VTT TECHNICAL RESEARCH CENTRE OF FINLAND LTD

Greening the Southern African Power Generation: New Technology Solutions for Conventional Production: Experiences in Co-firing Biomass Materials in Finland.

Fossil Fuel Foundation workshop July 29-30Marko Nokkala, Martti Aho

230/07/2015 2

Benefits of co-firing

§More green energy production (CO2 reduction)§ Longer lifeline of coal pit, compensation for quality reduction in

coal over time§ Possibility to increase efficiency of low caloric value lignite§ Substitution of fossil fuels with renewable sources of energy§ Potential exists in Southern Africa region due to variety of

feedstock of biomass available§ New fuel mix is expected to break even with existing fuel unit

price, but also to take into consideration the future price fluctuations to the extent the estimation is possible

330/07/2015 3

Generally about fluidised bed combustion

Source: Metso PowerMedium to large scale up to 100 MWe

Circulating Fluidized Bed (CFB)Bubbling Fluidized Bed (BFB)

*Enables co-firing of fuels with strongly varying combustion properties (wood- peat- coal)(in contrast to grate combustion)*Enables large power plant size range 3 – 1000 MW thermal*Good combustion stability independently to fuel quality variations (due to high sand mass)* Emission formation in the furnace (NO, SO2) is minimised due to low temperatures (again presence of sand helps) (in contrast to pulverised fuel combustion)

sandsandT T700 1000

700 1000Source: Metso PowerMedium to large scale up to 500 MWe

4

Risks of firing forest biomass in effective power plants (concerning also fluidised beds)

5

Heat transfersurface

Lack of protecting

compounds

Low ash content

R I

SKY COMPOUNDSALKALICHLORIDES

Cl releases corrosionä

BARK/FOREST RESIDUE

CASE 1. BARK/FOREST RESIDUE

Origin to thedamageswith wood-basedfuels

630/07/2015 6

Strong fouling of power plant superheaters

730/07/2015 7

Cl formation from alkali chlorides in deposits consuming metal

KCl, NaCl

HCl

Cl2

K2SO4, Na2SO4

FeO2

O2

SO2 + O2 + H2O

Superheater tube

8830/07/2015

Examples of superheater corrosion

9

Problem solution

10

Preventing alkali chloride deposition by destroying them before the superheter zone:

Na Cl

K Cl

HCl + XProtecting agents fromco-firing fuel

11

Peat,coal

SO2SO3

S

Stack

Forest biomass

K, Cl, (Na)

KClSO3 + KCl + =>K2SO4 + HCl

SO2, HCl, K2SO4

1230/07/2015 12

Additional power from coal

§ Alkali capture by alkali aluminium silicate formation:

§ Al2O3*2SiO2 + 2 KCl -> K2O*Al2O3*2SiO2 + 2HCl(g)

§ This can dominate in many cases (especially coal addition of low-sulphur coals.

to stack

1330/07/2015 13

Variation range of main ash constituents in coal ash(left) and minerals (right)

14

Recent experimental work in VTT (with industrialfunding) to find out maximum safe portion of forestbiomass when blended with fossil fuel

15

Methodology

16

VTT’s electrically stabilised fluidised bed reactors

20 kW BFB, height ≈ 5 m 60 kW CFB, height ≈ 8.5m

17

Inorganicvapors

Tackydeposit Fine fly

ash share

Vapour + ashsampling

Fine particleanalysis⇒Risky Cl in furnace

Example on samplingto analysis duringcombustion tests Deposit

sampling

deposit analysis => Cl deposition etc.

+ FluegasAnalysisHCl etc. 0

5

10

15

20

25

30

35

40

WA SA LA WB SB LB WA SA LA WB SB LB WA SA LA WB SB LB

60/40 60/40 60/40 60/40 60/40 60/40 T14 T14 T14 T14 T14 T14 T24 T24 T24 T24 T24 T24

wt-%in

deposits

Wood-peat blends

NaKClS

0

1

2

3

4

5

6

7

<0.03 0.03-0.26 0.26-0.62 0.62-10

Mas

s flo

w (m

g/Nm

36%

O2)

Fraktio (μm)

