Aerosol formation mechanisms
for different biomass fuels
newest results
Ingwald Obernberger
Institute for Resource Efficient and Sustainable Systems
Graz University of Technology
TEL.: +43 (316) 481300; FAX: +43 (316) 4813004
E-MAIL: Ingwald.Obernberger@tugraz.at
HOMEPAGE: http://RNS.TUGRAZ.AT
BIOENERGIESYSTEME GmbH
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Graz University of Technology
Contents
Introduction
Definitions and experimental set-up
Aerosol formation during biomass combustion - basic principles
Aerosol formation processes during combustion of
- chemically untreated wood fuels
- bark
- waste wood
- straw
- co-firing of wood and coal
Summary and conclusions
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Graz University of Technology
Introduction
During the last decade RNS, BIOS and ABC have been involved in several
national and international research projects focusing on aerosol formation
during biomass combustion and cofiring
The research was based on
- A considerable number of test runs
at combustion plants in all capacity ranges
(residential heating plants, pilot-scale testing plants, district heating plants,
CHP plants as well as power stations cofiring biomass)
utilising different biomass fuels
(chemically untreated wood fuels, bark, waste wood, straw, cofiring of wood)
- Modelling of the aerosol formation processes
From the results gained from this work aerosol formation processes could
be derived in dependence of the characteristics of the fuel utilised
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Experimental set-up for particle
characterisation tests
Fuel characterisation by its chemical composition
(Ca, Si, Mg, K, Na, S, Cl, Zn, Pb)
Definition of combustion conditions by the evaluation of the most relevant
plant operation parameters
(temperatures, flue gas composition, load etc.)
Aerosol particle characterisation
- Concentration in the flue gas and particle size distribution
- downstream the boiler: Berner-type low pressure-impactors
- in the furnace and boiler: high-temperature low-pressure impactor
- SEM/EDX and wet chemical analyses of the impactor samples
- SEM/EDX analyses of particles sampled with polycarbonate filters
downstream the boiler
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The high-temperatur
low-pressure impactor (HT-LPI)
Low-pressure impactor for particle sampling at temperatures up to about
1,000°C developed at the RNS/TU Graz
Determination of the concentration and particle size distribution of aerosols
in the hot flue gas
Possibility for subsequent chemical analyses of the particle samples
outer casing:
view into the furnace
furnace
T ~ 1,000°C
flue gas
pi
HT-LPI
HT-LPI after measurement
cooling
water
flue gas
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Definitions –
aerosols, coarse fly ashes, fly ash
Fly ashes
Aerosols
Coarse fly ashes
800
dm/dlog(dp) [mg/Nm³]
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wood chips
600
bark
waste wood
400
200
0
0.01
0.10
1.00
10.00
dp [9m ae.d.]
100.00
1,000.00
Data related to dry flue gas and 13 vol.% O2; results from grate-fired combustion systems
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Graz University of Technology
coarse fly ashes
KCl, K2SO4,
K2CO3 etc.
Aerosol and coarse fly ash
formation during biomass
combustion
CO2, H2O,
CO, CxHy
aerosols
Cooling of the flue gas
coarse fly ashes are
emitted
condensation
Gas phase reactions
(KCl, K2SO4, ZnO etc.)
