Developments in Lime Reactivation through Superheating of Ca(OH)2

By Vlatko Materić ; Robert Holt ; Stuart Smedley

Industrial Research Ltd, New Zealand

Carbonation

Dehydration

Lime-Limestone Thermochemical Cycle

Heat (>850C)

Diluted CO2 Concentrated CO2

Heat (500- 800C)

with Reactivation via Hydration

CaO

CaCO3

Carbonator CalcinerCa(OH)2

The Return Step Matters - Activity

Materic, Smedley Ind. Eng. Chem. Res 2011

Commercial Hydroxide - Heated in CO2Commercial Hydroxide - Dehydrated

Carbonating Ca(OH)2 directly always leads to higher activity than dehydrating it

Ca(OH)2 Ca(OH)2

65% CaCO3 88% CaCO3

Method used Morphology 39 cycles, 12 reactivations in BFB

No Reactivation

Calcination and Carbonation Cycles

Dehydration Method(standard)

Every 3 cycles, CaO is :

1. Steam Hydrated, 300°C → Ca(OH)2

2. Dehydrated in N2, 450°C → CaO

CarbonationMethod (super-heating)

Every 3 cycles, CaO is:

1. Steam Hydrated, 300°C → Ca(OH)2

2. Annealed in CO2, ≈ 540°C → Ca(OH)2/CaCO3

3. Dehydration triggered by → CaO/CaCO3(45%)removing CO2

The Return Step Matters – Attrition

Carbonating the hydroxide allows superheating → reduced attrition

Repeated Ca(OH)2 Superheating - effective

• Tested with 910°C calcinations, 3 Limestones, BFB and micro-CFB

• Superheating Ca(OH)2 improves sorbent performance • Sorbent Activity always higher than standard hydration method • Fragmentation comparable to No Reactivation in BFB and CFB

BFB Results - Over 39 Cycles and 12 Reactivations

No Reactivation,

16%

Standard Hydration,

36%

IRL's SD Reactivation,

56%0%

10%

20%

30%

40%

50%

60%

Average Activity - Lhoist Limestone

No Reactivation,

0.99%

Standard Hydration,

11.13%

IRL's SD Reactivation,

1.44%

0.00%

2.00%

4.00%

6.00%

8.00%

10.00%

12.00%

% Fines < 90 um - Lhoist

Superheating Model developed

• Hypothesis: CO2 gas leads to proton injection, stabilises Ca(OH)2.

• Ca(OH)2 is carbonated directly, CaO never forms→ Weakening avoided

• Model correlating Pore Size and Superheating behaviour was developed• Surprisingly, the model works!

Non-superheating Ca(OH)2Superheating Ca(OH)2

Dehydration Trigger (N2) Dehydration Trigger (N2)

Development Status

• Micro-CFB unit (20g limestone) for attrition testing• Simultaneous Impact, Abrasion and Chemical Attrition• Attrition Results Confirmed : S-heating = No Reactivation

• 3kg/day BFB to produce reactivated materials for CFB pilot tests - operational by December 2011

• A Stand-alone reactivation cycle concept developed

3kg/day Reactivator

Stand-alone Reactivation Cycle

Spent CaO

H2O / Air

CO2

Air Reactivated CaCO3 – 45%

Hydration300°C

Annealing+ Carbonation

540°C

Dehydration

540°C

H2O / Air

CO2 (recycle)

Calciner

Calciner

The particles carry the reaction heat through the process. Carbonation is a heat supply

Reactivator

300-540°C

Stand-alone Reactivator Integration

SpentCaO

45% CaCO3

Amended from Abanades et al., Environ. Sci. Technol. (2005) 39, 2861-2866

Stand-alone reactivator has no heat exchange with Ca Loop :

• Plugs in like Make-up

• Retrofittable

• Pre-, Post- combustion

(fp)(1-fp)

Heat to Power Cycle

Effect of Activity on Economics

15

17

19

21

23

25

27

29

31

33

35

0 10 20 30 40 50 60 70Solids activity (%)

Cost

of C

O2 A

void

ed

$/to

n

0

1

2

3

4

5

6

7

8

Solid

Flo

w (t/

min

) ; F

px 10

)

Total Cost $/tCO2

Solid Flow (tpm)

Fp (x10)

• How much savings does increased activity generate ?• Heat and mass balance model → Economic Model (IPCC/Abanades)• Increasing activity from 7.5% to 30% generates 11.4$ / t CO2 savings

→ With current assumption set

• Carbonator Capex change difficult to estimate – Fixed TCR + ?? $ / t.h-1

Economic Analysis

Economic Potential at 30% Activity11$ /tCO2

Economic Analysis

• What is the cost of increasing activity w/ different methods ?

