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
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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
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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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