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Comparison of Flame Stability under Air and Oxy-Fuel Conditions for an Aerodynamically Stabilized Pulverized Coal Swirl Flame

Martin Habermehl, Johannes Hees, Diego Zabrodiec, Reinhold Kneer 40th International Technical Conference on Clean Coal & Fuel Systems, May 31st – June 4th, 2015, Clearwater, Florida

Comparison of Flame Stability under Air and Oxy-Fuel Conditions for an Aerodynamically Stabilized Pulverized Coal Swirl Flame | Martin Habermehl | 40th International Technical Conference on Clean Coal & Fuel Systems | 2015, May 31st – June 4th | Clearwater, FL, USA

Introduction

Page 2

Different „modes“ of coal combustion:

Air Oxy-fuel (25 vol.-% O2) Oxy-fuel (21 vol.-% O2)

• What determines the stability of air and oxy-fuel flames?
• Possible parameters: flow velocities, momentum flows, oxygen concentrations,
• etc. etc.
• Investigation of flame stability conducted

Comparison of Flame Stability under Air and Oxy-Fuel Conditions for an Aerodynamically Stabilized Pulverized Coal Swirl Flame | Martin Habermehl | 40th International Technical Conference on Clean Coal & Fuel Systems | 2015, May 31st – June 4th | Clearwater, FL, USA

Experimental Setup: Combustion chamber

Page 3

4200 mm 2100 mm Ø400 mm

Displaceable burner port Observation ports (4x) Combustion chamber Flue gas quench Flue exit to the stack Coal CO2

(Carbon dioxide)

O2

(Oxygen)

Air supply Gas mixer

Comparison of Flame Stability under Air and Oxy-Fuel Conditions for an Aerodynamically Stabilized Pulverized Coal Swirl Flame | Martin Habermehl | 40th International Technical Conference on Clean Coal & Fuel Systems | 2015, May 31st – June 4th | Clearwater, FL, USA

Experimental Setup: Measurement techniques

Page 4

4200 mm 2100 mm Ø400 mm

Flue gas analyzer (NDUV/NDIR)

• Flue gas analyzer installed at

the stack

• Measurement of CO

concentration (dry) by non- dispersive infrared spectrography (NDIR)

• CO concentration used as an

indicator for stable combustion (low values  stable combustion) Flue gas analytics: CO content

NO CO O2 CO2

Refrigerant type dryer

Comparison of Flame Stability under Air and Oxy-Fuel Conditions for an Aerodynamically Stabilized Pulverized Coal Swirl Flame | Martin Habermehl | 40th International Technical Conference on Clean Coal & Fuel Systems | 2015, May 31st – June 4th | Clearwater, FL, USA

Experimental Setup: Measurement techniques

• Detection of UV radiation

emitted by excited OH* radicals at approx. 307 nm

• Indication of zones with high

reaction rates and heat release

• Mapping of flame structures
• Investigation of the entire flame:

 Placing the burner at different height positions  Recording multiple images and average them  Generate one chemiluminescence map of the entire flame by composing the averaged images

Page 5

CCD camera & intensifier UV filter

(λ=307 nm, Δλ=10 nm)

Optical access window

Investigation of OH* chemiluminescence

Recording system

Comparison of Flame Stability under Air and Oxy-Fuel Conditions for an Aerodynamically Stabilized Pulverized Coal Swirl Flame | Martin Habermehl | 40th International Technical Conference on Clean Coal & Fuel Systems | 2015, May 31st – June 4th | Clearwater, FL, USA

Experimental Setup: Measurement techniques

• Coal particles used as tracer

particles

• Measurement of axial and

tangential mean flow velocity components by two laser beam pairs of different wavelengths

• Positioning of the measurement

volume:

 Radial position adjusted by traversing a horizontally displaceable rack for the optics  Axial position adjusted by traversing the burner

• Only axial velocity components

considered in this study

Page 6

Laser Doppler Velocimetry

Ar Ion Laser Transmission

• ptics

Receiver

• ptics

Horizontally displaceable rack Evaluation and recording system Measurement volume

Comparison of Flame Stability under Air and Oxy-Fuel Conditions for an Aerodynamically Stabilized Pulverized Coal Swirl Flame | Martin Habermehl | 40th International Technical Conference on Clean Coal & Fuel Systems | 2015, May 31st – June 4th | Clearwater, FL, USA

