Stack Particle Size Analysis

Stack Testing Lab

In the coming months, MVA Scientific Consultants will post a series of the most common stack testing questions related to scanning electron microscopy (SEM) particle size analysis methods that we get from our clients. These questions and answers have helped many of our clients, and we feel they would benefit the stack testing community.

What is scanning electron microscopy (SEM) particle size analysis?
The most common analytical tool used to image stack emission particulate is the scanning electron microscope (SEM). For automated particle sizing, the image must contain sufficient contrast between the background and the particles that an image analysis algorithm is capable of differentiating between them. For automated image analysis systems, a “particle” is defined as a set of contiguous pixels all of which are brighter (or more rarely, darker) than the threshold brightness used to define the surrounding “background” pixels. Polycarbonate filters are ideal for this purpose.

Why should I use SEM particle size analysis for method 5 or method 202 stack samples?
It is now the case that many stack emissions have so low a particle count that the traditional method of particle size distribution (PSD) determination (i.e., collection by cascade impactor or cyclone followed by gravimetric analysis) is time prohibitive. The total sample collected during a four-hour incinerator trial burn may be as low as one milligram (0.015 grain).

SEM measurement of particle size distributions offers benefits over gravimetric techniques, allowing particle size determination of sample collections as small as 0.25 mg. Scanning electron microscopy methods are able to directly determine the number fraction of particles of a given size. In addition, the particle size bins chosen for data analysis and reporting are entirely customizable, which is important to engineering studies for control equipment design.

When should I use SEM particle size distribution methods rather than Method 201A or cascade impactors?
Cascade impactors and Method 201A cyclones cannot be used in wet stack conditions or conditions of cyclonic flow. It may also be impossible to use cyclones in close conditions where they do not fit or large ports are not available. Many sources are now so clean that it is impractical to collect enough material in the cyclones to produce accurate weights. And some conditions are simply too harsh for cyclones. Sample collection using Method 5 or a modified Method 5 followed by particle sizing using SEM provides an opportunity to determine particle size distributions that cannot be otherwise determined.

Will regulators accept PM10 and PM2.5 results from SEM particle sizing methods?
State agencies have shown a willingness to accept microscopy results when Method 201A cannot be used.

How much sample do I need to collect for SEM particle sizing?
It depends on the filter medium used. For standard Method 5 glass or quartz fiber filters, it is best to collect as much material as possible, since we will attempt to remove the particles from the filter for analysis. But if you cannot collect a “cake,” or can use polycarbonate filters (very much the best choice in most circumstances) in place of the glass or quartz fiber filters, particulate catches of less than one milligram (1mg) can be used for particle size determination.

Where can I obtain the appropriate polycarbonate filters for SEM particle sizing?
Many different vendors offer polycarbonate filters compatible with Method 5 sampling trains, here are just a few:

Sterlitech
Millipore
Thomas Scientific

Is it possible to identify the chemistry of the particles caught on the filter?
Yes. Scanning electron microscopy combined with energy dispersive x-ray spectrometry (SEM/EDS) provides both morphological and elemental composition data on individual particles collected on the filter, allowing the identification and estimated proportion of the common particle types to be determined. Carbon ash, aciniform soot, and aluminosilicate fly ash spheres are all common components of combustion sources. In some cases non-combustion sources are important. We have found sulfate crystals arising from emission control equipment, rust and other corrosion products, and fragments of degrading catalyst beds as significant contributors to the mass loading in stack samples.

Can you help me identify the constituents in a Method 202 or other back-half catch?
Yes, we look at a lot of back half catch filters. The normal approach is to first examine the residue under a microscope and see what we can determine just by looking at it. (We know this sounds terribly low tech, but you’d be amazed at what an experienced microscopist can tell just by looking.) This preliminary examination is then typically followed by either FTIR or SEM/EDS, or both. FTIR is good at identifying organic compounds in the back half catch filters, and we have a reference library of tens of thousands of reference spectra. It is our experience that the back-half catch residues, if organic, are usually a silicone or hydrocarbon oil, but other things certainly show up. SEM/EDS analysis will result in an identification of the elements present in the residue and is most useful for inorganic compounds. The elements found generally indicate which compounds are present. In these residue samples, it is most common to find sulfates/sulfites, but again other compounds occasionally show up. In cases where the microscopist recognizes that more than one type of material is present in the residue, the materials can be isolated and each analyzed separately by the appropriate technique(s).