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Measurement of Particle Size Distributions of Flue Dust by Means of Cascade Impactors
Atmospheric Pollution 1980, Proceedings of the 14th International Colloquium, Paris, France, May 5--8,1980, M.M. Benarie (Ed.), Studies in Environmental Science, Volume 8 0 Elsevier Scientific Publishing Company, Amsterdam,- Printed in The Netherlands
257
MEASUREMENT OF PARTICLE SIZE DISTRIBUTIONS OF FLUE DUST BY MEANS OF CASCADE IMPACTORS R. WIEDEMANN Lehrstuhl fur Thermische Kraftanlagen, Technische Universitat Miinchen, Munchen, Bundesrepublik Deutschland
INTRODUCTION This report deals with investigations on cascade impactors which were accomplished at an experimental plant of the Technical University of blunich and at some coal-fired power plants. These investigations were mainly orientated to practical applications. In the following some of the results and recornmodations will be shown.
Trajectory of a Particle to Small to Impact
Lines
Impaction Plate
Trajectory of an Impacted Particle
Fig. 1. Schematic plot of a single impactor stage.
Glass Fiber Filter
258
An impactoris a device to classify flue dusts with respect to their aerodynamic diameter. A solid-gaseous two phase flow is accelerated in a nozzle stage and is diverted in front of a following impaction plate (stagnation point flow). Coarse particles with a high inertial mass cannot follow the divertion and are impacted on the impaction plate, whereas fine particles follow the stream lines. Theoretically in each impactor stage a seperation into two particle size distributions is achieved. (Figure 1 ) . By connecting several nozzle stages and by increasing the mean flow velocity the cut-off points can be shifted to finer and iiner particle diameters, so-called "equivalent cut-off diameters". This separating process is governed by a dimensionless inertial parameter, the STOKES number. According to theoretical calculations the STOKES number describing this process is fairly constant for different nozzle dimensions and for a wide REYNOLDS number region. 1 1 1 .
1 . MEASUREMENTS AT AN EXPERIMENTAL PLANT
At an experimental plant installed at the Technical University of Munich some commercially available impactors were tested with respect to their applicability to reproduce correctly the particle size distribution of known, polydisperse flue dusts. By change of the dust concentration, the dust material and the gas flow rate various working conditions can be realized in a vertical duct of the plant (figure 2). The following criterions of performance of the impactors were examined at the experimental plant: 1 . Measurements should be reproducible. 2. Measured particle size distributions of known flue dusts should be recorded independently of the dust concentration and the flow rate through the impactor. 3. A measured particle size distribution should coincide with the particle size distribution of a known flue dust dosed into the experimental duct. The size distribution of the dosedin flue dust was determined by sedimentation analysis according to the Andreasen method. This analysis provides an aerodynamic diameter as well. The ratios of mean particle diameters of several different size distributions measured in the experimental plant should correspond tothe ratios, measured by means of the sedimentation analysis.
259
4. A great value was set on easy handiness. 5. The amount of wall losses should be as low as possible.
Fig. 2. Experimental plant for dust sampling at the Technical University of Munich All the conditions mentioned above could not be satisfied totally by any of the tested impactors. For measurements in the exhaust gas flow of coal-fired power plants a six stage cascade impactor with integrated nozzle-impaction plates was chosen. The research work at the experimental plant brought a lot of additional results and handling concepts which were considered for the measurements in stacks of power plants:
260 1.
2.
3.
4.
5.
6.
