research perspective - review of the current …€¢cha frorm co 2 atmosphere most likely higher...

Universität StuttgartProf. Dr. techn. G. Scheffknecht

Institut für Verfahrenstechnik und DampfkesselwesenInstitute of Process Engineering and Power Plant Technology

Research Perspective - Review of the

Current Understanding, Identifying

Research Gaps

Prof. Dr. Günter ScheffknechtInstitute of Process Engineering and Power Plant Technology IVD

University of Stuttgart

1st IEA GHG Oxyfuel Combustion ConferenceCottbus, September 9, 2009

Universität Stuttgart

2

Fundamental Principle of the Oxyfuel Process


3

Review of Current UnderstandingSituation Today

• Oxy-combustion has been investigated for a long time (also for other reasons than CCS)

• Increasing interest due to CO2 mitigation in the last 10 - 15 years

• Combustion evaluation in pilot scale in the lower MW rangedone on various places

• Various design studies for commercial plants have beenperformed

• With regard to coal PF as well as CFB firing technologies areavailable for oxyfuel combustion


44

NNCoal2009R22PJupiter Pearl plant, USA5

SCR FF70% dryNNCoal2008R10PBabcock&Wilcoxpilot plant, USA2

NNCoal2008NA30PDoosan Babcock, UK3

FFNYYCoal2010R30PCallide (CS Energy, Australia)6

SCR, ESP, FGDYYYCoal2015N250DJänschwalde9

NNCoal2013N50CFBJamestown/ Praxair Plant, USA8

YYCoal2016R100DYoungdong, South Korea10

No Demo/Pilot plant name

Scale(Demo/

Pilot plantMWe

New Retrofit Start up Main

Fuel

Electricitygeneration(Yes/No)

CO2Compression

(Yes/No)

CO2use/Seq

CO2purity

Gasclean up

1 Vattenfall pilotplant, Germany P (PC) 10 N 2008 Coal N Y Y (partial

by truck) 99,90% ESP, FGD

4 ALSTOM, Windsor, USA P 5 R 2009 Coal N N N FF, NID

7 CIUDEN, Spain P (PC/CFB) 10 N 2010 Coal Y N SCR FF

Overview Pilot and Demonstration Plants(Coal only)

Source: T. Wall, 2009, own updates


5

Scale-up over Time

Source: T. Wall, 2009


6

Presentation Outline

Recycle Rate / Oxygen Concentration

Pyrolysis & Char Combustion

Burner Aerodynamics / Flame Characterisation

NOx Emissions

Sulfur Chemistry

Radiation

Slagging, Fouling and Corrosion

Modelling

Fluidized Bed


7

Recycle Rate /

Oxygen Concentration


8

Recycle Rate and Oxygen Concentration

tadiabatic

CoalHeat Output

Flue Gas with Fly Ash

Bottom Ash

OxygenyO2, mix

tadiabatic

CoalHeat Output

Flue Gas with Fly Ash

Bottom Ash

OxygenyO2, mix

a) b)

Source: A. Kather, 2009


9

Effect of Different Recycle Rates

Source: J. Smart, 2008


10

Effect of Different Oxygen Concentrations (and Recycle Rates) on Flame Pattern

Source: J. Smart, 2008

Air

Oxyfuel

28% O2

Recycle rate 77%

Oxyfuel

38% O2

Recycle rate 66%


11

Pyrolysis & Char Combustion


12

Impact of Atmosphere on Pyrolysis Reactivity

Char gasification C+CO2 2CO is overruling pyrolysis attemperatures > 800°C



13

Impact of Atmosphere on Pyrolysis Gas Speciation

0

2

4

6

8

10

12

650 750 850 950 1050 1150

Pyrolysis temperature, °C

Gas

con

cent

ratio

ns [v

ol%

]

H2

CO

CH4

0

2

4

6

8

10

12

650 750 850 950 1050 1150

Pyrolysis temperature, °C

Gas

con

cent

ratio

ns [v

ol%

]

