Radiation and Convection Heat Transfer in Oxyfuel Combustion

transcript

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

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).

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

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

Driver for Studying Heat Transfer Distributions – Radiative and Convective

Schematic of CTF

IFRF Experimental Burner

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

Schematic of CTF Test Furnace

Oxyfuel System

Results: Radiative HT- South African coal – Dry RecycleRecycle

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

500 SAcoal/Air - 3% O2

Oxyfuel RR 65%

Oxyfuel RR 68%

Oxyfuel RR 70%

Oxyfuel RR 72%

Oxyfuel RR 75%

x k Oxyfuel RR 75%

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

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

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

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

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

New Build Retrofit Avoid

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

IFRF Burner – RR 77%; 28% Inlet O2

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)

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

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

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

Air Only

161820

Low O2 setting

Root Mid

6810121416

tion frequen

6810121416

tion frequen

0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5

Oscillat

Root Mid

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

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

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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Radiation and Convection Heat Transfer in Oxyfuel Combustion

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