Elemental and Organic Carbon in Flue Gas Particles of Various

Transcription

Elemental and Organic Carbon in Flue
Gas Particles of Various Wood
Combustion Systems
Project management:
Christian Gaegauf, dipl. Ing. ETH/SIA
[email protected]
Project team:
Martin Schmid, dipl. Ing. FH
[email protected]
Pierre Güntert, dipl. Ing. FH
[email protected]
Langenbruck, December 2005
¨Ökozentrum Langenbruck, CH-4438 Langenbruck
www.oekozentrum.ch
Elemental and Organic Carbon in Flue Gas Particles of Various Wood Combustion Systems
2
Content
Zusammenfassung
5
Messmethoden
5
Resultate
6
Diskussion
6
Schlussfolgerungen
7
1
Introduction
8
2
Materials and Methods
8
2.1
Flue gas sampling
2.2
Particle analysis
10
2.3
Carbon analysis
10
2.4
Wood combustion systems
11
3
Results
8
11
3.1
Coulometer
11
3.2
Particle size distribution
12
3.3
Emission factors
13
4
Discussion
14
4.1
EC- and OC-content in total particle emissions
14
4.2
Particle emissions in the different particle sizes
16
4.3
EC- and OC-emissions in the different particle classes
16
4.4
Products of incomplete combustion
18
4.5
Comparison of particle emissions
19
5
Conclusions
20
Acknowledgements
21
References
21
3
English text is based on the paper presented at the
8th International Conference on Energy for a Clean Environment, Lisbon 2005
Authors
C. K. Gaegauf, M. R. Schmid and P. Güntert
Centre of Appropriate Technology and Social Ecology, Langenbruck, Switzerland
Abstract
The airborne particulate matter (PM) in the environment is of ever increasing concern to authorities
and the public. The major fractions of particles in wood combustion processes are in the size less
than 1 micron, typically in the range of 30 to 300 nm. Of specific interest is the content of the
elemental carbon (EC) and organic carbon (OC) in the particles since these substances are known for
its particular potential as carcinogens. Various wood combustion systems have been analysed (wood
chip boiler, pellet boiler, wood log boiler, wood stove and open fire). The sampling of the particles
was done by mean of a multi-stage particle sizing sampler cascade impactor. The impactor classifies
the particles collected according to their size. The 7 stages classify the particles between 0.4 and 9
microns aerodynamic diameter. The analytical method for determining the content of EC and OC in
the particles is based on coulometry. The coulometer measures the conductivity of CO2 released by
oxidation of EC in the samples at 650°C. The OC content is determined by pyrolysis of the particle
samples in helium atmosphere.
Keywords wood combustion, smoke emission, particulates, elemental carbon, organic carbon
4
Zusammenfassung
Holzfeuerungen sind unter den stationären Anlagen eine relevante Emissionsquelle für Staub- und Feinpartikelemissionen. Von speziellem Augenmerk sind die Feinpartikel unter 10 µm (PM10), da sie über den menschlichen Atmungstrakt in den Körper penetrieren können. Wegen der Karzinogenität des organischen (Organic Carbon, OC) und elementaren (Elemental Carbon, EC) Kohlenstoffes ist der OC- und EC-Anteil in den Staubemissionen
für die Beurteilung der Umweltbelastung von Holzfeuerungen von Bedeutung.
In der vorliegenden Untersuchung wurden die Emissionsfaktoren sowohl des Gesamtstaubes (Total Particle Emissions, TPE) wie der Feinpartikelfraktionen verschiedener Holzfeuerungen ermittelt. Zusätzlich wurden die
Emissionsfaktoren der OC- und EC-Anteile im Gesamtstaub und den Feinpartikelfraktionen bestimmt. Für die zu
untersuchenden Holzfeuerungen wurde je ein typischer Vertreter der Kategorien Holzöfen, Stückholz-, Pellet- und
Schnitzelkessel ausgewählt und im Felde unter Praxisbedingungen gemessen. Die Staub- und Partikelemissionen
eines offenen Feuers lieferten Vergleichsdaten zu den untersuchten Feuerungssystemen mit geschlossenen Brennräumen.
Messmethoden
Gesamtstaub
Die Gesamtstaubmessung wurde auf gravimetrischer Basis mit Planfiltern durchgeführt. Die verwendeten
Planfilter basierten auf Quarzfasergewebe, damit sie für die EC/OC-Analytik tauglich waren.
Feinpartikel
Die Feinpartikel-Klassifizierungen wurden mit einem Kaskaden-Impaktor (Firma Andersen) durchgeführt.
Die Abstufung der Partikelklassen waren wie folgt gegeben: > 9 µm, 5.8 - 9 µm, 4,7 - 5.8 µm, 3.3 - 4.7 µm, 2.1 3.3 µm, 1.1 - 2.1 µm, 0.7 - 1.1 µm, 0.4 - 0.7 µm und < 0.4 µm.