The application of a pre-separator in form of a pre-impactor o r a pre-cyclone for the retardation of particles larger than 10 microns is essential, because coarse particles are reflected at the impaction plates and may be transported to the back-up filter. Because of this phenomenon a finer particle size distribution is faked. The application of a pre-separator shifts the size distribution curves to a coarser region. The comparison of the measured curves with those provided by the sedimentation analysis are pretty acceptable. Moreover a decrease of the wall losses could be observed by use of a pre-separator. F o r the measurements in stacks of power plants a pre-impactor was chosen because of its handiness and because of its low penetrability for particles larger 10 microns. The coating of the impaction plates with pretreated, punched-out glass fiber filter is recommended, expecially if the dust deposition will be physically or chemically examined afterwards. Generally speaking there is no significant change of results whether glas fiber filter are used o r not except the deposited dust has only a low adhesion to the uncoated impaction plates. The application of gooseneck probes is not recommended. Presumable in the bent of a gooseneck probe a preseparation of coarse particles occurs, and measured distributions are shifted to finer regions. The application of an impactor to measure the total amount of the dust concentration is not advisable because the wall losses may fluctate between 20 to 40 4, of the theoretically expected mass deposit. The wall losses of coarse particles are higher than those of fine particles. By a suitable design of the air intake funnel the wall losses can be reduced but at the same time a nonuniform loading of the first stages has to be taken into account. On the contrary to measurements of total dust concentrations isocinetic sampling conditions do not have to be strictly adjusted. The error made by non-isocinetic sampling becomes only significant for wide size distributions. On the other hand the impactor flow rate should be constant during sampling time, otherwise the magnitude of the "equivalent cut-off diameters" is changed permanently. The dust loading of a filter should not increase 8 mg, otherwise already impacted particles may be reintrained.
261 2 . MEASUREMENTS OF PARTICLE SIZE DISTRIBUTIONS
IN EXHAUST GASES OF
POWER PLANTS 2.1.
Description of the Measuring Positions Power Plant A: stone coal-fired Measurement were taken at a stone-coal fired power plant with an electrical power output of 320 MW. The measuring point was situated in the purified exhaust gas duct between the electro-static precipitor and the exhaust fan. The flow was directed vertically downwards. Disturbing obstructions, bents and pipebranchings were not in the vicinity of the measuring point. A second measuring point was situated in the crude exhaust gas duct just in front of the electro-static precipitor. 2 . 1 . 2 . Power Plant B: lignite-fired Power plant B was provided with a lignite fired Benson vessel and delivered a power output o f 600 MW. The position of the measuring point was similiar to those of power plant A, except that the flow was directed horizontally and the measuring point was situated very adversely with respect to the flow pattern just after a 90O-bent and in front of a pipe branching. Measurements in the crude exhaust gas were not taken. During measuring time both power plantswere operated under constant maximum electrical power output. In both cases measurements were taken only at some grid points of the cross sectional area. 2.1.1.
2.2.
Methode of Measurement The glass fiber filter for the coating of the impaction plates were submitted to following procedure: 1 . Dehydrating at a temperature o f 300 O C far two hours. 2 . Cooling off in a desiccating cylinder. 3. Weighing of the filter in weighing dishes. 4. Transport of the filter in weighing dishes and Petri bowls. After each measurement the impactor was washed in acetoneand cleaned. The impactor was run in a dust sampling train. A condensate collector and a drying tower were added to the impactor. The instantaneous flow rate was controlled by a flow meter; the sucked-off gas volume was recorded by a gas meter. As the flow rate was measured after passing the drying tower the humidity of the exhaust gas had to be taken in consideration for the adjustment of an isocinetic flow rate and for the evaluation of the results. Before starting the sampling train the impactor and pre-impactor were warmed up in the exhaust duct. After finishing the glass fiber filter were packed in weighing
262
dishes, dehydrated at a temperature of 1 1 0 "C, cooled-off in a desiccating cylinder and weighed. Changes of filter mass due to chemical reactions of sulphur dioxide and hydrofluoric acid were ignored. Dust depositions of the pre-impactor were swabbed out into a weighing dish and submitted to the same procedure as the filter. 2 . 3 . Evaluation of the Results
After determiningthe filter loading by weighing the "equivalent cut-off diameters d were calculated by trial-and-error method P according to the following equation:
lStk'=0,343 3
Volumenstrom
d
Furtikeldurchmesser (porticle diameter)
9
Portikeldichte
(particle density)
rl
dynom. Viskositat
Idynom. viscosity)
C
Cunningham Foktor (Cunninghorn factor)
St k
Stokes -Zohl
D
Dusendurchmesser (Nozzle diameter 1
A
Dusenfloche
(Nozzle c.s.orea)
n
Dusenonzohl
(number of nozzles)
(volume flux)
(Stokes number)
The actual parameters are the particle density, the exhaust gas temperature and the impactor flow rate. The mean nozzle diameter
263
of each stage was measured with help of a microscope. As magnitude for the particle density the unity density was chosen, because in general the particle density is unknown. In the following figures the cumulative residue of the distributions was plotted logarthmically versus the particle diameter.