• Methods for increasing activity can be compared with this method

• Reactivation costs and less than make-up in all cases tested → lower CaCO3 content of the feed (100% vs 45 %)

• Preliminary result : 30% activity w/ superheating costs 6$/tCO2

A 100 Mwth plant w/ Ca Looping would save 5$ / t CO2 → 1.2 M$ p.a. • Reactivator cost : 5.3 M$?• Carbonator cost : 7.5 M$?

Conclusion

• The return step of Ca(OH)2 to the Ca loop is important

• A superheating reactivator requires little interaction with the Ca Loop

• Economic modeling indicates that activity is worth increasing – economic data needed (anyone?)

• IRL is utimately seeking to transfer the superheating technology to a partner with the skill-set to develop it.

Model Assumptions

Technical:– Plant Size 100MW

– Coal 80%C,5%H2

– Base plant efficiency 0.368

– Charge Factor 0.8

– CO2 Capture efficiency 85%

– Carbonation Efficency 80%

– Oxygen cost 220 kj/mol

– Solid Transport 1 kW / t

– CaCO3 after reactivator 45 %

– Base Ca loop Activity 7.5 %

Economic:– TCR - Base Plant 542 $ / kWth

– TCR - Oxy-Fuel 990 $ / kWth

– TCR – Carbonator 81 $ /kWth + 22 k$ / t.h-1

– TCR - Reactivator 81 $ /kWth + 22 k$ / t.h-1

– Cost of Limestone 10 $ / t

(purchase + disposal)

– FOM,VOM,FCF etc.. As per IEA

Heat and Mass Balance Model Results -Make-up vs. Reactivation

CaO

CaCO3

Carbonator Calciner

Limestone100% CaCO3

Reactivator

45% CaCO3

Heat to Power Cycle

Cost of Make up- Increased O2 demand- Calcination heat not recovered- Additional Compression- Limestone Opex

Extra CO2 -make-up

Oxy-CombustionHeat

Purge30% CaCO3

Make-Up

Cost of Reactivation- Increased O2 demand (lower!)- Calcination heat not recovered (less!)- Reactivator Capex

14.2 $/tCO2

6.2 $/tCO2

Cost BreakdownMake-up –

7.25%Reactivation

– 7.25%

Fp 0.6 0.63

Efficiency 30% 32%

Capex increase ($/tCO2 avoided)

Oxy-fuel plant 1.8 0.7

Reactivator 2.9Opex increase ($/tCO2 avoided)

Calcin. heat losses 4.2 1.8Oxygen generation 1.0 0.5

Compression 1.7 0.5

Limestone 5.4Total 14.2 6.3

Heat and Mass Balance Model Results -The Cost of Reactivation, Breakdown

CaO

CaCO3

Carbonator Calciner

Limestone100% CaCO3

Reactivator

45% CaCO3

Heat to Power Cycle

Extra CO2 -make-up

Oxy-CombustionHeat

Purge30% CaCO3

Make-Up

Different Reactivation Techniques

Materic et al. Ind. Eng. Chem. Res 2011

Different Reactivation Techniques

Materic et al. Ind. Eng. Chem. Res 2011

Science Status – Model developed

• Hypothesis: CO2 gas leads to proton injection, stabilises Ca(OH)2.

• Ca(OH)2 is carbonated directly, CaO never forms→ Weakening avoided

• Model correlating Pore Size and Superheating behaviour was developed

Non-superheating Ca(OH)2Superheating Ca(OH)2

Dehydration Trigger (N2) Dehydration Trigger (N2)

CFB Attrition Tester

Main Attrition Sources :

• Impact (jet)

• Abrasion (cyclones)

• Chemical structure change

Solid Injection Area

CO2 / N2 Gas IN

Furnace

Zone

Furnace

Zone

Decreasing Frequency of Reactivation

Superheating is effective as long as CaO can be hydrated