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Aerosol formation in coal combustion processes
J. Aerosol Sci., Vol. 22, Suppl. 1, pp. $451-$454, 1991. printed in Great Britain.
0021-8502/91 S3.00 + 0.00 Pergamon Press plc
AEROSOL FORMATION IN COAL COMBUSTION PROCESSES Esko I. Kauppinen Technical Research Centre of Finland (VTT), Laboratory of Heating and Ventilation, Tekniikantie 4, SF-02150 Espoo, Finland.
KEYWORDS Coal combustion, pulverized and circulating fluidized bed processes, size distributions, formation mechanisms.
INTRODUCTION The understanding of aerosol formation in combustion processes is of importance when studying (i) the effect of particles on the heat transfer in the furnace (ii) erosion of heat exchanger tubes or turbine blades (advanced combined processes) (iii) gas cleaning methods and (iv) the environmental effects of aerosols emitted from the combustion processes (Sarofim et al., 1977). Combustion aerosol is generally an extremely complex and multicomponent aerosol system involving unburned fuel particles, soot, particles from fuel ash transformations, particles used for gas phase pollutant in-furnace capture and various compounds in the vapour phase. Major fraction of the aerosols from a well designed and operated pulverized coal combustion process originate from fuel ash transformations (vaporization, condensation, smelting, coalescense and agglomeration), which have been the focus of numerous studies (e.g. Sarofim et al., 1977; McElroy et al., 1982; Taylor and Flagan, 1982; Neville and Sarofim, 1982; Kauppinen and Pakkanen, 1990). Published results indicate a bimodal size distribution, with the fine mode at about 0.1 ]am and the large mode at about 10/am. More recently Helble et al. (1986) and Helble and Sarofim (1989) have shown experimental evidence for the bimodality for both suband supermicron size distributions. Modeling work of Jokiniemi and Kauppinen (1991) indicate, that alkali sulfates can also nucleate after the combustion process at about 600 °C gas temperature. Fluidized bed combustion and gasification processes are developed increasingly to create new, efficient and environmentally acceptable coal combustion technologies. The understanding of ash behaviour and aerosol formation is essential for the development of pressurized systems. However, relatively few studies on this topic can be found in literature.
This paper focuses on the aerosol formation in real scale, ambient pressure coal combustion processes, based on detailed number, mass and fly ash elemental size distribution measurements with differential electrical mobility analyzers and low pressure impactors (Hillamo and Kauppinen, 1991), respectively. Results from pulverized combustion are compared to those from circulating fluidized bed combustion for the combustion of bituminous coals.
$451
$452
E.I. KAUPPINEN
PULVERIZED COMBUSTION When measured after the electrostatic precipitator, ash matrix and trace elements size distributions from the combustion of bituminous coal I have four modes at about 0.05, 0.4, 2 and 15 pm (Fig.l). Similarly two submicron modes are seen on the number and mass size distributions measured simultaneously before and after the electrostatic precipitator for the combustion of bituminous coal II (Fig. 2). 0.05 pm mode results from the nucleation of vaporized ash matrix compounds of the boundary layer of the burning char particles. 0.5 pm mode has not been clearly observed before and its formation mechanism has not been clarified in detail so far. 0.5 pm mode particles readily penetrate through e.g. electrostatic precipitators and baghouses and are therefore of importance when environmental effects are considered.
CIRCULATING FLUIDIZED BED COMBUSTION Experimental results from the circulating fluidized bed combustion of bituminous coal III • (Kauppinen et al., 1991) show that at least three separate modes are formed in this modem combustion process. A very narrow mode (geometic standard deviation about 1.4) has been observed at about 0.025 pm and a wide mode (geometric standard deviation about 2.3) at about 0.3 pm in the number distributions. Elemental size distributions indicate, that ash matrix elements do not significantly vaporize, contradictory to their behaviour in pulverized combustion. High concentrations of halogens are found on both submicron modes, indicating nucleation and condensation of halogen species.
ACKNOWLEDGEMENTS This work has been funded by Academy of Finland, Maj and Tor Nessling Foundation, Ministry of Trade and Industry of Finland through research programs LIEKKI and SIHTI and by Ahlstrom Corporation, IVO and Tampella Power.
