Particle precipitation
in medium- and large-scale
biomass combustion plants
Thomas Brunner, Manfred Lixl
BIOENERGIESYSTEME GmbH
Inffeldgasse 21b, A-8010 Graz, Austria
TEL.: +43 (316) 481300; FAX: +43 (316) 4813004
E-MAIL: [email protected];
HOMEPAGE: http://www.bios-bioenergy.at
BIOENERGIESYSTEME GmbH
Inffeldgasse 21b, A-8010 Graz
Content
Characterisation of particles to be precipitated
Technological description of different dust precipitation
technologies
Evaluation of the aerosol precipitation efficiencies of different
dust precipitation technologies
Recommendations concerning the application of different
dust precipitation technologies in biomass combustion
plants
2
BIOENERGIESYSTEME GmbH
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Aerosols
Typical particle size distributions
of fly ashes formed during
biomass combustion
Coarse fly ash particles
dm/dlog(dp) [mg/Nm³]
600
500
400
300
200
100
0
0.01
0.10
1.00
10.00
xm [µm ae.d.]
100.00
1,000.00
3
BIOENERGIESYSTEME GmbH
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Dust precipitation technologies –
overview
Cyclones
Multi-cyclones
Flue gas condensation units
Electrostatic precipitators (ESP)
• dry ESP
• wet ESP
Baghouse filters
4
BIOENERGIESYSTEME GmbH
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Precipitation efficiencies of different
dust precipitation technologies
5
BIOENERGIESYSTEME GmbH
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clean gas
Cyclones and multi-cyclones
Principle:
precipitation by centrifugal forces
flue gas
Cut diameters:
~ 5 µm
Operation temperature:
- up to 1,300 °C
- usually 150 - 250°C
Operation pressure:
- up to 100 bar
- usually atmospheric
precipitated
dust
Only suitable for coarse fly ash precipitation
6
BIOENERGIESYSTEME GmbH
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Scheme of a multi-cyclone
clean
gas
c;eam gas out
c;eam gas out
clean
gas
dirty gas in
dirty gas
outlet tube
outlet tubes
spin vanes
spin vanes
dirty gas in
dirty gas
collected
dust
out
collected dust
out
collected dust out
collected
dust out
7
Multi-cyclones –
aerosol separation
BIOENERGIESYSTEME GmbH
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Measured particle size distributions of aerosols
upstream and downstream a multi-cyclone
(coarse fly ashes are not considered)
test 1
test 2
20
mg/Nm³
dirty gas
clean gas
15
10
5
dirty gas
clean gas
15
10
5
xm [µm ae.d.]
11.31
5.66
2.83
1.41
0.71
0.35
0.18
0.09
0.03
11.31
5.66
2.83
1.41
0.71
0.35
0.18
0
0.09
0
0.03
mg/Nm³
20
xm [µm ae.d.]
Almost no separation of aerosol particles
Explanations:
all data related to dry flue gas and 13 vol.% O2
ae.d.: aerodynamic particle diameter
underfeed stoker, full load
fuel: hardwood (wood chips and sawdust)
8
BIOENERGIESYSTEME GmbH
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Flue gas condensation units
Combined dust precipitation and
recovery of sensible and latent heat
Not applicable in all biomass
combustion units.
Constraints for application are:
- low temperature of return
- high moisture content of fuel
Particle separation efficiency
coarse fly ashes: almost 100%
aerosols: depends on amount of
condensed water vapour
Pre-separation of coarse fly ash
particles is recommended to avoid
plugging of tube pipes
Sludge/water separation is
necessary
Heat exchanger tubes can be
cleaned by water injection
9
Flue gas condensation units –
aerosol precipitation
BIOENERGIESYSTEME GmbH
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Measured particle size distributions of aerosols
upstream and downstream a flue gas condensation unit
11.31
1.41
0.71
0.35
11.31
5.66
2.83
0
1.41
0
0.71
10
0.35
5
0.18
20
0.09
10
0.18
30
0.09
15
40
0.03
mg/Nm³
20
0.03
mg/Nm³
25
dirty gas
clean gas
50
5.66
dirty gas
clean gas
30
xm [µm ae.d.]
test 2
60
2.83
test 1
35
xm [µm ae.d.]
Aerosol precipitation efficiency: test 1: 39%, test 2: 60%
Explanations: all data related to dry flue gas and 13 vol.% O2; ae.d.: aerodynamic particle diameter;
grate-fired combustion unit, 70% load; fuel: bark
10
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Electrostatic precipitators principle of dry ESP
Principle:
(1) Particles are charged and
(2) precipitated at collection
electrodes (plates)
(3) particles are removed from
electrodes by rapping
Required properties of dust:
Specific electric resistivity
between 107 and 1011 Ohm*cm
Operation temperature
up to 480°C, usually <250°C
Operation pressure
up to 20 bar, usually atmospheric
11
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Electrostatic precipitators plate and tube-type ESP
12
emission [mg/Nm³]
120
100%
100
90%
80
80%
60
70%
40
60%
20
50%
0
40%
50kV 30kV
50kV 35kV
50kV 35kV
separation efficieny
Dry electrostatic precipitators –
aerosol precipitation vs. voltage
BIOENERGIESYSTEME GmbH
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50kV 35kV
dirty clean clean dirty clean clean dirty clean clean dirty clean clean
gas gas gas gas gas gas gas gas gas gas gas gas
beech
Explanations:
MDF - dust
MDF - chips
waste wood
results from impactor measurements; only particles <1µm considered
all data related to dry flue gas and 13 vol.% O2
grate-fired combustion unit; filter current: <2 mA
13
BIOENERGIESYSTEME GmbH
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Electrostatic precipitators principle of wet ESP (I)
Principle:
• The flue gas flows vertically through
honeycomb-shaped collection
surfaces.
