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Ash and ash deposition for solid fuels
Bengt-Johan Skrifvars
Chemistry in Energy Technology, ÅA, 2020
Ash and ash deposition for solid fuels
Content
1. Ash related problems
Principles
Facts
2. Co-firing
3. Corrosion
4. Summary
Ash related problems
Why?
• Slagging, fouling and corrosion
most important single reason
for unscheduled shut downs of boilers
• Fireside deposits on heat exchanger tubes
- decreased heat transfer to steam/water side
- increased pressure drop in fluegas channel
- corrosion of heat exchanger tubes
• Emission problem
Trace elements, health risk
Amager Power Station:
Pendant SH after 1 week of Coal-Firing
/F.Frandsen DTU Ash Chemistry Course, October 1998/
• Depend on
- fuel ash content and type
- boiler type and operation
• Fireside deposits and corrosion
- all types of boilers
• Bed agglomeration
- fluidized bed boilers
• Trace emissions
- all types of boilers
Ash related problems
Ash and ash deposition for solid fuels
Content
1. Ash related problems
Principles
Facts
2. Co-firing
3. Corrosion
4. Summary
Bottom
ash
Pathways of ash forming elements entering a boiler
Transportation,
transformation,
reactions
Release of ash
forming elements
Slagging,
foulingFly ash
separation
Fuel
Air
Additives
Formation of a troublesome deposit:
Fuel
Formation of ash particles
Transportation of ash particles to a surface
Adhesion of ash particles to a surface
Densification of ash particles on a surface
Formation of a troublesome deposit:
Fuel
Formation of ash particles
Transportation of ash particles to a surface
Adhesion of ash particles to a surface
Densification of ash particles on a surface
• Ash = incombustible rest
• Quality & quantity depends on fuel
• Major elements:
Si, Al, Fe, Ti, Ca, Mg, Mn, P, Na, K, S, Cl
• Minor elements (trace elements, EU heavy metals):
As, Cd, Co, Cr, Cu, Hg, Mn, Ni, Pb, Sb, Tl, V
Ash-forming elements in a fuel
• Expressed often as weight-% oxides in ash
Si SiO2 Mn MnO
Al Al2O3 P P2O5
Fe Fe2O3 Na Na2O
Ti TiO2 K K2O
Ca CaO S SO3
Mg MgO Cl Cl
• Elements as oxides in ash is an assumption
everything in the fuel has oxidized
• If all elements have been analyzed
oxide sum = 100 %
Ash-forming elements in a fuel
Ash-forming elements in a fuel
• Minerals:
- included minerals
- excluded minerals
- Si, Al, Fe, Ca, Mg, Na, K, S
• Organically associated:
- Ca, Mg, K, Na, S
• Water soluble:
- Na, K, S, Cl
O
Na+O-
Included
mineals
Excluded
minerals
Org S
H2O-soluble
salts
Summary
FUEL ORGANICALLY MINERAL
ASSOCIATED PART
Brown coal 30 % 70 %
Bit. coal 15 % 85 %
Antrasite 5 % 95 %
Wood 100 % 0 %
Bark 98 % 2 %
Annual biom. 98 % 2 %
Oil 100 % 0 %
Waste derive 2-100 % 100-2 %
Ash-forming elements in a fuel
Formation of a troublesome deposit:
Fuel
Formation of ash particles
Transportation of ash particles to a surface
Adhesion of ash particles to a surface
Densification of ash particles on a surface
Formation of fly ash from coal /Flagan & Seinfield 1988/
Convective
transport
Pyrolysis Char burning and
fragmentation
Vaporization
Homogeneous
nucleation
Coagulation
Heterogeneous
condensation
Fly ash
0.1 -1 mm
Fly ash
1 - 100 mm
Mineral
inclusions
Excluded
minerals
Mineral
coalescence and
fragmentation
Mass-size distribution of fly ash from PC coal combustion/Flagan & Seinfield 1988/
0.1 10
Particle size dp
Ma
ss c
on
ce
ntr
atio
n fre
qu
en
cy
dm
/d(lo
gd
p)
Particles condensed
from gas phase
(aerosols)
Minerals
10.01
0.4
0.8
1.2
1.6
2.0
2.4
100
g
m3
mm
Mass concentration frequency, dm/d(logdp) ??
g/m3 ???
