An analysis of in-situ phase changes occurring in natural

hematite exposed to simulated high temperature redox gas

cycling encountered in chemical looping

Oxy-Combustion and Chemical Looping Program Review Omni William Penn Hotel

Pittsburgh, PA

August 25, 2017

Anna Nakano1,2, Jinichiro Nakano1,2, James Bennett1

1US Department of Energy, National Energy Technology Lab. 2AECOM Corporation, USA

2

Introduction

Bayham, S., Straub, D., and Weber, J., “Operation of the NETL Chemical Looping Reactor with Natural Gas and a Novel Copper-Iron Material,” NETL-PUB-20912; NETL Technical Report Series; U.S. Department of Energy, National Energy Technology Laboratory: Morgantown, WV, 2017.

Chemical Looping Reactor at NETL Morgantown, West Virginia

https://www.netl.doe.gov/newsroom/labnotes/labnotes-archive/01-2014More info on CLC:

Technical IssueOxygen carriers experience microstructural changes and degradation during CLC, contributing to attrition. Understanding of materials behavior is needed.

Objective of this part of workStudy in-situ microstructural characterization of hematite during oxidation/reduction cycles (no particle mixing).

3

Experimental Materials

Sample

XRF (mass.%) XRD

Fe2O3 MnO SiO2 Total ERR% Identified crystallinephases

Natural hematite 86.27 3.51 10.23 100.0 ±0.50 Fe2O3 (hematite)SiO2 (moganite)

Natural hematite- Primary crystalline phase = hematite.- Source = Wabush Mine,

Newfoundland/Labrador, Canada- Material crushed, particle size -48 US Mesh.

4

Experimental Test Procedures

Heating chamber

HT Confocal Scanning Laser Microscope – used to observe isothermal Redox gas cyclic exposures

• Temperature: 800oC and 1200°C • Gas switched between synthetic air (oxidation) and 10 vol.% CO-90 vol.% Ar (reduction) at a flow of 50 ml/min• Repeated 10 cycles -20 gas switches (1 cycle = 7.5 min each for oxidation + 10 min each for reduction)

5

Natural hematite analysis (800oC)

6

Real time surface morphology change at 800°C

Play back 8xGas switched at 11:00

Reduction. Cycle 1: 10CO-Ar

7

Hematite: Overall change with redox cycles (real color)

Before redox cycles

Gas switch # 1 (CO) Gas switch # 9 (CO) Gas switch # 19 (CO)

Gas switch # 2 (Air) Gas switch # 10 (Air) Gas switch # 20 (Air)

800°C

8

Before redox cycles

Gas switch # 1 (CO) Gas switch # 9 (CO) Gas switch # 19 (CO)

Gas switch # 2 (Air) Gas switch # 10 (Air) Gas switch # 20 (Air)

Hematite: Overall change with redox cycles (topology map)

800°C

9

Agglomeration at 800°C

300 µm

Before cycles00:00:00

300 µm

Cycle 3: 10CO-Ar00:40:00

300 µm

Cycle 6: 10CO-Ar01:32:30

300 µm

Cycle 9: 10CO-Ar02:25:00

300 µm

After 10 cycles: air02:50:00

1-2 4-5 7-8

7-8

SEM (cross-section)

10

Crack formation at 800°C

300 µm

Cycle 4: air01:05:32

300 µm

Before cycles00:00:00

300 µm

Cycle 6: air01:40:20

300 µm

Cycle 8: air02:15:26

300 µm

After 10 cycles: air02:50:20

1-3 5 6-7

6-7 9

11

SEM-WDS analysis: cross-sectionInner grains in each particle After 10 redox cycles at 800°C

Outer layer: hematite

Inner grains: magnetite

Grains inside the particle

Note: Dark grey area is remaining silica-based polishing/epoxy

Sintering of inner grains inside the particle

Outer layer formation around individual inner grains

Hematite particle

12

Volume change at 800°C

0

50

100

150

200

250

300

350

0 2 4 6 8 10 12 14 16 18 20

Volu

me

chan

ge (

%)

Gas switch

CO

Air

0

50

100

150

200

250

300

350

0 2 4 6 8 10 12 14 16 18 20

Volu

me

chan

ge (

%)

Gas switch

• Zig-zag volume change: decreases from reduction to oxidation, increases from oxidation to reduction – mainly due to hematite ↔ magnetite transitions (magnetite lattice > hematite lattice).

• Overall volume increase due to void formation.

Note: 3D scan in Air at 800oC was used as a reference 100% volume.

0

50

100

150

200

250

300

350

0 2 4 6 8 10 12 14 16 18 20

Volu

me

chan

ge (

%)

Gas switch

10CO-Ar

Air

13

Surface area, Area, and Roughness Changes at 800°C

• Surface area = topography area (including bumps and dents – 3 D).

• Area = overall area without bumps and dents – 2 D.

• Roughness = difference in max. heights and pits over surface

Note: 3D scan in air at room temperature before redox exposure was used as a reference for surface area and area.

• Increasing zig-zag trend with cycles• Surface area increase (25%) > area increase (21%) due to

the formation of surface bumps and dents.• Overall roughness increase due to structural

breakdown/void formation.

