Arthur H. Biermann

Application of surface-deposition models to size-fractionated coal fly ash

Stack fly ash emitted from a large, coal-fired power plant was separated in situ into 8 or 12 discrete fractions with inertial cascade impactors. The fractions, ranging in size from 0.1 to 50 μm and including three fractions with aerodynamic mass dia. ≥ 1 μm, were previously analyzed for up to 43 elements by instrumental neutron-activation analysis. In this report geometric and theoretical surface deposition models are used to estimate the depth of the surface layer as well as the concentrations of enriched elements in the surface layer. The results indicate that the thickness of the surface layer is small, even at particle sizes of 0.1 μm where it is estimated to be about 20% of the particle diameter. The behavior of the relative concentrations of surface-enriched elements as a function of particle size was consistent with the slip-flow regime of aerosol mechanics.

Atmospheric behavior of trace elements on particles emitted from a coal-fired power plant

Abstract Filter and cascade impactor samples of suspended particles were collected in-stack and at distances up to 64 km downwind in the plume of a large western coal-fired power plant equipped with both electrostatic precipitators (ESP) and venturi wet particulate scrubbers (VWS) to investigate modifications of the particulate signatures of minor and trace elements during transport. Samples were analyzed for 40 elements by instrumental neutron activation analysis. Precipitator malfunction during the experiment caused greater than normal emission of large particles, and concentrations of As, Zn, Sb, Mo, Ga, W, U, V and Ba in near-plume particles collected on filters were enriched relative to their concentrations in stack particles by factors of 1.4 to 2.5, presumably because of sedimentation of very large particles. Selenium was enriched by up to 6-fold (plume:stack). However, enrichment of elements in the plume relative to more typical in-stack particles were insignificant for all elements except Se, which was enriched 2.3-fold. Concentrations of Se on particles in the stack and plume suggest that most of the Se vapor in stack gases became associated with aerosol particles soon after emission. Thus although significant post-emission modifications of elemental signatures of particles may occur for poorly controlled plants, little change is expected for well-controlled plants equipped with ESPs except for Se. Source signatures measured for Se must account for vapor deposition. Impactor data showed a preferential decrease in the concentrations of the above elements in submicrometer particles; suggesting that either intermodal coagulation or size selective sampling losses were important. The impactor data further suggest that enrichment-particle-size profiles for VWS emissions were not conservative during transport.

Elemental particle-size emissions from coal-fired power plants: Use of an inertial cascade impactor

Coal-fly-ash particles collected on coated and uncoated impaction substrates were analyzed by scanning electron microscope (SEM) techniques in combination with instrumental neutron activation analysis (INAA) to verify the sizes of particles collected and their elemental composition to estimate the significance of bounce-off and reentrainment onto back-up filters and to evaluate wall and interstage losses for the University of Washington MK III Source Test Cascade Impactor. Particles were analyzed for a total of 39 elements. In samples collected downstream from an electrostatic precipitator (ESP), particles on upper impactor stages were much smaller than indicated by impactor 50% cut-off diameters given for the impactor, which resulted in significant differences in the measured aerosol-distribution parameters of the total particulate mass and of the mass of constituent elements. On the back-up filters, large particles (i.e. aerodynamic diameter ⪢ 2 μm) including those from bounce-off and reentrainment accounted for more than 90% of the total mass, 20–30% of the volatile species of Mo, As, Sb and Se, and all of the refractory elements. Wall and interstage losses of most elements were estimated to be about 40% by mass. Bounce-off and reentrainment onto back-up filters was only slightly reduced for small (aerodynamic diameter <2μm), wet particles collected after a wet scrubber and by use of coated impaction substrates. No significant wall and interstage losses were observed for particles collected downstream from the scrubber.

