A paper in the proceedings of a conference on fluidization 9 19–20 November 2014, Johannesburg, South Africa

IFSA 2014, Industrial Fluidization South Africa: 9–33. Edited by A. Luckos & B.C. North Johannesburg: Southern African Institute of Mining and Metallurgy, 2014

The future of Ca looping – A review of developments

D. Hanak, V. Manovic, E.J. Anthony Combustion and CCS Center, Cranfield University, Bedford, United Kingdom

Keywords: Ca looping, CO2 capture, pilot plant research

Abstract—CCS technology is essential if we are to meet goals of keeping CO2 levels at or below 450 ppm. Unfortunately, while current solutions in particular amine scrubbing offer are evidently technically feasible, there is a major need to reduce the cost and energy losses associated with CCS. One possible option is a second generation technology called Ca looping. This paper analyses current pilot plant research in order to provide an overview of where research is currently going and what yet remains to be done.

INTRODUCTION Novel CCS technologies, which avoid the expense and efficiency reductions associated with amine scrubbing, are urgently needed. A promising alternative to both oxy-combustion and solvent absorption is a second generation CO2 capture technology called calcium looping (CaL) that uses a calcium-based solid sorbent. Here, the main aim of this paper is to review recent developments in calcium looping technology for CO2 capture.

THE CALCIUM LOOPING PROCESS Use of calcium-based sorbent for CO2 absorption was patented in 1933.1,2 However, the calcium looping (CaL) process for post-combustion CO2 capture technology was first proposed by Shimizu et al.3 in 1999 (see Figure 1).

In the CaL process the flue gas from the air-blown boiler is fed directly to the carbonator and CO2 is removed from the flue gas stream in the form of solid CaCO3 particles.3 This process proceeds at a reasonably fast conversion rate for practical application2,4,5, but CO2 removal efficiency decreases rapidly with increasing temperature and ceases at ~775°C as the equilibrium partial pressure of CO2 exceeds the partial pressure in the flue gas above ~775°C.6 The optimal carbonator operating temperature ranges between 580°C and 700°C due to the trade-off between the reaction kinetics and the equilibrium driving forces.7,8

The CaCO3 is sent to another fluidised-bed reactor, the so-called calciner, in which it is calcined and CO2 is released.2 Calcination is conducted above 900°C to achieve rapid reaction without excessive sintering.9 As calcination is an endothermic, additional fuel must be burned in the calciner using O2.3,10,11 The calcination step consumes approximately 35% to 50% of the overall heat input to the system.12

Technology status There are numerous demonstrations of the technology worldwide: IFK (University of Stuttgart, 10 kWth), CanmetENERGY (75 kWth), La Pereda (CaOling project, 1.7 MWth), and Industrial Technology Research Institute Hsinchu (1.9 MWth) among others.13,14

10

Figure 1. Conceptual design of CaL process system for CO2 capture

Romano15 describes this process as similar to oxy-combustion technology but where only

some portion of coal is burned in pure oxygen environment. The main advantages of the CaL system over the solvent-based CO2 capture technologies are:

• Availability of a large amount of recoverable high-grade heat able to generate an additional high-pressure steam to produce surplus power output.10

• An efficiency penalty of 7% to 8%15, with the CO2 capture stage accounting for 2 to 3%, mainly due to the reduced oxygen requirement.16 This is comparable to the energy penalty of flue gas desulphurisation units (0.5-4%).17

• Application of the fluidised bed reactors, has been commercially proven for coal combustion systems.

• Natural limestone or dolomite as the CO2 absorber which is cheap5, environmentally benign18, and has a high CO2 absorption capacity.

Nevertheless, reduction in the sorbent CO2 carrying capacity19 on cycling operation, due to sintering, attrition and sulfation, appears to be the major challenge of this technology, and require a considerable amount of spent sorbent to be replaced, although reuse of this material reused in the cement production industry, might provide benefits in terms of reducing the CO2 production from both the power and cement industries.18

REVIEW OF CALCIUM LOOPING LABORATORY AND PILOT-SCALE EXPERIMENTS

A number of the more important pilot plant units are described in (Table 1).

Industrial Technology Research Institute The Industrial Technology Research Institute (ITRI) in Taiwan has developed a bench-scale 1 kWth pilot plant, which can be operated in either a batch or continuous mode (Table 2).20

The pilot plant developed at the ITRI was then modified by substituting the MB calciner with an air-fired rotary kiln calciner (RK) (Figure 1). This increased the pilot plant capacity to 3 kWth. The calciner has a length-to-diameter ratio of 18.5 and an inclination angle of 5°, based on the operating experience from the cement industry (Table 3). Such design corresponds to a residence time of approximately 30 minutes at a rotation speed of 1 rpm. Furthermore, liquefied petroleum gas is directly fired in the calciner using a 58 kWth burner. The gas enters the BFB carbonator through a perforated plate distributor composed of 96 holes of 1.5 mm in diameter. Although the carbonation reaction is exothermic, the carbonator was heated using an external heating system to balance the heat losses to the environment.14

Tabl

e 1.

Rev

iew

of d

esig

n an

d op

erat

ing

cond

ition

s of l

abor

ator

y- a

nd p

ilot-s

cale

cal

cium

loop

ing

faci

litie

s

Rese

arch

cen

tre

Size

(k

Wth

)

Car

bona

tor

C

alci

ner

Inle

t CO

2 co

nten

t (v

ol%

)

Max

. CO

2 ca

ptur

e le

vel

(%)

Type

* D

iam

eter

(m

) H

eigh

t (m

) Te

mpe

ratu

re

(°C

)

Type

* D

iam

eter

(m

) H

eigh

t (m

) Te

mpe

ratu

re

(°C

) In

dust

rial

Te

chno

logy

Re

sear

ch In

stitu

te

1 BF

B 0.

1 2.

560

0–70

0

MB

0.05

0.9

800–

900

12.5

99

3 BF

B 0.

1 2.

560

0–70

0

RK

0.27

550

0–10

0012

.599

1900

BF

B 3.

3 4.

265

0

RK

0.9

550

0–10

0012

.1–1

4.5

N/A

INC

AR-

CSI

C

30

CFB

0.

1 6.

556

8–72

2

CFB

0.

16

800–

1000

15–1

690

17

00

CFB

0.

65

1560

0–71

5

CFB

0.

7515

820–

950

12.5

90

Tech

nisc

he U

niv.

D

arm

stad

t 10

00

CFB

0.

59

8.66

650–

670

C

FB

0.4

11.3

5<1

000

12–1

2.6

92

IFK

at U

nive

rsity

of

Stu

ttgar

t 10

BF

B 0.

114

3.5

630–

700

C

FB

0.07

112

.485

0–90

015

97

200

CFB

0.

023

1065

0

CFB

0.

021

1087

5–93

015

N/A

200

CFB

0.

033

660

0–68

0

CFB

0.

