Bengt-Johan Skrifvarsusers.abo.fi/.../FPK_II_2018/2_Ash_and_Ash_Deposition_FPK_II_2018.pdf · size...

Ash and ash deposition for solid fuels

Bengt-Johan Skrifvars

FPK II, ÅA, 2018

Content

1. Ash related problems

Principles

2. Co-firing

3. Corrosion

4. Summary

Ash related problems

• 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

Content

Principles

2. Co-firing

3. Corrosion

4. Summary

Bottom

Pathways of ash forming elements entering a boiler

Transportation,

transformation,

reactions

Release of ash

forming elements

Slagging,

foulingFly ash

separation

Additives

Formation of a troublesome deposit:

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 %

• 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

Included

mineals

Excluded

minerals

H2O-soluble

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 %

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

Particles condensed

from gas phase

(aerosols)

Minerals

Mass concentration frequency, dm/d(logdp) ??

g/m3 ???

Particle size, mm

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

frequency

Let’s consider:

1000 particles

0 – 50 mm

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

Particle size, mm

frequency/s

10 20 30 40 50

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

Particle size, mm

fraction/s

10 20 30 40 50

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

Particle size, mm

ractio

10 20 30 40 50

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

Particle size, mm

tration

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

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

log(dp)

Mass-size distribution of fly ash from PC coal combustion/Jokiniemi & Kauppinen, 1995/

0.1 10

Particle size dp

~10 µm

Particle size-distribution measurements

with a low-pressure cascade impactor

••

/Berner 1972/

rp C(dp) v dp2

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

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

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/

• Diffusion- small particles (< 0.1 mm)- diffusion in direction of concentration gradient

(Fick´s law)

• Thermophoresis- small particles (< 5 mm)- diffusion in direction of temperature gradient

• Inertial impaction- large particles (> 5 mm)- dependent on gas velocity- angle of impact

The physics of thermophoresis

ash particle

tgas 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 heat gradient direction

• Impaction- large particles (> 5 mm)- dependent of gas velocity & particle mass

2-component phase diagram

5000 10050

NaCl Na2SO4

T100Liquid

NaCl(s)+ L Na2SO4(s)+L

NaCl(s)+Na2SO4(s)

“Lever-rule”

5000 10050

NaCl Na2SO4

Liquid

NaCl(s)+ LNa2SO4(s)+L

NaCl(s)+Na2SO4(s)

Amount of melt =

Amount of solid =

A - BC - B

A - CC - B

x 100 %

Bulk composition = A

Liquid composition = C

Solid composition = B

Amount of melt vs temperature

0500 600 700 800 900

Temperature, (°C)

85 mol-% Na2SO4

15 mol-% NaCl

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/

Tsteam

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/

500 550 600 650 700 750 800 850 900 950 1000

Temperature, oC

Melting behavior of different alkali salts

600 700 800 900 1000

Temperature, oC

T15 T70

Deposit equilibrium thickness

600 700 800 900 1000 600 700 800 900 1000 600 700 800 900 1000

T15 = 530oC, T70 = 690oC

Temperature, oC

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

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)

- Principles -

Content

Principles

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

- silicates, unsoluble rest

Total mineral matter

- all major ash-forming elements

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

Leached in H2O

Leached in Acetate

Leached in HCl

Rest fraction

Untreated fuel

267 g/kg 312 g/kg

/ÅA fuel database, 2006/

Wood Forest

Bark Const

Peat Coal Others

Ash-forming elements in fuels

Wood Forest

Bark Const

Peat Coal Others

Ash-forming elements in fuels

Ash-forming elements,

easily soluble

Ash-forming elements,

mineral part

18000S

fuel, m

Wood Forest

Bark Const

Peat Coal Others

Sulphur in fuels

18000C

rine in fuel, m

Wood Forest

Bark Const

Peat Coal Others

Chlorine in fuels

18000P

m in fuel, m

Wood Forest

Bark Const

Peat Coal Others

Potassium in fuels

Potassium,

easily soluble

Potassium,

mineral part

18000S

m in fuel, m

Wood Forest

Bark Const

Peat Coal Others

Sodium in fuels

Sodium,

easily soluble

Sodium,

mineral part

100000

m in fuel, m

Wood Forest

Bark Const

Peat Coal Others

Calcium in fuels

Calcium,

easily soluble

Calcium,

mineral part

100000

n in fuel, m

Wood Forest

Bark Const

Peat Coal Others

Silicon in fuels

Silicon,

easily soluble

Silicon,

mineral part

18000E

eavy m

ls in fuel, m

Wood Forest

Bark Const

Peat Coal Others

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

Content

Principles

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

Fuel A 50% Fuel B

/Hupa 2003; FBC plenary session/

0 10 20 30 40 50 60 70 80 90 100

Bark share in rice husk, weight-%

Depositio

/Skrifvars et al 2004; Energy & Fuels/

Co-firing, effect on slagging & fouling

Deposit probe measurements

Full-scale BFB

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

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 10 20 30 40 50 60 70 80 90 100

Share of straw in peat, weight-%

Depositio

/Theis 2006; Dr thesis /

Peat Straw

Cl & S in fuel vs deposition

- lab-tests -

2 KCl + SO2 + ½ O2 + H2O K2SO4 + 2 HCl

0.0 0.1 0.2 0.3 0.4 0.5

Molar ratio of Cl/S in fuel

Depositio

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

deposit,

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

Content

Principles

2. Co-firing

3. Corrosion

4. Summary

Compact steel-oxide layer

- protects from further oxidation

- requires the presence of oxygen

Traditional corrosion protection of steel

Does not work if

- the steel-oxide layer breaks

- oxygen is abscent

- the steel-oxide layer is porpous

Usually handleled by materials people only

Novel thinking needed

- challenging interface, steel-deposit-gas

- chemistry-physics-engineering

Carburization

Low melting

compounds

Molten

Sulfur

OXYGEN

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

s, (µ

0 5 10 15

Mean, median,

most frequent, max

- SEM analysis -

/Westen-Karlsson 2008, Lic. Thesis/

Synthetic

T0 = 834oC

(Na, K)2 SO4 + 0.0 p-% Cl

/Skrifvars et al, Corr. Sci. 2008/

140 Synthetic

T0 = 526oC

0.2% Cl

(Na, K)2 SO4 + 0.2 p-% Cl

140 Synthetic

T0 = 522oC

1.2% Cl

(Na, K)2 SO4 + 1.2 p-% Cl

100 200 300 400 500 600 700

Probe surface temperature, oC

posit,

%Deposit probe measurements, full-scale boilers

- Cl in deposit vs probe surface temperature-

/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

Content

Principles

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

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