K, Na, S, Cl,
Zn, Pb, Cd
coarse fly ashes are entrained
from the fuel bed, transported
with the flue gas and partly
precipitated in the furnace and
the boiler
Gas phase
burn-out
(CO2, H2O,
CO, CxHy)
CO, CxHy,
H2, etc
coagulation
nucleation
release of primary
particles, soot formation
bottom ash
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atmosphere
reducing
Institute for Resource Efficient
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Graz University of Technology
flue gas
temperature
Formation of inorganic aerosols –
main process steps with respect
to the biomass fuel used
biomass fuel
release of
SiO2 (waste wood, straw)
CaO (waste wood, bark,
wood)
release of elemental Zn
waste wood,
bark
wood, straw
release of alkali vapours,
S, Cl, Pb
all fuels
fragmentation
SiO2, CaO, particles
ZnO particle formation
K2SO4 particle formation
SiO2, CaO, ZnO, K2SO4
particles
coarse fly ash particles
heat exchanger surfaces
oxidising
Zn
ZnO
waste wood,
bark,
straw, wood
wood, straw, bark
alkali sulphates / chlorides
all fuels
heavy metal oxides
bark, wood, waste wood
heavy metal chlorides
waste wood
highly relevant
relevant
gas phase
reactions
(KCl, K2SO4 …)
nucleation
gas phase
reactions
cooling of
flue gas
minor relevance
condensation
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particles <1 9m [mg/Nm³]
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HT-LPI
BLPI
4
3
Aerosol formation during
the combustion of
chemically untreated wood (I)
• HT-LPI: measurements at 920 - 990°C
upstream boiler inlet
• BLPI:
measurements at ~200°C
downstream boiler outlet
• Aerosol emission at boiler outlet:
12 - 17 mg/Nm³ (dry flue gas, 13 vol% O2)
2
• Compounds relevant for aerosol formation
in the hot furnace:
K, S > Ca, Na, Zn
1
0
Ca
K
Na
Zn
Pb
S
Cl
•measurements performed at a 350 kWth grate
fired pilot-scale combustion plant
•fuel: spruce
•data related to dry flue gas and 13 vol.% O2
• Compounds relevant for aerosol formation
in the boiler:
K, Cl > Zn, Pb > S
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Aerosol formation during
the combustion of
chemically untreated wood (II)
Aerosol formation steps of relevance for
wood combustion
• Aerosol formation is dominated by alkali
salt formation.
• Formation of K2SO4, Na2SO4 and KCl.
•TEM-image of an aerosol sampled
downstream the boiler on a
polycarbonate filter
•sampling performed at a 440 kWth
grate fired combustion plant;
•fuel: beech
•dark zone: mainly K and S
•bright zone: mainly K and Cl
• K, which is not bound by Cl and S, is
present as KOH which subsequently forms
K2CO3.
• Nucleation of K- and Na-sulphates at
temperatures <1,000°C in the furnace and
the boiler.
• Condensation of KCl and minor amounts of
K2CO3 on already existing particles during
the cooling of the flue gas in the boiler.
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particles <1 9m [mg/Nm³]
Aerosol formation during
the combustion of
bark (I)
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BLPI
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• HT-LPI: measurements at 870 - 930°C
upstream boiler inlet
• BLPI:
12
10
measurements at ~200°C
downstream boiler outlet
• Aerosol emission at boiler outlet:
45 - 50 mg/Nm³ (dry flue gas, 13 vol% O2)
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6
4
• Compounds relevant for aerosol formation
in the hot furnace:
Ca, Zn, K, S
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Ca
K
Na
Zn
Pb
S
Cl
•measurements performed at a 350 kWth grate
fired pilot-scale combustion plant
•fuel: bark (spruce)
•data related to dry flue gas and 13 vol.% O2
• Compounds relevant for aerosol formation
in the boiler:
K, Cl, S > Zn > Pb, Na
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Aerosol formation during
the combustion of
bark (II)
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Aerosol formation steps of relevance for
bark combustion
• In the bark fuel Ca-oxalate structures are
embedded.
•SEM-image of Ca-containing particles in
charcoal from bark combustion
• During the heating of the fuel thermal
fragmentation of these structures takes
place.
• Submicron and supermicron CaO-particles
are formed which are entrained with the
flue gas.
• Aerosol formation is further dominated by
•SEM-image of a particle sampled downstream
the nucleation of K2SO4 at temperatures
the boiler (grate fired 440 kWth combustion plant) <1,000°C and condensation of K SO and
2
4
•Picture width: 2 9m
KCl at lower temperatures.