021

1087

5–93

015

–16

93

Ohi

o St

ate

Uni

v.

120

EB

N/A

N

/A45

0–65

0

RK

N/A

N/A

850–

1300

3–25

>90

Vie

nna

Uni

v. o

f Te

chno

logy

10

0 BF

B

0.28

**

265

0

CFB

0.

08**

585

0N

/AN

/A

CA

NM

ET

75

BF/M

B 0.

1 2–

558

0–72

0

CFB

0.

14.

5–5

850–

750

897

Cra

nfie

ld U

niv.

25

EB

0.

1 4.

360

0–65

0

BFB

0.16

51.

290

0–95

015

80

Tsin

ghua

Uni

v.

10

BFB

0.14

9 1

630

BF

B 0.

117

185

015

95

*BFB

– B

ubbl

ing

Flui

dize

d Be

d; C

FB –

Cir

cula

ting

Flui

dize

d Be

d; M

B –

Mov

ing

Bed;

EB

– En

trai

ned

Bed;

RK

– R

otar

y K

iln. *

*Equ

ival

ent d

iam

eter

est

imat

ed b

ased

on

cros

s-se

ctio

n ar

ea

12

PHInGO

Pilot-Desigcemeunit. higheholesheighsystem80–95diese

Tab

Parameter Height or lengnternal diame

Gas velocity (mOperating tem

Figure 1. 3 k

Tab

ParametBFB carbHeight (Internal Gas veloOperatinInventorRotary kLength (Internal RotationSolid resInclinati

-scale test fagn of the 1.9ent plant flue

Using data fer attainable s and an opeht were suspm is designe5%. Furthermel oil in the R

ble 1. Dimensi

gth (m) eter (m) m/s)

mperature (°C)

kWth experime

ble 2. Dimensi

ter bonator (m) diameter (m)

ocity (m/s) ng temperaturry (kg) kiln calciner (m) diameter (m)

nal speed (rpmsidence time (mion angle (°)

acility 9 MWth pilot e gas contaifrom cold mvelocities. T

en-area ratiopended at thed to operatemore, energy

RK calciner w

ons and opera

Car

ental facility at

ons and opera

3

re (°C)

m) min)

plant, whichning 20–25%

model tests, thThe design coo of 1.56%. Mhe top of thee with the avy for calcina

which require

ating conditio

rbonator 2.50.1

0.2–0.4600–700

t Industrial Te

ating conditio

3 kWth bench

060

h removes a% of CO221, whe perforatedonsists of a dMoreover, 36e carbonator

verage conveation is proes flue gas re

ns of the 1 kW

Calc

80

echnology Res

ns of the 3 kW

scale

2.50.1

0.2–0.400–700

8–15

50.271–3

15–355

a tonne of COwas based od plate gas d

double-layere6 water-cooler as a meansersion of 20–4ovided throuecirculation f

Wth ITRI pilot p

iner Cir0.9

0.05n/a

0–900

search Institut

Wth ITRI pilot p

1.9 MWth pi

10 000

O2 per hour on the 3 kWt

distributor wed perforateded double sts of tempera40% and CO

ugh direct ofor temperatu

plant

rculation pipe0

0.0025n/n/

te in Taiwan14

plant

ilot scale

4.2 3.3

0.2–0.4 650

0–17 000

5 0.9

1–5 10–15

5

from the Huth laboratory

was selected dd plate with teel jackets oature contro

O2 capture levoxy-combustiture control.

e 0.5 54 /a /a

ualien y-scale due to 6 mm

of 2 m l. The

vels of ion of A key

benefthus,

Test cTests85%vperfo

A to theover config

Test cAs inCO2, to thelimes

Thoperaof 6 correwas oregiocombunsuihighe

Th

whichbetweachiemakeinven

fit of such a increased us

campaign us were perfo

vol. air and 1orming the ex

57-h campaie calciner, wthe entire tiguration, the

campaign usn the previou

typical of coe carbonatorstone is fed tohe first test ation of the R

kg/h, the sponds to soobserved at

on (Figure 2)bustion chamitable for ener than 96%,

Figu

he 3 kWth pih was to keeen fresh so

eve the desire-up rate wntory in the

configuratiosable length

sing 1 kWth bormed to as5%vol. CO2,xperiments, iign of contin

was conducteime. Althoue MB carbon

sing 3 kWth bus case, the floal-fired powr at a controo the calcineperformed

RK calciner. usable lengorbent resid1 m from th. On the oth

mber causinndothermic cwhile the ca

ure 2. Temper

ilot plant waeep the CO2

rbent make-red CO2 cap

was lower the system wa

on is a more for calcinatio

bench-scale sess the COwhich is use

industrial grnuous operated and achie

ugh the preliator was clai

bench-scale lue gas enter

wer plants. Tolled rate of r, while inacusing the mIt showed thth for the cence time of

he combustioher hand, theng the operacalcination rrbonation co

rature distribu

as also used2 capture lev-up rate and

pture level, bhan the puras reduced

e uniform temon. The desir

facility O2 capture eed as the flu

rade limestontion, in whicheved CO2 caiminary resuimed to be d

facility ring the carb

The flue gas 47 L/min. T

ctive sorbent modified 3 khat if the calccalcination f 9 minutes.

on chamber ae temperaturating conditreaction. Neonversion wa

ution in the rot

d to run a 10vel above 85

d spent sorbebut maintainrge rate, two

and excessi

mperature dred calcinatio

efficiency froidising medine was calcinh 0.1 kg/h ofapture level ults indicateifficult to op

bonator contawas first preTo account fis purged fro

kWth facility ciner is fed wreaction is This is beca

and thus excre of 720°C wtions downs

evertheless, tas only 64%.1

tary kiln calcin

00-h long ca5%. Selectioent purge ratn appropriato effects weive degrada

istribution inon efficiency

om the mixtium in the caned at 850°C.f fresh limestin the carbo

d the practierate.14

ains approxieheated to 23for sorbent dom the systewas a batch

with fresh limapproximateause a tempcessive sinterwas observedstream of ththe calcinatio14

ner in the first

mpaign the n of the apte was showe process peere observed

ation of sorb

n the calciney is 80–95%.