EC/OC-Analytik
Die Bestimmung des EC/OC-Anteils in den Partikelemissionen erfolgte nach dem coulometrischen Standardverfahren (VDI 2465, Chemisch-analytische Bestimmung des elementaren Kohlenstoffes nach Extraktion und
Thermodesorption des organischen Kohlenstoffes). Das SUVA-Analyselabor in Luzern führte die Analysen durch. Es
wurden sowohl die Gesamtstaubproben wie die Feinpartikelfraktionen mit dem Standardverfahren auf den EC/OCGehalt hin untersucht. Bei der Schnitzelfeuerung wurde die EC/OC-Analytik bei sämtlichen Partikelklassen durchgeführt, bei den übrigen Anlagen nur in den drei Klassen >9 µm, 2.1 - 3.3 µm und <0.4 µm. Die Gesamtmenge des
organischen Kohlenstoffs (Total Carbon, TC) bestimmt sich aus der Summe von EC und OC.
Abgase
In Ergänzung zu den Staub- und Partikelmessungen wurde Kohlenmonoxid (CO) als gasförmiger Schadstoff im Abgas erfasst.
Vormessung
Die EC/OC-Emissionsuntersuchungen der Feuerungen erfolgt in einem zweistufigen Vorgehen. In Vormessungen wurden die vorgesehenen Messverfahren an einer Schnitzelfeuerung exemplarisch durchgeführt. Anhand
der Labor-Analysemethoden und der Datenauswertungen wurde das Messprozedere für die Hauptmesskampagne
festgelegt.
Für die coulometrischen Untersuchungen analysierte das SUVA-Labor den EC-Gehalt bei Analysetemperaturen von 650°C bzw. 800°C. Von den Filterproben wurde eine Filterhälfte mit Phosphorsäure behandelt, um die
Carbonate aus dem Filterstaub herauszulösen. Tabelle 2 zeigt, dass der EC-Anteil der unbehandelten Probe Faktoren höher ist als der EC-Anteil der behandelten. Dies ist darauf zurückzuführen, dass sich die Carbonate über
650°C verflüchtigen. Weiter kann festgestellt werden, dass mit Phosphorsäure behandelte Filterproben bei der EC5
Analyse auf dem Temperaturniveau 650°C und 800°C ähnliche EC-Werte aufweisen. Daraus lässt sich schliessen,
dass die EC-Analyse bei einer Coulometertemperatur von 650°C verlässliche Werte liefert. Damit ist bei dem Analysetemperaturniveau von 650°C die Vorbehandlung der Filterproben mit Phosphorsäure nicht erforderlich.
Resultate
Vormessung an Schnitzelfeuerung
Die Vormessung an der Schnitzelfeuerung über alle Impaktorstufen zeigte, dass die TC-Konzentration in
den Partikelgrössen-Klassen von >9 µm und <0.4 µm am höchsten ist (Fig. 6). Der EC-Anteil am Gesamtkohlenstoff
(TC) nimmt zu kleineren Partikelgrössen hin ab, der OC-Anteil zu.
Der prozentuale Anteil bezogen auf den Gesamtstaub liegt für EC bei 1%, für OC bei 3.1%. Der TC-Anteil
als Summe aus EC und OC erreicht im Gesamtstaub 4.1%. In den einzelnen Partikelgrössen-Klassen liegt der Anteil
für EC zwischen 1 - 5%, für OC zwischen 1.8 - 3.4%. Der TC-Anteil liegt bei allen Klassen unter 7%. Die Schnitzelfeuerung wies eine weitgehend vollständige Verbrennung auf, was sich aus den geringen CO-Werten im Abgas
zwischen 43 und 64 mg/m3n (@13% O2) schliessen lässt.
Hauptmesskampagne
Die EC/OC-Analyse wurde in der Hauptmesskampagne in den drei Impaktorstufen >9 µm, 2.1 - 3.3 µm und
<0.4 µm durchgeführt. Diese drei Klassen erwiesen sich auf Grund der Vormessungen als sinnvoll: >9 µm und
<0.4 µm hatten den höchsten Staub und TC-Anteil, die Impaktorstufe 2.1 - 3.3 µm liefert einen Anhalt der Werte
in den Zwischenklassen. Die Zusammenstellung der Emissionsfaktoren findet sich in Tabelle 3. In Tabelle 4 sind
die Emissionsfaktoren für den Gesamtstaub in den Partikelgrössenklassen PM10, PM2.5 und PM1 abgeschätzt. Die
prozentuale Aufteilung der Grössenklassen erfolgte mit dem Impaktor. Die Absolutwerte lieferte die Gesamtstaubmessung.
In Tabelle 5 sind Bereiche von Emissionsfaktoren aus diversen Feldmesskampagnen des Ökozentrums Langenbruck aufgeführt. Neben den auf die Brennstoffmasse bezogenen Emissionsfaktoren finden sich auch die auf
dem unteren Heizwert basierenden Emissionswerte. Als Anhaltspunkt für die gemessenen Emissionsfaktoren enthält die Tabelle auch die Staubgehalt-Anforderungen des Schweizer Qualitätssiegels für Holzfeuerungen sowie die
EURO-3- und EURO-4-Partikel-Grenzwerte für Dieselfahrzeuge differenziert nach Personen/Lieferwagen und Lastwagen.