40
80
120
160
200
Particle size, mm
Num
ber
frequency
10 20 30 40 50
/Hinds, W: Aerosol technology 2nd Ed. Wiley & Sons, 1999/
1000 particles
0 – 50 mm
size nbr
0 - 4 mm 104
4 - 6 mm 160
6 - 8 mm 161
8 - 9 mm 75
9 - 10 mm 67
10 - 14 mm 186
14 - 16 mm 61
16 - 20 mm 79
20 - 35 mm 90
35 - 50 mm 18
S 1000
Num
ber
frequency
Let’s consider:
1000 particles
0 – 50 mm
nbr
size nbr size
0 - 4 mm 104 26
4 - 6 mm 160 80
6 - 8 mm 161 80.5
8 - 9 mm 75 75
9 - 10 mm 67 67
10 - 14 mm 186 46.5
14 - 16 mm 61 30.5
16 - 20 mm 79 19.7
20 - 35 mm 90 6
35 - 50 mm 18 1.1
S 1000
20
40
60
80
100
Particle size, mm
Num
ber
frequency/s
ize
10 20 30 40 50
mm-1
1000 particles
0 – 50 mm
nbr nbr fract
size nbr size size
0 - 4 mm 104 26 0.026
4 - 6 mm 160 80 0.080
6 - 8 mm 161 80.5 0.0805
8 - 9 mm 75 75 0.075
9 - 10 mm 67 67 0.067
10 - 14 mm 186 46.5 0.0465
14 - 16 mm 61 30.5 0.0305
16 - 20 mm 79 19.7 0.0197
20 - 35 mm 90 6 0.006
35 - 50 mm 18 1.1 0.0011
S 1000
0.02
0.04
0.06
0.08
0.1
Particle size, mm
Num
ber
fraction/s
ize
10 20 30 40 50
mm-1
dN/d(dp) = f(dp)
1000 mg of particles
0 – 50 mmmass mass mass fract
size (mg) size size
0 - 4 mm 104 26 0.026
4 - 6 mm 160 80 0.080
6 - 8 mm 161 80.5 0.0805
8 - 9 mm 75 75 0.075
9 - 10 mm 67 67 0.067
10 - 14 mm 186 46.5 0.0465
14 - 16 mm 61 30.5 0.0305
16 - 20 mm 79 19.7 0.0197
20 - 35 mm 90 6 0.006
35 - 50 mm 18 1.1 0.0011
S 1000
0.02
0.04
0.06
0.08
0.1
Particle size, mm
Mass fra
ction/s
ize
10 20 30 40 50
1/mm
dm/d(dp) = f(dp)
1000 mg/Nm3 of particles
0 – 50 mm
conc conc/size
size (mg/Nm3) (mg/Nm3/mm)
0 - 4 mm 104 26
4 - 6 mm 160 80
6 - 8 mm 161 80.5
8 - 9 mm 75 75
9 - 10 mm 67 67
10 - 14 mm 186 46.5
14 - 16 mm 61 30.5
16 - 20 mm 79 19.7
20 - 35 mm 90 6
35 - 50 mm 18 1.1
S 1000
20
40
60
80
100
Particle size, mm
Concentr
atio
n f
requency/s
ize
10 20 30 40 50
mg/(Nm3·mm)
dc/d(dp) = f(dp)
Mass-size distribution of fly ash from PC coal combustion/Jokiniemi & Kauppinen, 1995/
0.1 10
Particle size dp
Ma
ss c
on
ce
ntr
atio
n fre
qu
en
cy
dm
/d(lo
gd
p)
10.01
1000
2000
3000
4000
5000
6000
100
mg
Nm3
mm
Mass concentration frequency, dm/d(logdp) =
- average mass of particles within a certain particle size range
- particle size range expressed on a log-scale
- may be treated mathematically as a frequency function
for ex. total mass concentration = total area under the curve
mass concentration within a certain range
f(log(dp)d(log(dp)))
g/m3 =
- should actually be
g/m3/1
since the term log(dp) is in the denominator
∫0
log(dp)
Mass-size distribution of fly ash from PC coal combustion/Jokiniemi & Kauppinen, 1995/
0.1 10
Particle size dp
Ma
ss c
on
ce
ntr
atio
n fre
qu
en
cy
dm
/d(lo
gd
p)
10.01
1000
2000
3000
4000
5000
6000
100
mg
Nm3
mm
Stk =
~10 µm
Particle size-distribution measurements
with a low-pressure cascade impactor
••
/Berner 1972/
rp C(dp) v dp2
9 m W
rp: particle density
C(dp): Cunningham slip factor
function
v: jet velocity
