Master´s thesis

We offer master´s theses about challenges of chemical and technical character withinpower and heat production.

As thesis worker, you will come in direct contact with issues of industrial relevance.

We represent an international research group (about 30 people) with cooperation withseveral international companies such as Valmet, Andritz, MetsäFibre, Fortum, UPM,International Paper and others.

Duration: 6 months

For more information contact prof. Leena Hupa, leena.hupa@abo.fi or Patrik Yrjas,patrik.yrjas@abo.fi. We are both located in Axelia II on the 4th floor in rooms B 417 (Leena)and B 416 (Patrik).

Combustion of solid fuels – an

introduction to ashes and related

problems

Patrik Yrjas

patrik.yrjas@abo.fi

2

Contents

Combustion technologies => fluidised beds

Ash formation and why is it worth studying?

Fly ash

How can it be so difficult?

Co-firing

What can we do?

Research methods

Empirical

Theoretical

3

Combustion technologies

Pulverized fuel combustionng

– mainly coal

Grate firing – today mainly

waste and biomass in smaller

scale

Fluidised beds – suitable for

several different types of fuels

4Sumitomo SHI FW

Fluidised beds Important in Finland, three world wide manufacturers

located here

Sumitomo SH FW (originates from Ahlström)

Andritz

Valmet

Proven technology; export

Suitable for both domestic and other CO2-neutral fuel

mixes

5

0

20

40

60

80

Nu

mb

er

of

co-f

irin

g in

itia

tive

s

Unknown

Grate

CFB

BFB

PF

(> 50 MWel)

IEA-Bioenergy, 2009

CYMIC multi-fuel boiler

TSE, Naantali, FinlandSteam 164 bar

555 C

390 MWth

(142 MWe, 244 MWheat)

Fuels: Wood biomass,

agrofuel, peat,

coal, SRF

Start-up: Fall 2017

Courtesy of Valmet Technologies Oy https://www.youtube.com/watch?v=8rzhZQ0nDhs

Ash formation - simplified

7

Fuel

(wood, coal, etc.)

Ash elements

(SiO2, Al2O3, Fe2O3, CaO, K2O, S, Cl, etc.)

Flue gases

(CO2, N2, O2, H2O, SO2, NO, HCl, etc.)

Air

(O2, N2)

Heat, Q

Ash formation – fly ash

Physical

transport

Pyrolysis Char combustion and

fragmentation

Evaporation

Homogeneous

nucleation

Coagulation

Heterogeneous

condensation

Fly ash

0.1 -1 µm

Fly ash

1 - 100 µm

Included

minerals

Excluded

minerals

Mineral

coalescens och

fragmentation

Why is ash then so important....?

9

70-90% of the ash becomes fly ash in FB

Ash is the most common reason for un-planned

shut-downs due to:

deposits on superheaters

corrosion (mainly due to alkali chlorides)

if ash melts, even if only partly, then always a risk for heavy

corrosion

bed-agglomeration

… which are dependent on:

ash amount and ash type (fuel, additives)

process conditions (temp., air staging, etc. osv)

Challenges in biomass combustion

Superheater corrosion is the major single reason for

efficiency limitations and operational problems

Bubbling fluidising

bed (BFB)

Valmet Technologies

Secondary air

Bottom-

ash

Transportation,

transformation,

reactions

Ash elements

released in bed

Deposits

on super-

heaters

Separation

of fly ash

Fuels

Additive 1

e.g. limestone

Additive 2

e.g. (NH4)2SO4

Primary air

Tertiary air

70-90%

Factors affecting corrosion

Material temperature

Steel composition

Deposit composition (connected to

ash but not directly the same)

Gas composition

Typical temperatures

Wall tubes and drum

Temperature of supersaturated steam

(depends on pressure) ~ 300°C

Material temperature at walls +50°C

Superheaters

Steam ~ 400°C → 550°C (…600°C?)

