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Bioenergia Portugal 2015
Portalegre, 29/05/2015
(Co-)combustão de Biomassa Residual
Mário Costa
Instituto Superior Técnico, Universidade de Lisboa, Lisboa
FÓRUM & MOSTRA TECNOLÓGICA, Escola Superior de Tecnologia e
Gestão, Portalegre, Portugal, 28 e 29 Maio 2015
Formação e emissão de partículas em
caldeiras domésticas alimentadas com
biomassa residual
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Bioenergia Portugal 2015
Portalegre, 29/05/2015
Motivation
Combustion of biomass is a well established thermal conversion
technique for generating heat and power...
... but this technique is still associated with high emissions of fine
particulate matter (PM)
Fine PM have a large impact on human health, being their capture
using conventional gas cleaning devices very difficult (and
expensive!)
Important to find ways to reduce their formation during the
combustion process, which requires a good understanding of the
formation mechanisms of fine PM during biomass combustion
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Particulate formation
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Bioenergia Portugal 2015
Portalegre, 29/05/2015
Domestic boiler
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Biomass characteristics
Parameter
Proximate analysis (% wt, ar)
Volatiles
Fixed Carbon
Moisture
Ash
Ultimate analysis (% wt, daf)
Carbon
Hydrogen
Nitrogen
Sulphur
Oxygen
Value
80.5
10.9
7.3
1.3
46.0
6.2
0.5
< 0.01
47.3
Parameter
Ash analysis (% wt, db)
SiO2
Al2O3
Fe2O3
CaO
SO3
MgO
P2O5
K2O
Na2O
Cl
Other oxides
Low heating value (MJ/kg)
Average dimensions (mm)
Diameter
Length
Value
20.9
6.2
21.6
26.2
0.3
4.3
4.2
11.5
2.5
0.04
2.3
17.1
6
18
6
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Bioenergia Portugal 2015
Portalegre, 29/05/2015
Flue-gas data (1/3)
PM emissions were dominated by
particles with sizes below 2.5 μm
and above 10 μm
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Flue-gas data (2/3)
 Scanning
electron microscope/energy dispersive x-ray detector
(SEM/EDS) analysis revealed the presence of ultra fine particles
composed mainly of O, K, Cl, Na and S …
 … that is, particles that resulted from the inorganic material in the fuel
ashes ...
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Bioenergia Portugal 2015
Portalegre, 29/05/2015
Flue-gas data (3/3)
 … and sub-micrometer sized particles/agglomerates contained mainly C,
but also O, Ca, Mg, K and P …
 … that is, particles that resulted from incomplete combustion (soot)
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Exhaust gas recirculation and air staging
In biomass combustion fine PM is produced from incomplete
combustion (i.e., soot) and from vaporization and condensation of
easily volatile ash elements
In principle, the presence of soot particles in the flue gas can be
minimized through combustion modifications.
Combustion modifications (e.g., temperature) may, however,
enhance the vaporization of the volatile ash elements, which may
originate higher ash based PM emissions
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Bioenergia Portugal 2015
Portalegre, 29/05/2015
Configurations
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Exhaust gas recirculation
Effect of the EGR on a) total PM, CO and
NOx emissions, and b) PM2.5 emissions
Effect of the EGR on the PM2.5 chemical
composition. a) 30% EGR, b) 60% EGR
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Bioenergia Portugal 2015
Portalegre, 29/05/2015
Air staging
Effect of the air staging on a) total PM, CO
and NOx emissions, and b) PM2.5 emissions
Effect of the air staging on the PM2.5
chemical composition. a) Sec. air/prim.
air = 0.06, b) sec. air/prim. air = 0.12
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Summary
In general flue gas recirculation has no beneficial impact on the
total PM and PM2.5 emissions. Presence of inorganics in the PM
decrease, but carbon (soot) increase.
Air stating may lower fine PM emissions, without adverse effects on
CO and NOx emissions. Presence of carbon in the PM decreases,
no changes observed in the inorganics.
Combination of the two strategies resulted in higher total PM and
PM2.5 emissions …
… Use of high flue gas recirculation rates combined with high levels
of air staging may result but poses operational problems (e.g.,
flame stability)
New burner/combustor designs are needed.