H2

CO

CH4

Pyrolysis using 100% N2 Pyrolysis using 100% CO2

H2

CO

CH4

H2

CH4

CO

Char gasification C+CO2 2CO is changing thepyrolysis gas speciation at temperatures > 800°C

Source: L. Al-Makhadmeh, 2009


14

Char Conversion - Different Results

Source: Wall, 2008; L. Al-Makhadmeh, 2009

• O2 diffusivity is lower slower reaction rates• Char faces higher O2 concentration• Char from CO2 atmosphere most likely higher BET surface

0

20

40

60

80

100

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5Residence time, s

Cha

r bur

nout

[wt %

daf

]

5_N2

5_CO2

15_N2

15_CO2

Initial oxygen conc. 5/15%


15

Reduction of Recycled NO on Chars

0

10

20

30

40

50

60

70

80

90

100

0 200 400 600 800 1000

Red

uctio

n of

recy

cled

NO

[%]

NO-inj [ppm]

KK-N2_8% O2/N2 KK-CO2_8% O2/N2KK-CO2_8% O2/CO2 LA-CO2_8% O2/CO2

KK_N2

LA_CO2

KK_CO2

KK_CO2

• Oxyfuel conditions enhance reduction of recycled NO

• Significant differences between various chars

Source: L. Al-Makhadmeh, 2009


16

Burner Aerodynamics /

Flame Characterisation


17

Ignition BehaviourRadiation Intensity over Injection Distance

Source: Molina & Shaddix, 2009

• Ignition deferred under oxyfuel conditions

• Higher variation indicates more unstable combustion


18

Ignition BehaviourFlame Propagation Velocity

• Flame propagation velocity in CO2/O2 largely decreases to 1/3-1/5 of that in N2/O2

Source: Okazaki, 2008


19

Stabilisation by Aerodynamic Measures

AirOxy

21% O2

Initial burner

AirOxy

21% O2

Burner with higher internal recirculation

Source: D. Toporov, 2008


20

Stabilisation by Individual Mixing

Air

Oxy

21% O2 totalO2 injecteddirectly

Oxy

33% O2 totalO2 mixedwith RFG


21

NOx Emissions


22

Nitrogen OxidesCourses of Axial Gas Concentration

0

150

300

450

600

750

0 0.5 1 1.5 2 2.5

Distance from burner [m]

NO

, HC

N, N

H3

[ppm

]

0

6

12

18

24

30

NO HCN NH3 O2 CO

0

150

300

450

600

750

NO

,HC

N [p

pm]

0

6

12

18

24

30

0

150

300

450

600

750

0 0.5 1 1.5 2 2.5


0

6

12

18

24

30

O2,

CO

[Vol

. %]

0

150

300

450

600

750

0

6

12

18

24

30

O2,

CO

[Vol

. %]

Staging Staging

Staging Staging

Air, KK OF27, KK

OF27, LAAir, LA


23

Nitrogen OxidesReduction Rate of Recycled NO

0

25

50

75

100

0.75 0.85 0.95 1.15

Burner stoichiometry

Red

uctio

nof

recy

cled

NO

[%]

KK,BR KK,BO LA,BR LA,BO

34

12

96

33

92

11

88

14

79

8

59

50


24

Nitrogen OxidesOxyfuel vs. Air Combustion

• NOx in recirculated flue gas is reduced in flames• No thermal NOx generation from N2

• Conversion ratio of N content within the fuel to NOxis more limited in oxy-fuel

• NOx Emission 1/4-1/3 in oxyfuel combustionSource: T. Wall, 2006; Okazaki, 2008


25

Sulfur Chemistry


26

In-furnace Desulfurization

Source: Okazaki, 2008


27

SulfurComparison Air vs Oxyfuel



28

SulfurCourses of Axial Gas Concentration

0

2000

4000

6000

0 0.5 1 1.5 2 2.5Distance from burner [m]