ture composarbonator. Pr.20 tone was suponator aboveicability of s

imately 15%v30°C and thedeactivation,em. h test to ev

mestone at thely 1.5 m, w

perature of 1ring occurs id at 2 m frohe calciner on efficiency

t test

main objectppropriate bawn to be critierformance. d: the total bent activity

13

er and

sed of rior to

pplied e 99%

such a

vol. of en fed , fresh

aluate he rate which 000°C in this

om the to be

y was

tive of alance ical to If the solid

y was

14

obserthe soaboveas thbecaucalcin

Wlevel solidsand amainphen

InstiThe ICSICcirculby spfluiditemp

Th

tempthe soThe

rved. These rolid inventoe 99%. Howe

his requires use insufficienation reactio

When the invevaried betw

s in the carba calcination

ntained at apnomenon in th

tuto NacioInstituto Nac

C) has devellating fluidis

pace constraiised using

peratures vary

Fig

he calciner iperature of 80

olids are dirrisers and t

resulted in rory through ever, operatimore heat

ent heat waon. entory in the

ween 80–100%bonator was 2n efficiency opproximatelyhe CaL syste

nal del Carcional del Cloped a 30 sed bed (CFBints to 3.2 m.a syntheticying within

gure 3. Process

is 6.5 m hig00–900°C. Earected throuthe seal loo

reduction of increase in tion under suand energys supplied t

e carbonator %. Under su2 h, with an of 75%. Morey 650°C. Thiem as the me

rbón—Conarbón - ConkWth pilot

B) reactors (F The carbona

c flue gas 568–722°C.22

s flow diagram

gh and has ach reactor igh a vertical

ops are equi

the CO2 capthe make-up

uch condition. Moreover, through LPG

r fluctuated auch operating

average coneover, the sois also show

ean particle s

nsejo Supensejo Superio

test facilityFigure 3). Thator, which icontaining

2

m of the 30 kW

the same is equipped wl standpipe ipped with

pture level. Op rate, impron increase the

the calcineG combustio

around the dg condition

nversion of soorbent tempewed particle size decrease

rior de Inveor de Investiy which conhe height of is 6 m high a3–25%vol. o

Wth pilot plant

nternal diamwith a high-eto the bubbsix heating

On the other oved the COe operating c

er efficiency on to sustain

desired valuethe average olids in the cerature in th

attrition to d from 248 to

estigacioneigaciones Ciensists of twthe CFBs wa

and 0.1 m intof CO2 and

at INCAR-CS

meter and isefficiency cycling fluidise. The heatin

r hand, increO2 capture lecost of the pr

dropped ton the endoth

e, the CO2 caresidence ti

carbonator ohe carbonatoo be an impo 188 μm.14

es Científicientíficas (INwo interconnas initially liternal diamed is operat

SIC23

s operated ayclone, from wed bed loop ng system i

ease in evel to rocess o 39% hermic

apture ime of of 30% or was ortant

cas NCAR-nected imited eter, is ted at

at the which seal.23 in the

carboexces

1.7 MUsingexperCSICdesigfired plantof 0.7achieCFBs

Input Flue gaCO2 coO2 conH2O cSO2 coN2 con*Nitro

Thand 8remoair-co

onator is onlss air to main

MWth pilot-scg the experrience for lar

C in partnersgned to proce

power plant comprises t75 m in the ceve gas velos.24

Tabl

from power pas flow rate (koncentration (ncentration (%concentration oncentration (ncentration* (%gen concentra

he typical op820–950°C, r

ovable bayonombustion en

ly used durintain the desi

cale test facilience gainedrge-scale CFship with ENess approxim

nt located intwo interconalciner and 0

ocities of 3–5

le 3. Operatin

plant kg/h) (%vol.)

%vol.) (%vol.) %vol.) %vol.) ation calculate

Figure 4. D

perating temrespectively.

net tubes. Thnvironment.

ing system sired tempera

lity d through r

FB combustorNDESA, Fos

mately 1% ofAsturias, Sp

nnected CFB 0.65 m in the5 m/s which

ng conditions o

Valu680–23

1257

0.74.

ed to balance t

Design of the 1

mperature wi. The tempe

he calciner isFresh limest

start-up. Fuature in the c

running the rs, a 1.7 MWster Wheelerf the flue gaspain (Error! reactors of 1

e carbonator h are simila

of the 1.7 MW

ue Pilot 300 Maxi2.6 Maxi5.5 O2 flo7.0 CO2 f07 Air fl83 Inven

the remaining

1.7 MWth pilo

indows for terature in ths directly firetone is contin

rthermore, ccalciner.22

30 kWth pWth pilot planr and HUNs produced i

Reference 15 m height (Figure 4). T

ar to those e

th pilot plant a

plant operatiimum coal flowimum fresh soow to calciner flow to calcinelow to calcinerntory in carbonconstituents.

t plant at INC

the carbonatohe carbonatoed with coalnuously fed

coal is comb

ilot plant annt was devel

NOSA. The Cn the 50 MWsource not fand with an

The diameterencountered

at INCAR-CSI

ng conditionsw (kg/h)

orbent flow (kg(kg/h)

er (kg/h) r (kg/h) nator (kg/m2)

CAR-CSIC25

or and calcinor can be col, either in oto the calcin

busted in 2–

and the indueloped by INCaL pilot plWe La Peredafound.). The

n internal diars were selec

d in the indu

IC24,25

s

g/h) 3

7060

) 10

ner are 600–ontrolled th

oxy-combustiner to mainta

15

6% of

ustrial NCAR-lant is a coal-e pilot ameter cted to ustrial

Value325300

300–60000–225000–250000–1000

–715°C rough ion or

ain the

16

desirefoundthe hloop loopi

Test cThe separresultcouldcarbocaptuthat t

Onsystem70–902.1 mparticlevels

MreporCO2. tempof 3 mshowkg/m

Test cArias380 hcomb

ReSánchif thecaptu660°C

Figure

CSIC2

Indecremaingas leand saroun

ed average d.). In both C

high-efficiencseals are de

ing rates betw

campaign uspreliminary

ration efficieted in unstab

d be kept cononator was ure levels wethere is a satin extension m stability im0%, which wm/s) and thcles, for whis dropped to

More experimrted by Rodr

Initially 20peratures for m/s. and un

wn to be achm2s.

campaign uss et al.25 noteh in the CO2 bustion in theepresentativehez-Biezma ee process is oure in the carC). Moreover

e 5. Represent26

n this test aseased close

ntaining propeaving the csorbent activnd 75%. Nev

carrying caCFB reactorscy cyclones, aesigned as bween, and th

sing 30 kWth

experimenency of the cble operationnstant only fooperated in

ere close to thisfactory soliof the risers

mproved. Thwas close to th

e solids circich the activ

o 65–75% for mental resultríguez et al.20 kg of limthe calciner

nder realistichievable for

sing a 1.7 MWes that the 1.capture mode calciner, (exe results froet al. (Figureoperated witrbonator is cr, the SO2 cap

tative steady-s

s no fresh lito the resid

per solid invealciner was vity caused

vertheless, th

apacity of sos the solids land then dir

bubbling fluihus the solid

h laboratory-ts conductecyclones (92–n of the systor a relativel

nearly isothe equilibriuid bed invents’ heights an

he modified she equilibriuculation ratevity is close

the solids cirts generated22 In their tes

mestone wasof 800–850°operating c

bed invento

Wth pilot pla.7 MWth pilode with CO2

xcess O2 of mom the expee 5).26 These rth the propeclose to the epture level of

state results ca

mestone wadual value oentories in oxreached at ththe CO2 ca

he authors cla

orbent witheaving the rrected to theidized beds.inventories i

-scale facilityed by Alon–97%) that ltem as the soly short timethermal condum values at tory and solind reconstrusystem opera

