Accurate particle size distribution (PSD) data can provide a significant amount of information about the nature of a particulate emissions source. One means of collecting accurate particle size distribution data uses an in-situ cascade impactor. Figure 1 shows the actual samples from a well-performed in-situ cascade impactor test. Each photo shows particulate matter (the tiny dark spots) deposited on ultra-pure quartz substrates that occupy each of the stages of a cascade impactor. The particle size of the particulate collected in each stage gets successively smaller moving from the left to the right in the top photo (Photo 1) and continuing from left to right in the bottom left photo (Photo 2). The lower right photo (Photo 3) is the final backup filter and contains the smallest particles. Figure 1: A University of Washington Mark V Cascade Impactor Collected Sample Cumulative Mass Percentage PSD Data Plot The initial information obtained from analyzing the samples in these photos is a gravimetrically determined set of particle size cuts (D50) called a particle size distribution. When sampled from an industrial process, the distribution can be used to determine how much particulate matter of each particle size cut that a new piece of air pollution control (APC) equipment would need to remove from the gas stream to meet a target emissions value. For a controlled process, the distribution can tell us how efficiently an existing APC device is operating. Figure 2 shows a cumulative mass plot of a spline-fitted PSD data set. Note the X-axis title of “Aerodynamic Diameter.” Particle size distribution data are presented in terms of either the aerodynamic or the Stokes diameter of the particles. The aerodynamic diameter is a mathematically derived parameter that allows the irregularly shaped particles to be represented as idealized spheres of unit density. The Stokes diameter is a similar construct that represents the particles as spheres having the same density and settling velocity as the actual particles. It is important to know which diameter basis is being represented when evaluating PSD data. The “mass mean diameter (MMD)” is another parameter typically used to evaluate PSD data. The MMD is the particle diameter at which half of the sample mass is comprised of particles with smaller diameters. In the case represented in Figure 2, the MMD is approximately 2.5 microns. The smaller the MMD, the more difficult it is for an APC system to remove the particles from the gas stream. This trend tends to go up exponentially as particle size decreases. Once the MMD is below about 5 µm, a significant investment of money and energy will be required to remove a substantial fraction of the entrained particulate matter. Figure 2: Cumulative Mass Percentage PSD Data Plot dM/dLogD PSD Data Plot Another common way to represent PSD data is the use of a dM/dLogD graph. The dM/dLogD plot presents the mass concentration on a linear Y-axis versus particle size on a logarithmic X-axis. By using a linear scale for the Y-axis, the area under the curve between any two particle sizes equals the total concentration of particulate existing between the diameters. This type of plot is particularly useful because it provides insight into the type and prominence of formation mechanisms of the captured particulate matter. Figure 3 is a dM/dLogD plot that shows a trimodal distribution, with the three distinct peaks reflecting the three modes of particle formation from a combustion process. Figure 3: dM/dLogD PSD Data Plot The peak on the far-right side centered at 20 µm is typical of “mechanically” generated particles. These are usually caused by processes such as grinding or crushing. In the combustion case, they are the larger pieces of material that have not fully combusted and still retain a significant portion of combustible content. The peak can also represent agglomerations of finer particles that act aerodynamically as larger particles. The large center peak of approximately 1 µm that dominates the particle size distribution is that of the inert products of combustion. This mass of material is approximately 3½ times as prevalent in the gas stream as that represented by the other two peaks. A substantial portion of this material would need to be removed to effectively “control” particulate emissions from this source. The third, leftmost, peak centered at 0.1 µm is generated by particles that have been formed by vapor phase condensation. Gas phase compounds have cooled enough to form solid material in the gas stream. It is typical for these to form in the sub-micron size range. These particles, if present in a significant quantity, present a significant APC challenge. Comparison of Process Time Related PSD Data Sets Multiple dM/dLogD plots taken at different times can present a picture of the particle size evolution during a process. Figure 4 shows the graph of two data sets from the same process. Run 1 was collected during the initial few minutes of combustion while Run 2 was collected after the combustion had fully developed. The shifts in the sizes and relative magnitudes of the peaks discussed above are markedly apparent and are indicative of changes in the particle size generation mechanisms as the combustion advances. Figure 4: Comparison of Process Time Related PSD Data Sets SEM Analysis In addition to regular gravimetric analysis of cascade impactor samples, another analytical tool that can be employed is scanning electron microscopy (SEM). SEM analyses can provide information about the specific morphology and actual size of the collected particulate matter. This tool provides a visual means to distinguish between individual particles and agglomerations of smaller particles acting like a larger particle in the gas stream. Figure 5 shows an SEM photograph of a collected sample of coal fly ash. Figure 5: SEM Photo of Coal Fly Ash Cenospheres Size Specific Chemical Speciation using ICP-MS In addition to SEM analysis, a variety of other analytical protocols can be applied to the individual recovered