dp: particle diameter
m: gas viscosity
W: jet diameter
~10 nm
Number-size and mass-size distribution of fly ash
from PC coal combustion /Jokiniemi & Kauppinen, 1995/
0.1 10
Particle size dp
Nu
mb
er
co
nce
ntr
atio
n fre
qu
en
cy
dN
/d(lo
gd
p)
10.01
1E+04
1E+05
1E+06
1E+07
1E+08
100
1
cm3
mm
Ma
ss c
on
ce
ntra
tion
freq
ue
ncy
dm
/d(lo
gd
p)
1000
2000
3000
4000
5000
6000
mg
Nm3
Particle size plots
- number-size distribution, mass-size distribution,
or concentration-size distribution
- expressed often as a frequency-per-size function
- x-axis particle size range often expressed on a log-scale
- y-axis numbers do not express directly number-, mass-,
or concentration values.
Particle size measurements
- Low pressure cascade impactor useful
- Gives mass vs size or number vs size information
~10 nm – 10 mm
Ash formation from other fuels,
indications from coal:
• High amount of organically associated minerals
a lot of sub-micron sized fly ash particles
• High amount of excluded minerals
a lot of larger fly ash particles
Formation of ash particles
Formation of a troublesome deposit:
Fuel
Formation of ash particles
Transportation of ash particles to a surface
Adhesion of ash particles to a surface
Densification of ash particles on a surface
Transport of ash particles to a surface
Diffusion
small particles
(< 0.5 - 5 mm)
Impaction
large particles
(> 0.5 - 5 mm)
/Hedley et al., Samms et al. 1966/
Transport of ash particles to a surface
• Diffusion- small particles (< 0.1 mm)- diffusion down the concentration gradient
(Fick´s law)
• Thermophoresis- small particles (< 5 mm)- diffusion down the temperature gradient
• Inertial impaction- large particles (> 5 mm)- dependent on gas velocity- angle of impact
The physics of thermophoresis
small
ash particle
Tem
pera
ture
Part
icle
net m
ovem
entgas molecule movements
gas molecule movements
Transport of ash particles in a boiler
Large particles
Small particles
Summary
• Diffusion- small particles (< 0.1 mm)- diffusion down the concentration gradient(Fick´s law)
- “Termophoresis” one diffusion mechanism in down-the-gradient direction
• Impaction- large particles (> 5 mm)- dependent of gas velocity & particle mass
Transport of ash particles to a surface
Formation of a troublesome deposit:
Fuel
Formation of ash particles
Transportation of ash particles to a surface
Adhesion of ash particles to a surface
Densification of ash particles on a surface
2-component phase diagram
600
700
800
900
5000 10050
NaCl Na2SO4
T0
T100Liquid
NaCl(s)+ L Na2SO4(s)+L
NaCl(s)+Na2SO4(s)
mol-%
oC
“Lever-rule”
600
700
800
900
5000 10050
NaCl Na2SO4
T0
T100
Liquid
NaCl(s)+ LNa2SO4(s)+L
NaCl(s)+Na2SO4(s)
A BC
Amount of melt =
Amount of solid =
A - BC - B
A - CC - B
x 100 %
x 100 %
Bulk composition = A
Liquid composition = C
Solid composition = B
mol-%
oC
Amount of melt vs temperature
100
80
60
40
20
0500 600 700 800 900
Temperature, (°C)
Am
ou
nt