2.4. Results Impactor measurements in the crude and purified exhaust gas of the stone coal fired power plant could be compared to sedimentation analyses of flue dust deposited in the pre- and final cleaner of the electro-static precipitor. In this case the equivalent cut-off diameters of the purified exhaust gas were evaluated with the flue dust density of the pre-cleaner (figure 3). As expected a significant decrease of the mean particle diameter of the flue dust is observed when passing the electro-static precipitor. The particle size distributions measured in the crude exhaust gas show a wide fluctuation mainly due to short sampling timeswhichhad to be chosen because of the high dust concentration. On the other hand these distributions overlap with those gained from the pre- and final cleaner. The particle size distributions in the purified exhaust gas are shifted clearly to finer regions. Evaluating the results by neglecting the mass deposited in the pre-impactor for all measurements in purified exhaust gas a narrow fluctuation field can be observed (figure 4). The results obtained at the lignite-fired power plant show a large scattering of the plotted points (figure 5). Presumable this is due to the adverse flow pattern caused by duct obstructions and to high fluctations of the fuel composition. The mean particle diameter in the purified exhaust gas of the stone coal-fired power plant turned out to be 3 , L microns, those of the lignite-fired power plant to 2 , 0 microns provided that the particle density is unity. Neglecting the pre-impactor massasmall scattering of the plotted points can be obtained similar to the proceeding case (figure 6). The wide fluctation field observed when taking into consideration the preimpactor mass is therefore due to high fluctations of the coarse particle concentration.
264
0 10
20
o lmpactw
Purified Exhaust Gas
0 Impactor
Crude Exhaust Gas
30 LO
5 50 0,
u)
?I 60 > 0,
-3 70
5 80
0
90 %
10 Pm Porticle Diometer dp
100
Fig. 3. Power plant A : stone coal-fired; comparison of the particle size distribution of flue dust out o f the crude exhaust gaq, the pre-cleaner, the final cleaner and the purified exhaust gas.
Yo
100
Od
1
10 Pm Particle Diometer dp ‘pp 1.0 g/cm3 1
100
Fig. 4. Power plant A: lignite-fired, evaluation without preimpactor mass.
265
0 10
3
0
1 Day
Axis 3
20
30 LO
4 50 3 LX 60 0
-
V
9Ar-r 100
L
0.l
1
10 Pm Partide Diameter dp
-
100
I Q =~i.0g/cm31
Fig. 5. Power plant B: lignite-fired, measurements in the purified exhaust gas.
0
POWER PLANT E
10
20
0 1 Day
V
0
Lignite
Axis 3
ZDay 3Day
30
3 LO
2
u)
a"
50
060
> .c
5 70 E
5 80 90 VO
100
0.1
1
10 Pm Particle Diameter dp
0
(pP = l . ~ g / c r n ~ ) Fig. 6. Power plant B: lignite-fired, evaluation without preimpactor mass.
266
3. CONCLUSION
For measurements of particle size distributions of flue dust a cascade impactor is very suitable if no great strain is put on high accuracy. The results of impactor measurements represent a time-averaged value. The application of a pre-separator if particles larger than 10 microns are expected is highly recommended. The impactor should not be used to determine the total dust concentration in an exhaust duct.