Diskussion
EC/OC-Gehalt im Gesamtstaub
Den geringsten Auswurf an Gesamtstaub (TPE) weist der Stückholzkessel auf. Der höchste hat das offene
Feuer gefolgt vom Cheminéeofen (Fig. 7). Die Anteile EC/OC in den Gesamtstaubpartikeln variieren. Feuerungen
mit schlechtem Ausbrand zeigen einen erhöhten EC-Anteil. Die Schnitzelfeuerung hat einen deutlich höheren
Anteil an OC gemessen am TC, das allerdings auf einem insgesamt sehr tiefen Niveau an TC (Fig. 9). Dies ist mit
der hohen Glutbetttemperatur in der Schnitzelfeuerung zu erklären, die die Verflüchtigung der mineralischen
Komponenten in der Asche erhöht (z.B. Carbonate) was zu salzartigen Partikeln führt.
Partikelauswurf und EC/OC-Gehalt in den Partikelklassen
Den höchsten Ausstoss an Partikeln haben alle Feuerungen in dem Bereich der Ultrafeinpartikeln <0.4 µm
(Fig. 11). Wie bei dem Gesamtstaub haben das offene Feuer und der Cheminéeofen den höchsten Staubauswurf in
den einzelnen Partikelklassen. Stückholz- und Pelletkessel liegen ähnlich. Der Kohlenstoffanteil (TC) ist bei offenem Feuer und Cheminéeofen deutlich am höchsten. Alle Kessel haben nur einen geringen Anteil an TC (Fig. 12).
Die EC-Emissionsfaktoren liegen in allen gemessenen Partikelklassen über denen von OC (Fig. 13, Fig.14).
6
Gesamtstaubauswurf und Partikelgrössenklassen
Die PM10-Partikel enthalten zwischen 83% und 98% der gesamten Staubmasse (Fig. 10). In der Grössenklasse PM2.5 ist dieser Anteil noch 73% bis 91%, bei PM1 61% bis 80% bezogen auf den gesamten Staubauswurf.
Das offene Feuer weist in den Grössenklassen PM2.5 und PM1 mit 73% und 61% den geringsten Anteil auf. Dies
lässt sich auf die unvollständige Verbrennung mit gröberen Verbrennungspartikeln zurückführen.
Einfluss der Verbrennung auf TC, EC und OC
Die Untersuchung am Stückholzkessel erlaubte die Unterscheidung von Betriebszuständen in zwei Testläufen. Ein Testlauf beinhaltete auch die Anfeuerphase des Kessels mit erhöhtem Anteil an Unverbranntem (Products
of incomplete combustion, PIC). Betrachtet man die TC bzw. EC/OC-Emissionen stellt man fest, dass sich sowohl
der Gesamtstaub wie der TC bei guter Verbrennung reduzieren. In Abbildung 15 (Fig. 15) zeigt sich, dass die OCEmissionen in etwa bei beiden Betriebszuständen (Anfeuern bzw. steady-state-Betrieb) gleich sind, die ECEmissionen aber deutlich bei besserer Verbrennung (CO low) zurückgehen.
Schlussfolgerungen
•
Bei allen untersuchten Feuerungen (Schnitzel-, Pellet- und Stückholzkessel sowie Cheminéeofen und offenes
Feuer) weisen Gesamtstaub (TPE) und Gesamtkohlenstoff (TC) die höchsten Emissionsfaktoren in der Partikelklasse <0.4 µm auf.
•
Zwischen 61% (offenes Feuer) und 80% (Stückholzkessel) des Gesamtstaubes werden mit der Partikelgrössenklasse PM1 emittiert. Die PM2.5-Partikel enthalten bereits 73% (offenes Feuer) bis 91% (Holzofen) des Gesamtstaubes, PM10 83% bis 98%.
•
Der Massenanteil des Gesamtkohlenstoffs (TC) im Gesamtstaub variiert zwischen 9% und 84%.
•
Der Anteil des elementaren Kohlenstoffes (EC) am Gesamtkohlenstoff (TC) liegt im Bereich von 20% - 70%,
der des organischen Kohlenstoffes (OC) im Bereich von 27% - 79%.
•
Es gibt keinen eindeutigen Zusammenhang für den Split von EC/OC im Gesamtkohlenstoff. Ein Einflussfaktor
kann die Verbrennungsgüte sein: bei erhöhtem Anteil an unverbrannten Produkten (PIC) im Abgas ist der ECAnteil am Gesamtkohlenstoff deutlich höher.
•
Das offene Feuer und der Cheminéeofen weisen für alle Werte (TPE, TC bzw. EC und OC) die höchsten Emissionsfaktoren auf. Die Gesamtstaubemissionen des offenen Feuers liegen um das 7-fache höher als die des
Stückholzkessels, der die tiefsten TPE-Wert aufwies.
•
Der Stückholzkessel erzielte unter den Praxisbedingungen im Feld die Staubemissionsanforderungen des
Schweizer Qualitätssiegels für Holzfeuerungen. Es gilt zu betonen, dass diese Anforderungen für Messungen
unter Prüflaborbedingungen mit klaren Prüfprozederes und standardisiertem Brennstoff gelten.
