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Stefan Åhman*, Wuyin Wang, Jörgen Grubbström09/09/2009

Flue Gas Condensers for Oxyfuel

POWER

Flue Gas Condenser for Oxyfuel - 1st Oxyfuel Combustion Conference, Cottbus 2009-09-09 P 2

Flue Gas Condenser forOxyfiring

A Flue Gas Condenser (FGC) plays an important role for drying & cooling of the flue gas prior to compression

Flue Gas Condensers previously used for recovery of low grade heat from fluegases when combusting waste, biofuel etc.

Boiler

Particulate control

Flue gas desulfurization

NOx controlCO2 compressor

Air sepa-ration unit

Flue gas condenser

Flue gas recirculation

CO2Air


Latent & Sensible Heats when cooling flue gas


Tube condenserVertical, straight-tube HEX – one pass tube side

Tube Condensor− Gas passing

downwards− Cooling water passing

in multi-stage cross flow upwards

− Essentially a counter current device

− Tubes/Tube sheet: Alloy Steel

− Shell: Pressure vessel steel

Gas in

Gas out

CW In

CW Out


Tube CondenserExample of Current Applications

Flue Gas Condenser at an IncinerationPlant

Heat delivered to the district heatingnet

Flue gas flows up to ~ 250,000 Nm3/h

Total plant efficiency = (El Power + Heat Power)/ Fuel Power may exceed 100 %, if based on Lower Heating Value (LHV) of fuel

Istalled at Waste- and Biofuelled plants


Condensing Scrubber

Condensing scrubber− Counter current packed

tower− Gas passing upwards

− Tower of FRP− Packing PP− External HEX of

Alloy SteelCW In

CW Out

Condensate

Gas in

Gas out


Condensing ScrubberExamples of Current Applications

Flue gas desulphurization by partial utilization of the alkalinityin sea water

As flue gas passes absorber, condensation of water occursparallell to the SO2 absorption

Flue gas flow per absorber up to3,500,000 Nm3/h


Cooling of Mixed Vapours with Noncondensing Gases

Similar calculation procedure for both types of condenser

Heat transfer driven by

Sensible heat: ξAckhg0(Tg – Tc) temperature difference

Latent heat ξStkg0λ(pv – pc) diffusion by partial

pressure difference

Total heat (gas): ξAckhg0 (Tg – Tc) + ξStkg

0λ(pv – pc)

Total heat (water): hi0 (Tc – tw) = U (Tg – tw) – Tube condenser

hl (Tc – TL) = U (Tg – TL) – Packed column

U is an over all heat transfer coefficient


Calculation procedureBasics

Mass transfer coefficients

kl = f(Re, Sc, packing & wetting properties)

kg = f(Re, Sc, packing & wetting properties)

Many correlations exist, e. g. Onda, Billet/Schultes, etc.

Packing constants matched to fit experimental data

By analogy between mass and heat transfer heat transfer coefficients are obtained:

hl = kl (ρl cp,l Kl/Dl)

hg = kg (ρg cp,g )2/3 (Kg/Dg)2/3

Total heat balance

hg (Tg – Tc) + kg λ(pv – pc) = hl (Tc – TL)


Calculation Procedure – Packed ColumnFlow sheet

If ( Tg out -Tg ) < eYes

Calculate wet gas enthalpy change in the Nth section, dH

Calculate hl , hg , kg at Nth stepusing appropriate correlations

Solve heat balance for Tc

Update Tg, TL

No

Choose appropriate dz

Z = Z + dz

Input Tg,in; Tg,out; TL,in; L; G; Z=0

Calculate TL,out; Tg=Tg in

Z; TL

Tg,out TL,in

Tg,in TL,out

dz


Calculation Procedure – Packed Column (Counter Current)Typical Output

0

10

20

30

40

50

60

70

80

90

100

Column height

Tem

pera

ture

, deg

C; P

erce

nt

Gas temp

Gas saturation temp

Liquid temp

% of total heat recovered

Gas water content, %


Calculation Procedure – Tube Condenser (Co Current) Typical Output

0

10

20

30

40

50

60

70

80

90

100

Condenser length, m

Tem

pera

ture

, deg

C; P

art o

f tot

al p

ower

tr

ansf

erre

d, %

t gas

t condensate

t water

% of total heat removed

Percent of total heat transferred per m

t gas

t condensate

t cooling water


Cooling of Mixed Vapours with Noncondensing Gases

Over all heat transfer coefficient U varies primarily with

• Water content of gas• Temperature• Flows

U varies in every point in the condenser

Rigorous calculation procedure required to take into account very varying gas conditions, due to phase change