REFERENCES
1
Marple, V. A.: A fundamental study of inertial impactors. Diss. Univ. of Minnesota
257
MEASUREMENT OF PARTICLE SIZE DISTRIBUTIONS OF FLUE DUST BY MEANS OF CASCADE IMPACTORS R. WIEDEMANN Lehrstuhl fur Thermische Kraftanlagen, Technische Universitat Miinchen, Munchen, Bundesrepublik Deutschland
INTRODUCTION This report deals with investigations on cascade impactors which were accomplished at an experimental plant of the Technical University of blunich and at some coal-fired power plants. These investigations were mainly orientated to practical applications. In the following some of the results and recornmodations will be shown.
Trajectory of a Particle to Small to Impact
Lines
Impaction Plate
Trajectory of an Impacted Particle
Fig. 1. Schematic plot of a single impactor stage.
Glass Fiber Filter
258
An impactoris a device to classify flue dusts with respect to their aerodynamic diameter. A solid-gaseous two phase flow is accelerated in a nozzle stage and is diverted in front of a following impaction plate (stagnation point flow). Coarse particles with a high inertial mass cannot follow the divertion and are impacted on the impaction plate, whereas fine particles follow the stream lines. Theoretically in each impactor stage a seperation into two particle size distributions is achieved. (Figure 1 ) . By connecting several nozzle stages and by increasing the mean flow velocity the cut-off points can be shifted to finer and iiner particle diameters, so-called "equivalent cut-off diameters". This separating process is governed by a dimensionless inertial parameter, the STOKES number. According to theoretical calculations the STOKES number describing this process is fairly constant for different nozzle dimensions and for a wide REYNOLDS number region. 1 1 1 .
1 . MEASUREMENTS AT AN EXPERIMENTAL PLANT
At an experimental plant installed at the Technical University of Munich some commercially available impactors were tested with respect to their applicability to reproduce correctly the particle size distribution of known, polydisperse flue dusts. By change of the dust concentration, the dust material and the gas flow rate various working conditions can be realized in a vertical duct of the plant (figure 2). The following criterions of performance of the impactors were examined at the experimental plant: 1 . Measurements should be reproducible. 2. Measured particle size distributions of known flue dusts should be recorded independently of the dust concentration and the flow rate through the impactor. 3. A measured particle size distribution should coincide with the particle size distribution of a known flue dust dosed into the experimental duct. The size distribution of the dosedin flue dust was determined by sedimentation analysis according to the Andreasen method. This analysis provides an aerodynamic diameter as well. The ratios of mean particle diameters of several different size distributions measured in the experimental plant should correspond tothe ratios, measured by means of the sedimentation analysis.
259
4. A great value was set on easy handiness. 5. The amount of wall losses should be as low as possible.
Fig. 2. Experimental plant for dust sampling at the Technical University of Munich All the conditions mentioned above could not be satisfied totally by any of the tested impactors. For measurements in the exhaust gas flow of coal-fired power plants a six stage cascade impactor with integrated nozzle-impaction plates was chosen. The research work at the experimental plant brought a lot of additional results and handling concepts which were considered for the measurements in stacks of power plants:
260 1.
2.
3.
4.
5.
6.