•
Die Emissionsfaktoren für Gesamtstaub von Stückholz- und Pelletkessel erreichen unter Praxisbedingungen
ähnliche Werte wie Dieselfahrzeuge mit EURO-4-Staubemissionsgrenzwerten. Auf Grund verschiedener Untersuchungen an Holzfeuerungen mit Partikelabscheider (Elektro- und Gewebefilter) kann gesagt werden, dass
sich mit solchen Filtersystemen TPE-Emissionsfaktoren im Reingas von 1 - 6 mg/MJ erzielen lassen.
•
Die Untersuchungen haben einen klaren Praxischarakter mit entsprechenden Zufälligkeiten für die erhobenen
Daten. Die ausgewiesenen Emissionsfaktoren sind statistisch nicht gesichert.
Von Interesse wäre eine wissenschaftliche Untersuchung, um die Prozessparameter in der Holzverbrennung zu
eruieren, die die TC- bzw. EC/OC-Bildung begünstigen bzw. senken.
7
1
Introduction
Life cycle assessments of wood combustion processes show many advantages over fossil fuels. There is
nevertheless a remarkable disadvantage looking at the wood combustion emission, particularly at particulate
emissions contributing to the total suspended particles in the ambient air. Airborne particulate matter (PM) in
the environment is of ever increasing concern to authorities and the public since it is known to be carcinogens
and respiratory irritants. The major fractions of the particles in the wood combustion process are in the size less
than 1 micron, typically in the range of 30 to 300 nm [1]. These nanoparticle fractions behave like gaseous effluents and penetrate deeply into the human respiratory tract [2]. The Swiss Federal Environment Protection Agency
(BUWAL) wanted to know more about the carbon fraction in the particles of wood fire exhausts. Of specific interest is the content of the elemental carbon (EC) and organic carbon (OC) in the particles of different size since
these substances are known for its particular potential as carcinogens.
The wood combustion systems examined are typical wood combustion systems such as wood chip boiler,
pellet boiler, wood log boiler, wood stove and an open fire. All but the open fire were tested in the field. Thus the
results reflect practice condition. The open fire has been tested on a test rig in the laboratory.
2
Materials and Methods
2.1
Flue gas sampling
2.1.1
In stack measurement
Three sampling lines are applied in the measuring section of the flue duct to determine particles, carbon
dioxide (CO2) and carbon monoxide (CO). The set up (Figure 1) allows the simultaneous detection of the effluents
during the test run.
Flue gas
TI
Dry gas
meter
Filter
Dryers Pumps
TI
TPE sampling train
Dry gas
meter
Impactor
Cooler
O2
O2
Impactor sampling train
CO
CO2
Gaseous effluents sampling train
Wood boiler/stove
Figure 1.
Set up for in stack emission measurement. Three sampling lines allow simultaneous analysis of total particle emissions (TPE), particle classes in an impactor and gaseous effluents.
The arrangement of the dust probes for the flat filter and the impactor is shown in Figure 2.
8
Figure 2.
Sampling probes for dust collection in stack. The flat filter is shown with filter and back up filter and the electric
heater. The third probe collects the gaseous effluents in the flue gases.
The flue gas analyses detects the emission concentration. For the conversion of the emission concentration to emission factors the specific gas volume per unit dry fuel is required. The specific flue gas volume was
calculated by carbon balance method which derives from CO2 and CO analysis in the flue gas [3].
2.1.2
Dilution tunnel
To analyse the particle emission of the open fire the method of constant volume sampling in a dilution
tunnel was applied [4]. Flue gas and ambient air are mixed in the dilution tunnel and kept at constant flow conditions. The sampling train for particles is installed in the measuring section of the dilution tunnel (Figure 3). The
particle sampling starts with the ignition of the fire and ends, when fuel is burnt. A weighing scale indicates the
end of a burn cycle when all fuel is fired. The emission factors can be determined by the particle mass sampled
over the entire burn cycle and the fuel mass burnt.
Dilution tunnel
TI
Filter
TPE sampling train
FI
Dry gas
meter
Exhaust
Dryers Pumps
TI
Dry gas
meter
Blower
Impactor
Impactor sampling train
Open fire
Scale
Figure 3.
Test rig with dilution tunnel to analyse the total particle emissions (TPE) and particle classes in an impactor of the
open fire.
9
2.2
Particle analysis
The sampling of the particle classes was done by means of a particle sizing sampler (Figure 4). The sampler used is a multi-stage, multi-orifice cascade impactor [5]. The impactor classifies the particles collected according to the aerodynamic dimensions which are a valuable measure to assess the particles lung penetrability
[2]. The 7 stages classify the particles between 0.4 and 9 microns aerodynamic diameter.
Figure 4.
Set up of impactor and handling of filter paper while loading the impactor stages. The paper is thermally conditioned for gravimetric particle analyses after loading.
Parallel to the impactor total particle emission (TPE) were measured in the flue gases. The TPE-filter
combines a filter and back up filter with a filter efficiency of 99.75%. The measurement uncertainty for the range
used (20 - 300 mg/mn3) is 2%.