100

105

110

115

120

125

130

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

Surf

ace

area

cha

nge

(%)

Number of gas switches

Surface Area

100

105

110

115

120

125

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

Area

cha

nge

(%)

Number of gas switches

5

10

15

20

25

30

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

Roug

hnes

s (µm

)

Number of gas switches

Roughness

Area

14

• Linear kinetics = interfacial• Velocity = 0.99 um/sec

y = 0.99x - 0.08R² = 0.9998

0

5

10

15

20

25

30

35

0 5 10 15 20 25 30 35

Tim

e (s

ec)

Distance (um)

Kinetics of surface phase transition at 800°C(Cycle 2: 10CO-90Ar)

Phases calculated by thermodynamics. Confirmation by SEM-WDS.

15

CO1 CO2 CO3 CO4 CO5 CO6 CO7 CO8 CO9CO10

04386

310372434

04488

304

342

380

0

10

20

CO1 CO2 CO3 CO4 CO5 CO6 CO7 CO8 CO9CO10

405060

528561594

P1 (s

ec)

Reduction

P2 (s

ec)

v (u

m/s

ec)

Del

ay (s

ec)

Air1 Air2 Air3 Air4 Air5 Air6 Air7 Air8 Air9 Air10

0.0

1.9

3.8

5.7

0.0

2.3

4.6

6.9

0

29

58

87

Air1 Air2 Air3 Air4 Air5 Air6 Air7 Air8 Air9 Air10

43.2

45.0

46.8

48.6

50.4

P1 (s

ec)

Oxidation

P2 (s

ec)

v (u

m/s

ec)

Del

ay (s

ec)

Kinetics at 800°C

• Delay = incubation time required for the onset of phase transition after gas switch

• v = velocity of surface phase transition

• P1 and P2 = time required for complete visual surface phase transition of the selected particles 1 and 2.

16

Natural hematite analysis (1200oC)

17

Natural hematite-1200oC

Reduction. Cycle 1 (10CO-Ar)

Play back 6x

Delay about 6 min before the front line was noted.Surface became rougher. Particles moved.

Reduction. Cycle 3 (10CO-Ar)

Play back 32x

Gas switched at 10:00 Gas switched at 45:00

Surface change‘front line’

Delay about 45 sec before the surface change was noted.Surface became darker and rougher. Particles moved, expanded, cracked.

Crack

18

Natural hematite, CSLM (1200oC)

Oxidation. Cycle 1 (Air)

Delay about 45 sec. Surface brightened and became more rough.

Oxidation. Cycle 3 (Air)

Play back 2x Play back 2xGas switched at 55:00Gas switched at 20:00

Delay about 45 sec. Surface darkened first and then brightened. Particles shrank.

19

300 µm 300 µm 300 µm

300 µm 300 µm 300 µm

Particle breakdown at 1200oC

Cycle 4: 10CO-Ar00:57:30

Before cycles00:00:00

Cycle 6: 10CO-Ar01:32:30

Cycle 8: 10CO-Ar02:07:30

1-3 5 7

Cycle 9: 10CO-Ar02:25:00

Cycle 10: 10CO-Ar02:42:30

7

20

Agglomeration at 1200oC

300 µm

Before cycles00:00:00

300 µm

Cycle 2: 10CO-Ar00:22:30

300 µm

Cycle 3: 10CO-Ar00:40:00

300 µm

Cycle 6: 10CO-Ar01:32:30

300 µm

Cycle 9: 10CO-Ar02:30:00

1 4-5

4-5 7-8300 µm

Just after 10 cycles: Air02:50:00

10

21

Volume change at 1200°C (color map)

Before redox cycles

Gas switch # 3 (CO) Gas switch # 7 (CO) Gas switch # 13 (CO)

Gas switch # 4 (Air) Gas switch # 8 (Air) Gas switch # 14 (Air)

8090

100110120130140150160170180

0 1 2 3 4 5 6 7 8 9 1011121314151617181920

Volu

me

chan

ge (%

)

Gas switch #

Orange: 10CO-ArBlue: Air

Note: Quartz window fogged after gas switch 15

22

SEM-WDS analysis: cross-section

After the redox gas cycles at 1200°C

Magnetite

Hematite

• The presence of interior magnetite implies 10 min reduction exposure was sufficiently long to induce phase transition at the center of the particle, at least at some point if not at later stages.

• At the last cycle, only surface grains transformed to hematite during 7.5 min air exposure.

Outer surface

Interior

23

Hematite surface change after redox exposures

White light images

Before redox exposure

After 10 redox cycles 800°C

After 10 redox cycles 1200°C

(General groups of hematite particles)Hematite surface turned reddish on cooling

Heavily sintered

24

• Materials behaviors of natural hematite (Fe2O3) particles at 800°C and 1200°C were investigated under redox gas cycles (synthetic air and 10CO-90Ar) using HT-CSLM

• During isothermal gas cycling, the hematite particles showed phase transitions across the surfaces corresponding to a hematite transforming to magnetite in 10CO-90Ar and magnetite to hematite when switching back to air (phases confirmed by SEM-WDS)

• In general, particle volumes, area, and roughness increased in transitioning to magnetite and decreased in transitioning to hematite, while, overall, all increased with cycles

• Layering around the particles was noted but full transition to magnetite throughout the particle indicates hematite would be suitable for CL application if appropriately processed

Conclusions

25

This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof.

Disclaimer

Acknowledgement

This technical effort was performed in support of the National Energy Technology Laboratory’s ongoing research under the RES contract DE-FE0004000.