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Removal of the aerosol formed in the electron beam dry scrubbing process by bag filters
J. Aerosol Sci., Vol. 19, No. 7, pp. 1397 - 1400, 1988 Printed in Great Britain
0021-8502/88 83.00. + 0.00 Pergamon Press plc
REMOVAL OF THE AEROSOL FORMED IN THE ELECTRON BEAM DRY SCRUBBING PROCESSBY BAG FILTERS H.-R. Paur, S. Jordan, W. Baumann Kernforschungszentrum Karlsruhe GmbH Laboratorium fi~r Aerosolphysik und Filtertechnik Postfach 3640, D-7500 Karlsruhe, FRG
Key Words:
Electron Beam, Flue Gas, Submicron Aerosols, Bag Filters
Introduction In the electron beam dry scrubbing (EBDS) process NOx and SOx are removed simultaneously by irradiation of exhaust gases with accelerated electrons. Upon addition of ammonia submicron aerosols, consisting of ammonium-nitrate and -sulfate are formed. These products are then collected by bagfilters and may be used as fertilizer. The process has been reviewed recently by Jordan et a1.(1988 a, b). The physical and chemical properties of the aerosol have been investigated by Jordan et al. (1986): The mass median diameters of this rather hygroscopic aerosol range from 0.5 up to 7.5 lam, depending on the absorbed dose. The mass concentrations in the irradiated flue gas are between 500 up to 1500 mg m-3. The mass Ioadings and the chemical composition depend substantially on dose, ammonia-concentration and relative humidity (Paur and Jordan, 1989, 1988). During the operation of larger EBDS pilot plants difficulties were encountered regarding the operation of the baghouse, because of a clogging of the filter bags. Frank et al. (1988) therefore suggested to combine an electrostatic precipitator with a baghouse in series in order to maintain stable operation conditions of the baghouse. Another approach to achieve improved performance of the baghouse is adding inert aerosol (e.g. fly ash, diatomaceous earth, lava dust) to the irradiated flue gas (Jordan et al., 1987, Frank et al., 1988). By this method the hygroscopic aerosol is 'diluted' and the dedusting of the filterbags becomes more efficient. We have performed a number of experiments at the pilot plant AGATE, in order to determine conditions for a stable baghouse operation in the EBDS-process. In the following we have compiled our results regarding removal efficiencies, filter media and operation conditions. From these data somebasic design concepts for the layout of larger filter units will be derived. Experimental Procedures The experiments were performed at the AGATE pilot plant, which is installed at the heavy oil fired steam generator of KfK. The plant has been described in detail by Jordan et al. (1986). The flue gas concentrations and the mass Ioadings have been measured according to our previously reported protocols 0Nienset al., 1987). The raw gas concentrations were: [SO2] = 500- 1000 ppmv; [NOx] = 200- 300 ppmv; [NH3] = 800- 1200 ppmv; [H20] = 12 - 16 Vol.%, mass flow = 300 m3/h. The filter used throughout the experiments was a bag filter (Riedel) equipped with 8 filterbags having a total surface of 9.78 m2. The face velocity was varied by operating the filter with 4 and 6 filterbags, corresponding to a filter surface of 4.89 m2 and 7.34 m2 resp.. Test run #9 was performed atthe RDK7 pilot plant in Karlsruhe. Unless otherwise stated the filterbags were coated with lava dust (mean particle size: 20 pm) which was injected before the filter vessel using heated pressurized air. The mass concentration of the dust was adjusted with a BEG 1000 (Palas) aerosol generator. Removal efficiencies for the EBDS-Aerosol Submicron ammonium salt aerosols released from production facilities give rise to violet clouds. This is due to the light scattering properties of these aerosols. In order to prevent this, the maximum mass concentration of aerosol in the cleaned offgas of a EBDS plant 1397
1398
H.-R.