All stage filters of the impactor were weighted for total particle emission to compare with the figures of
the TPE-filters. The impactor particle mass values were adapted to the total TPE-emission so they summed up to
equal mass of TPE-emissions. That implies that the impactor was solely used to define the fraction of the particle
mass in the impactor stages. The combination of the particle distribution fraction in the impactor and the TPE
lead to the mass distribution in the different particle classes.
2.3
Carbon analysis
Carbon in combustion particles occurs in the organic fraction as elemental carbon (EC) and as organic
carbon (OC). Different techniques are used to distinguish between EC and OC. The distinction between EC and OC
is usually made by coulometric or thermo gravimetric analysis [6]. The coulometric analysis gives the content of
EC for the non-volatile fraction and OC for the volatile fraction of organic carbon.
Coulometry is the VDI reference method for determining EC and OC (VDI, 1996). A filter sample is heated
in an oxygen flow to a temperature where all carbon is converted to CO2. The coulometer measures the conductivity of CO2 released by oxidation of EC in the samples at 650°C. The OC content is determined by pyrolyses in helium atmosphere. The total carbon (TC) is the sum of EC and OC analysed.
10
The EC and OC were analysed in the TPE-filters as well as in the impactor stage filters. The EC/OC-values
of the TPE-filter gives an answer of organic carbon from coarse (>10 µm) to ultra fine particles (< 1 µm) whereas
the EC and OC values of the impactor filters relates EC and OC to the various particle classes.
According to the SUVA-laboratories the accuracy of the coulometry is 2 - 7% depending on the filter load
(Eurachem-Guide).
2.4
Wood combustion systems
All wood combustion systems were tested in the field but the open fire, which was analysed in the laboratory of the institute. The heat output of the boilers and appliances were arbitrary according to the heat demand
of the heating system. The emission figures thus have real live character. The following wood combustion systems
were investigated for particles, EC, OC and TC resp.
Table 1
Wood combustion systems analysed in the field tests.
Type
Fuel
Heat output
Test site
Wood chip boiler
wood chips
70 kW
institute building
Pellet boiler
pellets
10-32 kW
apartment house
Wood log boiler
wood logs
35 kW
apartment house
Stove
wood logs
4 kW
living room
Open fire
wood logs
–
laboratory test rig
3
Results
3.1
Coulometer
Some pre-tests were performed to establish the coulometric analysis for the EC and OC content of the
particles from wood combustion. A crucial parameter is the temperature level of the coulometer when detecting
EC. To verify the optimal coulometer temperature two parts of the same and the one filter sample were measured
at temperatures of 800°C and 650°C resp. (Table 2).
Table 2
Verification of the temperature level of elemental carbon (EC) analytics in the coulometer. Phosphoric
acid treatment of the samples leaching out carbonates which evaporate at temperature over 650°C.
Samples
No.
Leaching
Coulometer
temperature
EC mass
[°C]
[µg]
Ia
untreated
800°C
2164
Ib
treated
800°C
346
IIa
treated
650°C
51
IIb
treated
800°C
63
One part of the sample was treated with phosphoric acid to leach the carbonates. Since carbonates
evaporate over 650°C this can lead to misinterpretation of EC content since part of the particle mass escapes with
the carbonates. The results show, that there is a considerable difference between leached and unleached samples
analysed by coulometer at 800°C. The difference of the two samples in EC content is due to evaporation of car11
bonates. If the sample is leached either 650°C or 800°C coulometer temperature show similar content of EC. After
the pre-test it has been decided to run the coulometer at a temperature of 650°C for all samples.
3.2
Particle size distribution
The multi-stage particle sizing sampler cascade impactor classifies the particles in 7 stages between 0.4
and 9 microns aerodynamic diameter. In a pre-test campaign the distribution of the particles were analysed in all
7 classes (Figure 5). The relevant classes in terms of particle emissions are those in the range of >9 µm and
<1.1 µm.
TPE-emissions in different particle classes
TPE concentration [mg/m3n @ 13% O2]
80
60
40
20
0
.0
>9
.0
-9
5.8
.8
.7
-4
-5
4.7
3.3
.3
-3
2.1
.1
-2
1.1
.1
-1
0.7
.7
-0
0.4
0.4
0-
Particle size [m]
Figure 5.
Total particle emission (TPE) of wood chip boiler measured in the various impactor stages.
There is a similar distribution as TPE for the TC emission (Figure 6). The major part of the said emission
are in the range of particles <1.1 µm.
EC-OC-TC-emissions in differen particle clases
2.50
TPE concentration [mg/m3n @ 13% O2]
OC
EC
TC
2.00
1.50
1.00
0.50
0.00
.0
>9
.0
-9
5.8
.8
-5
4.7
.7
-4
3.3
.3
-3
2.1
.1
-2
1.1
.1
-1
0.7
.7
-0
0.4
0.4
0-
Particle size [m]
Figure 6.
Distribution of EC, OC and TC emission in the different particle classes.
It was decided to analyse the impactor stages of >9 µm, 2.1-3.3 µm and <0.4 µm in the main measurement campaign.