• An emission electrode is located in the
centre of the honeycombs.
electrode
• Before entering the wet ESP, the flue
gas is cooled by a quenching process
and saturated with water.
collection
surface
• Increasing the amount of moisture in
the flue gas reduces the electrical
resistance of the dust, making it easier
to charge and remove the particles.
14
BIOENERGIESYSTEME GmbH
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Electrostatic precipitators principle of wet ESP (II)
Principle:
• Collection surfaces are configured as
circular pipes
electrode
• These collection surfaces are cooled on
the outside by a surrounding flow of
ambient air, which causes a layer of
condensed vapour on the inside of the
pipes
• Disadvantage: larger overall size
collection • Advantage: enhanced self-cleaning effect
surface
(as the condensate drains off, it creates a
film of water that carries away the separated
particles)
15
Wet ESP –
aerosol precipitation vs. filter current
emission [mg/Nm³]
60
<1 µm
total
efficiency
120%
50
100%
40
80%
30
60%
20
40%
10
20%
0
0%
0
50
100
precipitation
efficiency
BIOENERGIESYSTEME GmbH
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150
current [mA]
Explanations: results from impactor measurements; <1µm ... particles collected on impactor
stages with a cut diameter <1µm; total ... total amount of particles collected with the
impactor; all data related to dry flue gas and 13 vol.% O2;
grate-fired combustion unit; fuel: mixture of bark, wood chips and sawdust;
filter voltage: <60 kV
16
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Baghouse filters
Principle:
• Flue gas is filtered on the surface of filter media by cake generation
• The cake is periodically removed from the surface by pressurised air
Operation temperature
• up to 850°C (depending on filter material)
• usually in the range between 160 and 220°C
Operation pressure
• up to 50 bar
• usually atmospheric
Additional advantage
• Combination with dry sorption in order to decrease HCl, SOx, Hg and
PCDD/F emissions
17
Baghouse filters –
scheme
BIOENERGIESYSTEME GmbH
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pressurised air
(3-6 bar)
flue gas
outlet
valves
filter bags
flue gas
inlet
dust
18
Baghouse filters –
filter bag cleaning
BIOENERGIESYSTEME GmbH
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filtration
cleaning
clean gas
pressurised air
dirty gas
19
Baghouse filters –
aerosol precipitation
BIOENERGIESYSTEME GmbH
Inffeldgasse 21b, A-8010 Graz
300
2.4
dirty gas dm/dlog(dp)
[mg/Nm³]
250
clean gas
2.0
200
1.6
150
1.2
100
0.8
50
0.4
0
0.0
10.00
0.01
0.10
1.00
dp [µm ae.d.]
clean gas dm/dlog(dp)
[mg/Nm³]
dirty gas
Aerosol precipitation efficiency: >99%
Explanations: results from impactor measurements; all data related to dry flue gas and 13 vol.% O2;
grate-fired combustion unit; fuel: waste wood
20
Particle separation in biomass
combustion units –
results from measurements
BIOENERGIESYSTEME GmbH
Inffeldgasse 21b, A-8010 Graz
multicyclone
flue gas cond. unit
ESP 1
ESP 2
baghouse
filter
combustion technology
underfeed
stoker
moving
grate
moving
grate
underfeed
stoker
moving
grate
moving
grate
fuel
hardwood
bark
hardwood
hardwood
ind. waste
wood
waste
wood
load (% nominal capacity)
90
70
50
25
80 - 100
60 - 85
av. total fly ash, dirty gas
109
113
260
132
433
av. total fly ash, clean gas
74
24
<10
32
<2
⌧⌧⌧
⌧⌧⌧
⌧⌧⌧
⌧⌧⌧
⌧⌧⌧
coarse fly ash precipitation
⌧⌧
aerosols, dirty gas
22 – 35
60 – 110
26 – 36
90 – 120
35 – 58
72 – 98
aerosols, clean gas
22 - 35
36 – 55
22 - 26
<10
6–8
0.4 – 0.7
⌧⌧
⌧
aerosol precipitation
⌧⌧
Explanation: all data in mg/Nm³ related to dry flue gas and 13 vol.% O2
⌧⌧
⌧⌧⌧
21
Conclusions and
recommendations
BIOENERGIESYSTEME GmbH
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Best applicable precipitation technology with respect to the capacity
range and fuel used
Emission limit: >150 mg/Nm³
multi-cyclone
Emission limit: 50 mg/Nm³
flue gas condensation units,
dry ESP
Emission limit: 20 mg/Nm³
dry or wet ESP
Emission limit: 10 mg/Nm³
baghouse filters, wet ESP
Waste wood fired plants
highly efficient baghouse filters
Generally, a cyclone or multi-cyclone should be implemented upstream
an ESP, baghouse filter or flue gas condensation unit in order to
remove the coarse fly ash particles
22