Material

Radiation area+ 50°C

Convective area + 20-35°C

Economizers

Pre-heating of water max ~300°C

Air-pre-heaters

Pre heating of air (~120°C ±)

(Frandsen, doctoral thesis, DTU, 2011, CB 2008 - Hupa, Chirone et al., 2006)

Examples: deposits, corrosion and

an agglomerate due to unsuitable fuel mixes

Increased steam/material-temperatures

give higher efficiency but increases the

risk for corrosion

15

Net

ele

ctr

ic e

ffic

ien

cy (

%)

50 %

45 %

40 %

35 %

Coal

Biomass

Waste

DOUBLE

REHEAT

SINGLE

REHEAT

30 %

25 %

20 %

NO

REHEAT

15 %

10 %

350 400 450 500 550 600 650

Steam temperature (°C)

Vainikka, P., doctoral thesis, 2011

adopted from Dr.-Ing. Klaus Hein

Rule of thumb: 10°C increase gives about +2% in electrical efficiency

Standardised fuel characterisation

and prediction of ash behaviour

16

Analyses

• Moisture – and ash amount, volatiles, C,H,O,N,S

• Ash forming elements; Si, Al, Fe, Ti, Ca, Mg, Mn, P, Na, K, S, Cl

• Thermodynamic evaluations/calculations

• Trace elements; As, Hg, Cd, Pb, Cr, Zn, Cu, Ni, etc

• Different indices and empirical experiences

• Ash sintering test (ASTM, DIN)

IDT HT FTheight = 1/3 of height at HTheight = 1/2 of basetop disappeares

Standardised fuel characterisation

och ash behaviour prediction

Fuel

sample

Standardised analyses

Ash composition

A number of different indices

Standardised ash melting temp.

(ASTM, DIN)

Bed

agglomeration

(FB-boilers)

Emissions

Deposits

& corrosion

• originally developed for coal

fuels (not mixes)

17

Why is it so difficult then?

New, more demanding and cheaper fuels

Co-firing (fuel mixes) – positive and negative synergy effects

Laboratory ash versus ”real” ash from fuel mixes

Demands for higher efficiencies (less fuel = lower CO2/produced

electricity) - corrosion

Need for:

Prediction of the ash composition and its behaviour, deposit

formation and agglomeration tendencies from fuel analyses and

characterisation methods without full-scale tests (and a minimum

of laboratory tests).

18

Sustainable fuels, examples

Resources:

Biomass: wood, sawdust, forest residue, bark, (black

liquor)

Other biomass: agricultural residues (agro-fuels), energy

crops

Other sources: demolition wood, municipal waste,

industrial waste, sludges

100

90

80

70

60

50

40

30

20

10

0 100

90

80

70

60

50

40

30

20

10

0

0 10 20 30 40 50 60 70 80 90 100

CaO+MgO

K2O+Na2OSiO2

Kol

Main ash forming elements

ÅA fuel database 20

Coal

100

90

80

70

60

50

40

30

20

10

0 100

90

80

70

60

50

40

30

20

10

00 10 20 30 40 50 60 70 80 90 100

CaO+Mg

K2O+Na2O SiO2

KolTorv

ÅA fuel database 21

Coal

Peat

Main ash forming elements

ÅA fuel database 22

100

90

80

70

60

50

40

30

20

10

0 100

90

80

70

60

50

40

30

20

10

0

0 10 20 30 40 50 60 70 80 90 100

CaO+MgO

K2O+Na2O SiO2

KolTorv

TräGrotBarkAvfallsträ

Coal

Peat

Wood

Forest residue

Bark

Waste wood

Main ash forming elements

ÅA fuel database 23

100

90

80

70

60

50

40

30

20

10

0 100

90

80

70

60

50

40

30

20

10

0

0 10 20 30 40 50 60 70 80 90 100

CaO+MgO

K2O+Na2O SiO2

KolTorv

TräGrotBarkAvfallsträ

Jordbruksrester

Coal

Peat

Wood

Forest residue

Bark

Waste wood

Agricultural waste

Main ash forming elements

Ash chemistry of different fuel types

Coal => silicate based ash chemistry, Na, S and

Ca (in FBC when used for desulphurization)

Biomass => Ca, K, Na, S, and Cl (+Si in some

cases)

Agrofuels => Si, Ca, K, P, S and Cl

Waste fuels => .......... + Zn and Pb (+Br)