um value at 6es (0.8–2.3 kto the residurculation rat

d in the 30 sts, the rig ws loaded inC and the caonditions th

ories of 400

ant ot plant has b

capture levemore than 5%erimental trresults confirer solid invenequilibrium vf more than 9

aptured durin

as added to of 0.1. In axy-firing mohe calciner t

apture level aim that high

hin 0.1–0.7 (isers are sep

e double loop. Such desigin the CFB re

y so et al.23

led to lossesolid looping e. Nevertheleditions (±20a given carb

id looping rauction of theated with CO650°C, depenkg/m2s). Houal activity

te of 1.0–1.8 kkWth test f

was used witnto the systarbonator of

he actual COkg/m2 and

been operateels of 40–95%

%vol.), has beials were rerm the findinntory and sovalue at give95% was ach

ng testing of th

the system, addition, theode. Althougtemperature to drop bel

h CO2 captur

(Error! Referparated from p seals by grgn allows coeactors.25

indicated in in the solidrates and th

ess, this stud°C) and tha

bonator tempate. e high-efficieO2 capture lending on thewever, for tof 0.07–0.12,

kg/m2s.23 facility at INth flue gas cotem which

630–700°C, 2 capture levsolid loopin

ed for more %. Stable opeeen maintaineported for ngs from preorbent activiten temperatuhieved.

he 1.7 MWth pi

the activityere were sevh the CO2 puof 950°C, lo

low the equre levels can

rence sourcm the gas stre

ravity. The dontrol of the

nsufficiently ds inventory

he solids invedy showed that the actuaperature, pro

ency cycloneevels ranginge gas velocitythe highly c

2, the CO2 ca

NCAR-CSIC ontaining 20was operatand gas velo

vel of 70–90%ng rates of 0

than 1800 heration undened for 170 h

the first timevious studieity, the actuaures (above 9

ilot plant at IN

y of the limeveral issuesurity of 85%

ow solid inveuilibrium levn be achieved

ce not eam in double e solid

high y. This entory hat the l CO2

ovided

es, the g from y (2.0–cycled apture

were 0%vol. ted at ocities % was 0.5–2.2

h, with r oxy-.

me by es that al CO2 90% at

NCAR-

estone s with

in the entory vel, to d even

if higsystem

Arplantto higoperaafter conclbe re(95%)usingactivi

InstiDarmThe comp(Figuinternlimesclaimwhichin thcarboreactilance

Unconveexcha

ghly deactivam is operaterias et al.25 ht at INCAR-Cgh content ofating below

more than lusion was dached (over ), provided s

g the 30 kWt

ity are the m

tute of Emstadt 1 MWth pil

prises two inure 6). The cnal cooling tstone make-u

med to reduceh can be hea

he carbonatoonator, whichion, the pilot

e, or solid fue

Figure 6. Proc

nlike other peyors. Moreoangers, clean

ated sorbentd with the phas reportedCSIC. In the f non-calcineits desired te5 h of con

rawn as in th80%) even w

sufficient solth laboratory

most importan

Energy Sy

lot plant ernterconnectearbonator, istubes to conup is fed to e fuel and O

ated up to 35or. Preheateh is 11.35 mt plant is desel, which is in

cess flow diag

pilot plants, over, the flu

ned from fly a

t is used togroper solid i

d extensivelyinitial trials,

ed limestone emperature tinuous opehe study by Swhen sorbenlid inventory

y-scale facilitnt operating

ystems an

ected and ced CFB reacs 8.66 m hig

ntrol the temthe carbonat

O2 consumpti0°C using au

ed (up to 4m high and 0.

signed to comntroduced by

gram of the 1 M

the solids bue gas and thash in fabric

gether withinventory. y on the stea a CO2 captuin the solid of 920°C. Mo

eration withSánchez-Bieznt has residuy is availablety and proveparameters

nd Techn

commissionectors that argh and 0.59 mperature. Bytor, where ition in the caluxiliary elect450°C) oxyg.4 m in intermbust eithery a gravimet

MWth pilot pla

between the he CO2 streafilters.

high sulpha

ady state opure level of leinventory. Toreover, sorbout fresh limzma et al.26 thual activity ae. These resue that the soin the carbon

nology at

ed at Technre refractorym in internay contrast tot is heated tolciner.27 In adtric heaters, igen-enriched rnal diameter gaseous fuetric dosing sy

ant at Technisc

CFBs’ loop sam are subse

ate conversio

peration of thess than 40%

This was causbent activitymestone mahat high CO2

and for high ults confirm t

lid inventorynator reactor

Technisc

nische Unive lined to mal diameter, o other piloto 650°C. Sucddition, the sis used as a f

air is usedr. To mainta

el, using a gaystem.

che Universitä

seals are tranequently coo

ons, provide

he 1.7 MWth

% was reachesed by the ca

y dropped toake-up. The 2 capture levSO2 capture

trends determy and the so

r.

che Unive

ersität Darmminimise hea

is equippedt plants, the

ch configuratsynthetic flufluidising med to fluidisain the calcinas burner or

ät Darmstadt2

nsferred by oled down in

17

ed the

h pilot ed due alciner o 0.067

same vel can e level mined orbent

rsität

mstadt at loss d with e fresh tion is

ue gas, edium se the nation a bed

27

screw n heat

18

Test campaign The first tests performed using the 1 MWth pilot plant was conducted in July 2011. Since then, the facility has been operated for around 400 h to analyse CO2 capture using the CaL process. In the first campaign, lasting 72 h, continuous separation of CO2 from the 1300 Nm3/h synthetic gas comprising 10–12 vol% of CO2 was analysed.28 Throughout the campaign, the make-up flow was kept between 70–150 kg/h and the solid looping rate between 1500 and 3000 kg/h. The study revealed that lower CO2 capture levels are obtained if sintered CaO, which has been calcined at 1000°C before the test, is fed to the carbonator rather than fresh limestone. An increase of the CO2 capture level was observed when fresh limestone was fed to the carbonator. Despite poor performance of the calciner cyclone and low CO2 concentration in the flue gas caused by limited firing power in the calciner, the total CO2 capture level was more than 90%.

On increasing the power of the burners and the lances, as well as improving the cyclone performance, a second test campaign using propane firing in the calciner was carried out. These changes led to a significant reduction of the make-up rate. Moreover, the fluidisation air was enriched with 50% O2 to maintain desirable gas velocity in the calciner and to ensure nearly complete combustion of propane. Again, the carbonator was operated at 660°C and was supplied with the synthetic flue gas containing 12%vol. of CO2. To maintain a desired average CO2 capture level in the carbonator, the solid looping rate was kept at 2000 kg/h which corresponds to average Ca/C ratio of 11.6. The maximum CO2 capture level in the carbonator was found to be 85%, which is equivalent to 92% when oxy-combustion of propane in the calciner is considered.

