Radiative and Convective Heat Transfer in Oxy Coal CombustionTransfer in Oxy-Coal Combustion

John Smart Phil O‘Nions Gerry Riley Ed JamiesionJohn Smart, Phil O Nions, Gerry Riley, Ed Jamiesion

RWEnpower

RWEnpower’s OxyFuel facility

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Introduction

The work presented on radiative and convective heat transfer in oxy-coal combustion where coal is burnt, not in air but in a stream of 100% O2 diluted with recycled flue gas as comburent

The recycled flue gas can be either wet or dry dependent on where the recycled flue gas taken from in the system.

The recycled flue gas could be take wet from the outlet of the ESP (where the moisture content would be circa 18% by volume) or after an FGD system (where the moisture contentvolume) or after an FGD system (where the moisture content would be circa 8% by volume).

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Objectives of Current Experimental Programme

Simulate an oxyfuel recycle gas process using a once through combustion system – once through selected for flexibility

Both wet and dry recycle to be studied

Oxygen content in burner primary flow kept constant @ 21% v/v

Input parameters studied:

Recycle Ratio (RR) (65% 77%)- Recycle Ratio (RR) - (65% - 77%)

- Oxygen enrichment level (28% - 42 v/v)) dependent on RR and furnace exit Oand furnace exit O2

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Objectives of Current Experimental Programme (cont )(cont.)

Three coals to be studied – Two Russian and one South African

Co-firing of coal with biomass – Shea Meal and Sawdust (20% mass)

Measure heat transfer characteristics of flames compared to air –pradiative and convective

Carbon-in-Ash

Oxy-Overfire Air (OFA) to be Studied for heat transfer optimisation

Work performed on RWEn Combustion Test Facility (CTF) at DidcotDidcot

International Flame Research Foundation (IFRF) Burner

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Driver for Studying Heat Transfer Distributions – Radiative and Convective

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Schematic of CTF

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IFRF Experimental Burner

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Schematic of Once Through Oxy-Fuel System

Boiler SteamS

CoalPrimary AirBoiler Skid

HeatersMixing Skid

CO2Primary Flow

O2Vaporiser

Vaporiser

Secondary Flow

Tertiary Flow

Flow Control Skid

NOx/SOx

OXY OFA(Not used on this burner)

Secondary Air

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Secondary Air

Schematic of CTF Test Furnace

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Oxyfuel System

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Results: Radiative HT- South African coal – Dry RecycleRecycle

Furnace Heat Flux MeasurementsSouth African coal, Oxyfuel (3% O2)

500 SAcoal/Air - 3% O2

450

500

kW/m

2

Oxyfuel RR 65%

Oxyfuel RR 68%

Oxyfuel RR 70%

Oxyfuel RR 72%

Oxyfuel RR 75%

350

400

Hea

t Flu

x k Oxyfuel RR 75%

250

300

Rad

iativ

e H

200

50

0 500 1000 1500 2000 2500 3000 3500Axial Distance from Burner mm

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Axial Distance from Burner, mm

Normalised Convective & Radiativeheat fl R ssian Coal Dr Rec cleheat flux – Russian Coal - Dry Recycle

Dry Oxyfuel Operation Normalised to Air OperationPeak Radiation Flux Convective heat transfer and calculated flame temperaturePeak Radiation Flux, Convective heat transfer and calculated flame temperature

Russian coal

1.4

1.6

1.4

1.6Normalised Flame Temperature (calculated)Peak Normalised Heat Flux (measured)Normalised Convective HTC (measured)

Measured Convective Heat Transfer Coefficient indicates 74% Recycle is "Air-equivalent"

1.2

diab

atic

er

atur

e 1.2

adia

tive

and

Hea

t Flu

xMeasured Peak Radiative data indicates 74% Recycle is "Air-equivalent"

New Build Retrofit Avoid

0.8

1

orm

alis

ed A

dla

me

Tem

pe

0.8

1

Nor

mal

ised

Ra

Con

vect

ive

H

0.4

0.6

No F

0.4

0.6

N

Calculated dry oxyfuel adiabatic flame temperatures are equivalent to air at 69% recycle

1360% 65% 70% 75% 80%

Effective Recycle Ratio

IFRF Burner - RR 66%, 38% Inlet O2

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IFRF Burner – RR 77%; 28% Inlet O2

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Flame Animations (South African Coal) ( )

• Images for different simulated recycle rates under low O2 settings

70% rr, Total flow 656.99kg/h Sec 501kg/h@29.0% O2(time: 13:14)

68% rr, Total flow 615.71kg/h Sec 457kg/h@31.6% (time: 13:44)

65% rr, Total flow 554.74kg/h Sec 400kg/h@35.8% O2(time: 14:19)

72% rr, Total flow 709.04kg/h Sec 552kg/h@26.5%O2(time: 12:41)(time: 13:14)(time: 13:44)(time: 14:19) (time: 12:41)

• Images for different simulated recycle rates under high O2 settings

1665% rr, Total flow 567.69kg/h Sec 412kg/h@38.0% O2(time: 14:36)

75% rr, Total flow 806.57kg/h Sec 650kg/h@25.4% O2(time: 12:29)