PAUR et al.
should be maintained below 10 m g / m 3 Table 1 liststhe removal efficiencies for EBDS aerosol under a variety of experimental conditions such as filter media and face velocities. The removal efficiencies usually were in the range between 99 up to 99.9% regardless of the filter media used. Within the experimental error no significant effect was found due to the face velocities nor the amount of added inert dust exept in run #12 which was performed w i t h o u t adding lava dust to the aerosol laden flue gas. This decreased the removal efficiencies significantly. From these results it can be concluded, that the removal of the aerosol occurs mainly on the filter cake, rather than on the filter fabric itself.
Tab. 1 : Summary of the test results !Run
filter
face velocity
madditi v
Cm 2)
C=2)
q
#
medium 1)
cm/sec
kg/h
mg/m3
mg/m3
%
1
PTFEM
1.08
3.3
360 + 48
1
99.8
2
PTFEM
1.08
2.1
554
2
99.7
3
PTFEM
t.45
3.0
799 + 106
4
99.5
4
PTFEM
1.45
1.6
677 + 3
8
98.7
5
PTFEM
1.45
0.9
1149
5
99.5
6
PTFEM
2.17
3.6
921 _+ 53
2
99.8
7
PTFEM
2.18
2.4
723 -+ 102
7
99.0
8
PTFEM
2.18
0.9
n.d.3)
n.d.
n.d.
9
DT-NF
0.57
n.d.
1121 + 77
< 1
<99.8
10
DT-NFA
1.08
0.9
517 + 25
1
99.8
11
DT-NFPU
1.08
0.4
838 + 90
1
99.9
12
DT-NF
1.08
0
897 +_ 52
139
82-92
1) PTFEM = PTFE-membrane on teflon fabric (PTFE/PTFE 280); DT-NF = Dralon T needled felt (DT-DT 551); DT-NFA = Dralon T needled felt with acid protection (DT-DT 551 + CS 17); DT-NFPU = Dralon T needled felt with PU coating (GMD 4570-MF) 2) mass concentration of EBDS-aerosol before (Cm) and after (c®) the bagfilter 3) n.d. = not determined
Time dependence of the differential pressure The differential pressure of a aerosol filter may be expressed as the sum of the differential pressures of the dedusted filter and the filter cake. tn the case of stable filter operation the differential pressure of the filter after dedusting(Ap rain,) should stay constant. For the sake of comparability of different test runs we use the term dedusting efficiency EA, which is given by equation (1): APnmx -- AP'ntn I%1 EA -
with
APmax -
APo
Apmax = maximum differential pressure before dedusting Apmin = differential pressure of the dedusted filter and Ap0 = differential pressure of the clean filter before the test run.
The condition for stable filter operation becomes thus EA = const.
(1)
Removal of the aerosol formed in the electron beam dry scrubbing process
1399
80
~60 C CU
uJ r-
"U
o20
1
2
3
4
Cumulated Filter Load [kg rn"2] n ~
Fig. 1 : Dedusting efficiency in test runs with on-line cleaning as a function of the cumulated filter load
Filter tests with on-line dedustin.q Fig. 1 shows the dedusting efficiencies from test runs #1, 3 and 6 as function of the cumulated specific filterload. The filterload was calculated from the sum of the masses (kg/h) of lava dust and EBDSaerosol (see Tab. 1) and the runtime of the dedusting pressure pulse. The specific filterload was obtained by dividing the cumulated mass by the available filter surface (see section experimental procedures). The data show that even at the lowest face velocity a stable filter operation was not achieved. Obviously the differential pressure of the filter fabric increases due to the incorporation of hygroscopic submicron particles, which cannot be removed by pressure pulses (5 bar). A chemical analysis of the water extract from a plugged filter bag (run # 6), which had been thoroughly cleaned by pressurized air after removal from the filter vessel, revealed that the mass of incorporated water soluble material amounted only to 2 up to 7 g/m2. The extracted material consisted almost completely of pure ammonium sulfate. The highest amounts of soluble material were found in the samples from filter areas, which had been completely dedusted by the pulse jet. Not uniform face velocities across the filter surface area promotes clogging at these areas.This result leads to the conclusion that an improved filter operation may be expected by not exposing the fully dedusted filter bags to the aerosol laden flue gas. The loss of dedusting efficiency must be therefore due to the re-attachment of submicron particles to the unprotected filter fabric during or shortly after the pressure pulse. The same conclusion can be drawn from the observation, that the differential pressure curve raises very steeply shortly after dedusting and levels off as soon as a filter cake has been built up. In order to improve the dedusting efficiency the filter fabric should be coated with inert dust prior to exposing it to the EBDS-aerosol. Comparing test runs #3 and #4, which were performed with the same face velocity, but with different amounts of inert lava dust added to the flue gas, shows, that doubling the amount of additiv improves the filter performance also. This may be attributed to a faster re-building of a filter cake during the dedusting phase, which protects the filter fabric from getting clogged. Further improval of the filter performance may therefore be expected by raising the amount of inert additiv. Filter tests with off-line dedustinq The on-line test runs show rather high initial dedusting efficiencies (see fig. 1). In a second set of experiments the off-line dedusting efficiencies of two filter media (PTFEM and DTNF) were determined. Table 2 summarizes the results.
