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Comparison of particle emissions from different combustion systems
J AerosolSci.
Vol. 31, Suppl. I, pp. $620-$621,2000
Pergamon www.elsevier.com/locate/jaerosci
S e s s i o n 7 D - C o m b u s t i o n aerosols I COMPARISON OF PARTICLE EMISSIONS FROM DIFFERENT COMBUSTION SYSTEMS H. BURTSCHER Fachhochschule Aargau, University of Applied Science, CH-5210 Windisch, Switzerland
Keywords: combustion particles, diesel soot, spark ignition engine, oil burner
INTRODUCTION Depending on the fuel used and on the kind of combustion (explosive combustion in an engine, stationary combustion in heater-devices) particles in the exhaust may differ significantly, not only in number and size, but also in structure and composition. Whereas particles, arising from combustion of gaseous or liquid fuels (gas, petrol, diesel) mainly consist of carbonaceous material, which results from incomplete combustion, and sulfates, a large fraction of particulate emission from the combustion of solid fuels as wood or coal arises from incombustible material (fly ash). Particles belonging to this fraction usually are much bigger (>llam) and will not be considered here. The submicron particles mainly differ in their content of volatile material (hydrocarbons, sulfates), size and concentration. The content of volatile material also has a strong effect on the structure. Almost always emitted particles are agglomerates. If the volatile fraction is high, agglomerates may coalesce and from spherical particles, otherwise grape- or chainlike structures are formed. In the following some examples for the properties of particles from combustion of petrol, diesel (in engines and in a stove) and will be given. INSTRUMENTATION Size distributions are measured by the Scanning Mobility Particle Sizer (SMPS). To obtain information on particle mass m, a low pressure impactor (LPI) is used in series with a DMA. The DMA classifies particles according to their mobility b, the LPI according to mb. This allows to determine the mass as function of the particle size (in terms of the mobility diameter d). From this relation a fractal-like dimension dfcan be calculated (Schleicher et al., 1995) m = const • d df. A thermodesorber TD allows to remove the volatile fraction (material which can be vaporized at temperatures below 400°C). Further, a photoelectric aerosol sensor (PAS) is applied, which selectively responds to material from incomplete combustion. When operating this sensor with a low phOton energy, it is selectively sensitive to polycyclic aromatic hydrocarbons. At higher photon energy its signal is mainly determined by elemental carbon. Other materials as sulfates show no signal at all. For some measurements a pulsed laser is used for photoemission, which is triggered by the ignition of the engine with a variable delay. This allows measurements of the emission as function of time after the ignition (Burtscher and Siegmann, 1994). The total particle mass is determined by a Tapered Element Oscillating Microbalance (TEOM), the elemental carbon (EC) concentration by an Aethalometer. From these measurements emission factors and the EC-fraction can be derived. RESULTS The particle sizes are similar in all cases. Most particles are in the size range from 20 to 200 nm. Table 1 gives a summary of emission factors and EC/TM ratio. The determination of the elemental carbon concentration has been done with the factory calibration of the Aethalometer. This value has to be
$620
Abstracts of the 2000 European Aerosol Conference
$621
regarded as an estimate, as the calibration depends on the particle properties and will not be correct for all cases, errors up to about 50% have to be expected. This explains the BC fractions greater 100%. Air/fuel ratio 2.1
BC/TM [%], 20°C 4
BC/TM [%], 280°C 45
Emission factor 2 l04
SI, nom load
1.8
2
3
2 10-5
Oil stove lean
1.3
13
30
1 10.5
Oil stove rich
1.1
70
83
1 10.4
SI, idling
Diesel idling
5.8
30
100
2 10.4
Diesel, 30% load
3.9
45
120
2 10.4
Diesel, 60% load
2.5
88
120
4 10.4
Diesel, nom load
1.9
100
120
1 10"3
Table 1: black carbon to total mass ratio and emission factor for different combustion systems.
Some other characteristic results are: • The size distribution of particles from the oil burner has two modes, one in the ultrafine range, which is dominant under lean conditions, one in the range of 100nm, which increases, when the combustion becomes richer. When heating the TD, the size of particles in the small mode decreases significantly already at low temperature whereas particles in the larger mode are not significantly influenced by the TD. This shows, that the two modes consist of completely different material. We assume that the ultrafine node contains unburned fuel, the larger mode mainly elemental carbon. • Particles in the exhaust of the spark ignition engine are very small and have a high content of volatile material. Their fractal like dimension df decreases from 3 to 2.2., when heating the thermodesorber. A hypothesis for this observation is that due to the high content of liquid material, droplets are formed, when the exhaust gas cools. These droplets incorporate a nonvolatile agglomerate, formed already in the hot part immediately after the combustion. When the volatile part is removed by the TD, this nonvolatile part becomes 'visible'. • For diesel engines a significant difference in emissions from turbo loaded engines and naturally aspirated engines is found, for example in the content of volatile material. The loaded engine has a very low content of volatiles under all operating conditions investigated, for the naturally aspirated it is high at low load and becomes much smaller at high load. ACKNOWLEDGMENTS I want to thank H.C. Siegrnann for his support for this work and for many helpful discussions. Ch. Haglin, S. Ktinzel, A. Leonardi, U. Matter, M.A. Schwaab and D. Steiner were involved in doing the measurements, presented here. REFERENCES Burtscher, H. and H.C. Siegmann (1994). Monitoring PAH-emissions from combustion processes by photoelectric charging. Combust. Sci. Technol. 101, 327-332. Schleicher, B., S. Kiinzel and H. Burtscher (1995). In-situ measurement of size and density of submicron aerosol particles. J. Appl. Phys., 78, 4416-4422.