The application of a pre-separator in form of a pre-impactor o r a pre-cyclone for the retardation of particles larger than 10 microns is essential, because coarse particles are reflected at the impaction plates and may be transported to the back-up filter. Because of this phenomenon a finer particle size distribution is faked. The application of a pre-separator shifts the size distribution curves to a coarser region. The comparison of the measured curves with those provided by the sedimentation analysis are pretty acceptable. Moreover a decrease of the wall losses could be observed by use of a pre-separator. F o r the measurements in stacks of power plants a pre-impactor was chosen because of its handiness and because of its low penetrability for particles larger 10 microns. The coating of the impaction plates with pretreated, punched-out glass fiber filter is recommended, expecially if the dust deposition will be physically or chemically examined afterwards. Generally speaking there is no significant change of results whether glas fiber filter are used o r not except the deposited dust has only a low adhesion to the uncoated impaction plates. The application of gooseneck probes is not recommended. Presumable in the bent of a gooseneck probe a preseparation of coarse particles occurs, and measured distributions are shifted to finer regions. The application of an impactor to measure the total amount of the dust concentration is not advisable because the wall losses may fluctate between 20 to 40 4, of the theoretically expected mass deposit. The wall losses of coarse particles are higher than those of fine particles. By a suitable design of the air intake funnel the wall losses can be reduced but at the same time a nonuniform loading of the first stages has to be taken into account. On the contrary to measurements of total dust concentrations isocinetic sampling conditions do not have to be strictly adjusted. The error made by non-isocinetic sampling becomes only significant for wide size distributions. On the other hand the impactor flow rate should be constant during sampling time, otherwise the magnitude of the "equivalent cut-off diameters" is changed permanently. The dust loading of a filter should not increase 8 mg, otherwise already impacted particles may be reintrained.
261 2 . MEASUREMENTS OF PARTICLE SIZE DISTRIBUTIONS
IN EXHAUST GASES OF
POWER PLANTS 2.1.
Description of the Measuring Positions Power Plant A: stone coal-fired Measurement were taken at a stone-coal fired power plant with an electrical power output of 320 MW. The measuring point was situated in the purified exhaust gas duct between the electro-static precipitor and the exhaust fan. The flow was directed vertically downwards. Disturbing obstructions, bents and pipebranchings were not in the vicinity of the measuring point. A second measuring point was situated in the crude exhaust gas duct just in front of the electro-static precipitor. 2 . 1 . 2 . Power Plant B: lignite-fired Power plant B was provided with a lignite fired Benson vessel and delivered a power output o f 600 MW. The position of the measuring point was similiar to those of power plant A, except that the flow was directed horizontally and the measuring point was situated very adversely with respect to the flow pattern just after a 90O-bent and in front of a pipe branching. Measurements in the crude exhaust gas were not taken. During measuring time both power plantswere operated under constant maximum electrical power output. In both cases measurements were taken only at some grid points of the cross sectional area. 2.1.1.
2.2.
Methode of Measurement The glass fiber filter for the coating of the impaction plates were submitted to following procedure: 1 . Dehydrating at a temperature o f 300 O C far two hours. 2 . Cooling off in a desiccating cylinder. 3. Weighing of the filter in weighing dishes. 4. Transport of the filter in weighing dishes and Petri bowls. After each measurement the impactor was washed in acetoneand cleaned. The impactor was run in a dust sampling train. A condensate collector and a drying tower were added to the impactor. The instantaneous flow rate was controlled by a flow meter; the sucked-off gas volume was recorded by a gas meter. As the flow rate was measured after passing the drying tower the humidity of the exhaust gas had to be taken in consideration for the adjustment of an isocinetic flow rate and for the evaluation of the results. Before starting the sampling train the impactor and pre-impactor were warmed up in the exhaust duct. After finishing the glass fiber filter were packed in weighing
262
dishes, dehydrated at a temperature of 1 1 0 "C, cooled-off in a desiccating cylinder and weighed. Changes of filter mass due to chemical reactions of sulphur dioxide and hydrofluoric acid were ignored. Dust depositions of the pre-impactor were swabbed out into a weighing dish and submitted to the same procedure as the filter. 2 . 3 . Evaluation of the Results
After determiningthe filter loading by weighing the "equivalent cut-off diameters d were calculated by trial-and-error method P according to the following equation:
lStk'=0,343 3
Volumenstrom
d
Furtikeldurchmesser (porticle diameter)
9
Portikeldichte
(particle density)
rl
dynom. Viskositat
Idynom. viscosity)
C
Cunningham Foktor (Cunninghorn factor)
St k
Stokes -Zohl
D
Dusendurchmesser (Nozzle diameter 1
A
Dusenfloche
(Nozzle c.s.orea)
n
Dusenonzohl
(number of nozzles)
(volume flux)
(Stokes number)
The actual parameters are the particle density, the exhaust gas temperature and the impactor flow rate. The mean nozzle diameter
263
of each stage was measured with help of a microscope. As magnitude for the particle density the unity density was chosen, because in general the particle density is unknown. In the following figures the cumulative residue of the distributions was plotted logarthmically versus the particle diameter.
