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Fuel ash behavior –

importance of melting

Why is ash melting important?

Bed agglomeration in fluidized bed boilers

Bed behavior in BL recovery boilers

Deposit formation and build up

Corrosion of superheaters

0

200

400

600

800

1000

1200

Tem

pera

ture

[°C

Na2S (1180)

K2SO4 (1069)

K2CO3 (901)

K2S (948)

Na2SO4 (884)

Na2CO3 (858)

KCl (771)

NaCl (801)

Na2S2O7 (402)

K2S2O7 (415)

NaOH (320)

KOH (404)

NaHSO4 (187)KHSO4 (197)

Pure substance

melting points

Chemistry in Combustion Processes II

- Hupa

400 500 600 700 800 900

Initialcone

Firstmelting

Sticky Radicaldeformation

Completemelting

Temperature, Co

Ash Melting at Increasing Temperatures


- Hupa

0

10

20

30

40

50

60

70

80

90

100

500 550 600 650 700 750 800 850 900 950 1000

Temperature, oC

Am

ou

nt

of

me

lt, w

-%Melting behavior of different alkali salts

T15

T70

0

10

20

30

40

50

60

70

80

90

100

400 600 800 1000

Me

lt f

rac

tio

n [

Wt%

]

T [ C]

T0 T15

T70 T100

Percentage Melt vs Temperature for an Alkali Salt Mixture

T0 First meltingtemperature

T15 Stickytemperature

T70 Flow temperature

T100 Complete melting temp.

Chemistry in Combustion Processes II - Hupa

Chemistry in Combustion Processes II - Hupa

550

950

0 20 40 60 80 100

Composition [wt-% NaCl]

Tem

peratu

re [

°C] LIQUID

Na2SO4(s) + NaCl(s)

85

15

70

884

T100 838

T70 820

T15 648

T0 628

800

L + NaCl(s)

4%

628

Percentage of molten phase – “lever rule”

Sticky ash and T15


- Hupa

Entrained Flow Particle ReactorUniversity of Toronto

25.4

mm

Gasbrännare

Provmatning

Sond

Ugn

Partiklar

VCR

Våg

9 m

Mullitrör


- Hupa

Stickiness of Salt Particles vs.

Temperature and Composition

0

0.02

0.04

0.06

0.08

0.10

600 700 800 900 1000

Temperature ( oC)

Dep

osi

tio

n R

ate

(m

g/g

/cm

2/m

in)

2 mol% Na2Cl2

5%10%

20%

Na2SO4 - NaCl blandningar


- Hupa

Stickiness of Partially Molten ParticlesEntrained Flow Reactor Tests in Toronto

0

0.02

0.04

0.06

0.08

0.10

0.12

0 10 20 30 40 50 60 70 80 90 100

Fraction molten phase (wt-%)

De

po

sitio

n (

mg

/g-c

m2-m

in)


- Hupa

Sticky ash and T15F

urn

ace

ga

s te

mp

era

ture

(°C

)

Ash particles stick

upon impact

T15 5 mole-% K/(Na+K)

Location of sticky area in boiler depends on

furnace gas temperature and ash melting behavior

(Honghi Tran)

NaHSO4 can form due to

cool lower furnace, high

sulfidity

Can extend range to lower

temperatures

Sticky

Ash particles – can be assumed to have same T as furnace gas

Ash

particles

do not stick

Flowing ash deposit and T70

Air Cooled Probes

after Exposure in

Flue Gases

Probe Surface Temp 500 C

Flue Gas Temp 950 C

1 min 10-20 min 60+ min

Deposit has started

growing

Deposit continues

growing

Deposit stops

Growing in thickness

Temperature around a tube

wall and steady-state deposit

thickness

Tube wall

Tsteam = 500 °C

Tgas = 1000 °C

Flue-gas-to-steam heat transfer

- Clean tube

570 °C 600 °C

Heat flux

110 kW/m2


- Clean tube

Convection ←Conduction← Radiation

& convection


- Tube with deposit

Tube wall

Steam

500 °C

Flue gas

1000 °CHeat flux

Ash deposit

Temperature

profile ?

Tube wall

(5 mm)

Tsteam = 500 °C

Tgas = 1000 °C


- Tube with deposit

560 °C580 °C

Case Heat flux

Clean 110 kW/m2

Deposit 95 kW/m2

680 °C

Ash deposit (1mm)

Thermal conductivities λ (W/m-K)

Steel: 20

Deposit: 1

Conduction heat flux q = λ·ΔT/Δx

ΔT/Δx = q/λ

110 kW/m2 95 kW/m2

Deposit thickness and surface temperature

95 kW/m2 82 kW/m2

1 mm deposit 2 mm deposit

Note how deposit surface temperature increases as deposit grows in thickness

0

10

20

30

40

50

60

70

80

90

100

400 600 800 1000

Me

lt f

rac

tio

n [

Wt%

]

T [ C]

T0 T15

T70 T100

Percentage Melt vs Temperature for an Alkali Salt Mixture





Tube wall

(5 mm)

