j. Aerosol Sci., Vol. 22, Suppl. 1, pp. $467-$470, 1991. Printed in Great Britain.
0021-8502/91 S3.00+ 0.00 Pergamon Press pie
AEROSOLS FROM CIRCULATING FLUIDIZED BED COAL COMBUSTION Esko I. Kauppinen ~, Terttaliisa M. Lind t, Jari J. Eskelinen ~, Jorma K. Jokiniemi2, Willy Maerthau~, Oddvar Roysed, Marit Vadset4, Harri Vilokki 5 and Reijo Kuivalainen s )Technical Research Centre of Finland (V'IH'), Laboratory of Heating and Ventilation, Tekniikantie 4, SF-02150 Espoo, Finland, : Nuclear Engineering Laboratory, 3University of Gent, Institut for Nuclear Sciences, Proeftuinstraat 86, B-9000 Gent, Belgium, 4Norwegian Institute for Air Research, Postboks 64, N-2001 LillestrOm, Norway, 5A. Ahlstrom Corporation, Research and Development, SF-48601 Karhula, Finland.
KEYWORDS Coal combustion, circulating fluidized beds, size distributions, elemental analysis.
INTRODUCTION The understanding of aerosol formation and ash behaviour (evaporation, condensation and mineral particle transformations) is needed when studying e.g. fouling and slagging, high-temperature-high-pressure gas cleaning systems, conventional flue gas cleaning methods and possible adverse environmental effects of emitted aerosols. Most of the research efforts of aerosol formation in coal combustion has been so far concentrated on pulverized combustion process ( e.g. Kauppinen and Pakkanen, 1990; McElroy et al., 1982), although fluidized bed systems are increasingly being adapted to coal combustion. In this study, characteristics of aerosols from athmospheric circulating fluidized bed combustion of low sulphur ( S content 0.5 %) Venezuelan coal in 81 MW capasity boiler have been studied experimentally.
Aerosol mass size distributions in the size range 0.01-70 grn were measured with the combination of in-situ Berner-type low pressure impactor and cyclone sampler with estimated Stokes cut diameter of 5.4 )am to pievent overloading of upper impactor stages (Hillamo and Kauppinen, 1991).. Size distributions of particles collected by the cyclone were determined by Coulter and Bahco wind sieve methods. The impactor samples were also examined with scanning electron microscope, to observe the shape of the particles collected. Number size distributions in the size range 0.01-5 grn were determined by diluting and cooling the aerosol with two stage ejector-based dilution system and measuring the number spectra with differential mobility and aerodynamic particle size analyzers. Both impactor and dilution unit samples were collected before the electrostatic precipitator at the gas temperature of about 125 *C. In addition, aerosol gas composition, total and size fractionated mass concentrations and coal, lime and bottom ash characteristics were measured. Coal and lime size distributions were measured by mechanical s i e v i n g , lime size spectra by the combination of mechanical and Bacho wind sieving and Coulter methods. Coal ash mineral particle size distributions were measured by Coulter and Bacho methods after evaporating carbon at 815 *C,
E . I . KAUPPINEN et af.
Aerosol elemental size distributions in the size range 0.01-5 prn were determined by analyzing impactor samples with chemical and physical methods. Poreless polycarbonate (Nuclepore) films (thickness about 15 pro) greased with a thin, homogeneous layer of Apiezon L vacuum grease (50-360 }ag/stage) were used as collection substrates. In order to estimate the effect of vapour phase aerosol components reacting with foil material and collected panicles on the size distributions determined by analysing impactor samples, so called in-situ blank impactor samples were collected through two high efficiency quartz fiber falters. Impactor samples, blank and greased films and in-situ blanks have been analyzed at the University of Gent, Belgium by multicomponent PIXE and INAA methods which require no sample treatment. In addition, samples have,been analyzed by multicomponent ICP-MS-method at NILU, Norway after dissolving the sample in the heated teflon bomb at concentrated nitric acid.