SO2,

H 2S

[ppm

]0

6

12

18

24

30

O2

[Vol

. %]

SO2 H2S SO2+H2S O2

0

2000

4000

6000

0 0.5 1 1.5 2 2.5Distance from burner [m]

SO2,

H2S

[ppm

]

0

6

12

18

24

30

O2

[Vol

. %]

(SO2+H2S)max

(SO2+H2S)max

Air, LA

OF27, LA


29

Sulfur – Speciation in SubstoichiometricCombustion Zone

0

25

50

75

100

KK,0 KK,3000 LA,0 LA,3000

Vol

umet

ric s

hare

of S

O2 a

nd H

2S [%

]

SO2 H2S

0

25

50

75

100

KK,0 KK,3000 LA,0 LA,3000

Vol

umet

ric s

hare

of S

O2 a

nd H

2S [%

]Air OF27

Special attention needed for low rank coals with high sulfur content?


30

Radiation


31

• Higher CO2 (and higher H2O) partial pressure let expect a higher radiation intensity

• This is confirmed for gaseous fuels • In coal flames this might be overruled by particle

radiation and temperature effectsSource: Anderson, 2008


32

Slagging, Fouling and Corrosion


33

Ash behaviour

• Higher CO2 partial pressure expected to affect the transformations of coalminerals, particularly carbonates such as calcite CaCO3 and siderite FeCO3.

• Ash deposition in the boiler may be affected.• Fly ash properties might change

Source: F. Wigley, 2008


34

Deposit Formation – Various Approaches forDeposit and Corrosion Investigations

Formation of„Origin“ Samples

Further Exposure toGas Atmosphere

in Laboratory

SamplePreparation


35

25% Chromium Material after Exposure in an Oxyfuel Atmosphere

Fe Ni

O S


36

Conclusion

• Higher sulphur intrusion due to higher SO2 concentration• Tendency towards carbonate formation• Effect of higher CO2 concentration on the formation of the

(protective) oxide scales: scale appears less stable and lesscompact compared to the air case and, therefore, a higher gas diffusivity towards the base material occurs.

Deposit behaviour, cleanability (soot blowing) and corrosionresistance need to be verified in pilot plants


37

Modelling


38

CFD ModellingMain Issues• Extended homogeneous and heterogeneous reaction schemes required:

(R1) CmHn + m/2 O2 → m CO + n/2 H2(R2) CmHn + m H2O → m CO + (m+n/2) H2(R3) CO + ½ O2 ↔ CO2(R4) H2 + ½ O2 ↔ H2O(R5) CO + H2O ↔ CO2 + H2

(R6) C + ½ O2 → CO(R7) C + H2O → CO + H2(R8) C + CO2 → 2 CO

• Radiation ModellingGas absorptionSootScattering…

• NOx chemistry (especially reduction via char)

• Validation required

• High importance for scale-up


39

CFD ModellingValidationExtended chemistry modellingdue to increasedconcentrations of CO2 and H2O (important as well for stagedair combustion)

0

10

20

30

40

50

60

70

80

90

100

0 0.5 1 1.5 2 2.5


O2,

CO

, CO

2[v

ol. %

dry

]

O2 measuredCO measuredCO2 measured

O2 simulatedCO simulatedCO2 simulated

Simulationvs measurement

(DTF 20 kWth)


40

Fluidized Bed


41

Fluidized Bed Combustion

The majority of the items presented so far are related to PF firing. Fluidized bed combustion offers further opportunities.

• External heat exchanger in the primary loopA higher heat duty in the primary loop leds to significantlylower recirculation rates (with corresponding high oxygenconcentrations in the oxygen-RFG mixture)

• Opportunity fuels (high ash fuels, slurry, anthracite, biomass, RDF, ...

• Direct desulfurisation• ...

research perspective - review of the current …€¢cha frorm co 2 atmosphere most likely higher...

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