Over all heat transfer coefficientas function of water content of gas

Evaluation of full scale data

0

70

140

210

280

350

420

490

560

0,00 2,00 4,00 6,00 8,00 10,00 12,00 14,00 16,00 18,00 20,00

Water content of gas, % vol

Ove

r all

heat

tran

sfer

coe

ffici

ent,

W/m

2, d

egC


Tube condenser vs. Packed column

Packed column

Two pinch points

Tower: dT outlet gas / inletrecirculating water

HEX: dT cooling water / recirculating water

Tube condenser

One pinch point

HEX: dT cooling water / condensate

A detailed analysis show slightly less costs for the tube condenser. However, cost is much influenced by actual prices for alloy steel, so analysis should be revisited for

every project


Lab pilotPacked Column

Purpose

Simulation of Oxyfuel conditions by addition of CO2

Vary gas flow, composition, temperatures

Secondary effects; SO2/SO3 removal etc.


Packed column - Water removal

0,00

10,00

20,00

30,00

40,00

50,00

60,00

70,00

80,00

90,00

100,00

Column height

Wat

er re

mov

al e

ffici

ency

, %

Increasing L/G


Water removal as a function of packingheight with L/G as parameter



Influence of CO2 content of gas on water removal efficiency

CO2 impact on water removal efficiency@ different L/G

82

84

86

88

90

92

94

96

0 10 20 30 40 50 60 70

CO2 (%)

Wat

er re

mov

al e

ffic

ienc

y (%

)


Evaluation of dataModelling supplemented by evaluation of HTU values

For heat transfer the NTU (number of transfer units) can be expressed as

NTU =

Including correction for Stefan flow this expression transforms into

This is used for evaluation of experimantal data.

The height of a transfer unit is then

HTU = Z / NTU, where Z is the height of the column

∫ −1

2)/( HHdH i

))/ln(/()()/(1

2 giggigi ppPppHHdH∫ −⋅−

Condensing scrubber, NOG

0,0000

0,0100

0,0200

0,0300

0,0400

0,0500

0,0600

0,0700

0,0800

0,0900

0,0 100,0 200,0 300,0 400,0 500,0 600,0 700,0 800,0

Enthalphy, kJ/kg dry gas

1/(i-

i*) 1/(i-i*)Low limitHi limit

NTU

Condensing scrubber

0,0

100,0

200,0

300,0

400,0

500,0

600,0

700,0

800,0

0,0 10,0 20,0 30,0 40,0 50,0 60,0 70,0 80,0

Temperature, deg C

Enht

alpy

, kJ/

kg d

ry g

as

Sat curveOp line

Enthalpy used as driving force


Scale up

0,00

0,35

0,70

1,05

1,40

1,75

0,00 3,00 6,00 9,00 12,00 15,00

Liquid to Gas Ratio

HTU

Lab Pilot

3 MW Pilot

Comparision of data from lab- and field pilot (~5 MW)

Lab scale data correlateswell with field pilot and full scale data

As expected, only minor impact of Oxyfuelconditions on heat transfer


SO2 Removal

NaOH may be added for

control of SO2 emission &

corrosion protection

SO2 absorption is much

enhanced at high pH, but

CO2 absorption rate is also

increasing …


0

0,5

1

1,5

2

2,5

3

3,5

4

4,5

4,00 4,20 4,40 4,60 4,80 5,00 5,20 5,40 5,60 5,80 6,00

pH

NaO

H a

dded

for C

O2

neu

tral

izat

ion,

kg/

h

10 % CO240 % CO2

40 % CO2

10 % CO2

100,000 Nm3 dry gas/h0,4 l/Nm3 condensate50 deg C

SO2 Absorption/Neutralisation

Increase of CO2 content from 10 % to 40 % increasesdissolved CO2 in the condensate

NaOH consumption for neutralization will increase


Condenser - Issues for Oxyfuel

Heat transferTarget is moisture removal and cooling, rather than heat recovery

Oxyfuel conditions can be readily adjusted for Mass transfer - SO2/SO3/HCl/HF/ash/aerosols

- Alkali consumption will influence range of application for SO2

removal- Influence on mass transfer of other species will be minor,

if any- SO2/SO3 removal tests in progress


Conclusion

Alstom has experience from condensing scrubbers and heat exchangers that is relevant for Oxyfiring conditions

The unusually high water content requires rigorous sizingprocedures

Supplementary studies in lab- and medium scale pilots confirmsizing methods and tools

The high CO2 content will have some (limited) impact on alkali consumption for adjustment of condensate pH

Performance of flue gas condenser will impact downstream process steps

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