of
me
lt, w
-%
85 mol-% Na2SO4
15 mol-% NaCl
450oC
Tsteam
Tsticky
Tfluegas1000oC
Deposit at its initial growth
Tsticky , “sticky temperature”
• silicates, “glas, slag”:
viscosity < 105 dPa s/Walsh et al 1990/
• low-viscous melt:
melt amount > 15 %/Backman et al 1987/
450oC
Tsteam
Tflow
Tfluegas1000oC
Deposit at its “equilibrium thickness”
Tflow , “flow temperature”
• silicates, “glas, slag”:
viscosity < 105 dPa s/Walsh et al 1990/
• low-viscous melt:
melt amount > 70 %/Backman et al 1987/
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
600 700 800 900 1000
10
8
6
4
2
0
Temperature, oC
De
po
sit
th
ick
ne
ss
, m
m
T15 T70
Deposit equilibrium thickness
600 700 800 900 1000 600 700 800 900 1000 600 700 800 900 1000
T15 = 530oC, T70 = 690oC
10
8
6
4
2
0
Temperature, oC
De
po
sit
th
ick
ne
ss
, m
m
T15 = 850oC, T70 = 900oC T15 = 710oC, T70 = 830oC
Deposit equilibrium thicknesses
for various compositions
• Amount of melt dominating reason for
- large impacting particles (> 10 mm)
- front side of tubes (wind side)
• Physical & physico-chemical forces important for
- small particles (< 1 mm)
- electrostatic forces, van der Waal’s forces
- around the tube (both wind & lee)
• Chemical reactions in the deposit sometimes
important
- Ca particles reactive with SO2 & CO2
Adhesion of ash particles to a surface
Formation of a troublesome deposit:
Fuel
Formation of ash particles
Transportation of ash particles to a surface
Adhesion of ash particles to a surface
Densification of ash particles on a surface
Densification of ash particles
Sintering
- small amount of “freezing” melt,
partial melting
- slow flowing of amorphous “glas”-phase
viscous flow sintering
- chemical reactions between particles and gas
- solid particles “growing” together
solid state sintering
Summary
Large effect from
• used fuel (what is fed into the boiler)
• ash composition (how the feed behaves thermally)
• flow field (where the particles go)
• temperature (amount of melt)
Ash related problems
- Principles -
Ash and ash deposition for solid fuels
Content
1. Ash related problems
Principles
Facts
2. Co-firing
3. Corrosion
4. Summary
“Opportunity fuels”
Annual biomasses
Forrest residues & prunings
Agricultural rests, -shells
Olive residues
Sorted wastes
Sludges
Coal slurry, pet coke
Others
MBM (meat and bone meal)
Solid animal excrement
ÅA fuel database
Analyzed per 2006
18 wood bark fuels
46 wood based fuels
(trunk, forest residues, construction residues)
11 annual biomass fuels
18 peats
15 coals
37 other
(sorted wastes, sludges, biomass-
based wastes, chicken litter)
TOT 145 fuels
Analyzing ash forming elements in a fuel
Conventional
- done on the ash of the fuel
- ashing + element analysis from ash
- all other elements except S & Cl
- ashing affects the analysis
Advanced
- done directly on fuel
- dissolving of fuel + element analysis of solution
- all elements
- no ashing
For ex.