1400
H.-R. PAUR et al.
Tab. 2: Offline Dedusting efficiencies PTFEM [in %]
DTNF [in %]
pulse jet t)
99.4
94.6
pressurized air 2)
0.5
5.1
washing 3)
0.1
0.3
100 %
100 %
Dedusting method
sum
1) The preweighed filter bags were submitted to 5 bar pressure pulses after a 6 h test run with EBDS-Aerosol and lava additiv (2 kg/h) 2) Preweighed samples of a pulse-jet-cleaned filter bag were thoroughly dedusted by pressurized air to constant weight. 3) Preweighed samples of the filter (cleaning see footnote 2) were extracted with 500 ml of dest. water, dryed and weighedagain. The offline dedusting efficiencies are considerably higher than the online efficiencies. This hold especially for the PTFE-membrane filters, which may be cleaned to more than 99% by the pulse jet method. After complete dedusting by pressurized air the amount of incorporated water soluble material was determined. For both filter fabrics only minor amounts of soluble material was found. This is in accordance to the conclusion drawn above, that the clogging of the filterbags occurs only during or shortly after the dedusting pulse. PTFE membrane filters are superior to DTNF filters with respect to the completeness of dedusting. Conclusions It has been shown that the removal efficiencies for the EBDS aerosol are between 99% and 99.9%. Regardless of the filter media used, the face velocities or the amount of inert additives the concentrations of ammonium salts in the offgas are below 10 mg/m3. The dedusting efficiencies of the aerosol filters are poor in on line cleaning test runs. This is due to incorporation of sticky submicron aerosols during the dedusting phase. Improved dedusting efficiencies have been observed, when the aerosol filters were subjected to offline cleaning. For larger EBDS plants operating with fabric filters offline cleaning seems to be mandatory. With respect to dedusting it is advisable to use membrane filter media. Sufficient amounts of inert additives (above 10 g/m3) improve also the filter performance. Acknowledqement The authors thank Mrs. S. Manegold, Mr. H. Stiefel and Mr. K. Woletz for their able technical assistance. The stimulating discussions with Professor Dr. W. Schikarski, Mr. W. Lindner and Dr. Seeck are appreciated. This work was partly funded by PEF (Projekt Europ~isches Forschungszentrum for MaP~nahmen zur Luftreinhaltung) under research contract No. 86•006•3. References Frank, N., Hirano, S. and Kawamura, K., (1988), Radiat. Phys. Chem. 31, 57-82 Jordan, S. (1988 a); Radiat. Phys. Chem. 31,21-28 Jordan, S., Paur, H.-R. and Baumann W. (1987), Third International Conference on Electrostatic Precipitation, AbanoNenice, Italy, October 25-29. Jordan, S., Paur, H.-R., Cherdron, W. Lindner, W., (1986), J. Aerosol Sci.L 17, 669-675 Jordan, S., Paur, H.-R., Schikarski, W., (1988 b), Physik in unserer Zeit, 19, 8-16 Paur, H.-R., Jordan S., (1989), J. Aerosol Sci., 20 (in press) Paur, H.-R., Jordan, S., (1988), Radiat. Phys,Chem. 31,9-13 Wiens, H., Paul H.-R., Jordan, S., (1987), Aktuelle Aufgaben der Mel3technik in der Luftreinhaltung, S. 315-327
0021-8502/88 83.00. + 0.00 Pergamon Press plc
REMOVAL OF THE AEROSOL FORMED IN THE ELECTRON BEAM DRY SCRUBBING PROCESSBY BAG FILTERS H.-R. Paur, S. Jordan, W. Baumann Kernforschungszentrum Karlsruhe GmbH Laboratorium fi~r Aerosolphysik und Filtertechnik Postfach 3640, D-7500 Karlsruhe, FRG
Key Words:
Electron Beam, Flue Gas, Submicron Aerosols, Bag Filters