•Composition (atom%): 36% Ca, 2.2%S, 2.2%K,
1.8%Zn, 1.5%Mg, 1.2% Pb, 53.1% O
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particles <19m [mg/Nm³]
25
HT-LPI 1
HT-LPI 2
HT-LPI 3
BLPI
20
15
10
5
0
Ca
Si
K
Na Zn
Pb
S
Cl
•measurements performed at a 44 MWth grate fired
CHP plant (steam boiler);
•fuel: waste wood (A1-A4, German classification)
•data related to dry flue gas and 13 vol.% O2
Aerosol formation during
the combustion of
waste wood (I)
• HT-LPI1: measurement at 907°C
(1st duct)
• HT-LPI2: measurement at 802°C
(2nd duct)
• HT-LPI3: measurement at 400°C
(downstream superheater)
• BLPI:
measurements at ~180°C
downstream the economiser
• Aerosol emission at boiler outlet:
~80 mg/Nm³ (dry flue gas, 13 vol% O2)
• Compounds relevant for aerosol
formation upstream superheater:
Zn > K, S > Na, Pb > Ca, Si
• Compounds relevant for aerosol
formation downstream superheater:
Cl, K, Pb
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mole%
< 1.0
3.8
4.8
6.1
20.3
6.4
2.7
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wt%
<1.0
3.5
6
4.5
43.1
6.6
5.1
29.1
Aerosol formation during
the combustion of
waste wood (II)
Aerosol formation steps of relevance for
waste wood combustion (I)
• Under reducing conditions in the fuel bed
elemental Zn is released from the fuel to the
gas phase.
• As soon as the atmosphere becomes
oxidising ZnO is formed.
• Due to the high Zn-concentrations in waste
wood and the low saturation vapour pressure
of ZnO a high number of ZnO particles is
formed at high temperatures (>1,000°C).
•SEM-image and EDX-analyses of aerosols
sampled with the HT-LPI in the 2nd duct of
the boiler (802°C)
•sampling performed at a 44 MWth grate
fired CHP plant
• These ZnO particles provide enough surface
for condensation and therefore, the further
nucleation of other compounds (e.g.: K2SO4)
is low.
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Aerosol formation during
the combustion of
waste wood (III)
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point
1
2
atom%
atom%
K
8.7
13.1
Na
4.4
S
Aerosol formation steps of relevance for
waste wood combustion (II)
3.0
Cl
36.6
44.6
Zn
12.6
10.4
Pb
25.2
6.5
O
12.6
22.5
•SEM-image and EDX-analyses of aerosols
sampled with polycarbonate filters downstream
the boiler
(grate fired 440 kWth combustion plant)
• In the hot furnace (hot water boilers)
respectively upstream the superheater
(steam boiler) alkali-sulphates as well as
Pb-compounds (presumably oxide) start to
condense on the ZnO particles at
temperatures below ~1,000°C.
• In the convective path of steam boilers as
well in the cooler regions of hot water
boilers mainly condensation of Pb-chlorides,
KCl and PbO takes place.
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particles <1 9m [mg/Nm³]
120
HT-LPI
BLPI
100
80
60
Aerosol formation during
the combustion of
straw (I)
• HT-LPI: measurement at 1,058°C
• BLPI: measurements at ~150°C
downstream the economiser
• Aerosol emission at boiler outlet:
~320 mg/Nm³ (dry flue gas, 13 vol% O2)
• Compounds relevant for aerosol
formation in the boiler:
Si, K, S, Cl
40
20
0
Ca
Si
K
S
Cl
• measurements performed at a 118 MWth grate
fired CHP plant (steam boiler)
• fuel: straw (wheat)
• data related to dry flue gas and 13 vol.% O2
• Compounds relevant for aerosol
formation downstream superheater:
K, S, Cl
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Aerosol formation during
the combustion of
straw (II)
Aerosol formation steps of relevance for
straw combustion
• Straw contains high amounts of Si. Suband supermicron SiO2 particles are
formed by thermal fragmentation of Sistructures during the heating of the fuel.
• Moreover, SiO can be formed during
combustion and be released from the fuel
matrix to the gas phase. SiO is then
oxidised to SiO2 and nucleates in the hot
zones of the furnace (>1,000°C).