CFB-MTH

Na

K

S

Cl

Risky Cl

Cl in HCl

lee

side

wind

Mass flowmg/Nm3 6% O2

18

Fuels under investigation

19

Fuel Stem wood chips

Forest residue

1 60/40 blend

Forest residue

2 Finnish

peat Russian

coal S-A coal

Ash (550 ºC) wt% 1.1 3.7 2.1 4.6 10.2 11.4 16.1

Ash (815 ºC), wt% 0.9 3.3 1.9 4.3 10.1 10.9 15.4

LHV, MJ/kg 19.35 19.33 19.34 19.54 19.62 29.19 27.8

C, m-% k-a 49.8 49.9 49.8 51.5 50.9 72.3 70.4 H, m-% k-a 6.2 6.0 6.1 6.0 5.5 5.1 3.95 N, m-% k-a 0.32 0.47 0.38 0.64 2.16 2.24 1.61 S, m-% k-a 0.020 0.030 0.024 0.05 0.22 0.30 0.59

Cl, wt% 0.006 0.012 0.0084 0.017 0.025 0.007 0.030 molar S/Cl 3.7 2.8 3.2 3.2 9.7 47.4 21.7

Safe molar S/Cl > 3 ???!

20

Results

2130/07/2015 21

Mass flow of corrosive Cl (in KCl + NaCl), and mass flows of na, K and Cl in the furnace for two forest biomass with S/Cl ≈ 3.2

0

1

2

3

4

5

6

7

<0.03 0.03-0.26 0.26-0.62 0.62-10

Mas

s flo

w (m

g/Nm

36%

O2)

Fraktio (μm)

CFB-60/40

Na

K

S

Cl

0

1

2

3

4

5

6

7

<0.03 0.03-0.26 0.26-0.62 0.62-10

Mas

s flo

w (m

g/Nm

36%

O2)

Fraktion (μm)

CFB-MTH

Na

K

S

Cl

Condensed alkali chlorides

=FR

2230/07/2015 22

Concentration of key elements in deposits A = Tprobe about 450 C b= Tprobe > 500 C, W= wind, S = side, L = lee, CFB, 100% forest biomass

0

5

10

15

20

25

30

35

40

60/40 F.R 60/40 F.R 60/40 F.R 60/40 F.R 60/40 F.R 60/40 F.R

wt%in

deposit

Na

K

Cl

S

CFB, 100% forest biomass

A, W

A, S

A, LB, W

B, S

B, L

F.R contains ≈ 2 x as much Cl and S as wood 60/40, but S/Cl is ≈3.2 in both forest biomass

Lee side

Inorganicvapors

Tackydeposit

W ind side

50o from W ind side β=50°

Lee side

Inorganicvapors

Inorganicvapors

Tackydeposit

W ind side

50o from W ind side β=50°

≈

>

<

2330/07/2015 23

Cl deposition for forest residue/peat blends ”FR” contains more Cl than ”60/40” but S/Cl is ≈ 3.2 in both forest biomass

T means peat, number means energy % in blend

0

2

4

6

8

10

12

14

WA SA LA WB SB LB WA SA LA WB SB LB WA SA LA WB SB LB

60/40 60/40 60/40 60/40 60/40 60/40 T14 T14 T14 T14 T14 T14 T24 T24 T24 T24 T24 T24

Clwt-%

in deposit

CFB: wood "60/40" and peat (T)

§ ’

0

2

4

6

8

10

12

14

WA SA LA WB SB LB WA SA LA WB SB LB WA SA LA WB SB LB WA SA LA WB SB LB

F.R. F.R. F.R. F.R. F.R. F.R. T17 T17 T17 T17 T17 T17 T27 T27 T27 T27 T27 T27 T44 T44 T44 T44 T44 T44

Clwt-%

in deposit

CFB Forest residue (FR) - peat (T)

85 ≈80% wood ≈75> 75 < 75

2430/07/2015 24

Conclusions

§ Combustion of forest biomass alone can result in high Cl concentrations in deposits⇒metal damages at superheaters with Tmetal > 450 C

§ Wood can be safely co-fired with peat and coal at energy portion of > 70%. TypicalCoal has stronger protection power against Cl deposition than typical peat

§ The exact upper limit is dependent on fuel compositions , superheater materials and the accuracy of determining/knowing the blend composition by the plant operators

§ Combustion tests are recommended to determine maximum safe wood portions

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