In the third campaign, the calciner was fired with the pulverised coal and the fluidisation air enriched by 45–50% O2. The temperature in the carbonator reached 670°C. The same amount of synthetic flue gas was used as in second campaign. It was found that to reach the same CO2 capture level in the carbonator (85%), the solid looping ratio needed to be increased to approximately 2800 kg/h (Ca/C=17.2). Moreover, it was found that the CO2 concentration in the clean gas was close to the equilibrium CO2 concentration at the current operating temperature. The CO2 capture level dropped to approximately 60% when the temperature in the carbonator dropped to 610°C.27

Institute of Combustion and Power Plant Technology at University of Stuttgart The Institute of Combustion and Power Plant Technology (Institut für Feuerungs-und Kraftwerkstechnik, IKF) at University of Stuttgart has developed a calcium looping pilot plant based on 10 kWth dual fluidised beds (DFB) that can be operated in continuous mode.29 The practicability and stability of the DFB system, which is composed of a CFB riser and a bubbling fluidised bed (BFB) (Table 5), was first analysed using a down-scaled cold model.57,59

Table 4. Dimensions and operating conditions for the 10 kWth DFB pilot plant

Parameter CFB BFB Height (m) 12.4 3.5 Internal diameter (m) 0.071 0.114 Gas velocity (m/s) 4 – 6 < 1.2 Operating temperature (°C) 630–700 850–900

In the 10 kWth IFK facility, the CFB and the BFB can each be operated as the carbonator or calciner. A benefit of operating the BFB as the carbonator and the CFB as the calciner is improved process control. A novel feature of this DFB system is control of the calcium circulation rate between the beds by varying the cone valve opening and the BFB absolute pressure. Due to heat losses to the environment, the CFB, the BFB and the solid circulation system are electrically heated.30 The temperature in the calciner can be raised by direct natural gas combustion in oxygen-enriched air (40%vol. O2), if the electric heating system capacity is insufficient.29

200 kWth pilot-scale dual fluidized bed facility To investigate the long-term performance of the process under real combustion conditions, a 200 kWth pilot plant was designed at the IFK. The pilot plant design includes a CFB calciner

operaeitherFigurregimvalveconfigcontr

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Test cThe tused condistate carbomaxim

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ating in the fr under a turre 7).31,32 A

me, required es for controguration wit

rolled throug

Fig

he fast fluidipped with a d chips). Althgy demand itions in the ternal diamegned to operence time.31

meter28, is fluioved via a wabed region. T

he system duinsulating co

campaign ustests were co

to clean theitions. The eoperation w

onator was omum of 97%he effect of ysed and comn temperatur

campaign user et al.31 repo

recorded an

fast fluidisatrbulent or fasdesign invoa redesign o

olling the loth the carbongh the L-valv

gure 7. Comp

ised CFB calcstaged oxid

hough the firin the calcincalciner. In

eter), the heatrate under lThe fast flui

idised with tater-cooled hThe facility h

uring start-uponcrete and r

sing a 10 kWnducted usine synthetic fexperiments with a minooperated at

%. the carbona

mpared withre were achie

sing a 200 kWort that more

nd the facilit

tion regime ast fluidised flving two s

of the solid ciooping ratiosnator operative which is d

arison of 200 k

ciner, whichdant supply ring system ner, flue gasthe turbulent is removedlower fluidisidised CFB che flue gas a

heat exchanghas a 400 kWp. To minimrefractory m

Wth DFB facilng the Germflue gas havrevealed tha

or make-up 660°C, the

ator operatinh the equilibreved by Ca/C

Wth DFB facie than 600 h ty was found

and a reconffluidisation rsymmetric Circulation sys in both CFing in the tur

directly fed fr

kWth DFB pilo

h is 10 m in hfor oxy-comis designed s recirculati

nt fluidised bd only using sation velocicarbonator, wand the heat ger in the denWth gas burn

mise the heat material resist

lity man limestone

ving 15%volat it was relof fresh sorachievable C

ng temperaturium data (FC ratio highe

ility of successfu

d to be hydr

figurable CFregime ( CFBs, which ystem througFBs indepenrbulent regimrom the CFB

ot plant config

height and 0mbustion of s

to handle upon is used

bed carbonatthe fluidisedities than th

which is 10 mreleased du

nse bed regioner that is us

loss in the sant to abrasi

e originating. CO2 to simlatively strairbent to accCO2 capture

ure and the igure 8). Theer than 14.30

ul operation orodynamicall

FB carbonato

operate in h implement

ndently.32 Come, the solidcalciner.31

gurations at IF

.021 m in intsolid fuel (cop to 70%vol.to achieve rtor (6 m in hd bed heat exhe CFB carbom high and 0e to the exoton and a bayed to genera

system the reion.32

g from Swabimulate coal-fghtforward

count for atte level excee

CO2 capture best CO2 ca

of the 200 kWly stable. Th

or that can op

the fast fluntation of twoonversely, fo

d circulation

FK33

ternal diameoal, wood p. of O2 to merealistic ope

height and 0.xchanger. It ionator and

0.023 m in inthermic reactyonet cooler ate hot gas toeactors were

ian Alb whicfired power to achieve strition. Wheeded 90%, w

re level werapture levels

Wth pilot planhe tests perfo

19

perate

idised o cone or the rate is

eter, is pellets, eet the rating 033 m is also lower

nternal tion is in the o heat

e lined

ch was plant

steady en the with a

e also s for a

nt has ormed

20

mainsynth

Figure

HturbuNevereactitempstudydesuldeter(Figulevel,

Figure

IFK33

nly aimed ahetic flue gas

e 8. Effect of th

owever, theulent fluidiseertheless, theion takes pla

perature (620y revealed thlphurisation rmined by thure 9). This m, leading to e

e 9. Effect of

at reaching s containing 1

he carbonator

e desired temed bed carbe CO2 captuace in the de0–650°C) havhat higher CO

unit, fed tohe equilibriummeans that lowenergy saving

f the carbonat

steady state14% CO2.

r temperature

mperature ofbonator, whiure level is ense region. ve a minor eO2 capture leo the carbonm calculationwer looping g in the rege

tor temperatur

e conditions

on the CO2 ca

f 650°C is oile the tempmaintained It was also

effect on theevels are obtanator. Thesens revealing ratios would

enerator.31

re on the CO2

s under var

apture level da

observed onlperature is r

above 90%,found that f

e carbonatorained with ife were found

a good gas-sd be required

2 capture leve

riable tempe

ata from the 10

ly in the dereduced in t, indicating fluctuations r efficiency. wet flue gas

d to closely solid contactd to reach de

el data from th

erature and

0 kWth DFB at

ense region othe upper rthat most oin the carboFurthermor

s, such as fro follow the t in the carboesired CO2 ca

the 200 kWth D

with

t IFK30

of the egion. of the onator re, the om the

trend onator apture

DFB at

OhioOhio Carbothe OReactby th

InFi

reactooccur980°Cmultisorbe

Test cThe Cnaturwas u

Innot acarboamou

Thlevel perfoachiesuch challe

VienReseareform

o State UnivState Univer

onation – CaOhio State Ction Separati

he Ohio State n the 120 kWt

igure 10) theor operated rs in an elecC. The calciniple cycles.38

ent is fed dire

Figu

campaign CCR sub-piloral gas in theused to main

n the once-thraffect the COonator must unt of active he study clea

between 40ormance of geved a CO2 c

small partienging and,

nna Universarchers at Vming (SER)

versity rsity has pioalcination ReCarbonation ion for CO2 University.