substrates to get size-specific chemical speciation. This can be used to correlate one or more specific compounds in the gas stream to each particle size cut-point. This can then be related back to the particle generation mechanism for each cut-point. Figure 6 is a dM/dLogD plot of a metal coating process. After gravimetric determinations were made, each substrate was analyzed for a selection of target metals by ion coupled plasma - mass spectroscopy (ICP-MS). The result is a separate dM/dLogD plot for each target metal. As expected, the metals in the exhaust gas stream undergo vapor phase condensation to form submicron particles. These ultrafine particles present both a potential health risk as well as a significant challenge for emissions control. Figure 6: dM/dLogD plot Showing Size Specific Chemical Speciation using ICP-MS XRD Analysis of a Single Impactor Substrate Another tool is the use of X-ray diffraction (XRD) analysis of the collected sample on each substrate. XRD provides information about the crystallographic structure and chemical composition of the sample. Figure 7 is an XRD analysis of a substrate that was part of an impactor sample collected downstream of a venturi scrubber demister. This information was used to determine how much gypsum from the scrubber liquor was being lost because of subpar demister performance. Figure 7: XRD Analysis of a Single Impactor Substrate The examples presented herein demonstrate some of the information that can be generated from in-situ cascade impactor sampling. This information can help end-users assess the efficacy of their APC equipment or specify new equipment in a more informed and capital sensitive way. The author of this article was involved with the development of the Pilat, University of Washington impactor as well as other PSD tools. For further information on determining particle size distribution in any type of gas stream, contact email@example.com
Overview Determining the particle size distribution (PSD) of the filterable particulate matter (FPM) contained within a gas stream is often a necessary and valuable sampling tool. A PSD can be used diagnostically to evaluate the performance of an air pollution control device or to gather information to design a new one. It can be used to assess the nature of a product containing process gas stream with an eye towards recovery and re-use. In some instances, it can be used for compliance purposes by application of California Air Resources Board (CARB) Method 501. The most important decision to make is whether the PSD will be obtained using an in-situ device, which sizes the FPM as it is withdrawn from the gas stream at as-found conditions, or with a secondary method that evaluates the PSD of an extracted mass sample, usually in a laboratory. There are two primary reasons for doing extractive PSD work. The first, and most prevalent reason, is that the expertise to perform in-situ sampling is of limited extent in the industry. In-situ PSD sampling is technically challenging and is performed much less frequently than standard particulate matter source sampling methods. The second reason to use extractive sampling is that the gas stream cannot safely or reasonably be accessed with an in-situ device. A long-held particulate sampling tenet is, “Measure it while it’s moving.” The morphology and density of the particle have a strong effect on its behavior in the gas stream in addition to its actual physical size. The gas stream factors of temperature, pressure, velocity and moisture also come into play. With an extractive method, these factors are unavailable during the measurement process and can only be predicted to varying degrees of certainty. The two primary in-situ PSD devices are cyclones and cascade impactors. Cyclones The five-stage cyclone sampler developed by Southern Research Institute for the U.S. E.P.A is shown in Figure 1. Figure 1: EPA/SoRI Five-Stage Cyclone Sampler Each cyclone provides an individual particle size cut commonly referred to as a D50. The sizing range of the device shown is 10 to 1 micron. The sample is withdrawn isokinetically so the size distribution collected is representative of the PSD in the gas stream. Impactors Cascade impactors sample isokinetically and separate the sampled aerosol particles into size increments by inertial impaction of the particles on to a collection surface, which can be a lightweight greased steel foil, ultrapure quartz microfibre filter paper, Teflon™, or some other type of appropriate substrate material. This occurs at successive stages through the impactor, hence the name “cascade.” The Pilat, University of Washington, Mark V cascade impactor is shown in Figure 2. Figure 2: Pilat, University of Washington, Mark V Cascade Impactor Within this compact stainless-steel casing, 11 different D50’s ranging from ~20 to 0.1 microns are available. By changing stage configuration and adjusting sample rate the size distribution can be customized to a specific need. Cascade impactors are almost always used with a precutter head attached that enables sampling to be conducted in a standard, perpendicular to the gas stream orientation as shown in Figure 3. The precutter also can remove heavier loading of large sized particles that could overload the impactor. A special application for wet droplet PSD sampling is also possible with the precutter head. This application allows PSD sampling and analysis of saturated gas stream environments with droplets present such as downstream of a wet scrubber. Both droplet size and particulate size determinations are possible. The Pilat, University of Washington impactors have been considered the standard for in-situ PSD measurements and were the basis of the CARB 501 Method. Figure 3: Cascade impactor with precutter head The author of this blog post was involved with the development of the Pilat, University of Washington impactor as well as other PSD tools. For further information on determining particle size distribution in wet or dry gas streams, contact firstname.lastname@example.org.