Tsteam = 500 °C

Tgas = 1000 °C

Steady-state deposit thickness

550 °C570 °C

Case Heat flux

Clean 110 kW/m2

1 mm dep. 95 kW/m2

2 mm dep. 82 kW/m2

Steady-state

deposit 79 kW/m2

750 °C

Ash deposit (2.3mm)

T70

0

10

20

30

40

50

60

70

80

90

100

400 600 800 1000

Me

lt f

rac

tio

n [

Wt%

]

T [ C]

T0 T15

T70 T100

Ash ”Sticky range”



T70 Flowtemperature


Sticky range

Sticky ash and T15F

urn

ace

ga

s te

mp

era

ture

(°C

)

Ash particles stick

upon impact

T15 5 mole-% K/(Na+K)

Location of sticky area in boiler depends on


(Honghi Tran)



sulfidity


temperatures

Sticky


Ash

particles

do not stick

Fu

rna

ce

ga

s te

mp

era

ture

(°C

)

Lim

ited

to s

teady-s

tate

thic

kness

Sticky

rang

e

De

po

sit

ca

n

keep

gro

win

g

Does n

ot

stick

T70T15 5 mole-% K/(Na+K)

(Honghi Tran)



sulfidity


temperatures


Ash melting and deposit growthLocation of sticky range / region where deposits can grow in a boiler depends on


Steady-state

deposit thickness

Deposit can grow

5 mole-% K/(Na+K)

(Honghi Tran)

Steady-state

deposit thickness

Deposit can grow

T (°C)

T (°C) T (°C) T (°C)

Fu

rna

ce

ga

s te

mp

era

ture

(°C

)

Lim

ited

to s

teady-s

tate

thic

kness

Sticky

rang

e

De

po

sit

ca

n

keep

gro

win

g

Does n

ot

stick

T70T15 What would happen to the

position of the sticky range

in the superheater region in

case the furnace gas

temperatures were to

increase ?

Consider two scenarios

1) furnace gas temperature at

the bullnose level increases

2) Deposit growth in the

superheater region becomes

for some reason higher or

alternatively the superheater

region sootblowing becomes

less efficient; either reason

resulting in thicker deposits




Fu

rna

ce

ga

s te

mp

era

ture

(°C

)

Lim

ited

to s

teady-s

tate

thic

kness

Sticky

rang

e

De

po

sit

ca

n

keep

gro

win

g

Does n

ot

stick

T70T15 Is the ”stickyness” criterion

the only relevant reason for

deposit build-up, i.e. are

there ash deposits on heat

transfer surface after the

sticky region ?

In case there are ash

deposits after the sticky

region, is there a need to

clean/sootblow in these

areas ?




High temperature corrosion

and T0

Corrosion test with alkali salt

deposits on steel at 550 C0 % molten phase in

deposit

5 % molten phase in

deposit

High temperature corrosion and T0

Tflue gas

Tste

am

180 kW/m2

180 kW/m279 kW/m2

79 kW/m2

T0 = 540 °C

T0

T0 T0

T0

Fuel ash melting - summary


- Hupa

Fuel ash melting – important for understanding

ash-related issues and for boiler design and

operation

Bed agglomeration in fluidized bed boilers

Bed behavior in BL recovery boilers

Deposit formation and build up

Corrosion of superheaters

Thermodynamic modeling of ashchemistry and melting

0

10

20

30

40

50

60

70

80

90

100

400 600 800 1000

Me

lt f

rac

tio

n [

Wt%

]

T [ C]

T0 T15

T70 T100







Experim

enta

lM

odelin

g

Melting behavior

Corrosion experiments

Well defined

conditions,

synthetic ash

Laboratory Pilot / Full scale

Corrosion probes

Actual boiler ash

Melting behavior

(Stable species ~ ash corrosivity)

Synthetic ash

Melting behavior

(Stable species ~ ash corrosivity)

Accurate prediction of ash

composition and conditions at steel

surface???

Understanding

boiler

• ash chemistry

and melting

• corrosion

Thermodynamic modeling successfully utilized in understanding BL smelt chemistry and melting

Predicting Ash Chemistry

Reactive

Inert

Thermo-

dynamic

equi-

librium

Chemical

compositionFuel

sample 1

Chemical

fractionation

&

SEM

&

Lab testsDecision how to divide fuel elements into

”reactive” and ”inert” fractions ?

• Some info can be drawn from e.g.

Fuel fractionation data

• Based on trial-and-error

(comparison of predictions to exp data)

40Mueller, Skrifvars, Backman, Hupa (2003) in ”Progress in CFD”

Thermodynamic modeling - Summary

Succesfully used for predicting smelt and

dust chemistry in black liquor combustion

For biomass combustion, additional

process or fuels specific parameters often

needed

Element speciation, release / reactivity

Parametric studies to give general

understanding of chemistry

fuel ash behavior importance of meltingusers.abo.fi/maengblo/cet_2020/3_importanceof... ·...

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