Introduction In the electron beam dry scrubbing (EBDS) process NOx and SOx are removed simultaneously by irradiation of exhaust gases with accelerated electrons. Upon addition of ammonia submicron aerosols, consisting of ammonium-nitrate and -sulfate are formed. These products are then collected by bagfilters and may be used as fertilizer. The process has been reviewed recently by Jordan et a1.(1988 a, b). The physical and chemical properties of the aerosol have been investigated by Jordan et al. (1986): The mass median diameters of this rather hygroscopic aerosol range from 0.5 up to 7.5 lam, depending on the absorbed dose. The mass concentrations in the irradiated flue gas are between 500 up to 1500 mg m-3. The mass Ioadings and the chemical composition depend substantially on dose, ammonia-concentration and relative humidity (Paur and Jordan, 1989, 1988). During the operation of larger EBDS pilot plants difficulties were encountered regarding the operation of the baghouse, because of a clogging of the filter bags. Frank et al. (1988) therefore suggested to combine an electrostatic precipitator with a baghouse in series in order to maintain stable operation conditions of the baghouse. Another approach to achieve improved performance of the baghouse is adding inert aerosol (e.g. fly ash, diatomaceous earth, lava dust) to the irradiated flue gas (Jordan et al., 1987, Frank et al., 1988). By this method the hygroscopic aerosol is 'diluted' and the dedusting of the filterbags becomes more efficient. We have performed a number of experiments at the pilot plant AGATE, in order to determine conditions for a stable baghouse operation in the EBDS-process. In the following we have compiled our results regarding removal efficiencies, filter media and operation conditions. From these data somebasic design concepts for the layout of larger filter units will be derived. Experimental Procedures The experiments were performed at the AGATE pilot plant, which is installed at the heavy oil fired steam generator of KfK. The plant has been described in detail by Jordan et al. (1986). The flue gas concentrations and the mass Ioadings have been measured according to our previously reported protocols 0Nienset al., 1987). The raw gas concentrations were: [SO2] = 500- 1000 ppmv; [NOx] = 200- 300 ppmv; [NH3] = 800- 1200 ppmv; [H20] = 12 - 16 Vol.%, mass flow = 300 m3/h. The filter used throughout the experiments was a bag filter (Riedel) equipped with 8 filterbags having a total surface of 9.78 m2. The face velocity was varied by operating the filter with 4 and 6 filterbags, corresponding to a filter surface of 4.89 m2 and 7.34 m2 resp.. Test run #9 was performed atthe RDK7 pilot plant in Karlsruhe. Unless otherwise stated the filterbags were coated with lava dust (mean particle size: 20 pm) which was injected before the filter vessel using heated pressurized air. The mass concentration of the dust was adjusted with a BEG 1000 (Palas) aerosol generator. Removal efficiencies for the EBDS-Aerosol Submicron ammonium salt aerosols released from production facilities give rise to violet clouds. This is due to the light scattering properties of these aerosols. In order to prevent this, the maximum mass concentration of aerosol in the cleaned offgas of a EBDS plant 1397
1398
H.-R.
PAUR et al.
should be maintained below 10 m g / m 3 Table 1 liststhe removal efficiencies for EBDS aerosol under a variety of experimental conditions such as filter media and face velocities. The removal efficiencies usually were in the range between 99 up to 99.9% regardless of the filter media used. Within the experimental error no significant effect was found due to the face velocities nor the amount of added inert dust exept in run #12 which was performed w i t h o u t adding lava dust to the aerosol laden flue gas. This decreased the removal efficiencies significantly. From these results it can be concluded, that the removal of the aerosol occurs mainly on the filter cake, rather than on the filter fabric itself.
