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.