Transformation of Pyrite and Its Implication on Ash Deposition during

Oxy-coal Combustion

Dunxi Yu

Weizhi Lv, Jianqun Wu

Lanlan He, Minghou Xu

State Key Laboratory of Coal Combustion (SKLCC) Huazhong University of Science & Technology (HUST)

Wuhan, Hubei, China

Ponferrada, Spain, 9th – 13th September 2013

Motivation

Design & operation of oxyfuel systems

Prediction & optimization Model validation &development

Fundamental understanding of the impacts of oxyfuel combustion

Ash deposition

Combustion Pollutant

emissions ……..

Impacts of ash deposition Ash deposition

Slagging: on walls in the radiant section Fouling: on convection tube sections

Impacts Reduced heat transfer Impedance of gas flow Physical damages Corrosion ……

Reduced efficiency, Unscheduled outages, Reduced availability, Costly modifications, etc

Slagging processes Mineral transformation and ash formation

Mineral properties (Chemical form/mineralogy, Association with carbon, Particle size distribution…)

Time-temperature-atmosphere history

Ash transport phenomena Fluid Dynamics

interaction with heat surfaces Particle properties (size, composition, fusion, etc) Surface properties (deposit properties)

Role of pyrite in ash deposition

Pyrite: FeS2

Major Fe-containing mineral in coal

Minor Fe-containing minerals such as siderite (FeCO3), ankerite (CaFe(CO3) 2)

Major source of Fe that contributes to ash

slagging High content of Fe in ash deposits is often

observed

Excluded pyrite Pyrite transformation

Included pyrite

McLennan et al.， Energy&Fuel，2000

Factors affecting pyrite transformation

Gas chemistry Time-temperature history Availability of oxygen Occurrence of pyrite in coal ……

Objective of this work

Does replacing N2 in air by CO2 have effects on pyrite transformation?

What are the implications of the effects on ash deposition?

Coal experiments (DTF)

Excluded/included pyrite experiments

(DTF)

Excluded pyrite experiments (TGA)

Work organization

Observation of the impact of oxyfuel combustion on iron transformation

Specially designed to see if oxyfuel combustion affects pyrite transformation

Mechanisms? The role of CO2

Coal experiments (DTF)

Excluded/included pyrite experiments

(DTF)

Excluded pyrite experiments (TGA)

Coal properties

Ash composition (wt %)

Na2O MgO Al2O3 SiO2 P2O5 SO3 K2O CaO TiO2 Fe2O3

SH 2.11 0.79 15.19 30.35 0.57 13.73 0.33 21.12 0.46 15.35

CF 0.99 3.12 14.30 45.79 0.62 12.98 0.76 6.07 0.39 14.98

Proximate analysis (wt%, ad) Ultimate analysis (wt%, ad)

M VM A FC C H N S O(by

difference)

SH 5.67 32.45 7.23 54.65 70.63 7.45 0.90 0.40 7.73

CF 8.07 37.67 14.94 39.32 63.94 5.59 0.96 0.25 6.25

SH: bituminous coal; CF: lignite coal. Particle size: <150 µm

Quantification of iron minerals

0

10

20

30

40

50

60

Mo

le f

ract

ion

of

Fe

in m

iner

al, %

Pyrite Siderite Iron oxide

Iron minerals

SH

CF

Obtained by Mössbauer spectroscopy

Experimental Facility: DTF (drop tube furnace) Atmosphere： AIR , 21%O2/79%N2, OXY21, 21%O2/79%CO2 Gas flow rate: 4L/min Feeding rate: 0.3g/min Wall temperature: 1300ºC Resid. time: ~2s Samples: Bulk ash Analysis: Mössbauer spectroscopy

Results – Speciation of Fe in ash For each coal, three phases were identified:

Magnetite (Fe3O4) Hematite (Fe2O3)

Decomposition and oxidation Fe-glass (Fe-Si-Al)

Interactions between Fe and alumino-silicates

The result suggests that replacing N2 by CO2

does not change Fe-containing phases in ash

Results – Fe partitioning

1. Hematite: OXY21 > AIR

2. Magnetite: OXY21 < AIR

3. Fe-glass: similar content

Replacing N2 by CO2 seems to favor the formation of hematite, indicating CO2 plays a role in iron transformation

Coal experiments (DTF)

Excluded/included pyrite experiments

(DTF)

Excluded pyrite experiments (TGA)

Properties of pyrite mineral

Commercial sample

Elemental composition of pyritesample

0

10

20

30

40

50

60

Na Ca Al Si S K Fe

wt

%

Quartz Pyrite

Sample preparation

Excluded pyrite sample Original pyrite sample was crushed and sieved

into 38-45µm to simulate the excluded pyrite in the coal

Synthetic char with included pyrite

Original pyrite sample was crushed and sieved into 0-10µm, which was dispersed in synthetic char