RESULTS AND DISCUSSION Ultimate and proximate analysis of the coal used in this study are presented in Table 1. Aerosol total mass loading, as measured with isokinetic filter sampler, varied in the range 15-35 g/Nm 3. Typical gas composition during short impactor sampling periods was about 4.3 % O:, 14.5 % CO2, 140 ppm CO, 80 ppm SO: and 180 ppm NO~. Aerosol number size distributions ('Fig.1.) were trimodal with modes at about 0.02, 0.3 and 2 )am. Geometric standard deviations of submicron modes were about 1.4 and 2.7, respectively. Number concentrations of 0.02 )am mode particles varied strongly during experiments, being 1 to 2 orders of magnitude lower than those reported from the pulverized coal combustion process (Flagan and Taylor, 1981). Number concentrations of 0.3 }am mode particles were stable. Mass size distributions (Fig.1.) peaked at about 30 }am with no gTavimetrically detectable mass below 0.1 }am. Mass size distribution results are very repeatable. The small mean size, narrow size spectra and large concentration variations of the 0.02 pm mode panicles suggest they are formed in the homogeneous nucleation process of vaporized ash companents. This is supported by the measured coal ash mineral panicle number size spectra which peaked at about 4 }am, far above 0.02 pan mode size range.
P~r~ic!e N u m o e r 1.8r.005
. . . . . .
: ~ 40000
........ A P S
Cyclone precut|er/ Coulter i!
Cycloneprecut.~er/ , ~ w,nd sieve
iI ,oooo 0.01
O= , ~/,fTl
\ [ flA~x~\[~
/d,m l O
Figure 1. Number and mass size distributions of circulating fluidized bed coal combustion aerosols.
Size distributions of ash matrix elements Ca, A1, Na, K and Fe, as well as halogens C1, I and Br (Fig.2.), were determined in the size range 0.01 - 5 }am from the INAA- and ICP-MS-analyses results of impactor samples by the simple cut-point inversion method. No clear mode is observed in the ash matrix element size distributions below 0.I )am. This indicates, that these elements do not significantly vaporize in the circulating fluidized bed combustion, contradictory to their behaviour in the pulverized coal combustion (Kauppinen and Pakkanen, 1990). Halogen spectra show a clear mode below 0.1 )am, indicating nucleation of halogen species after the combustion process. High halogen concentrations are also found on in-situ
Aerosols from coal combustion Differential ~I / Nm3
Figure 2. Ca, A1, Na, K, Fe, C1, I and Br differential size distributions for the aerosol penetrating the impactor pre-cyclone. Mass Ca
o.o.~ "...... ~.
00,00;' ~" O.~S
. ~.0to O0.CI~I •
~ oJDo'7 ~. ;.:::::
• o, [
,1 | ~Omo~4
Figure 3. Ca, A1, Na, K, Fe, C1, I and Br mass fraction size distributions for the aerosol i~rl~tl~firlg thc impactor pre-cyclone.
E.I. KAUPPINENet al.
blank samples indicating the existence of halogen vapours at 125"C. Mass fraction size distributions of these elements were also determined (Fig.3.). Mass fractions of halogens show a significant increase with decreasing particle size indicating vaporization of these elements in the process. Table 1. Ultimate and proximate analysis of coal (dry basis) used in this study. moisture ash : volatiles fixed carbon
8.1 4.3 37.3 58.4
% % % %
C H N S O
80.8 5.42 1.46 0.49 7.5
% % % % %
C1 total CI" Na acid soluble K acid soluble
0.03 <0.01 0.01 0.02
% % % %
Ash components N~O K.O CaO MgO SiO,_ A1,O3 Fe:O3 P:O5 TiO, SO3
0.67 1.0 8.0 5.3 42.2 22.5 6.7 0.11 1.1 9.2
% % % % % % % % % %
ACKNOWLEDGEMENTS This study has been funded by Ministry of Trade and Industry of Finland through the combustion research program LIEKKI.
REFERENCES Fiagan, R.C. and Taylor, D.D. (1981) Laboratory Studies of Submicmn Particles from Coal Combustion. 18th Symp. (Int.) on Combustion, Combust.Inst., Pitlsburgh, PA, pp. 1227-1235. Hillamo, R.E. and Kauppinen, E.I. (1991) On the Performance of the Bemer Low Pressure Impactor. Aerosol Sci. Technol., 14, 1. Kauppinen, E.I and Pakkanen, T.A. (1990) Coal Combustion Aerosols: A Field Study. Envir. Sci. Technol., 24, pp. 1811-1818. McElroy, M.W., Cart, R.C., Ensor, D.S. and Markowski, G.R. (1982) Size Distribution of Fine Particles from Coal Combustion. Science, 215, 13.