2.4. Results Impactor measurements in the crude and purified exhaust gas of the stone coal fired power plant could be compared to sedimentation analyses of flue dust deposited in the pre- and final cleaner of the electro-static precipitor. In this case the equivalent cut-off diameters of the purified exhaust gas were evaluated with the flue dust density of the pre-cleaner (figure 3). As expected a significant decrease of the mean particle diameter of the flue dust is observed when passing the electro-static precipitor. The particle size distributions measured in the crude exhaust gas show a wide fluctuation mainly due to short sampling timeswhichhad to be chosen because of the high dust concentration. On the other hand these distributions overlap with those gained from the pre- and final cleaner. The particle size distributions in the purified exhaust gas are shifted clearly to finer regions. Evaluating the results by neglecting the mass deposited in the pre-impactor for all measurements in purified exhaust gas a narrow fluctuation field can be observed (figure 4). The results obtained at the lignite-fired power plant show a large scattering of the plotted points (figure 5). Presumable this is due to the adverse flow pattern caused by duct obstructions and to high fluctations of the fuel composition. The mean particle diameter in the purified exhaust gas of the stone coal-fired power plant turned out to be 3 , L microns, those of the lignite-fired power plant to 2 , 0 microns provided that the particle density is unity. Neglecting the pre-impactor massasmall scattering of the plotted points can be obtained similar to the proceeding case (figure 6). The wide fluctation field observed when taking into consideration the preimpactor mass is therefore due to high fluctations of the coarse particle concentration.
264
0 10
20
o lmpactw
Purified Exhaust Gas
0 Impactor
Crude Exhaust Gas
30 LO
5 50 0,
u)
?I 60 > 0,
-3 70
5 80
0
90 %
10 Pm Porticle Diometer dp
100
Fig. 3. Power plant A : stone coal-fired; comparison of the particle size distribution of flue dust out o f the crude exhaust gaq, the pre-cleaner, the final cleaner and the purified exhaust gas.
Yo
100
Od
1
10 Pm Particle Diometer dp ‘pp 1.0 g/cm3 1
100
Fig. 4. Power plant A: lignite-fired, evaluation without preimpactor mass.
265
0 10
3
0
1 Day
Axis 3
20
30 LO
4 50 3 LX 60 0
-
V
9Ar-r 100
L
0.l
1
10 Pm Partide Diameter dp
-
100
I Q =~i.0g/cm31
Fig. 5. Power plant B: lignite-fired, measurements in the purified exhaust gas.
0
POWER PLANT E
10
20
0 1 Day
V
0
Lignite
Axis 3
ZDay 3Day
30
3 LO
2
u)
a"
50
060
> .c
5 70 E
5 80 90 VO
100
0.1
1
10 Pm Particle Diameter dp
0
(pP = l . ~ g / c r n ~ ) Fig. 6. Power plant B: lignite-fired, evaluation without preimpactor mass.
266
3. CONCLUSION
For measurements of particle size distributions of flue dust a cascade impactor is very suitable if no great strain is put on high accuracy. The results of impactor measurements represent a time-averaged value. The application of a pre-separator if particles larger than 10 microns are expected is highly recommended. The impactor should not be used to determine the total dust concentration in an exhaust duct.
REFERENCES
1
Marple, V. A.: A fundamental study of inertial impactors. Diss. Univ. of Minnesota