Environ. Sci. Technol. 1990, 24, 18 1 1- 18 18
Coal Combustion Aerosols: A Field Study Esko I . Kaupplnen" Technical Research Centre of Finland, Laboratory of Heating and Ventilating, Lampomiehenkuja 3, SF-02150 Espoo, Finland Tuomo A. Pakkanen Finnish Meteorological Institute, Air Quality Laboratory, Sahaajankatu 22 E, SF-00810 Helsinki, Finland
Mass and elemental size distributions of aerosols emitted in the particle size range 0.01-11 pm (Stokes diameter) from a boiler firing pulverized, bituminous coal from Poland were measured by using in situ particle size classification with an in-stack compressible flow lowpressure impactor. Samples were collected after the electrostatic precipitator. Mass and elemental size distributions of Na, Mg, Al, Si, S, K, Ca, Ti, Fe, V, Mn, Cu, Zn, Sr, Cd, and Pb were bimodal. Geometric mass mean diameters of the fine and coarse modes were about 0.05 and 2 pm, respectively. Correspondinggeometric standard deviations were 1.4 and 1.8. About 5 % of the particle mass and particle-bound Na, Mg, Al, Si, Ti, Fe, Mn, and Zn were found in the fine-particle mode. S, Ca, V, Cu, Sr, Cd, and P b were enriched in the fine-mode particles; i.e., -80% of the particle-phase S, 17% of Ca, 23% of V, 22% of Cu, 11% of Sr, 34% of Cd, and 9% of P b were in the submicron fine-mode particles. 1. Introduction Coal combustion is an important source of particulate emissions to the atmosphere. As the result of many field and laboratory studies (1-61, coal combustion aerosol size distributions have been found to be bimodal. The fineparticle mode around 0.1 pm, which is believed to be formed via nucleation of vaporized ash components and growth via coagulation and heterogeneous condensation, has received considerable attention. The submicron particles have been found to be enriched with many toxic trace elements (4-11). Submicron particles also have a higher probability to penetrate through common flue gas cleaning equipments, like electrostatic precipitators and baghouses, than the bulk of coarse fly ash particles formed in the combustion process (4, 10, 12). Once emitted in the atmosphere, fine particles have long residence times and a high probability of penetrating into the alveolar regions of lungs when inhaled. The potential environmental impact of nucleation-condensation-generated fine-mode particles may further be enhanced by their relatively high water solubility and surface concentration of toxic elements (13).
In laboratory-scale combustion experiments of various pulverized US. coals, the composition of the fine mode has been found to vary considerably, depending mainly on the properties of mineral matter in the parent coal and on combustion conditions (9, 14, 15). Available fine-mode composition size distribution data, i.e., element mass fraction of the aerosol particles as the function of particle size, measured at field combustion systems is more limited in volume and accuracy, because particle size has mainly been measured with low-pressure impactors lacking detailed laboratory calibration (4, 10, 11). As neutron activation analysis has mainly been used to analyze the low-pressure impactor samples, submicron size distribution data for the highly toxic trace elements like P b and Cd have not been reported. Discrepancies up to a factor of 2 between fine-mode mean sizes measured simultaneously 0013-936X/90/0924-1811$02.50/0
Table I. Properties of Bituminous Coal from Poland (Dry Basis)
moisture, 3' % ash content, VC volatile matter, '3 nonvolatile matter, YC sulfur, % heating value, MJ/kg
10.6 15.5 26.5 47.4 0.86 24.5
with in-stack low-pressure impactors and with electrical mobility analyzers ( E M ) from diluted combustion gases have been reported (4,111. However, accurate composition and size distribution data are needed when the various modes of ash behavior and the enrichment of fine-mode particles with toxic, volatile species are to be determined (7). Combustion aerosol characteristics have been shown to depend on the properties of mineral matter in the parent coal. Coal properties can vary widely, depending strongly on the origin of the coal. Because most of the combustion aerosol composition data has been measured for combustion of US.coals, accurate measurements of combustion aerosol properties and emissions from the combustion of coals having different origins are needed. This paper describes studies to characterize the properties of the particles emitted from the combustion of pulverized, bituminous coal originating from Poland. Combustion aerosol characteristics were measured after the flue gas cleaning system (electrostatic precipitator), because the main emphasis of this study was to accurately measure the stack emissions of particle mass and various matrix and trace elements of the coal ash as a function of particle size. Particles were size classified in situ with a calibrated, in-stack low-pressure impactor. Because greased substrate weight loss by evaporation during sampling has been reported to be the main difficulty associated with submicron combustion aerosol mass size distribution measurements using high-velocity inertial impactors (11, 16), the stability of substrates used during in-stack sampling was carefully evaluated by sampling filtered flue gas with the low-pressure impactor. Composition (element mass fraction) size distributions were determined from measured differential size distributions. Possible particle formation mechanisms and behavior of elements during the combustion process are discussed on the basis of measured aerosol size distribution data. 2. Process Description
