Research Perspective - Review of the Current Understanding, Identifying Research Gaps

Transcription

1 Institut für Verfahrenstechnik und Dampfkesselwesen Institute of Process Engineering and Power Plant Technology Prof. Dr. techn. G. Scheffknecht Research Perspective - Review of the Current Understanding, Identifying Research Gaps Prof. Dr. Günter Scheffknecht Institute of Process Engineering and Power Plant Technology IVD University of Stuttgart 1 st IEA GHG Oxyfuel Combustion Conference Cottbus, September 9, 2009

2 Fundamental Principle of the Oxyfuel Process 2

3 Review of Current Understanding Situation Today Oxy-combustion has been investigated for a long time (also for other reasons than CCS) Increasing interest due to CO 2 mitigation in the last years Combustion evaluation in pilot scale in the lower MW range done on various places Various design studies for commercial plants have been performed With regard to coal PF as well as CFB firing technologies are available for oxyfuel combustion 3

4 Overview Pilot and Demonstration Plants (Coal only) No Demo/Pilot plant name Vattenfall pilot plant, Germany Babcock&Wilcox pilot plant, USA Doosan Babcock, UK ALSTOM, Windsor, USA Jupiter Pearl plant, USA Callide (CS Energy, Australia) 7 CIUDEN, Spain Jamestown/ Praxair Plant, USA Jänschwalde Youngdong, South Korea Scale (Demo/ Pilot plant P P P P CFB D D MW e New Retrofit R NA R R N N R Start up Main Fuel Coal Coal Coal Coal Coal Coal Coal Electricity generation (Yes/No) N N N Y N Y Y CO 2 Compression (Yes/No) P (PC) 10 N 2008 Coal N Y N Y N Y CO 2 use/seq Y (partial by truck) N N N Y Y CO 2 purity 70% dry Gas clean up 99,90% ESP, FGD SCR FF P 5 R 2009 Coal N N N FF, NID P (PC/CFB) 10 N 2010 Coal Y N SCR FF FF SCR, ESP, FGD Source: T. Wall, 2009, own updates 4

5 Scale-up over Time Source: T. Wall,

6 Presentation Outline Recycle Rate / Oxygen Concentration Pyrolysis & Char Combustion Burner Aerodynamics / Flame Characterisation NO x Emissions Sulfur Chemistry Radiation Slagging, Fouling and Corrosion Modelling Fluidized Bed 6

7 Recycle Rate / Oxygen Concentration 7

8 Recycle Rate and Oxygen Concentration a) b) Flue Gas with Fly Ash Oxygen y O2, mix t adiabatic Coal Heat Output Bottom Ash Source: A. Kather,

9 Effect of Different Recycle Rates Source: J. Smart,

10 Effect of Different Oxygen Concentrations (and Recycle Rates) on Flame Pattern Air Oxyfuel 28% O 2 Recycle rate 77% Oxyfuel 38% O 2 Recycle rate 66% Source: J. Smart,

11 Pyrolysis & Char Combustion 11

12 Impact of Atmosphere on Pyrolysis Reactivity Char gasification C+CO 2 2CO is overruling pyrolysis at temperatures > 800 C Source: T. Wall,

13 Impact of Atmosphere on Pyrolysis Gas Speciation Gas concentrations [vol%] Pyrolysis using 100% N 2 Pyrolysis using 100% CO 2 CO H2 H 2 H2 6 6 CO CO 4 CH4 CO 4 CH4 H CH 4 CH Gas concentrations [vol%] Pyrolysis temperature, C Pyrolysis temperature, C Char gasification C+CO 2 2CO is changing the pyrolysis gas speciation at temperatures > 800 C Source: L. Al-Makhadmeh,

14 Char Conversion - Different Results 100 Char burnout [wt % daf] _N2 5_CO2 15_N2 15_CO2 Initial oxygen conc. 5/15% Residence time, s O 2 diffusivity is lower slower reaction rates Char faces higher O 2 concentration Char from CO 2 atmosphere most likely higher BET surface Source: Wall, 2008; L. Al-Makhadmeh,

15 Reduction of Recycled NO on Chars Reduction of recycled NO [%] KK-N2_8% O2/N2 KK-CO2_8% O2/N2 KK-CO2_8% O2/CO2 LA-CO2_8% O2/CO2 KK_CO 2 KK_CO 2 KK_N 2 LA_CO NO-inj [ppm] Oxyfuel conditions enhance reduction of recycled NO Significant differences between various chars Source: L. Al-Makhadmeh,

16 Burner Aerodynamics / Flame Characterisation 16

17 Ignition Behaviour Radiation Intensity over Injection Distance Ignition deferred under oxyfuel conditions Higher variation indicates more unstable combustion Source: Molina & Shaddix,

