Emissions testing methods explained

Some methods indicate higher emissions compared to EPA Method 25A, which is the method that most ethanol plant permits have relied upon
By D. Howard Gebhart | May 01, 2002
The controversy regarding ethanol plant emissions has evolved in part from changes in the emissions sampling methods. Some methods indicate higher emissions compared to USEPA Method 25A, which is the method that most ethanol plant permits have relied upon.

Method 25A is commonly employed for volatile organic compound (VOC) testing because it is simpler and less expensive. Method 25A measures total hydrocarbon concentrations using a heated flame ionization detector, calibrated against methane. Since many VOCs do not give the same instrument response as methane, errors occur where such compounds are present in the emissions. Method 25A also does not provide any information on VOC speciation.

More recently, some ethanol plant testing has used a modified USEPA Method 5 sampling train, which collects emissions in water impingers for later laboratory analysis. Four impingers in series are used, with one impinger spiked with a 1% sodium bisulfite solution for capture of acetaldehyde. The impinger solutions are analyzed in a laboratory for the target VOC compounds using gas chromatography with a flame ionization detector (GC/FID), except for formaldehyde, which uses ultraviolet spectrophometry. This method provides data on VOC composition, but is limited to those compounds that are water soluble.

A producer that has applied these two methods side-by-side at a CO2 scrubber noticed a difference of about 100% in the measured VOC levels, with Method 25A yielding the lower result. This difference may vary depending on the exact composition of the VOC emissions.

Additional VOC test methods are available, including direct sampling of the stack emissions using GC/FID. Because variation in the results should be expected from different emissions sampling methods, it is important that any emission limits specified in a permit be matched to the intended compliance sampling method. Producers may need to conduct additional emissions sampling in order to assess whether modifications to existing permits are needed and determine the required level of allowable emissions.