72% rr, Total flow 722.64kg/h Sec 567kg/h@28.9%O2(time: 12:54)

70% rr, Total flow 670.91kg/h Sec 516kg/h@31.3%O2(time: 13:27)

68% rr, Total flow 624.70kg/h Sec 470kg/h@33.9%O2(time: 14:04)

Flame Imagesg

• Temperature profiles for different simulated recycle rates under lower O settingslower O2 settings

65% RR: Sec.f 368kg/h@34.8% 68%RR: Sec.f 62% RR: Sec.f 322kg/h@39.4% g @(time: 15:18, 29-10) 422kg/h@30.5%

(time: 15:05, 29-10)

62% RR: Sec.f 322kg/h@39.4%(time: 12:32, 30-10)

75% RR: Sec.f 600kg/h@22.1%(ti 13 41 29 10)

72% RR: Sec.f 513kg/h@25.5%( )

17Note: Images and temperature profiles shown here are averaged for 10 instantaneous readings over about 2 minutes.

(time: 13:41, 29-10) (time: 14:18, 29-10)

Flame oscillation frequency for different recycle ratios and total flows and their comparison to the air-firing (Russian Coal)

161820

Hz)

Air Only

161820

(Hz

Low O2 setting

Root Mid

6810121416

tion frequen

cy (H

6810121416

tion frequen

cy 

0246

0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5

Oscillat

Root Mid

0246

60 65 70 75 80

Oscillat

R l ti (%)20

High O2 setting

O2 (%)

Note: 1) The data points are average values of 10 instantaneous readings. The “error

b ” i di t th t d d i ti f th d t

Recylce ratio (%)

101214161820

equen

cy (Hz) Root Mid

bar” indicates the standard variation of the data.

2) Low O2%: 22.1(75rr), 25.5(72rr), 32.9(68rr), 34.8(65rr) and 39.4(62rr)

3) High O2%: 24.5(75rr), 28.0(72rr), 30.5(68rr), 37.1(65rr) and 41.7(62rr)

0246810

Oscillation fre

18

0

60 65 70 75 80Recylce ratio (%)

Conclusions (Dry recycle data) Air operation radiative heat flux found to be equivalent to 72 – 75%

recycle ratio (due to different radiative properties of carbon dioxide compared to nitrogen)

Radiative heat flux peak shifts downstream as recycle rate increases

Convective Heat Transfer equivalent to air at 74% recycle ratio (main factors here are temperature and mass flow)

Working range exists (there is a recycle ratio for which both Working range exists (there is a recycle ratio for which both radiative and convective transfer can be reasonable matched between air and oxyfuel operation. It is therefore possible to design a boiler for efficient operation in both oxyfuel and air conditions).

Flame stability decreases with increasing recycle ratio

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Stop Press

Latest results - Wet/Dry comparison

Radiative Heat FluxRussian coal B, 18%H2O, 3% O2

550

450

500

550

x kW

/m2

Russian B/Air, 3%O2 - dryOF 65% RR - dryOF 72% RR - dryOF 75% RR - dryOF 68% RR - wetOF 72% RR - wet

350

400

Hea

t Flu

x OF 75% RR - wetOF 65% RR - wet

250

300

Rad

iativ

e

2000 500 1000 1500 2000 2500 3000 3500

Axial Distance from Burner, mm

R

21

,

Latest results – Wet (8%) / Wet (18%) comparison

Radiative Heat Flux

Russian coal B, 3% O2 550

450

500

550

kW/m

2 OF 68% RR, H2O 18% - wet

OF 68% RR, H2O 8% - wet

350

400

450

eat F

lux

250

300

350

diat

ive

He

200

250

0 500 1000 1500 2000 2500 3000 3500

Rad

22Axial Distance from Burner, mm

Wet (18%) and Dry Recycle Normalised Peak Radiative and Convective Heat FluxRadiative and Convective Heat Flux

Normalised Peak Radiative and Convective Heat FluxRussian coal, 3%O2,

Dry v's Wet combustion1.4

nd Peak radiative - dry

Peak radiative - wet

1.0

1.2

conv

ectiv

e an

diat

ive

HF

Convective - dry

Convective - wet

Li (P k di ti

0.6

0.8

Nor

mal

ised

cpe

ak ra

d

0.460% 65% 70% 75% 80%

Effective Recycle ratio

N

Oxyfuel operation normalised to Air operation

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Acknowledgements

The combustion test facility conversion to oxyfuel was financed by RWE npower

The experimental programmes are co-funded by RWE npower and:

The European Commission Research Fund for Coal and Steel -BOFCom: Contract No RFCR CT 2006 0001BOFCom: Contract No. RFCR-CT-2006-0001

The UK Technology Strategy Board - Oxycoal-UK: Contract No. TPC/00/00404/00/00TPC/00/00404/00/00

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Thank you for your attention.

Questions?

Background – Recycle Ratio

Recycled Flue Gas (mRFG)

ASUAir

WetRecycle

DryRecycle

ASU Boiler

N2Fuel

CO2 - Rich Product(mPFG)

N2H20

MRFGRFGRR = ----------------------

MRFG + MPFG

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