12
3.3
Emission factors
The emission factors are calculated from standardised particle concentration at 13% oxygen measured in
the flue gas. The emission factors are based on the dry fuel mass. Since the wood combustion systems were tested
in the field the emission figures have real live character. They give some indication of the EC/OC characteristic
even though the data are not statistically proven. Each wood combustion systems was tested over two runs. The
data in Table 3 are mean values of the two test runs. The figures are given as TPE, TC, EC and OC derived of TPEfilter measurement as well as impactor measurement in the particle classes of > 9 µm, 2.1 - 3.3 µm and < 0.4 µm.
Table 3
Emission factors for total particle, total carbon, elemental carbon and organic carbon of the wood combustion
systems analysed based on fuel dry.
TPE: total particle emissions, PE: particle emission, TC: total carbon, EC: elemental carbon, OC: organic carbon.
The measurement uncertainty for TPE is 2%, for EC/OC 4.5%.
Emission factors
Wood
combustion
system
Wood chip
boiler
Pellet boiler
Wood log
boiler
Wood stove
Open fire
Particle class
PE
TC
EC
OC
[µm]
[g/kg]
[g/kg]
[g/kg]
[g/kg]
TPE
1.450
0.129
0.027
0.101
%
100%
8.9%
1.9%
7.0%
>9
0.138
0.010
0.007
0.003
2.1-3.3
0.042
0.002
0.0004
0.001
< 0.4
0.704
0.027
0.003
0.024
TPE
0.825
0.593
0.401
0.192
%
100%
71.9%
48.6%
23.3%
>9
0.028
0.009
0.007
0.003
2.1-3.3
0.048
0.024
0.020
0.004
< 0.4
0.387
0.157
0.145
0.012
TPE
0.621
0.463
0.197
0.265
%
100%
74.5%
31.8%
42.7%
>9
0.029
0.010
0.005
0.004
2.1-3.3
0.024
0.009
0.005
0.004
< 0.4
0.336
0.096
0.067
0.029
TPE
2.201
2.162
1.501
0.661
%
100%
83.9%
62.3%
21.6%
>9
0.066
0.037
0.032
0.005
2.1-3.3
0.096
0.067
0.061
0.006
< 0.4
1.126
0.770
0.737
0.033
TPE
4.520
2.724
1.998
0.727
%
100%
60.3%
44.2%
16.1%
>9
0.365
0.154
0.094
0.059
2.1-3.3
0.293
0.133
0.080
0.052
< 0.4
1.476
0.857
0.574
0.283
The airborne particle are classified according to their size. The commonly used classes are PM10
(< 10 µm), PM2.5 (< 2.5 µm) and PM1 (< 1 µm). With the gravimetric data of the impactor stage filters it is possible to estimate the emission factors of the particle classes PM10, PM2.5 and PM1. The share of the impactor
13
stages were summed up according to the PM-class. PM1 was calculated from the impactor stages <0.4 µm, 0.40.7 µm and 0.7-1.1 µm. For the PM2.5 the stages 1.1-2.1 µm and 2.1-3.3 µm were added. The stages 3.3-4.7 µm,
4.7-5.8 µm and 5.8-9 µm were additionally used to get PM10 figures.
Table 4
PM-emission factors of the wood combustion systems analysed based on fuel dry.
TPE: total particle emissions; PM10: particles with an aerodynamic diameter <10 µm, PM2.5: aerodynamic diameter
<2.5 µm, PM1: aerodynamic diameter <1 µm.
Emission factors
Wood
combustion
system
Wood chip
boiler
Pellet boiler
Wood log boiler
Wood stove
Open fire
Particle class
TPE
PM10
PM2.5
PM1
[g/kg]
1.45
1.29
1.135
0.997
%
100%
83.3%
78.2%
68.8%
[g/kg]
0.825
0.805
0.774
0.638
%
100%
97.6%
90.2%
77.3%
[g/kg]
0.659
0.63
0.582
0.526
%
100%
95.6%
88.2%
79.8%
[g/kg]
2.2
2.12
2
1.75
%
100%
96.3%
90.8%
79.6%
[g/kg]
4.52
4.0
3.3
2.7
%
100%
89.4%
72.7%
60.8%
4
Discussion
4.1
EC- and OC-content in total particle emissions
The TPE-emission factors show lowest values for wood log boiler followed by pellet and wood chip boiler
(Figure 7). The open fire and the wood stove have the highest emission factors.
The correlation of EC and OC in the TPE is varying: the domination of elemental carbon for the open fire
and the stove indicates a combustion with products of incomplete combustion (PIC). These test runs show also
high emission factors for carbon monoxide (CO). The middle field of the EC- and OC-emission are for wood log and
pellet boiler. They burned with fairly complete combustion emitting little EC and OC. The emission characteristic
of wood chip boiler indicates a complete combustion at a high combustion temperature. The salty components of
the minerals in the ash evaporate due to elevated charcoal bed temperature as carbonates. The particles are aerosols from these condensed inorganic carbon components. The low content of EC and OC demonstrates the complete combustion in the wood chip boiler.