Synergy effects between fuels in fuel mixes

Physical effects - sand blasting, dilution

Chemical effects - reactions

25

Depositio

n r

ate

, g/m

2h

Fuel A, weight-%

negative synergy

positive synergy

Exemple on physical synergy effect

rice husk and barkTest rig at the University of Toronto

26

100% bark 36% bark + 64% rice husk

100% rice husk 64% rice husk, 36% bark,100% bark

deposit = 13 mg deposit = 10 mgdeposit = 195 mg

Rate of build-up with different rice

husk/bark mixes

(prof. H.Tran, Toronto, Canada)

0

50

100

150

200

250

0 16 36 60 84 100

På

sla

g, m

g

Bark, vikt-%

Deposit,

mg

Bark, weight-%

Example on chemical synergy effects

bark + sludgeTest rig 20 kW, BFB,

VTT, Jyväskylä

29

To stack

Sampling port

Sampling port

Sampling port

Gas cooling

Bagfilter

Gas probe

Observationport

Cyclone

Gas sample

Temperature control

Tertiary air optional

Tertiary air optional

Tertiary air (preheated)

Fuel container 2Fuel container 1

Secondary air(preheated)

Nitrogen

Air

Additivecontainer

Primary gas heating

Heating zone 2/Cooling zone 2

Heating zone 3

Heating zone 4

Heating zone 1/Cooling zone 1

BEDmade of quarz

PC control and data logging system

Obervation port

Obervation port

Obervation port

Obervation port/Deposit probe

Deposit probe

Sludge has high ash and high contents

of sulphur and alumina silicates.

Bark Slam

Aska, 550oC, vikt-% 3.30 55.60

C vikt-% 51.40 23.00

H 5.90 3.40

N 0.40 2.60

S 0.036 1.19

Cl 0.021 0.076

LHV, MJ/kg d.s. 19.27 9.14

LHV, MJ/kg a.r. 19.03 8.98

SiO2 vikt-% 7.71 25.74

Al2O3 1.75 7.20

Fe2O3 1.03 32.40

TiO2 0.07 0.70

MnO 2.08 0.07

CaO 40.11 6.72

MgO 4.09 1.64

P2O5 3.58 15.37

Na2O 1.00 1.08

K2O 6.83 1.45

SUM, % 68.25 92.37

Element i askan som oxider

HCl and SO2 emissions and Cl in deposits

30

0

10

20

30

40

50

60

70

80

90

100

100% Bark 2%

Sewage

sludge

4%

Sewage

sludge

6%

Sewage

sludge

8%

Sewage

sludge

Gaseo

us S

O2 a

nd

HC

l em

issio

ns,

mg

/MJ (

LH

V d

.s.)

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

Avera

ge C

l in

dep

osit

s (

valu

es f

rom

win

d, 50 d

eg

., a

nd

lee),

wt-

%

HCl

SO2

Cl

Al2O3∙2SiO2(s) + 2KCl(g) + H2O(g) => K2O∙Al2O3∙2SiO2(s) + 2HCl(g)

2KCl(g) + SO2(g) + ½ O2(g) + H2O(g) => K2SO4(s)+ HCl(g)

Yrjas et al., 2009

How predict behaviour of fuel mixes?

Bed

agglomeration

Standardised fuel analysis

? ? ?

Fuel

2

Emissions

Deposits

& corrosion

Fuel

3

Fuel

1

?

?

?

31

Chemical fractionation

32

Today we have about 250 fuels in the ÅA database

(fractionation data, C, H, O, N, S, ash, and heat values)

0

2000

4000

6000

8000

10000

12000

14000

16000

18000

1 5 9 13 17 21 25 29 33 37 41 45 49 53 57 61 65 69 73 77 81

mg

/kg

Rest Fraction

Leached in HCl

Leached in Acetate

Leached in H2O

Coal Peat Wood Bark For.Res. Agr.Res.

K

33

0

500

1000

1500

2000

2500

3000

3500

4000

4500

1 5 9 13 17 21 25 29 33 37 41 45 49 53 57 61 65 69 73 77 81

mg/k

g

Rest Fraction

Leached in HCl

Leached in Acetate

Leached in H2O

Coal Peat Wood Bark For.Res. Agr.Res.