High bituminous coal imported from Poland was burned in a pulverized coal boiler equipped with 16 burners on four levels at the corners of the furnace. The average properties of the Polish coal (dry basis) are given in Table I. Coal consumption was 17.2 kg/s. The boiler was operated at fuel-lean combustion conditions, i.e., flue gas O2 concentration of -4.5%. Flue gases were cleaned with an electrostatic precipitator (ESP), which had eight electrically independent sections. The ESP was designed to achieve 99.7% overall collection efficiency for the inlet
0 1990 American Chemical Society
Environ. Sci. Technol., Vol. 24, No. 12, 1990
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Table 11. Summary of the Coal Combustion Aerosol Low-Pressure Impactor Sample Characteristics, Including Types of Substrates, Sampling Times, Impactor Inlet and Outlet Temperatures, and Analysis Methods
sample
substrate film greasg APL APL APL
mgrsasa,o
Irg
tSlb min
Tin: O C
TO"t,d OC
SST'
Gf
analysis GFAAP
PIXEh
SSBLPI 2 116 SSBLPI 4 X 112 " SSBLPI 6 110 X X X Alk SSBLPI 8 X 115 N' APL 100-785 SSBLPI 10 117 X X X SSBLPI 14 see Table I11 X 120 X The amount of the grease on the film. *Sampling time. Gas temperature at the impactor inlet. Gas temperature a t the impactor outlet. 'Substrate stability test: 'Gravimetric analysis. #Graphite furnace atomic absorption spectroscopy. Particle-induced X-ray emission analysis. Mylar film. Apiezon L vacuum grease. Aluminum film. 'Polycarbonate (poreless Nuclepore) film. Mi Alk
220-13 10 180-480 400-840
15 5 21 10 5 30
mass concentration of 14 g/Nm3 (1Nm3 = 1 m3 of dry gas at 1 atm and 0 "C).
116 114 116 119 117 123
X
STACK
1
3. Experimental Section 3.1 Sampling Methods. Size-classified coal combustion aerosol samples were collected in-stack from the duct after the ESP before the main blower with an ll-stage,multijet, compressible-flow low-pressure impactor (BLPI, Hauke 25/0.015) (17, 18). The average gas velocity in the duct was measured with a Pitot tube. The diameter of the impactor sampling nozzle was chosen to equalize the gas velocity at the nozzle tip and that measured by the Pitot tube. The calibration of BLPI low-pressure stages is described in detail elsewhere (19). Thin aluminum (Al), Mylar (M), and polycarbonate (poreless Nuclepore, NP) films were used as impaction substrates. In order to prevent particle bounce, films were greased with a thin, homogeneous layer of Apiezon L (APL) vacuum grease. Films were greased by generating grease aerosols and impacting grease particles on the rotating films with a radial slit impactor (19). Before sampling, the greased substrates were baked in the oven 22 h at 125 "C, in order to increase substrate weight stability during in-stack sampling. The sampling system is shown schematically in Figure 1. Before sampling BLPI was allowed to heat-up in the duct with the inlet facing downstream. When the BLPI outlet temperature reached the inlet temperature, the impactor was rotated to face upstream and sampling started by opening the regulating valve and adjusting the downstream pressure to 83 mbar. As the process gas pressure was 1 atm, this sampling method assured the BLPI stage operating absolute gas pressures to be the same as those used in the experimental calibration, Le., fixing the sample flow rate and individual stage pressure ratios. Substrate weight stability during sampling was evaluated by collecting one impactor sample (SSBLPI 14; see Table 11) through two high-efficiency 47-mm quartz fiber filters (Munktell MK 360, manufactured by Stora Kopparberg, Grycksbo, Sweden). Filters collected aerosol particles, allowing only the gaseous part of the aerosol to enter BLPI stages. Pulverized coal, furnace bottom ash, and ESP-collected fly ash samples were collected regularly during the sampling period. 3.2. Analytical Techniques. Gravimetric analysis of impactor samples was carried out by weighing the substrates carefully before and after sampling on a Mettler Me 3030 microbalance in a clean, almost constant humidity laboratory room. Before gravimetric analysis the substrates were exposed to an ion stream generated by an a-active source, in order to reduce the effects of electrical
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Environ. Sci. Technol., Vol. 24, No. 12, 1990
VACUUM
TEMPERATURE MEASUREMENT
Figure 1. Schematic diagram of the in-stack low-pressure impactor sampling system. The behavior of the impactor is controlled by regulating the impactor downstream absolute gas pressure to give the overall impactor outlet-to-inlet pressure ratio the value (0.083) used in impactor calibration.
