Respirable Aerosols from Fluidized Bed Coal Combustion. 2. Physical Characteristics of Fly Ash Robert L. Carpenter, George J. Newton”, Simon J. Rothenberg, and Phillip B. DeNee Inhalation Toxicology Research Institute, Lovelace Biomedical and Environmental Research Institute, P.O. Box 5890, Albuquerque, N. Mex. 871 15
The respirable fraction of the exhaust aerosol produced by an experimental 18-in. atmospheric pressure fluidized bed coal combustor was sampled as part of a program to assess the potential inhalation hazard associated with this emerging technology. Aerosol sampling instrumentation and the basis for its choice are described. Respirable fly ash aerosol parameters at four locations in the exhaust cleanup system are reported, as is the penetration of system cleanup devices by the respirable aerosol. Particle morphology was examined by transmission and scanning electron microscopy. Data on the real and aerodynamic particle size were used to calculate particle density. The specific surface area of the fly ash was determined as was the adsorption and desorption half-time of water vapor. These data were determined for fly ash particles obtained when the combustor was operated using a western low-sulfur subbituminous coal, an eastern high-sulfur bituminous coal, lignite, lignite refuse, and Paraho oil shale. The data show that the fly ash escaping the fluidized bed exhaust cleanup system has a mass median aerodynamic diameter on the order of 2 pm and consists of unfused particles having high specific surface areas. Water vapor adsorption times are long, implying that surface adsorption onto this ash is diffusion controlled. Particle densities were consistent with these observations, ranging from 0.8 to 6.1 g/cm3. This paper describes some physical characteristics of respirable fly ash aerosols emitted from an experimental 18-in. i.d. (45.7 cm) atmospheric pressure fluidized bed coal combustor (FBC) located a t the Morgantown Energy Technology Center (METC),Morgantown, W. Va. Sampling instrumentation characteristics and the basis for instrument choice are also reported. The work reported herein is a portion of a comprehensive research effort to assess potential inhalation hazards of fluidized bed coal combustion. These data are also useful in engineering assessme‘nt and development of improved FBC control technology. A description of the FBC and sampling system is reported in the accompanying paper (1). Subsequent papers will describe organic and trace elemental characteristics of FBC fly ash, organic constitution of FBC flue gas, and surface chemical composition of FBC fly ash. Preliminary results of this effort have been reported (2-5). Department of Energy research and development reports describing nonrespirable FBC airborne effluent composition, fuel-mineral characterization, FBC operating characteristics, mineral composition of solid waste streams, and major constituents of these streams are available from METC (6-10). Figure 1 is a schematic of the METC atmospheric pressure fluidized bed combustor. The FBC was sampled a t the four positions shown. A more complete description of the combustor and sampling positions has been reported (1).
Materials and Methods Aerosol Sampling Instrumentation. Table I lists sampling insQuments used, sampling flow rates, and types of samples obtained. This battery of instruments was selected to: (a) determine aerosol aerodynamic size distributions in the 0.5-10 wm regime; (b) compare count and aerodynamic size distributions to obtain information on the polydense nature of collected particles; (c) obtain transmission and scanning electron microscope samples to determine particle morphol854
Environmental Science & Technology
ogy and elemental composition by X-ray energy spectroscopy; (d) obtain sufficiently large samples for spark source mass spectrometry, X-ray fluorescence spectroscopy, and surface area determination by adsorption measurements; and (e) obtain samples for organic constituent determinations. The Lovelace Multi-Jet Impactor (LMJ) ( 1 1 )and the Sierra Radial Slit Jet Impactor (SRSJ) (Sierra Instruments, Inc., Carmel, Calif.) were chosen as the main size-selective samplers. Criteria used for selection were: low wall losses, operation within the Marple (12) regime (S/W and T/W = 1-5 and flow Reynolds numbers of 500-3000), leak-proof operation, and suitable sample collection capacity. The choice of these impactors does not imply that other impactors were unacceptable. Although many impactors can separate particles and have been widely used by other investigators, the fact that they do not satisfy Marple’s criteria suggests that extensive calibration is required. Cascade impactors operated within the Marple regime can be calibrated easily using computational methods. This approach permits accurate stage calibrations under any operating conditions within the impactor design range. Reliance in the field on calculated calibrations requires laboratory validation of that calibration. Equally important is a laboratory demonstration that all impactors used follow the same calibration curves. T o ensure that computed Calibration factors would be accurate, a comparison was made among the laboratory standard Mercer (13),LMJ, and SRSJ impactors. The impactors simultaneously sampled a uranine-CsC1 aerosol (10 mg/mL uranine, 10 mg/mL CsC1, pH 11). The agreement is shown in Figure 2. Agreement among these three impactors indicates that well-designed, correctly operated impactors can be confidently adapted to field sampling situations using computed stage characteristics. Aerosol collection with cascade impactors involves compromises in factors leading to a choice of collection substrates. Trade-offs made for the METC field studies were unusual. The rationales for the choices made are as follows: Collection substrates for cascade impactors are often hard surfaces coated with a silicone grease that acts as a cushion rather than a “glue” (14) to reduce particle bounce effects. Chemical analyses (spark source mass spectrometry and X-ray fluorescence spectrometry) and the requirement for large samples led to the conclusion that application of silicone greases was not acceptable for this sampling effort. Many investigators have used fiber filter material for collection substrates. Consequently, tests of available filter media were made to determine the best compromise collection surface. Wall losses, an unexpected adverse effect, were seen using all filter media in these impactors while sampling the uranine-CsC1 test aerosol. Use of filter media also compromised particle size distribution data. Filter media caused wall losses ranging from 7 to 18%. The same aerosol collected on brass shim stock (0.005 in.) coated with Dow-Corning Anti-Foam A resulted in wall losses of 3 to 5% for the LMJ and 5 to 10% for the SRSJ. Although many investigators have described particle bounce-off and its correction using greasy substrates, many of these studies were conducted using polystyrene latex spheres, which may not be representative of coal combustion effluent aerosols. A series of tests evaluated uncoated 5-mil brass shim stock as a collection surface. For both the LMJ and SRSJ, wall losses
0013-936X/80/0914-0854$01 .OO/O @ 1980 American Chemical Society
Table 1. Samplers Used on Effluent Streams from METC FBC saimpiing device
Lovelace multi-jet cascade impactor (LMJ) Sierra radial slit jet cascade impactor (SRSJ) Mercer cascade impactor (MI) (laboratory
flow rate, L/min
total sample size
comments
0.5
400 mg 500 mg 1.0 mg
eight aerodynamic size fractions 0.6 to -10 p m seven aerodynamic size fractions 0.6 to -10 p m eight aerodynamic size fractions 0.3 to 5.0 p m
0.3