REFERENCES
Helble, J., Neville, M. and Sarofim, A.F. (1986) 21th Symp.(Int.) on Combustion, Combustion Inst., pp. 411-417. Helble, J. and Sarofim, A.F. (1989) Combustion and Flame 76, 183-196. Hillamo, R.E. and Kauppinen, E.I. (1991) Aerosol Sci. Technol. 14, 33-47. Jokiniemi, J.K. and Kauppinen, ET (1991) Behaviour of alkaline species in coal combustion and gasification. 1991 European Aerosol Conference. Kauppinen, E.I. and Pakkanen, T.A. (1990) Environ. Sci. Technol. 24, 1811-1818. Kauppinen, E.I., Lind, T.L., Eskelinen, J.E., Jokiniemi, J.K., Maenhaut, W., Royset, O, Vadset, M., Vilokki, H. and Kuivalainen, R. (1991) Aerosols from circulating fluidized bed coal combustion. 1991 European Aerosol Conference. McElroy, M.W., Carr, R.C., Ensor, D.S. and Markowski, G.R. (1982) Science 215, 13-19. Neville, M. and Sarofim, A.F. (1982) 19th Symp.(Int.) on Combustion, Combustion Inst., pp. 1411-1449. Sarofim, A.F., Howard, J.B. and Padia, A.S. (1977) Combust. Sci. Technol. 16, 187-204. Taylor, D.D. and Flagan, R.C. (1982) Aerosol Sci. Technoi. 1, 103-117. Wolfenbarger, J.K. and Seinfeld, J.H. (1990) J. Aerosol Sci. 21, 227-247.
A e r o s o l f o r m a t i o n in
a)
coal
c o m b u s t i o n processes
$453
Mg
Na
AI
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250 ,
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Fig. 1.
Differential ash matrix and trace element continuous size distributions for pulverized combustion of bituminous coal I. Measured with low pressure inertial impactor downstream the electrostatic precipitator. Inversion is carried out with the method based on constrained regularization (Wolfenbarger and Seinfeld, 1990).
$454
E.I.
KAUPPINEN
BLPI
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a)
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Fig. 2.
10
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Differential number and mass size distributions for pulverized combustion of bituminous coal II, measured a) before and b) after the electrostatic precipitator. Number and mass distributions are measured with differential mobility analyzers (DMA) and low pressure impactors (BLPI), respectively. A cyclone with the Stokes cut diameter of about 5.4 ~am has been used at the impactor inlet.
0021-8502/91 S3.00 + 0.00 Pergamon Press plc
AEROSOL FORMATION IN COAL COMBUSTION PROCESSES Esko I. Kauppinen Technical Research Centre of Finland (VTT), Laboratory of Heating and Ventilation, Tekniikantie 4, SF-02150 Espoo, Finland.
KEYWORDS Coal combustion, pulverized and circulating fluidized bed processes, size distributions, formation mechanisms.
INTRODUCTION The understanding of aerosol formation in combustion processes is of importance when studying (i) the effect of particles on the heat transfer in the furnace (ii) erosion of heat exchanger tubes or turbine blades (advanced combined processes) (iii) gas cleaning methods and (iv) the environmental effects of aerosols emitted from the combustion processes (Sarofim et al., 1977). Combustion aerosol is generally an extremely complex and multicomponent aerosol system involving unburned fuel particles, soot, particles from fuel ash transformations, particles used for gas phase pollutant in-furnace capture and various compounds in the vapour phase. Major fraction of the aerosols from a well designed and operated pulverized coal combustion process originate from fuel ash transformations (vaporization, condensation, smelting, coalescense and agglomeration), which have been the focus of numerous studies (e.g. Sarofim et al., 1977; McElroy et al., 1982; Taylor and Flagan, 1982; Neville and Sarofim, 1982; Kauppinen and Pakkanen, 1990). Published results indicate a bimodal size distribution, with the fine mode at about 0.1 ]am and the large mode at about 10/am. More recently Helble et al. (1986) and Helble and Sarofim (1989) have shown experimental evidence for the bimodality for both suband supermicron size distributions. Modeling work of Jokiniemi and Kauppinen (1991) indicate, that alkali sulfates can also nucleate after the combustion process at about 600 °C gas temperature. Fluidized bed combustion and gasification processes are developed increasingly to create new, efficient and environmentally acceptable coal combustion technologies. The understanding of ash behaviour and aerosol formation is essential for the development of pressurized systems. However, relatively few studies on this topic can be found in literature.
This paper focuses on the aerosol formation in real scale, ambient pressure coal combustion processes, based on detailed number, mass and fly ash elemental size distribution measurements with differential electrical mobility analyzers and low pressure impactors (Hillamo and Kauppinen, 1991), respectively. Results from pulverized combustion are compared to those from circulating fluidized bed combustion for the combustion of bituminous coals.
$451
$452
E.I. KAUPPINEN
PULVERIZED COMBUSTION When measured after the electrostatic precipitator, ash matrix and trace elements size distributions from the combustion of bituminous coal I have four modes at about 0.05, 0.4, 2 and 15 pm (Fig.l). Similarly two submicron modes are seen on the number and mass size distributions measured simultaneously before and after the electrostatic precipitator for the combustion of bituminous coal II (Fig. 2). 0.05 pm mode results from the nucleation of vaporized ash matrix compounds of the boundary layer of the burning char particles. 0.5 pm mode has not been clearly observed before and its formation mechanism has not been clarified in detail so far. 0.5 pm mode particles readily penetrate through e.g. electrostatic precipitators and baghouses and are therefore of importance when environmental effects are considered.