Tab. 1 : Summary of the test results !Run
filter
face velocity
madditi v
Cm 2)
C=2)
q
#
medium 1)
cm/sec
kg/h
mg/m3
mg/m3
%
1
PTFEM
1.08
3.3
360 + 48
1
99.8
2
PTFEM
1.08
2.1
554
2
99.7
3
PTFEM
t.45
3.0
799 + 106
4
99.5
4
PTFEM
1.45
1.6
677 + 3
8
98.7
5
PTFEM
1.45
0.9
1149
5
99.5
6
PTFEM
2.17
3.6
921 _+ 53
2
99.8
7
PTFEM
2.18
2.4
723 -+ 102
7
99.0
8
PTFEM
2.18
0.9
n.d.3)
n.d.
n.d.
9
DT-NF
0.57
n.d.
1121 + 77
< 1
<99.8
10
DT-NFA
1.08
0.9
517 + 25
1
99.8
11
DT-NFPU
1.08
0.4
838 + 90
1
99.9
12
DT-NF
1.08
0
897 +_ 52
139
82-92
1) PTFEM = PTFE-membrane on teflon fabric (PTFE/PTFE 280); DT-NF = Dralon T needled felt (DT-DT 551); DT-NFA = Dralon T needled felt with acid protection (DT-DT 551 + CS 17); DT-NFPU = Dralon T needled felt with PU coating (GMD 4570-MF) 2) mass concentration of EBDS-aerosol before (Cm) and after (c®) the bagfilter 3) n.d. = not determined
Time dependence of the differential pressure The differential pressure of a aerosol filter may be expressed as the sum of the differential pressures of the dedusted filter and the filter cake. tn the case of stable filter operation the differential pressure of the filter after dedusting(Ap rain,) should stay constant. For the sake of comparability of different test runs we use the term dedusting efficiency EA, which is given by equation (1): APnmx -- AP'ntn I%1 EA -
with
APmax -
APo
Apmax = maximum differential pressure before dedusting Apmin = differential pressure of the dedusted filter and Ap0 = differential pressure of the clean filter before the test run.
The condition for stable filter operation becomes thus EA = const.
(1)
Removal of the aerosol formed in the electron beam dry scrubbing process
1399
80
~60 C CU
uJ r-
"U
o20
1
2
3
4
Cumulated Filter Load [kg rn"2] n ~
Fig. 1 : Dedusting efficiency in test runs with on-line cleaning as a function of the cumulated filter load
Filter tests with on-line dedustin.q Fig. 1 shows the dedusting efficiencies from test runs #1, 3 and 6 as function of the cumulated specific filterload. The filterload was calculated from the sum of the masses (kg/h) of lava dust and EBDSaerosol (see Tab. 1) and the runtime of the dedusting pressure pulse. The specific filterload was obtained by dividing the cumulated mass by the available filter surface (see section experimental procedures). The data show that even at the lowest face velocity a stable filter operation was not achieved. Obviously the differential pressure of the filter fabric increases due to the incorporation of hygroscopic submicron particles, which cannot be removed by pressure pulses (5 bar). A chemical analysis of the water extract from a plugged filter bag (run # 6), which had been thoroughly cleaned by pressurized air after removal from the filter vessel, revealed that the mass of incorporated water soluble material amounted only to 2 up to 7 g/m2. The extracted material consisted almost completely of pure ammonium sulfate. The highest amounts of soluble material were found in the samples from filter areas, which had been completely dedusted by the pulse jet. Not uniform face velocities across the filter surface area promotes clogging at these areas.This result leads to the conclusion that an improved filter operation may be expected by not exposing the fully dedusted filter bags to the aerosol laden flue gas. The loss of dedusting efficiency must be therefore due to the re-attachment of submicron particles to the unprotected filter fabric during or shortly after the pressure pulse. The same conclusion can be drawn from the observation, that the differential pressure curve raises very steeply shortly after dedusting and levels off as soon as a filter cake has been built up. In order to improve the dedusting efficiency the filter fabric should be coated with inert dust prior to exposing it to the EBDS-aerosol. Comparing test runs #3 and #4, which were performed with the same face velocity, but with different amounts of inert lava dust added to the flue gas, shows, that doubling the amount of additiv improves the filter performance also. This may be attributed to a faster re-building of a filter cake during the dedusting phase, which protects the filter fabric from getting clogged. Further improval of the filter performance may therefore be expected by raising the amount of inert additiv. Filter tests with off-line dedustinq The on-line test runs show rather high initial dedusting efficiencies (see fig. 1). In a second set of experiments the off-line dedusting efficiencies of two filter media (PTFEM and DTNF) were determined. Table 2 summarizes the results.