Experimental Setup: Swirl Burner Oxy-3

Page 7

T S S T Sta Sta

P – Primary flow: coal dust + carrier gas S – Secondary flow, swirled T – Tertiary flow Sta – Staging flow

P

Comparison of Flame Stability under Air and Oxy-Fuel Conditions for an Aerodynamically Stabilized Pulverized Coal Swirl Flame | Martin Habermehl | 40th International Technical Conference on Clean Coal & Fuel Systems | 2015, May 31st – June 4th | Clearwater, FL, USA

Very schematic sketch of design flame shape

Page 8

Internal recirculation backflow Conical dispersing swirl combustion zone

Comparison of Flame Stability under Air and Oxy-Fuel Conditions for an Aerodynamically Stabilized Pulverized Coal Swirl Flame | Martin Habermehl | 40th International Technical Conference on Clean Coal & Fuel Systems | 2015, May 31st – June 4th | Clearwater, FL, USA

Settings and Investigated Cases Local Oxygen Ratio at Burner vicinity

• Parameter characterizing the near burner

flow field: Local oxygen ratio λlocal:

𝜇𝑚𝑝𝑑𝑏𝑚 = 𝑛 𝑃2,𝑢𝑠𝑝𝑣𝑕ℎ 𝑐𝑣𝑠𝑜𝑓𝑠 𝑛 𝑃2,𝑠𝑓𝑟𝑣𝑗𝑠𝑓𝑒

• Takes only the flows through the burner

into account

• Remaining oxygen provided by staging

flow

Page 9

• Influenced by the Oxygen content xO2 and the molar mass MOxydizer of the oxidizer

𝜇𝑚𝑝𝑑𝑏𝑚 = 𝑦𝑃2 ⋅ 𝑁𝑃2 𝑁𝑃𝑦𝑧𝑒𝑗𝑡𝑓𝑠 ⋅ 𝑛 𝑃𝑦𝑧𝑒𝑗𝑡𝑓𝑠 𝑢𝑠𝑝𝑣𝑕ℎ 𝑐𝑣𝑠𝑜𝑓𝑠 𝑛 𝑃2

• Applied values: λlocal = 0.6; 0.8; 1.0

Comparison of Flame Stability under Air and Oxy-Fuel Conditions for an Aerodynamically Stabilized Pulverized Coal Swirl Flame | Martin Habermehl | 40th International Technical Conference on Clean Coal & Fuel Systems | 2015, May 31st – June 4th | Clearwater, FL, USA

Settings and Investigated Cases Oxidizer Composition

• Investigated Oxidizer Compositions:

 „Reference“ case with air (21 vol.-% O2) (AIR)  Oxy-fuel case with 21 vol.-% O2 and 79 vol.-% CO2 (OXY-21)  Oxy-fuel case with 25 vol.-% O2 and 75 vol.-% CO2 (OXY-25)

• Reasoning for these parameters:

 OXY-21: Same volumetric flow rates, velocities and O2 content as for the AIR case  OXY-25: Same momentum flow rates P

as for the AIR

case

Page 10

λlocal AIR OXY-21 OXY-25 1.0 1.000 1.438 1.003 0.8 0.640 0.920 0.642 0.6 0.360 0.518 0.361

Normalized total momentum flow through the burner

𝑄 = 𝑛 ⋅ 𝑤 =

𝑛 2 𝜍⋅𝐵𝑑𝑠𝑝𝑡𝑡

Comparison of Flame Stability under Air and Oxy-Fuel Conditions for an Aerodynamically Stabilized Pulverized Coal Swirl Flame | Martin Habermehl | 40th International Technical Conference on Clean Coal & Fuel Systems | 2015, May 31st – June 4th | Clearwater, FL, USA

Observations and Results Stability assessment from flue gas analytics

• Stable combustion:

 Low values of CO concentration within the flue gas (single digit range)  Continuous and stable trend of the CO concentration

• Instable combustion:

 Raised and fluctuating values of CO concentration within the flue gas (100 – 200 ppm)  Sporadic distinct CO concentration overshoots

Page 11

λlocal AIR OXY-21 OXY-25

1.0 stable stable stable 0.8 stable stable Stable 0.6 instable stable instable

Comparison of Flame Stability under Air and Oxy-Fuel Conditions for an Aerodynamically Stabilized Pulverized Coal Swirl Flame | Martin Habermehl | 40th International Technical Conference on Clean Coal & Fuel Systems | 2015, May 31st – June 4th | Clearwater, FL, USA