Vol. 31, Suppl. I, pp. $620-$621,2000
Pergamon www.elsevier.com/locate/jaerosci
S e s s i o n 7 D - C o m b u s t i o n aerosols I COMPARISON OF PARTICLE EMISSIONS FROM DIFFERENT COMBUSTION SYSTEMS H. BURTSCHER Fachhochschule Aargau, University of Applied Science, CH-5210 Windisch, Switzerland
Keywords: combustion particles, diesel soot, spark ignition engine, oil burner
INTRODUCTION Depending on the fuel used and on the kind of combustion (explosive combustion in an engine, stationary combustion in heater-devices) particles in the exhaust may differ significantly, not only in number and size, but also in structure and composition. Whereas particles, arising from combustion of gaseous or liquid fuels (gas, petrol, diesel) mainly consist of carbonaceous material, which results from incomplete combustion, and sulfates, a large fraction of particulate emission from the combustion of solid fuels as wood or coal arises from incombustible material (fly ash). Particles belonging to this fraction usually are much bigger (>llam) and will not be considered here. The submicron particles mainly differ in their content of volatile material (hydrocarbons, sulfates), size and concentration. The content of volatile material also has a strong effect on the structure. Almost always emitted particles are agglomerates. If the volatile fraction is high, agglomerates may coalesce and from spherical particles, otherwise grape- or chainlike structures are formed. In the following some examples for the properties of particles from combustion of petrol, diesel (in engines and in a stove) and will be given. INSTRUMENTATION Size distributions are measured by the Scanning Mobility Particle Sizer (SMPS). To obtain information on particle mass m, a low pressure impactor (LPI) is used in series with a DMA. The DMA classifies particles according to their mobility b, the LPI according to mb. This allows to determine the mass as function of the particle size (in terms of the mobility diameter d). From this relation a fractal-like dimension dfcan be calculated (Schleicher et al., 1995) m = const • d df. A thermodesorber TD allows to remove the volatile fraction (material which can be vaporized at temperatures below 400°C). Further, a photoelectric aerosol sensor (PAS) is applied, which selectively responds to material from incomplete combustion. When operating this sensor with a low phOton energy, it is selectively sensitive to polycyclic aromatic hydrocarbons. At higher photon energy its signal is mainly determined by elemental carbon. Other materials as sulfates show no signal at all. For some measurements a pulsed laser is used for photoemission, which is triggered by the ignition of the engine with a variable delay. This allows measurements of the emission as function of time after the ignition (Burtscher and Siegmann, 1994). The total particle mass is determined by a Tapered Element Oscillating Microbalance (TEOM), the elemental carbon (EC) concentration by an Aethalometer. From these measurements emission factors and the EC-fraction can be derived. RESULTS The particle sizes are similar in all cases. Most particles are in the size range from 20 to 200 nm. Table 1 gives a summary of emission factors and EC/TM ratio. The determination of the elemental carbon concentration has been done with the factory calibration of the Aethalometer. This value has to be
$620
Abstracts of the 2000 European Aerosol Conference
$621
regarded as an estimate, as the calibration depends on the particle properties and will not be correct for all cases, errors up to about 50% have to be expected. This explains the BC fractions greater 100%. Air/fuel ratio 2.1
BC/TM [%], 20°C 4
BC/TM [%], 280°C 45
Emission factor 2 l04
SI, nom load
1.8
2
3
2 10-5
Oil stove lean
1.3
13
30
1 10.5
Oil stove rich
1.1
70
83
1 10.4
SI, idling
Diesel idling
5.8
30
100
2 10.4
Diesel, 30% load
3.9
45
120
2 10.4
Diesel, 60% load
2.5
88
120
4 10.4
Diesel, nom load
1.9
100
120
1 10"3
Table 1: black carbon to total mass ratio and emission factor for different combustion systems.
Some other characteristic results are: • The size distribution of particles from the oil burner has two modes, one in the ultrafine range, which is dominant under lean conditions, one in the range of 100nm, which increases, when the combustion becomes richer. When heating the TD, the size of particles in the small mode decreases significantly already at low temperature whereas particles in the larger mode are not significantly influenced by the TD. This shows, that the two modes consist of completely different material. We assume that the ultrafine node contains unburned fuel, the larger mode mainly elemental carbon. • Particles in the exhaust of the spark ignition engine are very small and have a high content of volatile material. Their fractal like dimension df decreases from 3 to 2.2., when heating the thermodesorber. A hypothesis for this observation is that due to the high content of liquid material, droplets are formed, when the exhaust gas cools. These droplets incorporate a nonvolatile agglomerate, formed already in the hot part immediately after the combustion. When the volatile part is removed by the TD, this nonvolatile part becomes 'visible'. • For diesel engines a significant difference in emissions from turbo loaded engines and naturally aspirated engines is found, for example in the content of volatile material. The loaded engine has a very low content of volatiles under all operating conditions investigated, for the naturally aspirated it is high at low load and becomes much smaller at high load. ACKNOWLEDGMENTS I want to thank H.C. Siegrnann for his support for this work and for many helpful discussions. Ch. Haglin, S. Ktinzel, A. Leonardi, U. Matter, M.A. Schwaab and D. Steiner were involved in doing the measurements, presented here. REFERENCES Burtscher, H. and H.C. Siegmann (1994). Monitoring PAH-emissions from combustion processes by photoelectric charging. Combust. Sci. Technol. 101, 327-332. Schleicher, B., S. Kiinzel and H. Burtscher (1995). In-situ measurement of size and density of submicron aerosol particles. J. Appl. Phys., 78, 4416-4422.