Testing of industrial sources to measure filterable particulate matter equal to or less than a nominal aerodynamic diameter of 2.5 micrometers (PM2.5) emissions was codified when in 2009 EPA added a PM2.5 cyclone to the existing PM10 cyclone in Method 201A. However, this method could not be used in “wet” stacks, i.e., those with water drops present. Instead, EPA recommended that EPA Method 5 in conjunction with Method 202 be used which guaranteed a degree of positive bias in most test results for those sources. Subsequent studies of smelt dissolving tanks and lime kilns using dilution testing methods estimated that ~25% of the measured particulate using Method 5 is greater than FPM2.5. 

Wet particle sizing technology, which included the ability to determine a filterable PM2.5 fraction, began officially when in 1994 Southern Research Institute developed an addendum to the particle size distribution method using cascade impactors that they had developed for the California Air Resources Board (CARB) in 1988, CARB 501. Testing of venturi scrubber emissions from a coal fired boiler and an absorber outlet downstream of an ESP on a heavy fuel oil fired boiler using the CARB 501a wet testing protocol indicated that the greater than FPM2.5 portion of the FPM emissions was in the 12 – 30% range. 

As the ambient FPM2.5 standards have continued to ratchet down there was a desire among industry technical groups to develop a method specifically for the measurement of wet stack FPM2.5. This desire manifested itself in 2016 when the American Petroleum Institute (API) and the National Council for Air and Stream Improvement (NCASI) supported such a project. The test method was submitted to the EPA and included into the Other Test Method (OTM) category as Other Test Method 36 (OTM-36). This method’s equipment was a direct modification of CARB 501a with the substitution of a FPM2.5 heated cyclone for CARB 501a’s heated cascade impactor. 

In 2016, a technical review of OTM-36, by CleanAir Engineering and followed up by NCASI, revealed a significant design flaw in the method’s in-stack precollecter. This precollecter was supposed to provide a droplet cut size of 12 microns but it was revealed that the actual operational cut size was ~5 microns which gave a marked negative bias to the results. This resulted in EPA removing the method from their website. 

CleanAir Engineering used their expertise and familiarity with particle size distribution calculations and testing to develop a new precollecter that provided a droplet cut size of ~12 microns over a wide range of saturated stack conditions while still giving a cut size of 2.5 microns for the heated cyclone. After reviewing the design data, NCASI performed some computational fluid dynamic evaluations of CleanAir’s adjustable precutter nozzle (APN), and the results showed the APN performed within the desired parameters. 

The next step is to perform field trials on wet sources side by side with EPA Methods 5 and 202 to see if a reduction in emissions reporting is achievable.