Observations and Results Intensity maps of OH* chemiluminescence emissions for AIR (relative intensity units)

Page 12

• Main reaction

zone attached to the burner quarl

• Conical shape
• f the reaction

zone identifiable

• Prolonged

flame for decreased local oxygen ratio

• Cylindrical

vortex instead

• f conical

expanding swirl

• Significantly

increased length of the reaction zone

Comparison of Flame Stability under Air and Oxy-Fuel Conditions for an Aerodynamically Stabilized Pulverized Coal Swirl Flame | Martin Habermehl | 40th International Technical Conference on Clean Coal & Fuel Systems | 2015, May 31st – June 4th | Clearwater, FL, USA

Observations and Results Mean axial velocities measured by LDV for AIR

Page 13

Internal recirculation backflow Extending conical swirl region No recirculation backflow at x = 100 mm No extending swirl

Comparison of Flame Stability under Air and Oxy-Fuel Conditions for an Aerodynamically Stabilized Pulverized Coal Swirl Flame | Martin Habermehl | 40th International Technical Conference on Clean Coal & Fuel Systems | 2015, May 31st – June 4th | Clearwater, FL, USA

Observations and Results Intensity maps of OH* chemiluminescence emissions for OXY-25 (relative intensity units)

Page 14

• Main reaction

zone attached to the burner quarl

• Conical shape
• f the reaction

zone identifiable

• Reaction zone

size increased compared to AIR

• Change of

flame structure very similar to the air case

Comparison of Flame Stability under Air and Oxy-Fuel Conditions for an Aerodynamically Stabilized Pulverized Coal Swirl Flame | Martin Habermehl | 40th International Technical Conference on Clean Coal & Fuel Systems | 2015, May 31st – June 4th | Clearwater, FL, USA

Observations and Results Intensity maps of OH* chemiluminescence emissions for OXY-21 (relative intensity units)

Page 15

• Main reaction

zone lifted off from the burner quarl

• Compact

coherent reaction zone

• Main reaction

zone lifted off from the burner quarl

• No significant

change of the flame structure as for the other cases

Comparison of Flame Stability under Air and Oxy-Fuel Conditions for an Aerodynamically Stabilized Pulverized Coal Swirl Flame | Martin Habermehl | 40th International Technical Conference on Clean Coal & Fuel Systems | 2015, May 31st – June 4th | Clearwater, FL, USA

Observations and Results Intensity maps of OH* chemiluminescence emissions for OXY-21 (relative intensity units)

Page 16

Internal recirculation backflow

Comparison of Flame Stability under Air and Oxy-Fuel Conditions for an Aerodynamically Stabilized Pulverized Coal Swirl Flame | Martin Habermehl | 40th International Technical Conference on Clean Coal & Fuel Systems | 2015, May 31st – June 4th | Clearwater, FL, USA

Interpretation: Change of flame structure

Page 17

Flow separation from the conical burner quarl wall Breakdown of internal recirculation backflow zone Conical expanding swirl replaced by a cylindrical non- expanding swirl

Comparison of Flame Stability under Air and Oxy-Fuel Conditions for an Aerodynamically Stabilized Pulverized Coal Swirl Flame | Martin Habermehl | 40th International Technical Conference on Clean Coal & Fuel Systems | 2015, May 31st – June 4th | Clearwater, FL, USA

Conclusions

Page 18

• Transition between the two observed flame patterns seems mainly

governed by the momentum flow through the burner

• Different momentum flows caused by two effects

 Increased molecular weight of CO2 in comparison to N2  Increase of O2 concentration in the oxidizer streams results in reduced overall mass flows for same oxygen ratio λ

• Balance of these two effects causes similar flame characteristics and

change of flame pattern for the AIR and OXY25 cases

• The increased momentum flow for the OXY21 cases sustains the

intended flow field pattern

• Stability of the investigated flame mainly determined by aerodynamic

effects

• Main parameter for similar flame behavior: momentum flow

Thank you for your attention!

Dipl.-Ing., B.E. Martin Habermehl habermehl@wsa.rwth-aachen.de Institute of Heat and Mass Transfer RWTH Aachen University www.wsa.rwth-aachen.de

Deutsche Forschungsgemeinschaft DFG (German Research Foundation), Collaborative Research Center SFB/Transregio 129 „Oxyflame“ Acknowledgements