 150 180
Time after charging limestone [s]
ln(1-
X) [-
]Average temperature 1216KAverage temperature 1233KAverage temperature 1251K
k:rate const. [s-1]
Slope: k
Assumed reaction rate expression
−×=
TPeq
20474exp10137.4 7
Driving force of CaCO3 decomposition: Difference between equilibrium pressure (Peq ) and CO2 partial pressure around the particle (P).
dX/dt=k’(Peq-P)(1-X).
The decomposition rate is assumed to be given by the driving force (Peq- P), unreacted fraction of the solid (1 – X), and rate constant (k’), as:
Determination of activation energy
For whole temperature range: Ea=193kJ/mol (within the range of literature value)
-4.5
-4
-3.5
-3
-2.5
0.00079 0.0008 0.00081 0.00082 0.00083T-1 [K-1]
ln(k
/(Peq
-P))
[-]
Run 1Run 2Run 3
Conclusion • A method to measure CaCO3
decomposition rate under Ca-L process conditions (in fluidized bed) was proposed.
• The reaction order with respect to solid conversion and activation energy were determined.
• Bedmaterial size had only minor influence of decomposition rate.