18 Ignition Behaviour Flame Propagation Velocity Flame propagation velocity in CO 2 /O 2 largely decreases to 1/3-1/5 of that in N 2 /O 2 Source: Okazaki,

19 Stabilisation by Aerodynamic Measures Initial burner Air Oxy 21% O 2 Burner with higher internal recirculation Air Oxy 21% O 2 Source: D. Toporov,

20 Stabilisation by Individual Mixing Air Oxy 33% O 2 total O 2 mixed with RFG Oxy 21% O 2 total O 2 injected directly 20

21 NO x Emissions 21

22 Nitrogen Oxides Courses of Axial Gas Concentration NO HCN NH3 O2 CO NO,HCN [ppm] Air, KK Staging OF27, KK Staging O 2, CO [Vol. %] NO, HCN, NH 3 [ppm] Air, LA Staging OF27, LA Staging O 2, CO [Vol. %] Distance from burner [m] Distance from burner [m] 22

23 Nitrogen Oxides Reduction Rate of Recycled NO KK,BR KK,BO LA,BR LA,BO Reduction of recycled NO [%] Burner stoichiometry 23

24 Nitrogen Oxides Oxyfuel vs. Air Combustion NO x in recirculated flue gas is reduced in flames No thermal NO x generation from N 2 Conversion ratio of N content within the fuel to NO x is more limited in oxy-fuel NO x Emission 1/4-1/3 in oxyfuel combustion Source: T. Wall, 2006; Okazaki,

25 Sulfur Chemistry 25

26 In-furnace Desulfurization Source: Okazaki,

27 Sulfur Comparison Air vs Oxyfuel Source: T. Wall,

28 Sulfur Courses of Axial Gas Concentration SO2 H2S SO2+H2S O (SO 2 +H 2 S) max SO 2, H 2 S [ppm] Air, LA O2 [Vol. %] SO2, H2S [ppm] OF27, LA O2 [Vol. %] (SO 2 +H 2 S) max Distance from burner [m] Distance from burner [m] 28

29 Sulfur Speciation in Substoichiometric Combustion Zone SO2 H2S Volumetric share of SO 2 and H 2 S [%] KK,0 KK,3000 LA,0 LA,3000 Air Volumetric share of SO 2 and H 2 S [%] OF27 KK,0 KK,3000 LA,0 LA,3000 Special attention needed for low rank coals with high sulfur content? 29

30 Radiation 30

31 Higher CO 2 (and higher H 2 O) 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 effects Source: Anderson,

32 Slagging, Fouling and Corrosion 32

33 Ash behaviour Higher CO 2 partial pressure expected to affect the transformations of coal minerals, particularly carbonates such as calcite CaCO 3 and siderite FeCO 3. Ash deposition in the boiler may be affected. Fly ash properties might change Source: F. Wigley,

34 Deposit Formation Various Approaches for Deposit and Corrosion Investigations Formation of Origin Samples Sample Preparation Further Exposure to Gas Atmosphere in Laboratory 34

35 25% Chromium Material after Exposure in an Oxyfuel Atmosphere Fe Ni O S 35

36 Conclusion Higher sulphur intrusion due to higher SO 2 concentration Tendency towards carbonate formation Effect of higher CO 2 concentration on the formation of the (protective) oxide scales: scale appears less stable and less compact compared to the air case and, therefore, a higher gas diffusivity towards the base material occurs. Deposit behaviour, cleanability (soot blowing) and corrosion resistance need to be verified in pilot plants 36

37 Modelling 37

38 CFD Modelling Main Issues Extended homogeneous and heterogeneous reaction schemes required: (R1) C m H n + m/2 O 2 m CO + n/2 H 2 (R2) C m H n + m H 2 O m CO + (m+n/2) H 2 (R3) CO + ½ O 2 CO 2 (R4) H 2 + ½ O 2 H 2 O (R5) CO + H 2 O CO 2 + H 2 (R6) C + ½ O 2 CO (R7) C + H 2 O CO + H 2 (R8) C + CO 2 2 CO Radiation Modelling Gas absorption Soot Scattering NO x chemistry (especially reduction via char) Validation required High importance for scale-up 38

39 CFD Modelling Validation Extended chemistry modelling due to increased concentrations of CO 2 and H 2 O (important as well for staged air combustion) O 2, CO, CO 2 [vol. % dry] Simulation vs measurement (DTF 20 kw th ) O 2 measured CO measured CO 2 measured O 2 simulated CO simulated CO 2 simulated Distance from burner [m] 39

40 Fluidized Bed 40

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 loop A higher heat duty in the primary loop leds to significantly lower recirculation rates (with corresponding high oxygen concentrations in the oxygen-rfg mixture) Opportunity fuels (high ash fuels, slurry, anthracite, biomass, RDF,... Direct desulfurisation... 41