th sub-pilot pe CaO or Ca

at 450–650°ctrically-heat

ned sorbent i8 Nevertheleectly to the c

ure 10. Process

ot plant was e stoker and

ntain the fluerough test, WO2 and SO2 be considersorbent in th

arly indicates0 to 100% ground lime

capture level icles from ttherefore, th

sity of TechVienna Univ

process for

neered the coeaction (CCR

Ash Reacti(CaRS-CO2)

plant ( a(OH)2 can b°C to allow ted rotary kis then hydraess, some dealciner.

s flow diagram

used to clead containing e gas temperaWang et al.38

capture levred when dehe system. s that applicafor Ca/C r

e (d50=600 μof 9% Neve

the gas strehe process fea

hnology versity of Tr biomass g

oncept of simR) process. Thivation (OSCprocess37 – b

be fed to thfor CO2 an

kiln calciner ated using seactivated s

m of the Ohio S

an the flue ga12.5%vol. of

ature at 650°note that the

vels. In addieciding on th

ation of the Catios betweμm) and pulertheless, as peam in indasibility of th

Technology gasification,

multaneous Che process w

CAR) procesboth process

he carbonatod SO2 captuoperated at

team to impsorbent (2–10

State Universi

as producedf CO2 and 14C prior to bee presence oftion, the amhe purge fra

Ca(OH)2 (d50

en 0.5 to 1lverized gropointed out ustrial-scale

his approach

have been another pos

CO2 and SO2

was based onss34-36 and thses develope

or, which is ure. The sorbt a maximum

prove the sor0%) is purg

ity sub-pilot p

through air-450 ppmv ofeing fed to thf fly ash in th

mount of fly action to ma

0=3 μm) resu.7, which isund lime (dby Dean et areactors m

needs more

exploring sssible applic

2 capture usinn a combinathe Calcium-

ed at and pat

an entrainerbent regenem temperaturbent activityged and the

plant38

r-firing of coaf SO2. Naturhe carbonatorhe carbonatory ash enterinaintain the p

ults in CO2 cas superior td50=18 μm) wal.18, separat

may be extrexamination

sorption-enhcation of the

21

ng the tion of -based tented

d bed ration ure of y over

fresh

al and ral gas r. r does

ng the proper

apture to the which tion of emely n.

hanced e CaL

22

procewhichhigh Figuroperatakesmaterare trconceheat heat t

FigureTechn

Test cThe condifor th700°C

ThproceachieCO2 rg/Nm

ThKoppgasifiTherecombsolid mater

ess.39 Their 1h is 2 m higand has a cro

re 11).40 In thates at tempes place at aprial with satiransported bept was thenand power pto the local c

e 11. Processnology42

campaigns 100 kWth laitions with ihe carbonat

C.39 This washe analysis cess, in whichevable. This iremoval fromm3 to 0.3–0.9 he first test cpatz et al.43 ier and theefore, the gabustion temp

looping raterial. The tes

100 kWth duagh and has aoss-section ahe conventioeratures of 8

pproximatelyisfactory res

between bothn successfullplant in Gue

community.41

s flow diagr

aboratory-scin-situ CO2 ction reactions achieved thconducted bh olivine is is, due in pam the gasifieg/Nm3 was

campaign usiSuch opera

CFB combasification teperature wases at low levst campaign

al fluidised a cross-sectioarea of 0.005 onal solid fe850–900°C, wy 920°C. For istance to atth reactors vily used for essing, Austr1

am of the 1

ale facility capture usingn, the BFB rhrough reducby Pfeifer et

used as thert to an equi

er. Moreover observed. ing 8 MWe G

ating mode bustors to amperature w

s dropped frovels. Moreov

results reve

bed gasifier on area of 0.0m2 ( eedstock gas

while the air-this system

trition and mia two loop sdevelopmenria, that deli

100 kWth lab

has been mg limestone reactor temp

cing the solveal.39 has rev

he bed materilibrium shifr, a simultane

Guessing CHrequires a t

allow for efwas reducedom 950°C to

ver, the oliviealed that th

facility cons062 m2, and

sification pro-combustion

m, olivine wamoderate tar seals which

nt of the biovers 1.8 MW

boratory-scale

modified to (Figure 12). perature waent looping rvealed that crial, higher ft in the wateeous reducti

HP in SER opetemperature fficient carb

d from 850–9o 850°C. Thisne was subshe content o

sists of a BFa CFB comb

ocess the BFof fuel in th

as shown to cracking actare fluidisedmass-fired 8

W of electricit

plant at Vie

operate unTo achieve

as reduced rates betweencompared toH2 yields u

er-gas shift ron of the tar

erating moddifference

onation-calc900°C to 650s was achievestituted withof H2 and C

FB gasifier rebustor that i

FB gasifier rhe CFB combbe a suitabl

tivity.39 The d with steam8 MWth comity and 4.5 M

enna Univers

nder SER prproper condto approximn the reactor

o the convenup to 70%voreaction causr content fro

de was reportbetween the

cination reac0–750°C, whied by keepin

h limestone aCH4 in the s

eactor, is 5 m

eactor bustor le bed solids

m. The mbined MW of

sity of

rocess ditions mately rs.43 ntional ol. are sed by m 4–8

ted by e BFB ctions. ile the ng the as bed syngas

decredrivinwith be im

Fi

Figureusing

Figuretypica

eases with inng force for temperature

mproved overigure 13).

e 12. Schematcalcium loopi

e 13. Comparal range at the

ncrease of thethe carbona

e. The qualityr the conven

tic representating process43

ison of the sy pilot plant43

e gasificationtion reactiony of the syngtional proces

tion of the sor

ngas composi

n temperaturn, as the equgas producedss, for the 10

rption enhanc

ition in conve

re. This was uilibrium pard under SER 0 kWth labor

ced reforming

ntional and S

explained byrtial pressureoperating matory-scale p

process with

ER gasificatio

by reduction e of CO2 incr

mode was fouplant (

in-situ CO2 c

on processes a

23

of the reases und to

capture

and the

24

CanmCanmin semand acalcin

Deand cThe carbom in fluidirefracelectrtemploopisolidsThe callowcarbo

Test cThe 7(ErrosynthThe fwas ocombThe tlevel captucarbofoundperfohighe

metENERGmetENERGYmi-continuouanalysed usiner and a car

Figure

epending oncan be operacarbonator

onation proce internal diaise both stagctory and inric heaters t

perature.7,45 Ting that inclus rate througcalcined sor

ws the carboonator for SO

campaign us75 kWth pilo

or! Referenchesised mixtufluidising airoperated undbustion of biotests perform

of 97% withure level wasonator. Mored to occur af

ormance andest CO2 captu

GY has developus mode to ding process

rbonator ope

e 14. Schemat

n the applicatted under aiwas dividedesses. Therefameter. Suchges. To min

nsulation mathat are us

The CanmetEudes a 45° “gh the convebent enters

onated sorbeO2 capture.

