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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
Chemistry in Combustion Processes II
- 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
Chemistry in Combustion Processes II
- Hupa
-
Entrained Flow Particle ReactorUniversity of Toronto
25.4
mm
Gasbrännare
Provmatning
Sond
Ugn
Partiklar
VCR
Våg
9 m
Mullitrör
Chemistry in Combustion Processes II
- 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
Chemistry in Combustion Processes II
- 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)
Chemistry in Combustion Processes II
- 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
-
Flue-gas-to-steam heat transfer
- Clean tube
Convection ←Conduction← Radiation
& convection
-
Flue-gas-to-steam heat transfer
- 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
Flue-gas-to-steam heat transfer
- 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
T0 First meltingtemperature
T15 Stickytemperature
T70 Flow temperature
T100 Complete melting temp.
-
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”
T0 First meltingtemperature
T15 Stickytemperature
T70 Flowtemperature
T100 Complete melting temp.
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
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
-
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)
NaHSO4 can form due to
cool lower furnace, high
sulfidity
Can extend range to lower
temperatures
Ash particles – can be assumed to have same T as furnace gas
Ash melting and deposit growthLocation of sticky range / region where deposits can grow in a boiler depends on
furnace gas temperature and ash melting behavior
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
Ash particles – can be assumed to have same T as furnace gas
Ash melting and deposit growthLocation of sticky range / region where deposits can grow in a boiler depends on
furnace gas temperature and ash melting behavior
-
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 ?
Ash particles – can be assumed to have same T as furnace gas
Ash melting and deposit growthLocation of sticky range / region where deposits can grow in a boiler depends on
furnace gas temperature and ash melting behavior
-
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
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Fuel ash melting - summary
Chemistry in Combustion Processes II
- 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
Thermodynamic modeling of ashchemistry and melting
T0 First meltingtemperature
T15 Stickytemperature
T70 Flow temperature
T100 Complete melting temp.
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Thermodynamic modeling of ashchemistry and melting
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”
-
41
-
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
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