Reaerosolization of Fluidized Spores in Ventilation Systems

ABSTRACT This project examined dry, fluidized spore reaerosolization in a heating, ventilating, and air conditioning duct system. Experiments using spores of Bacillus atrophaeus, a nonpathogenic surrogate for Bacillus anthracis, were conducted to delineate the extent of spore reaerosolization behavior under normal indoor airflow conditions. Short-term (five air-volume exchanges), long-term (up to 21,000 air-volume exchanges), and cycled (on-off) reaerosolization tests were conducted using two common duct materials. Spores were released into the test apparatus in turbulent airflow (Reynolds number, 26,000). After the initial pulse of spores (approximately 1010 to 1011 viable spores) was released, high-efficiency particulate air filters were added to the air intake. Airflow was again used to perturb the spores that had previously deposited onto the duct. Resuspension rates on both steel and plastic duct materials were between 10−3 and 10−5 per second, which decreased to 10 times less than initial rates within 30 min. Pulsed flow caused an initial spike in spore resuspension concentration that rapidly decreased. The resuspension rates were greater than those predicted by resuspension models for contamination in the environment, a result attributed to surface roughness differences. There was no difference between spore reaerosolization from metal and that from plastic duct surfaces over 5 hours of constant airflow. The spores that deposited onto the duct remained a persistent source of contamination over a period of several hours.

Simulated Workplace Protection Factor Study of Powered Air-Purifying and Supplied Air Respirators

A study protocol was developed to obtain simulated workplace protection factor (SWPF) data for eleven models of powered air-purifying respirators (PAPRs) and supplied-air respirators (SAR) with hoods and helmets. Respirators were tested in a chamber that allowed the simulation of 12 exercises, including 2 exercises of interest to the pharmaceutical industry. Each respirator was tested by 12 volunteers, and a total of 144 sets of test results were obtained for each device. The testing protocol allowed SWPFs up to 250,000 to be measured (limit of quantification). Median SWPFs for all respirators, except one SAR, were at or above this reporting limit. Lower fifth percentiles were above 100,000, except for one SAR previously noted. An assigned protection factor (APF) was estimated for each respirator by dividing the lower fifth percentile by a safety factor of 25. APFs ranged from 6000-10,000 for PAPRs (including one loose-fitting PAPR) and 3400-10,000 for SARs, with one exception. This SAR had a lower fifth percentile of less than 20 and an estimated APF of 1. Results indicated that most respirators tested could provide a high degree of protection for workers, although one National Institute for Occupational Safety and Health-approved SAR provided minimal, if any, protection. Direct testing in a simulated workplace seems the only method that will assure employers of choosing an adequate SAR. This may be true for other classes of respirators. Furthermore, the historical approach of establishing APFs for classes of respirators, rather than individual models, may not provide adequate protection to the wearer. This is also a serious problem for regulatory agencies seeking to promulgate respirator standard provisions such as APFs for classes of respirators.

Enhancement of air filtration using electric fields.

Although polarized electrostatic air filters are efficient air filtrating devices, their main disadvantages are difficulty in collecting conductive particles or in operating at relative humidities above 70%. We describe here a new filter design that eliminates these problems. A nonconductive media, normally a glass fiber mat, is placed between two insulated conductive screens. As the voltage across the screens is increased, the penetration of particles decreases exponentially. Increasing the electric field from 0 to 10 kV/cm will decrease the mass penetration from 60% to less than 10% of a polydispersed 0.8 micrometer ammd(sigma g = 2.0) sodium chloride aerosol. The experimental effects of face velocity, particle charge and size, packing density, fiber size, and screen insulation mirror the theoretical effects of these variables on particle penetration.

Measurement of Aerosol Concentration as a Function of Size and Charge

We have developed an aerosol diagnostic system that measures the number concentration of aerosols as a function of particle size and charge. This system differs from other methods that measure aerosol concentration as a function of either particle size or particle charge. Previous methods treated only one of the two variables as a parameter. Our system treats both particle size and particle charge as variables in a three-dimensional representation of the aerosol distribution. Size-charge analysis was accomplished by first fractionating the aerosol according to the electrical mobility of the particle, then determining the particle-size distribution of each mobility fraction using either a Climet optical counter or an ASASX-P laser spectrometer. By accumulating a particle-size distribution of each mobility fraction, we could determine by mathematical techniques the aerosol number concentration as a function of both particle charge and size. Use of an LSI-11/23, interfaced to the aerosol instrumentation, all...