sing a 75 kWot plant has e source nure of air an

r was first prder three heaomass and bi

med by Lu et hin the first s found to deeover, a confter 25 cycles

d demonstraure level (98

ped a 75 kWt

demonstratesimulation rated under

ic of the 75 kW

tion the calciir, oxygen-end into two fore, dependh configurat

nimise the heaterials. Mored during t

ENERGY pilo“T” that colleying line tothe carbona

ent to be dir

Wth pilot planbeen operat

ot found.). nd CO2, to areheated to 2ating modesituminous coal.7 confirmeseveral cycl

ecrease, whicnsiderable drs. This confirtes the need

8%) was reac

th pilot-scale e the processby Hughes either movin

Wth pilot plan

iner is 4.5–5 nriched air an

stages to sding on the tetion allows eat losses in

reover, each the start-upot plant has lects solids fo the carbonator throughrected to th

nt ted under cDuring the

achieve CO2

250°C prior ts: electric heaoal with flueed that the Cles. As the sch is associatrop in CO2

rms the impd for fresh sched within t

dual fluidise feasibility. Tet al.44 The

ng or bubblin

nt developed a

m in height nd oxy-firing

separate the est performethe air or a

n the systemreactor is eq

p and somea novel solidfrom the cal

nator is contrh the L-valvhe calciner o

ontinuous me tests, the concentratioo being fed t

ating, biomase gas recycle. CaL system csuperficial gated with lowcapture leve

pact of sorbensorbent makthe temperat

ed bed systeThe system w

pilot plant ng bed condi

t CanmetENE

and 0.1 m ing modes wit

combustioned it is 2–5 mair and supem, the reacto

quipped wittimes to cod transport slciner-piercedrolled using e. Similarly,

or to the sec

mode for mocarbonator

ons of approto the carbonss combustio

an operate was velocity w

wer residenceel to approxnt deactivatike-up. It wature window

em which opwas first descomprises a

itions (Figur

ERGY44

n internal diath flue gas ren/sulphation

m in height anerheated steaors are linedth three 4.5–ontrol the rsystem for sod distributora solenoid

, the 45° “Tcond stage o

ore than 50 was fed w

oximately 15nator. The caon in air, and

with a CO2 cawas increasee time of gas ximately 72%ion on the pras shown thw of 580–600

perates signed a CFB re 14).

ameter ecycle. n and nd 0.1 am to

d with –5 kW eactor orbent r. The valve.

T” line of the

hours with a 5–16%. alciner d oxy-

apture ed, the

in the % was rocess at the °C for

25

fresh sorbent and approximately 700°C after 20 cycles. When the temperature dropped below 500°C, the reaction rate slowed down significantly. This was reflected in the off-gas CO2 concentration of approximately 9–10%vol. of CO2, which reflects a drop of CO2 capture level.

Table 5. Operating conditions of the 75 kWth CanmetENERGY pilot plant

Parameter Minimum Maximum CFB Calciner Bed temperature (°C) 850 950 Initial sorbent inventory (kg) 4.5 5.0 Sorbent make-up batch (kg) 0.3 0.5 Biomass consumption rate (kg/h) 4.0 7.6 Coal consumption rate (kg/h) 2.6 5.8 Air flow rate (air-firing mode) (kg/h) 8.0 14.4 Oxygen flow rate (oxy-firing mode) (kg/h) 5.2 7.7 Moving or bubbling FB carbonator Bed temperature (°C) 580 720 Air flow rate (slpm) 40 100 CO2 flow rate (slpm) 7.5 19.0 CO2 concentration at inlet (%vol.) 15.0 16.5 Air flow for solid conveying (slpm) 35 55

Cranfield University A 25 kWth pilot-scale CaL rig developed at Cranfield University consists of an entrained bed carbonator, which is 4.3 m high and 0.1 m in internal diameter, and a BFB calciner 1.2 m high and 0.165 m internal diameter (Figure 15). The desired operating temperature in the carbonator (600°C) and the calciner (900–950°C) are maintained using electric heating elements.46 The calciner is directly heated by oxy-combustion of natural gas. The resulting flue gas is used as the fluidising medium. The the flue gas generated by air-combustion of natural gas (8%vol. CO2), is used as the fluidising medium in the carbonator. The optimal operating gas input rates are given in Table 6. The solid input to the calciner is achieved using a screw-feeder with a maximum feeding rate of 1.6 kg/h. Here, the solid looping rates are controlled via two fluidised loop seals, at the top and bottom of the calciner.47 The facility has two cyclones at the top of the carbonator to ensure that the sorbent lost with the flue gas is minimised.46

Table 6. Optimal gas composition in 25 kWth Cranfield University pilot plant46

Fluidizing gas Carbonator Calciner CO2 volumetric flow rate (l/min) – 40 O2 volumetric flow rate (l/min) – 16 Air volumetric flow rate (l/min) 150 – CH4 volumetric flow rate (l/min) 22.5 11

Test campaign A main objective of the test campaign conducted using the 25 kWth pilot-scale facility at Cranfield University was maximisation of the CO2 capture level by modification of the process configuration and the operating conditions.

The study revealed that by changing the carbonator temperature, the CO2 capture levels of within 50% and 70% were reached within the temperature windows of 650–700°C and 600– 650°C, respectively. Such improvement of the efficiency for a system operated at lower temperature is attributed to the chemical equilibrium, as reducing the temperature reduces the equilibrium CO2 content and in turn increases the equilibrium CO2 capture level.

26

It and wresultexplait wapossito flu

Thmodiimpleand t

was found twas seen forts from anot

ained by incras found thaible that suchuidise leadinghe maximumifications resementation otemperature.