14
Figure 7.
Emission factors of total particle emission (TPE). The elemental (EC) and organic carbon (OC) was analysed from
TPE-filters.
The content of TC as a fraction of TPE is in the range of 60% to 80% (Figure 8). Just the wood chip
boiler is an exemption with a content of less than 9%. The EC fraction is between 30% and 60% of the TPE, OC
10% to 40%.
Fraction of EC/OC/TC based on TPE
100%
Percentage [%]
80%
60%
40%
20%
0%
Wood chip
boiler
Pellet boiler
Wood log
boiler
EC
Figure 8.
OC
Stove
Open fire
TC
Fraction of elemental carbon (EC), organic carbon (OC) and total carbon (TC) based on total particle emissions
(TPE).
The share of EC and OC in the TC is indicated in Figure 9. The content of EC in the TC spreads in the
range of 21% - 74%, the one of OC 26% - 78%.
15
Fraction of EC/OC based on TC
100%
Percentage [%]
80%
60%
40%
20%
0%
Wood chip
boiler
Pellet boiler
Wood log
boiler
EC
Figure 9.
4.2
Stove
Open fire
OC
The share of elemental (EC) and organic carbon (OC) in the total carbon (TC) for total particle emission (TPE).
Particle emissions in the different particle sizes
PM10 contains 83% (wood chip boiler) up to 98% (pellet boiler) of the total particle mass (Figure 10).
PM2.5 holds 73% (open fire) to 91% (stove) of TPE. PM1 still includes 61% (open fire) to 80% of total particle
mass. This indicates that the mayor part of dust particles are of ultra fine size in the flue gas of wood combustion
processes.
Figure 10.
The particle emission in the particle sizes PM10, PM2.5 and PM1 as fraction of total particle emissions (TPI)
The open fire with more products of incomplete combustion shows a higher content of coarser particles.
The particle emission of PM1 are just 61%, PM2.5 just 73% of the total particle emission.
4.3
EC- and OC-emissions in the different particle classes
In all tests the impactor stages >9 µm, 2.3-2.5 µm and <0.4 µm were analysed for EC, OC and TPE. Figure
11 summarises the emission factors of the classes for total particles.
16
Emission factor [g/kg fuel dry]
TPE-emission in different particle classes
1.6
1.4
1.2
1.0
0.8
0.6
Wood chip boiler
Pellet boiler
Wood log boiler
Stove
Open fire
0.4
0.2
0.0
PC >9
Figure 11.
PC 2.3-2.5
PC 0-0.4
Total particle emission (TPE) in different particle classes.
The open fire has highest TPE-emission in all classes. All wood combustion systems have most TPEemission in the ultra fine fraction of particle classes <0.4 µm.
Highest TC-emission occurs like the TPE-emission in the particle class <0.4 µm (Figure 12).
TC-emission in different particle classes
1.0
0.8
0.6
Wood chip boiler
Pellet boiler
Wood log boiler
Stove
Open fire
0.4
0.2
0.0
PC >9
Figure 12.
PC 2.3-2.5
PC 0-0.4
Total carbon emission (TC) in different particle classes.
When EC- and OC-emission are compared over the various classes EC dominates the emission (Figure 13).
17
EC-emission in different particle classes
1.0
0.8
0.6
Wood chip boiler
Pellet boiler
Wood log boiler
Stove
Open fire
0.4
0.2
0.0
PC >9
Figure 13.
PC 2.3-2.5
PC 0-0.4
Elemental carbon emission (EC) in different particle classes.
OC is emitted at a lower level than EC in all classes (Figure 14).
OC-emission in different particle classes
1.0
0.8
0.6
0.4
Wood chip boiler
Pellet boiler
Wood log boiler
Stove
Open fire
0.2
0.0
PC >9
Figure 14.
4.4
PC 2.3-2.5
PC 0-0.4
Organic carbon emission (OC) in different particle classes.
The test runs with the wood log boiler comprised two phases: one test was with a start phase including
the ignition of the wood logs. The later was a phase under normal operating conditions. The start phase had periods with incomplete combustion. Thus the CO emission factor was 117 g/kg fuel dry (10'400 mg/m3n @ 13% O2).
The value for CO under normal operating conditions was 35 g/kg (3'050 mg/m3n @ 13% O2). Total carbon (TC) is
clearly reduced (Figure 15). Where OC remains almost identical EC is low for the combustion with little product of
incomplete combustion (PIC).
18
1
0.8
0.6
0.4
0.2
0
Test run 1 CO high
TPE
Figure 15.
4.5
Test run 2 CO low
EC
OC
TC
Elevated products of incomplete combustion (PIC) in a test run is indicated by high carbon monoxide (CO) and
low PIC with low CO resp.
Comparison of particle emissions
In Table 5 the total particle emissions are compared with other particle sources. Since the emissions
measured in the combustion system has some arbitrary character, there are ranges given. All data derive from
field measurements done by our institute. The European standards for Diesel vehicles are listed as emission factors based on net heating value of fuel. This allows the comparison of wood emission with Diesel engines as an
other source of fine particles.