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Bioenergia Portugal 2015
Portalegre, 29/05/2015
Formação de depósitos durante a cocombustão de carvão com biomassa
residual
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Motivation
Annual increase in coal utilization is expected to be 1.8% until 2030 (IEA); during
this period it will represent about ¼ of the electrical energy demand
Biomass potential for energy utilization in EU between 243 and 316 Mtoe in 2030
Co-firing: slagging and fouling can reduce the heat transfer in heat exchangers
Co-firing: biomass fuels present high percentages of inorganic matter; high levels of
alkali metals can produce sulphates and chlorides (KCl, NaCl, Na2SO4, K2SO4);
ashes with low melting point; high levels of Cl (corrosion)
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Bioenergia Portugal 2015
Portalegre, 29/05/2015
Co-firing
Advantages
Co-firing can be undertaken with existing power plants, and thus has high power
generation efficiency
Short term implementation on large scale; if well-managed, technical risks are low
It is attractive in terms of the capital investment requirement and generation cost
It can increase the amount of renewable energy and thereby reduce the CO2
emissions
Almost any biomass material can be used (e.g., wood, olive and palm oil waste,
cereal and straw). Thus it is attractive in terms of security of supply
Disadvantages
Biomass is a poor fuel: it contains O2 and a substantial amount of moisture
Elements present in the ashes can cause severe corrosion problems
Biomass can be expensive: it depends on subsidies at present
May not be sustainable
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Large-scale furnace
Burner gun
Quarl
Ignitor burner
Water out
Water out
Cooling water
Refractory
cement
60o
Secondary air
ºC
Ø 120
Termocouple
type J
Ø 600
150
50
20
Primary air
+ biomass
Water out
Ø 56
300
Primary air + coal
Ø12
Ø16
Ø32
Dimensions in mm
Ø55
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Bioenergia Portugal 2015
Portalegre, 29/05/2015
Fuel characteristics
Parameter
Proximate analysis (% wt, as received)
Volatiles
Fixed Carbon
Moisture
Ash
Ultimate analysis (% wt, as received)
Carbon
Hydrogen
Nitrogen
Sulphur
Oxygen
Ash analysis (% wt, dry basis)
SiO2
Al2O3
Fe2O3
CaO
SO3
MgO
TiO2
P2O5
K2O
Na2O
Cl
Other oxides
Ash fusibility
Shrinking temperature (ºC)
Initial deformation temperature (ºC)
Softening temperature (ºC)
Hemispherical temperature (ºC)
Fluid temperature (ºC)
Heating value
High heating value (MJ/kg)
Low heating value (MJ/kg)
Particle size
% under 100 mm
% under 500 mm
% under 1000 mm
Coal
Sawdust
Straw
Olive stones
Peach stones
44.6
51.4
1.7
2.3
63.7
21.2
12.5
2.6
64.9
11.5
8.9
14.7
57.8
19.7
9.4
13.1
66.4
19.3
13.3
1.0
79.3
5.9
1.9
0.5
8.4
46.6
6.3
0.9
< 0.02
31.1
58.7
0.5
1.1
< 0.02
16.1
43.2
5.6
1.9
< 0.02
26.8
45.49
6.26
0.73
< 0.02
33.22
34.9
20.5
21.9
3.1
12.8
0.9
1.5
0.1
1.1
2.3
0.9
6.1
4.4
1.9
50.3
2.3
12.0
0.0
7.2
10.2
2.7
2.9
34.3
7.7
2.9
24.1
2.6
3.7
0.4
3.4
15.1
0.8
3.5
1.5
30.4
10.6
9.9
9.9
0.7
7.2
0.9
7.4
18.7
2.9
0.7
0.7
5.5
2.3
6.0
12.1
2.4
12.3
0.3
25.7
30.4
1.9
1.1
n.i.
1400
1440
1447
1454
921
1158
n.i.
1284
1319
915
1304
n.i.
1382
1390
925
1138
n.i.
1307
1317
781
1035
n.i.
> 1500
> 1500
35.0
33.6
18.3
17.0
19.0
18.8
17.5
16.4
18.1
16.2
69.9
100
100
34.7
67.0
95.1
16.9
46.0
92.7
21.3
49.8
94.1
16.1
58.0
100
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Ash deposition rate
Deposition rate (g/m2h)
200
180
160
140
Sawdust
Olive stones
Coal
120
100
80
60
40
20
0
10
20
30
50
100
Solid fuel in the blend (thermal fraction %)
20
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Bioenergia Portugal 2015
Portalegre, 29/05/2015
Chemical analysis of
representative ash deposits
Concentration (% wt)
a)
50
45
Coal
40
10% sawdust
20% sawdust
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• Coal + sawdust co-firing
• High content of Si and Al
SiO2 e Al2O3 high melting
temperatures
30% sawdust
50% sawdust
30
25
20
15
10
5
0
Si
Al
Fe
K
Cu
Ca
Mg
Ti
S
Na
Element
Concentration (% wt)
b)
50
45
Coal
40
10% olive stones
20% olive stones
35
• Coal + olive stones co-firing
• High content of K
K2O and K2SO4 have low
melting temperatures
30% olive stones
30
50% olive stones
25
• High content of S
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Formation of sulfates
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10
5
0
Si
Al
Fe
K
Cu
Ca
Mg
Ti
S
Na
P
Element
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Concluding remarks
The intense use of wood derived fuels in domestic and industrial
combustion processes is putting an enormous pressure on the forest
To meet the existing scenarios, we must increase the use of alternative
biomass fuels, in particular herbaceous materials and agricultural
residues
... but such residues can cause a number of problems because of the
presence of alkali metals and chlorine and other ash related impacts as
well as corrosion on the metallic surfaces and particulate matter emissions
…
… which may limit the variety of biomass fuels that can actually be used in
(co-)firing processes
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