Figure 1

that in the bar particle sizther study crease in the sat below 125h small particg to bed agglm CO2 captsulting fromof air shaker47

15. Process flo

atch runs thezes between carried out isurface-to-vo μm, the COcles would hlomeration.49

ture level ofm the cold m

s and heatin

ow diagram of

e optimum C125–250 μm

in the same olume ratio aO2 capture lehave been of 9 f 80% in th

model, tempeng elements t

f the 25 kWth

CO2 capture lm (Figure 16)

group by Kas the particlevel drasticaGeldart’s gr

he carbonatoerature and to the loop s

pilot plant at

level, which . This was in

Kavosh.48 Sucle size was really droppedoup C, and t

or was reachPSD optimieals to enhan

Cranfield Uni

was close ton agreementch behavioueduced. How

d below 50%thus were di

hed after thisation, as wnce solids tra

iversity47

o 90%, t with

ur was wever,

%. It is ifficult

he rig well as

ansfer

TsinThe interc0.149transfinjectcompcalcinheate

Test cIn thsorbewas kBFBs(850°level the C

Th0.7%vaveraleavinmeancarbo

ghua Univedual fluidis

connected BF m and 0.117ferred betwetion nozzles pensate for hner, each reaers are used f

campaign e test campa

ent to clean kept betwee. When the dC), the CO2 of 89% in th

CO2 fraction ihe carbonatovol., which cage conversing the carbo

n size of theonator, respe

Figure 1

ersity sed bed syFBs (Figure 7, respectiveeen the bedare used to

heat losses aactor was equfor heating th

aign performthe syntheti

en 30–36 kg/desired temp

fraction in he carbonatorincreased to aor was operacorresponds ton in the ca

onator was 16e particles w

ectively.

16. Effect of pa

ystem develo17). The intely, and the h

ds through tho transport tand to mainuipped with he flue gas, t

med, dolomiic flue gas co/h which coperatures wethe clean gar. Increasing approximateated at 640°to a CO2 caparbonator rea6.2%. Feng ewas reduced

article size on t

oped at theernal diametheight of eache cyclones the solids frntain desired

four 2.5 kWthe risers, the

ite (d50=0.5 montaining 12

orresponded ere achieved as was 1.2%vg the operatinely 10%vol d°C and the rpture level of

ached 70.4%et al.50 noted d to 0.16 mm

the CO2 captu

e Tsinghua ters of the cach bed is 1 m

and the dorom the bedd temperatu

Wth electric hee cyclones an

mm) was us2.1–14.5%volto the invenin the carbo

vol., which cng temperatu

due to chemicresulting CO95%. Under

% and the avthat after 7 h

m and 0.42

ure level47

University arbonator an

m. In this systowncomers.

to the riserre in the ca

eaters. Also, nd the downc

sed as a sourl. CO2. The sntory heightnator (630°Ccorresponds ure of the carcal equilibriu

O2 fraction insuch operat

verage conveh of continuomm in the

consists ofnd the calcintem, the soliIn addition,r in each BFarbonator anadditional ecomers.50

urce of the nsolid loopint of 0.3 m inC) and the ca

to a CO2 carbonator to 6um limitationn the clean gting conditioersion of paous operatio

e calciner an

27

f two ner are ds are solid

FB. To nd the lectric

natural g rate

n both alciner apture 680°C, ns. gas to ns the rticles

on, the nd the

28

ImprEnhaplantplantwasteotherimprosteamregenin the

ProcAlthothe bimpro

IntegAs thfor oxdesigfrom tempused charabe tesefficie

Figure 17. Pr

roved heat ancement of t is regardedt output on e heat availrwise would ovement wo

m cycle. Morneration can e CaL proces

cess configough most obasic configuove the over

grated comhe main sourxy-fuel comb

gn in which an external

perature fluidas a fluid

acterised by sted in practency of the in

rocess flow di

ALTER

integrationthe heat int

d as a viable capture plaable in the be cooled u

ould stem froreover, heat also improv

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gurations fof the studiesuration propall process e

mbustor andrce of the effbustion in ththe heat forl source (Figdised bed codising medihigher thermice. Neverthentegrated po

iagram of the 1

RNATIVE PR

n egration degoption for r

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using the utilom generatioexchanger n

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mal efficiencyeless, this co

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gree betweenreduction of on.21 This cae plant andlity streams. on of an addnetwork desnce. An optim

ng the proces

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alty in the CAbanades et t regeneratioheir study aelled with an

calciner. Ay and no req

onfiguration wy 1.6% point

ator-scale plan

CONFIGURA

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an be achievd the CO2 co

The main soditional amosigns for enhmal design os performan

ovement the CaL pro

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CaL process sal.16 have pron is supplieassumed than air and fueAlthough suquirement fowas reported

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ATIONS

apture systec load impo

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configurationof the oxygen

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a University50

em and the posed on the p

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2 capture conns, which a

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29

Figure 18. Heat transfer integrated calciner and combustor16

Indirect heat transfer using solid carrier In another configuration proposed by Abanades et al.16 the heat requirement of the calcination reaction is supplied using a solid heat carrier (Figure 19). The process involves a circulating fluidised bed combustor that is fired with an air and fuel mixture. The solid bed material, which is a dense material such as Al2O3 or sintered CaO, is heated in the combustor, and then sent to the calciner to drive the calcination. Continuous separation of these particles based on density differences would be required, but if this configuration is feasible it could produce efficiency improvement amounting to 2.2% points.

Figure 19. Indirect heat transfer from combustor to the calciner16

Heat recovery fluidised bed Martínez et al.52 have proposed another CaL process configuration that aims to preheat the CaCO3 particles before they enter the calciner (Figure 20). In this configuration, the sensible heat of the CO2 stream leaving the calciner is recovered to preheat the solid particles from the carbonator in an additional heat recovery fluidised bed. Although the flue gas acts as a fluidising medium and mixes with the partially carbonated sorbent particles, no carbonation reaction occurs in the heat recovery bed. This is because the fast carbonation reaction cannot take place as the active CaO is covered with the CaCO3 layer formed in the carbonator.53 Although in this configuration the heat required by the calcination process is supplied via oxy-

30

fuel cpoint

CalcAn exloopi( Figurreactogaseoand ccapabrequiconsi

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cium and chxperimental ing with the

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uthors compthe heat for the metal ox

ormer beingerall system rocess configges of develo

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mal efficiencymption of 9%

additional he

le Anthony54 exerised coal c

dition to the ceneration is polid sorbent cn carrier. Thce as cryogenere proposed

-looping cycle

y of the CaLpoints.

eat recovery be

xplored integcombustion p

carbonator aprovided by consists of bherefore, thisnic air separ

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an air of the oxide

tion is longer

must be

31

CONCLUSIONS The CaL process, using two interconnected fluidised beds, appears to be a suitable option for fossil-fuel power plants resulting in efficiency penalty of only 2 to 3%, mainly due to the reduced energy requirement for O2 separation. Extensive pilot plant testing of CaL process concepts have confirmed the process feasibility and practicability for CO2 capture from fossil-fuel power plants. Moreover, the performance of the pilot plants along with the operating conditions extensively reported in the literature and can serve as a basis for process modelling and validation. Finally, the literature survey suggests that the energy requirement for sorbent-based CO2 capture plants can be enhanced by improved heat integration between CO2 capture and the power plant, the use of process intensification equipment or advanced process configurations for the CO2 capture plant.

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