selective leaching (=chemical fractionation)
various microscopic methods (SEM/EDS, CCSEM)
others
/Benson & Holm 1985, Baxter 1994, Zevenhoven 2001/
Water leachible
- alkali sulfates/carbonates/chlorides
Buffer solution leachible
- organically associated
Acid leachible
- carbonates/sulfates
Rest
- silicates, unsoluble rest
Total mineral matter
- all major ash-forming elements
H2O
NH4Ac
HCl
Stepwise leaching
/Benson & Holm 1985, Baxter 1994, Zevenhoven 2001/
Water leachible
- alkali sulfates/carbonates/chlorides
Buffer solution leachible
- organically associated
Acid leachible
- carbonates/sulfatesRest
- silicates, unsoluble rest
Total mineral matter
All major ash-forming elements
Stepwise leaching
Easily soluble
Mineral part
Stepwise leaching
Coal Peat Bark Wood AB Other0
20
40
60
80
100
120
140
Ash form
ing e
lem
ents
, g/k
g d
ry fuel
Leached in H2O
Leached in Acetate
Leached in HCl
Rest fraction
Untreated fuel
267 g/kg 312 g/kg
/ÅA fuel database, 2006/
0
5
10
15
20
25
30
Ash
-fo
rmin
g e
lem
en
ts, w
eig
ht-
% d
.b.
Wood Forest
res.
Bark Const
res.
An.
biom.
Peat Coal Others
/ÅA fuel database, 2006/
Ash-forming elements in fuels
Ash
fo
rmin
g e
lem
en
ts, g
/kg
d.b
.
0
50
100
150
200
250
300
0
5
10
15
20
25
30
Ash
-form
ing e
lem
ents
, w
eig
ht-
% d
.b.
Wood Forest
res.
Bark Const
res.
An.
biom.
Peat Coal Others
/ÅA fuel database, 2006/
Ash-forming elements in fuels
Ash-forming elements,
easily soluble
Ash-forming elements,
mineral part
Ash
fo
rmin
g e
lem
en
ts, g
/kg
d.b
.
0
50
100
150
200
250
300
0
2000
4000
6000
8000
10000
12000
14000
16000
18000S
ulp
hu
r in
fu
el, m
g/k
g d
.b.
Wood Forest
res.
Bark Const
res.
An.
biom.
Peat Coal Others
/ÅA fuel database, 2006/
Sulphur in fuels
0
2000
4000
6000
8000
10000
12000
14000
16000
18000C
hlo
rin
e in
fu
el, m
g/k
g d
.b.
Wood Forest
res.
Bark Const
res.
An.
biom.
Peat Coal Others
/ÅA fuel database, 2006/
Chlorine in fuels
0
2000
4000
6000
8000
10000
12000
14000
16000
18000P
ota
ssiu
m in
fu
el, m
g/k
g d
.b.
Wood Forest
res.
Bark Const
res.
An.
biom.
Peat Coal Others
/ÅA fuel database, 2006/
Potassium in fuels
Potassium,
easily soluble
Potassium,
mineral part
0
2000
4000
6000
8000
10000
12000
14000
16000
18000S
od
ium
in
fu
el, m
g/k
g d
.b.
Wood Forest
res.
Bark Const
res.
An.
biom.
Peat Coal Others
/ÅA fuel database, 2006/
Sodium in fuels
Sodium,
easily soluble
Sodium,
mineral part
0
10000
20000
30000
40000
50000
70000
80000
90000
100000
60000
Ca
lciu
m in
fu
el, m
g/k
g d
.b.
Wood Forest
res.
Bark Const
res.
An.
biom.
Peat Coal Others
/ÅA fuel database, 2006/
Calcium in fuels
Calcium,
easily soluble
Calcium,
mineral part
0
10000
20000
30000
40000
50000
70000
80000
90000
100000
60000
Sili
co
n in
fu
el, m
g/k
g d
.b.
Wood Forest
res.
Bark Const
res.
An.
biom.
Peat Coal Others
/ÅA fuel database, 2006/
Silicon in fuels
Silicon,
easily soluble
Silicon,
mineral part
0
2000
4000
6000
8000
10000
12000
14000
16000
18000E
U h
ea
vy m
eta
ls in
fu
el, m
g/k
g d
.b.