1400
H.-R. PAUR et al.
Tab. 2: Offline Dedusting efficiencies PTFEM [in %]
DTNF [in %]
pulse jet t)
99.4
94.6
pressurized air 2)
0.5
5.1
washing 3)
0.1
0.3
100 %
100 %
Dedusting method
sum
1) The preweighed filter bags were submitted to 5 bar pressure pulses after a 6 h test run with EBDS-Aerosol and lava additiv (2 kg/h) 2) Preweighed samples of a pulse-jet-cleaned filter bag were thoroughly dedusted by pressurized air to constant weight. 3) Preweighed samples of the filter (cleaning see footnote 2) were extracted with 500 ml of dest. water, dryed and weighedagain. The offline dedusting efficiencies are considerably higher than the online efficiencies. This hold especially for the PTFE-membrane filters, which may be cleaned to more than 99% by the pulse jet method. After complete dedusting by pressurized air the amount of incorporated water soluble material was determined. For both filter fabrics only minor amounts of soluble material was found. This is in accordance to the conclusion drawn above, that the clogging of the filterbags occurs only during or shortly after the dedusting pulse. PTFE membrane filters are superior to DTNF filters with respect to the completeness of dedusting. Conclusions It has been shown that the removal efficiencies for the EBDS aerosol are between 99% and 99.9%. Regardless of the filter media used, the face velocities or the amount of inert additives the concentrations of ammonium salts in the offgas are below 10 mg/m3. The dedusting efficiencies of the aerosol filters are poor in on line cleaning test runs. This is due to incorporation of sticky submicron aerosols during the dedusting phase. Improved dedusting efficiencies have been observed, when the aerosol filters were subjected to offline cleaning. For larger EBDS plants operating with fabric filters offline cleaning seems to be mandatory. With respect to dedusting it is advisable to use membrane filter media. Sufficient amounts of inert additives (above 10 g/m3) improve also the filter performance. Acknowledqement The authors thank Mrs. S. Manegold, Mr. H. Stiefel and Mr. K. Woletz for their able technical assistance. The stimulating discussions with Professor Dr. W. Schikarski, Mr. W. Lindner and Dr. Seeck are appreciated. This work was partly funded by PEF (Projekt Europ~isches Forschungszentrum for MaP~nahmen zur Luftreinhaltung) under research contract No. 86•006•3. References Frank, N., Hirano, S. and Kawamura, K., (1988), Radiat. Phys. Chem. 31, 57-82 Jordan, S. (1988 a); Radiat. Phys. Chem. 31,21-28 Jordan, S., Paur, H.-R. and Baumann W. (1987), Third International Conference on Electrostatic Precipitation, AbanoNenice, Italy, October 25-29. Jordan, S., Paur, H.-R., Cherdron, W. Lindner, W., (1986), J. Aerosol Sci.L 17, 669-675 Jordan, S., Paur, H.-R., Schikarski, W., (1988 b), Physik in unserer Zeit, 19, 8-16 Paur, H.-R., Jordan S., (1989), J. Aerosol Sci., 20 (in press) Paur, H.-R., Jordan, S., (1988), Radiat. Phys,Chem. 31,9-13 Wiens, H., Paul H.-R., Jordan, S., (1987), Aktuelle Aufgaben der Mel3technik in der Luftreinhaltung, S. 315-327