charge on the weighing results. Na, Mg, Al, K, Ca, V, Mn, Fe, Ni, Cu, Zn, Cd, and Pb contents of impactor samples were determined with the graphite furnace atomic absorption spectrometer (GFAAS). Before analysis, 1/4 of the substrate was wetashed in a solution containing 2 mL of 65% HN03, 2 mL of 37% HCI, and 0.5 mL of 30% HzOz. Following the evaporation of liquid by heating, 10 mL of deionized H20 was added and the sample was kept in an ultrasonic bath for 30 min. The resulting solution was analyzed for its metal content with GFAAS. Impactor samples were analyzed by PIXE (particle-induced X-ray analyses) by Element Analysis Corp. at Tallahassee, FL. A detailed description of the PIXE method, including sample radiation, measurement of the X-ray spectra, and the method to calculate the concentrations of elements in the sample from the measured spectrum, is given elsewhere (20). Mg, Al, Si, S, K, Ca, Ti, V, Mn, Fe, Ni, Cu, Zn, Sr, and P b were detected by PIXE. As the surface mass concentration of the particles on the impactor substrate vary depending on the location relative to jets, the quantitative elemental analysis of the Berner-type low-pressure impactor samples is difficult with surface-sensitive methods like PIXE. Mg, Al, K, Ca, V, Mn, Fe, Ni, Cu, Zn, and P b concentrations were measured with both GFAAS and PIXE. A detailed comparison of the PIXE and GFAAS results is given elsewhere (21).Pixe results were lower than those of GFAAS. Typically the ratio of PIXE/GFAAS increased with increasing mass of the element in the sample and decreasing energy of the X-ray peak used to determine the PIXE result. In ad-
Table IV. Concentrations of Matrix Elements and Trace Elements in Coal, Furnace Bottom Ash, and Fly Ash Collected by the Electrostatic Precipitatorn
Table 111. Results of the Substrate Stability Tests"
stage
filmb
1 6 2 3 7 8 11 4 9 5 10
N N N N N N N A1 A1 M M
substrate APL' concn, pg
substr mass change, pg
400 100 400 100 880 440 450 250 360
coal
-18 -2 1 -3 1 +1 +1
-5 +4 +6 +8 +37 +33
" In which the weight change of the substrate is measured when filtered flue gas is sampled with the impactor. Before sampling the substrates were baked in the oven 22 h at 125 OC. Pure polycarbonate (N) films were not baked. N, polycarbonate (poreless Nuclepore) film; Al, aluminum film; M, Mylar film. CApiezonL vacuum grease. dition, the PIXE/GFAAS ratio was found to be the function of the impactor sample deposit spot geometry. Therefore the PIXE results for Si, S, Ti, and Sr were adjusted by using the GFAAS/PIXE ratio of the neighboring elements, i.e. mSi
=
l/Z(RAI
+ RK)mSi,PIXE
ms = ~/zZ(RAI + RK)~S,PIXE mTi mSr
=
%(RCa
HgC
Pb
+ RV)mTi,PIXE
=
mi,GFAAS/mi,PIXE
0.07 0.19 1.8 2.8 0.49 0.12 0.23 0.54 0.10 0.66 41 31 132 6.5 21
36 33 1.4 2.4 1.0 12 93 38 0.4 0.1 19
fly ash
0.98 1.7 8.4 24 0.24 0.64 1.5 2.9 0.40 5.4
0.20 0.95 8.5 27 0.09 ndb 1.5 2.8 0.40 5.8
110 121
809 7.8 87 87 71 11
0.8