Furfuryl alcohol 80℃ 10min 0-10µm pyrite particle

20nm carbon nano-particles 4-Toluene sulfonic acid

N2

125℃ 10h

N2

N2

550℃ 1h 15-45µm synthetic char

grinding

sieving

Mixed synthetic char

synthetic char

synthetic char

200℃ 6h

(Synthetic char preparation, method after Sensor C.L et al. 1984)

Experimental Conditions similar to those of coal experiments

Facility: DTF (drop tube furnace) Atmosphere： AIR , 21%O2/79%N2, OXY21, 21%O2/79%CO2

Gas flow rate: 4L/min Feeding rate: 0.18g/min Wall temp.: 1400ºC Resid. time: ~2s Samples: Bulk ash Analysis: XRD

Results – Speciation of Fe Excluded pyrite Included pyrite

1. Two phases were identified: hematite and magnetite

2. Replacing N2 by CO2 does not change Fe-containing phases in products of excluded/included pyrite

▲Hematite ◆Magnetite

Results – Fe partitioning

Excluded pyrite: (1) hematite > magnetite; (2) hematite increases while magnetite decreases, when switching from AIR to OXY21

Included pyrite: (1) magnetite > hematite; (2) magnetite increases while hematite decreases, when switching from AIR to OXY21

Excluded pyrite Included pyrite

Coal experiments (DTF)

Excluded/included pyrite experiments

(DTF)

Excluded pyrite experiments (TGA)

Experimental

Temperature Flow rate（L/min）

Mass (g) Residence

time (min)

900℃

2 0.5g

8

800℃ 8

700℃ 10

600℃ 15

500℃ 15

400℃ 15

Facility： TGA Sample： Excluded pyrite sample Atmosphere: 100% CO2

Mass loss M

ass

loss

, g

Temperature, ºC

Mass loss increases with increasing temperature. Indicating that increasing temperature enhances the release of sulfur.

Mineralogy △ Quartz SiO2 ☆ Pyrrhotite Fe1-xS ★ Pyrite FeS2 ◆ Magnetite Fe3O4

Temperature, ºC

HSC calculation

1. 400-500 ºC pyrite no significant decomposition 2. 600 ºC pyrite & pyrrhotite significant decomposition 3. 700-800 ºC pyrrhotite stable phases 4. 900 ºC pyrrhotite & Magnetite commencement of oxidation

Gas evolution 400℃ 600℃ 500℃

700℃ 800℃ 900℃

1. The production of SO2 and COS indicates that CO2 participates in pyrite transformation.

2. The increase in mass loss with increasing temperature is due to the enhanced release of sulfur, that is oxidized to SO2 by CO2.

Mechanisms Bhargava et al. (Fuel 2009) observed the formation of magnetite &

hematite > 800ºC during heating of pyrite in CO2. It was attributed to the oxidation by O2 from CO2 dissociation.

2CO2

Temperature, ºC

0

0.01

0.02 G

as p

rod

uct

ion

, mo

l HSC calculation

Mechanisms

Appreciable CO2 dissociation is expected only when the temperature >1200ºC.

The formation of magnetite during heating pyrite in CO2 at 900ºC is most likely due to direct reactions between CO2 and pyrite or its products.

Discussion on DTF results Excluded pyrite Included pyrite

1. Oxidizing condition 2. High level of CO2 in OXY21

case promotes the oxidization of pyrite to hematite

1. More reducing condition for the OXY21 case due to reactions between CO2 and carbon matrix

2. The oxidation of pyrite to hematite is delayed

Implications of the findings

For coals enriched with excluded pyrite, replacing N2 by CO2 will lead to a lower content of Fe in the deposits

Because a larger fraction of hematite are expected, which has a high melting point and is less likely to stick to the waterwall.

Implications of the findings

For coals enriched with included pyrite, replacing N2 by CO2 will lead to a higher content of Fe in the deposits

Because delayed oxidation of pyrite are expected, which will result in a longer duration of the melt phase and favors the deposition of iron phases on the waterwall.

Conclusions Does replacing N2 in air by CO2 have effects

on pyrite transformation? YES. Excluded pyrite: enhanced hematite formation

Due to oxidation of pyrite by CO2 Included pyrite: enhanced magnetite formation

Due to more reducing conditions caused by CO2 induced gasification reactions

Conclusions What are the implications of the effects on

ash deposition? Excluded pyrite enriched coals: lower Fe in ash

deposits Included pyrite enriched coals: higher Fe in ash

deposits

Issues need to be addressed in the future Reaction mechansims between CO2 and pyrite Kinetics Effects of gas impurities (H2O, SO2, etc.) ……

Acknowledgements

Program for New Century Excellent Talents in University (No. NCET-11-0192)

National Key Basic Research and Development Program of China (No. 2013CB228501)

Specialized Research Fund for the Doctoral Program of Higher Education (No. 20110142110075)

Thank you

additional slides

Fegley and Hong

Mechanochemical milling-induced

Aylmore et al.

Bhargava et al.