Table 5.
Emissions of various particle sources. Diesel vehicles emission factors are based on European emission standards
(EURO-3 and Euro-4). There are also the requirements of the Swiss quality label for wood heat boilers and appliances.
TPE-Emission factor
based on fuel net
heating value
TPE-Emissions
Swiss quality label
requirement
TPE-Emission
[mg/MJ] 4)
[mg/mn3]
@ 13% O2
[mg/mn3]
@ 13% O2
Wood chip boilers
42 - 85
62 - 125
90
Pellet boilers
19 - 48
28 - 71
60
Wood log boilers
16 - 39
24 - 57
60
Wood log boilers
with flue gas filter
1 - 5.8
1.5 - 8.5
Wood stoves
24 -129
35 - 190
100
Open fire
265
390
-
Light duty vehicles
(EURO 3) 1)
22.7
-
-
Heavy duty vehicles
(lorries, EURO-3) 2)
11.4
-
-
Light duty vehicles
(EURO-4) 1)
11.4
-
-
Heavy duty vehicles
(lorries, EURO-4) 2)
2.3
-
-
Source
1) Diesel fuel consumption: 6 liter/100 km
2) Diesel fuel consumption: 205 g/kWh
3) specific flue gas volume: 11.6 mn3/kg @13%O2
4) net heating value of wood: 17 MJ/kg
19
In Table 5 the total particle emissions are compared with the requirement of the Swiss quality label for
wood heat boilers and appliances. The Swiss label requirements give some indications what the threshold values
of clean air requirements for areas suffering from air pollution could be.
Wood log and pellet boilers have the potential to achieve similar emission values for TPE as Diesel vehicles according to EURO-3 standard. Wood chip boilers with dust filter systems such as electrostatic precipitators
or fabric filters [7] have TPE-emission lower than EURO-3 and -4 from Diesel vehicles.
5
Conclusions
The distribution of total particle emission (TPE) over the particle classes show highest emission factors in
the particle class <0.4 µm. This is also true for total carbon (TC) emission as the sum of elemental (EC) and organic carbon (OC). The content of TC in the TPE varies between 9% up to 84%. The share of EC and OC fraction in
the TC is in the range of 20% to 70% for EC and 27% to 79% for OC. There is no correlation of the EC and OC
share in the TC: there are values measured for higher EC content as OC and visa versa. The crucial parameters seem
to be the grade of complete combustion and charcoal bed temperature for EC and OC formation. Little products of
incomplete combustion also result in low EC-emission factors.
PM10 contains 83% (wood chip boiler) up to 98% (pellet boiler) of the total particle mass. PM2.5 holds
73% (open fire) to 91% (stove) of TPE. PM1 still includes 61% (open fire) to 80% of total particle mass. This
indicates that the mayor part of dust particles are of ultra fine size in the flue gas of wood combustion processes.
The open fire and the wood stove have the highest emission factors for TPE, EC and OC. The average TPEemission factor of the open fire is 7 fold higher than lowest emission of wood log boiler. The wood log boiler
achieved in the field the requirements of the Swiss quality label. It must be stressed that the requirements are
tailored to emission of type tests under test lab condition.
Wood log and pellet boilers can achieve TPE-emission values in the range of EURO-3 Diesel vehicles. The
TPE-emission figures of the wood chip boilers and wood log appliances are higher. The particle emissions of wood
combustion can be significantly reduced by dust filter systems such as electrostatic precipitators or fabric filters.
20
Acknowledgements
The analytical work of the EC and OC coulometry is greatly acknowledged to the labs of the Swiss National Accident Insurance Fund (SUVA), Analytical Section.
The Swiss Federal Environment Protection Agency (BUWAL) granted the research work.
References
[1] Hueglin C., Gaegauf C., Kuenzel S., Burtscher H.:Characterization of Wood Combustion Particles. Morphology,
Mobility and Photoelectric Activity. Environmental Science and Technology, 1997.
[2] Wells, W.F.: Airborne Contagion and Air Hygiene, Harvard University Press, Cambridge, Massachusetts, 1955.
[3] P.E. Denyer, D. Wilkins: Further investigations into the use of a flue loss method for measurement of the thermal efficiency of wood-burning roomheaters, CRE Group Ltd., Gloucestershire GB, F&AT Report no. 70, 1994.
[4] C. K. Gaegauf, Y.Macquat: Comparison of draft ISO- and CEN-standard to determine efficiency and emission of
solid fuel burning appliances, 1st World Conference and Exhibition on Biomass for Energy and Industry, Sevilla, 2000.
[5] Andersen 1 ACFM Particle sizing sampler.
[6] Physical characterization of particulate emissions from diesel engines: a review H. Burtscher, Journal of Aerosol Science, Elsevier, in press.
[7] C. K. Gaegauf, M.P. Schmid, Andres Jenni: Wirksamkeit neuer Abscheidetechniken in Holzfeuerungsabgasen
bezüglich Feinpartikeln, Final Report of Swiss Environment Protection Agency (BUWAL), 2003.
21

Elemental and Organic Carbon in Flue Gas Particles of Various

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