Wood Forest
res.
Bark Const
res.
An.
biom.
Peat Coal Others
/ÅA fuel database, 2006/
EU heavy metals in fuels
Summary
”Opportunity fuels”
• challenging, from an ash point-of-view
• don not necessarily increase the ash amount
• increase clearly ash aggressiveness
High Chlorine, Alkali (Na, K), Calcium
• sulphur, silicon as in conventional fuels
• heavy metals higher than
in conventional fuels
Opportunity fuels
( )
Ash and ash deposition for solid fuels
Content
1. Ash related problems
Principles
Facts
2. Co-firing
3. Corrosion
4. Summary
Contractual fuels in the large FBC
deliveries 2001-2002
• Wood based 16/23
• Peat 10/23
• Coal 8/23
• Sludges 8/23
• Pet Coke 4/23
Co-firing vs single-fuel-firing 20 vs 3
/Hupa 2003; FBC plenary session/
Co-firing, effect on slagging & fouling
Sla
gg
ing &
Fou
ling
Fuel A 50% Fuel B
/Hupa 2003; FBC plenary session/
0
10
20
30
40
50
60
0 10 20 30 40 50 60 70 80 90 100
Bark share in rice husk, weight-%
3h10h
Depositio
n,
g/m
2h
/Skrifvars et al 2004; Energy & Fuels/
Co-firing, effect on slagging & fouling
Deposit probe measurements
Full-scale BFB
0
10
20
30
40
50
60
70
80
90
100
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
Thermal share
Depositio
n,
g/m
2h
Full-scale deposit probe measurements
- Deposition vs fuel mix -
Biomass Peat or Coal
/ÅA deposit probe measurements, 2006/
Peat/straw co-firing
lab-scale drop-tube tests
0
20
40
60
80
100
120
140
160
0 10 20 30 40 50 60 70 80 90 100
Share of straw in peat, weight-%
Depositio
n,
g/m
2h)
/Theis 2006; Dr thesis /
Peat Straw
72
Cl & S in fuel vs deposition
- lab-tests -
2 KCl + SO2 + ½ O2 + H2O K2SO4 + 2 HCl
0
20
40
60
80
100
120
140
160
0.0 0.1 0.2 0.3 0.4 0.5
Molar ratio of Cl/S in fuel
Depositio
n,
g/m
2h Peat-straw
Peat-bark
/Theis 2006; Dr thesis /
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
Thermal shareBiomass Peat or coal
0
5
10
15
20
25
30
35
40
Cl in
deposit,
weig
ht-
%
Wind
Lee
Full-scale deposit probe measurements
- Cl i deposit vs fuel mix -/ÅA deposit probe measurements, 2006/
Summary
• seldom a linear ash behavior
• co-firing worse than single-fuel-firing if fuel ashes
cause a melt when mixed together
• silicate-based ashes may function
as “cleaning agents”, through an erosive effect
• sulphur may stop chlorine from getting into deposits
Co-firing
Ash and ash deposition for solid fuels
Content
1. Ash related problems
Principles
Facts
2. Co-firing
3. Corrosion
4. Summary
Compact steel-oxide layer
- protects from further oxidation
- requires the presence of oxygen
Steel
Traditional corrosion protection of steel
Does not work if
- the steel-oxide layer breaks
- oxygen is abscent
- the steel-oxide layer is porpous
Steel
Traditional corrosion protection of steel
Usually handeled by materials people only
Novel thinking needed
- challenging interface, steel-deposit-gas
- chemistry-physics-engineering
Traditional corrosion protection of steel
Carburization
Mo
lten
sa
lts
co
rros
ion
Su
lfid
ati
on
Low melting
compounds
Molten
salts
Carb
on
Oth
er
low
me
ltin
g
co
mp
ou
nd
s
(Ni-
P,
V2O
5,
Mo
O3,
etc
.)