•SEM-image and EDX-analyses of aerosols
sampled with the HT-LPI at ~1,060°C
•Area 1 (atom%): 20% Si, 11% K, 4% P, 2.5% S,
rest: O
•Area 2 (atom%): 5% Si, 30% K, 10% S, rest: O
• Subsequently K2SO4 nucleation starts
upstream the superheater followed by
KCl condensation in and downstream the
superheater.
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HT-LPI
BLPI
particles <1 9m [mg/Nm³]
12
10
8
6
Aerosol formation
during cofiring of
coal and sawdust (I)
• HT-LPI: measurements at ~1,000°C
• BLPI: measurement at ~150°C
downstream the air heater
• Aerosol emission at boiler outlet:
~55 mg/Nm³
• Compounds relevant for aerosol
formation:
Si, Al, Ca, S, Fe
4
2
0
Ca Si Mg K Na Zn Mn S
P Ti Fe Al
• Due to low biomass contribution,
aerosol formation is dominated by
coal combustion.
•measurements performed at a 500 MWth pf
combustion power plant
•fuel: coal (96%) and sawdust (4% related to NCV)
•data related to dry flue gas and 13 vol.% O2
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100%
Al
Fe
Ti
P
S
Mn
Zn
Na
K
Mg
Si
Ca
[mol%]
80%
60%
40%
20%
De
tai
l2
De
tai
l1
0%
•SEM-image and EDX-analyses of aerosols
sampled with the HT-LPI at ~1,000°C
•Cofiring of coal and sawdust
Aerosol formation
during cofiring of
coal and sawdust (II)
Aerosol formation steps of relevance for coal
combustion and cofiring of coal and woody
biomass
• 2 types of particles are formed in the furnace:
•Particles originating from the fragmentation of
minerals contained in the coal:
main elements involved: Si, Al, Fe
particle size: major share ~1 9m and larger.
•CaSO4 originating from the release of Ca from
the coal (at flame temperatures of 1,500°C and
higher), oxidation to CaO, nucleation and finally
sulphation of the CaO particles.
particle size range: <<1 9m.
• Alkali-compounds either react with alumosilicates or condense on the surfaces of the
CaSO4 and alumosilicate particles.
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Influence of the fuel used on the
mass of aerosols formed
1,000
Particles <1 9m (ae.d.) [mg/Nm³]
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wood chips
bark
waste wood
straw
1,000
10,000
100,000
concentration of aerosol forming elements in the fuel K+Na+S+Cl+Zn+Pb
[mg/kg d.b.]
• Emissions at boiler outlet
• Summary of the data gained during the test runs mentioned in the previous slides
grey regions: experiences from other projects
• Particle emissions related to dry flue gas and 13 vol% O2
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Conclusions
The mass of aerosols formed during the combustion of biomass
strongly depends on the concentrations of aerosol forming species in
the fuel and on the release of these elements to the gas phase.
The single processes governing aerosol formation as well as the
locations in the furnace and the boiler where they take place, depend on
the elements and compounds involved in the process and therefore
depend on the fuel used.
During cofiring of woody biomass with coal at low biomass ratios,
aerosol formation has turned out to almost exclusively depend on the
characteristics of the coal used.
The application of the high-temperature impactor has turned out to
provide valuable results in order to track aerosol formation processes
along the streamline of the flue gas through a combustion plant.
The results from these measurements are applied to check and calibrate
aerosol formation models and to improve their prediction preciseness.
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Graz University of Technology
ACKNOWLEDGEMENTS
The financial support of the
European Commission (FP5 and FP6)
is gratefully acknowledged
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Institute for Resource Efficient
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Graz University of Technology
Thank you for your attention
Contact details:
Prof. Dr. Ingwald Obernberger
Inffeldgasse 21b, A-8010 Graz, Austria
TEL.: +43 (316) 481300; FAX: +43 (316) 4813004
Email: obernberger@bios-bioenergy.at
HOMEPAGE: http://www.bios-bioenergy.at
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