P

34

Coal

35

0

5000

10000

15000

20000

25000

30000

35000

Si Al Fe Ti Mn Ca Mg P Na K S Cl

mg/k

g

161 Rest Fraction

161 Leached in HCl

161 Leached in Acetate

161 Leached in H2O

Bark

36

0

2000

4000

6000

8000

10000

12000

Si Al Fe Ti Mn Ca Mg P Na K S Cl

mg/k

g

67 Rest fraction, analysed

67 Leached in HCl

67 Leached in Acetate

67 Leached in H2O

Prediction of ash behaviour in fuel mixes

Fuel

2

Chemical

fractionation

&

lab. methods

(SEM, others)

Reactive

Inert

Chemical

thermo-

dynamics

CompositionFuel

1

Chemical

fractionation

&

lab. methods

(SEM, others)

Ash compounds

Melting behaviour

%-melt

+ Practical experience

Knowledge and solutions

37

40th Anniversary International Recovery Boiler Conference, Porvoo, Finland, May 12-14, 2004 - Mikko Hupa

Fraction of Molten Phase vs. Temperature

0

20

40

60

80

100

500 600 700 800 900

Temperature [°C]

Pe

rce

nta

ge

Liq

uid

Ph

as

e [

wt

%]

T0

T100

Challenging fuel mixes

Sumitomo SHI FW´s fuel system:

Courtesy of Sumitomo SHI FW

Challenging fuel mixesValmet´s fuel system:

Empirical use of fractionation results

combined with deposit measurements Full-scale deposits measurements

SEM/EDX analyses of Cl in deposits

Based on the well known reactions between S and K

which reduce the formation of chlorides, while forming

sulphates and releasing Cl to the flue gas as HCl:

2KCl/NaCl + SO2 +H2O +1/2O2=> K2SO4/Na2SO4+ HCl

• Short term, in-situ deposit sampling

• Surface temperature regulated probe

60 – 200 cm

41

Common to predict the risk for Cl in deposits with the S/Cl ratio in the fuel.

Not always valid for biofuels, due to varying amounts of active sulphur binding species, e.g Ca-compounds present in the fuel:

CaO +SO2 +1/2 O2=> CaSO4

ref. Yrjas et al. 18th FBC, 2005

Co-firing of peat, coal, bark, wood chips,

and forest residue in a large scale CFB

42

Co-firing of peat, coal, bark, wood chips,

and forest residue in a large scale CFB

S/(Ca+K2+Na2)reactive

molar ratio calculated

separately for every

fuel mixture and

plotted against the

amount of chlorine in

the deposits.

0.0

0.5

1.0

1.5

2.0

2.5

0.00 0.20 0.40 0.60 0.80 1.00

S/(Ca+K2+Na2)reactive, molar ratio

Ch

lori

ne

in

de

po

sit

s, w

t-%

350°C, 50 deg. side

350°C, lee side

350°C, wind side

Yrjas et al.18th FBC43

(Ca+Na2+K2)/S molar ratio as a

funct. of Cl in deposits and SO2

0.0

0.5

1.0

1.5

2.0

2.5

0 1 2 3 4 5 6 7 8

(Ca+Na2+K2)/S

Cl in

dep

osit

s (

350C

, 50

deg

.)

0

20

40

60

80

100

120

140

160

SO

2 p

pm

, 6%

O2

ClSO2

EU-Biomax project

Agglomeration very hard to predict – mainly

based on experience and laboratory tests

45

Bed height: 5 cm

Five thermocouples

Pressure drop measured

with a mikromanometer

Instrumental and

flue gas outlet

Combustion

chamber

Fuel

feeding

Tube furnace

Pre-heater

Bed net

(105 µm)

Air feed

Experiments with SiO2 and addition of KCl

46

Defluidisation after 12 g

(50 mg/min) or 3.1 weight-%

of the bed weight

Agglomerates of SiO2 and KCl

Scanning electron microscope/energy

dispersive x-ray analyzer (SEM/EDXA)

SEM; a method for high resolution

imaging of surfaces

EDXA; a method for elemental

analysis

KCl K2CO3

Sevonius et al., 2014

Tricks to decrease corrosion risks

Co-firing with sulphur and/or aluminasilicate rich

fuels

Use of additives containing sulphur

Fuel pre-treatments (heat, washing....)

Use right materials at each boiler location

Improved/better materials (improvement vs. price)

Design

QuizWhich boiler is a dedicated waste firing boiler?

Energy statistics app

App:

https://webstore.iea.org/key-world-energy-

statistics-2019