CIRCULATING FLUIDIZED BED COMBUSTION Experimental results from the circulating fluidized bed combustion of bituminous coal III • (Kauppinen et al., 1991) show that at least three separate modes are formed in this modem combustion process. A very narrow mode (geometic standard deviation about 1.4) has been observed at about 0.025 pm and a wide mode (geometric standard deviation about 2.3) at about 0.3 pm in the number distributions. Elemental size distributions indicate, that ash matrix elements do not significantly vaporize, contradictory to their behaviour in pulverized combustion. High concentrations of halogens are found on both submicron modes, indicating nucleation and condensation of halogen species.
ACKNOWLEDGEMENTS This work has been funded by Academy of Finland, Maj and Tor Nessling Foundation, Ministry of Trade and Industry of Finland through research programs LIEKKI and SIHTI and by Ahlstrom Corporation, IVO and Tampella Power.
REFERENCES
Helble, J., Neville, M. and Sarofim, A.F. (1986) 21th Symp.(Int.) on Combustion, Combustion Inst., pp. 411-417. Helble, J. and Sarofim, A.F. (1989) Combustion and Flame 76, 183-196. Hillamo, R.E. and Kauppinen, E.I. (1991) Aerosol Sci. Technol. 14, 33-47. Jokiniemi, J.K. and Kauppinen, ET (1991) Behaviour of alkaline species in coal combustion and gasification. 1991 European Aerosol Conference. Kauppinen, E.I. and Pakkanen, T.A. (1990) Environ. Sci. Technol. 24, 1811-1818. Kauppinen, E.I., Lind, T.L., Eskelinen, J.E., Jokiniemi, J.K., Maenhaut, W., Royset, O, Vadset, M., Vilokki, H. and Kuivalainen, R. (1991) Aerosols from circulating fluidized bed coal combustion. 1991 European Aerosol Conference. McElroy, M.W., Carr, R.C., Ensor, D.S. and Markowski, G.R. (1982) Science 215, 13-19. Neville, M. and Sarofim, A.F. (1982) 19th Symp.(Int.) on Combustion, Combustion Inst., pp. 1411-1449. Sarofim, A.F., Howard, J.B. and Padia, A.S. (1977) Combust. Sci. Technol. 16, 187-204. Taylor, D.D. and Flagan, R.C. (1982) Aerosol Sci. Technoi. 1, 103-117. Wolfenbarger, J.K. and Seinfeld, J.H. (1990) J. Aerosol Sci. 21, 227-247.
A e r o s o l f o r m a t i o n in
a)
coal
c o m b u s t i o n processes
$453
Mg
Na
AI
700 r
250 ,
2OO
250
i
200
5OO tSO
a00
O0
3O0
I
150 t
O0
2OO 50
1O0
Z 0
=
0.01
o.1
I
0
O.Ol
1o
o.t
I
IO
o.1
Ca
10
I
tO
t
10
I
10
Fe
150
500
t
:200
E 400
150 I O0
300 I00 200 t00 0 O.Ot
0.I
I
0.01
I0
O.t
1
0.01
I0
0.1
D p , lain
b)
V
3o
Mn
Cu
15
40 3O 2O 10 c' . ' O.Ol
m
L,. 0 1
I
, 10
,
,
,?
0
0.01
0,1
0.5O
10
O.25
0 o.1
oi
I
1o
Pb
t.O0
20
o.o|
10
Cd
Zn 40
I
0.00
~ o.oi
o.1
" I
",'. lo
o.oi
0.1
Dp, pm
Fig. 1.
Differential ash matrix and trace element continuous size distributions for pulverized combustion of bituminous coal I. Measured with low pressure inertial impactor downstream the electrostatic precipitator. Inversion is carried out with the method based on constrained regularization (Wolfenbarger and Seinfeld, 1990).
$454
E.I.
KAUPPINEN
BLPI
DMA
a)
2ooo
1oo0ooooo !
18oo 16oo
10000000 1200 E
1000000 8oo 6oo 100000 4oo
10000
. . . . . . . . . . . . . .
0.01
0.1
1
0.01
10
Particle Diameter in pm
b)
0.1
1
10
Particle Diameter In pm
1000000
loo~
~o
10oo0
lOOO 0.01
. . . . . 0.1
M
't. 1
Particle Diameter in pm
Fig. 2.
10
0.01
0.1
L, 1
10
Particle Diameter in pm
Differential number and mass size distributions for pulverized combustion of bituminous coal II, measured a) before and b) after the electrostatic precipitator. Number and mass distributions are measured with differential mobility analyzers (DMA) and low pressure impactors (BLPI), respectively. A cyclone with the Stokes cut diameter of about 5.4 ~am has been used at the impactor inlet.