Sulfur
OXYGEN
O2
CO/CO2
H2/H2O
/Salmenoja 2001, Dr Thesis/
High-temperature corrosion mechanisms
Fe Cl S
Corrosion caused by Chlorine
/Salmenoja 2001, Dr Thesis/
ÅA, laboratory-scale corrosion tests
Before
Heat treatment After
Heat treatment
For SEM-
analysis
/Westen-Karlsson 2008, Lic. Thesis/
Cross-section, mm
Co
rro
sio
n layer
thic
kn
es
s, (µ
m)
0
200
0 5 10 15
100
Mean, median,
most frequent, max
ÅA, laboratory-scale corrosion tests
- SEM analysis -
/Westen-Karlsson 2008, Lic. Thesis/
Synthetic
ash
T0 = 834oC
0% Cl
10% K
0
20
40
60
80
100
120
140C
orr
os
ion
la
ye
r th
ick
ne
ss
, m
m
ÅA, laboratory-scale corrosion tests
(Na, K)2 SO4 + 0.0 p-% Cl
/Skrifvars et al, Corr. Sci. 2008/
0
20
40
60
80
100
120
140 Synthetic
ash
T0 = 526oC
0.2% Cl
10% K
ÅA, laboratory-scale corrosion tests
(Na, K)2 SO4 + 0.2 p-% Cl
/Skrifvars et al, Corr. Sci. 2008/
Co
rro
sio
n la
ye
r th
ick
ne
ss
, m
m
0
20
40
60
80
100
120
140 Synthetic
ash
T0 = 522oC
1.2% Cl
10% K
ÅA, laboratory-scale corrosion tests
(Na, K)2 SO4 + 1.2 p-% Cl
/Skrifvars et al, Corr. Sci. 2008/
Co
rro
sio
n la
ye
r th
ick
ne
ss
, m
m
0
5
10
15
20
25
30
35
40
100 200 300 400 500 600 700
Probe surface temperature, oC
Cl in
de
posit,
weig
ht-
%Deposit probe measurements, full-scale boilers
- Cl in deposit vs probe surface temperature-
Wind
Lee
?
/ÅA deposit meaurements database, 2006/
Summary
• alkali chlorides enhance corrosion strongly
• already a small amount of Cl increases corrosion
• sulphur may stop chlorine to get into the deposit
• increase of steam temperature very challenging
Corrosion
Ash and ash deposition for solid fuels
Content
1. Ash related problems
Principles
Facts
2. Co-firing
3. Corrosion
4. Summary
Strong influence of
• fuel fired (what is fed into the boiler)
• ash composition (how the element react to ash in the boiler)
• flow fields (where the ash particles flow/impact)
• temperature (amount of melt in the ash/deposit)
Summary 1(4)
From ash-forming elements to deposits
• challenging, from an ash point-of-view
• don not necessarily increase the ash amount
• increase clearly ash agressiveness
High Chlorine, Alkali (Na, K), Calcium
• sulphur, silicon as in conventional fuels
• heavy metals higher than
in conventional fuels
New fuels, ”Opportunity fuels”
( )
Summary 2(4)
Summary 3(4)
Co-firing
• seldom a linear ash behavior
• co-firing worse than single-fuel-firing if fuel ashes
cause a melt when mixed together
• silicate-based ashes may function
as “cleaning agents”, through an erosive effect
• sulphur may stop chlorine from getting into deposits
Corrosion
• alkali chlorides enhence corrosion strongly
• already a small amount of Cl increases corrosion
• sulphur may stop chlorine to get into the deposit
• increase of steam temperature very challenging
Summary 4(4)
Dr. Mischa Theis
@Bayer, Germany
Ms. Micaela Westén-Karlsson
@Finnsementti
Assoc. Prof. Flemming Frandsen,
@Technical University of Denmark
Prof. Rainer Backman
@ University of Umeå
Acknowledgments
Prof. Mikko Hupa
Mr. Tor Laurén
Mr. Linus Silvander
Dr. Johan Werkelin
Dr. Patrik Yrjas
Dr. Maria Zevenhoven
@Åbo Akademi University