3M Thermal Oxidizer VOC Destruction Efficiency Testing Experiences - The Case against EPA Method 25 and for Method 25A

摘要

When testing for thermal oxidizer (TO) destruction efficiency (DE) of organic compounds, EPA regulations require the use of EPA Method 25, whenever the TO exhaust contains more than 50 ppm as carbon. The stated justification is concern over the flame ionization analyzer (FIA) response factors with Method 25A, due to the likelihood that the TO exhaust contains different organic compounds than the inlet. For capture testing under EPA Method 204, similar reasoning led to the specification for the use of the FIA, since essentially the same solvents are emitted from all the coating bay exhausts. To demonstrate compliance with the Paper and Other Web MACT (NESHAP 40 CFR Part 63 Subpart JJJJ), 3M independent contractors recently completed capture efficiency testing at US coating plants, totaling 27 coating bays and DE retesting on 26 associated thermal oxidizers. Almost always, since the TO exhaust carbon levels were unknown, 3M paid the testing consultants to provide both Method 25 and 25A equipment and the extra personnel to perform both test procedures simultaneously. This added up to $11,000 per TO DE test, when sampling was also done with Method 25. Even if not used, part of the cost for the Method 25 equipment still had to be paid, since it was leased and the cost of any extra personnel. All the testing consultants used by 3M had their own Method 25A equipment. Once the contractor had his equipment set-up on site, the TO exhausts were checked for ppm carbon at the lowest allowable combustion chamber operating temperature. With the outlet information in hand, a decision was then made whether to use one or both test methods. This decision was also based on the type of TO. Almost all new installed 3M TO(s) are the regenerative type (RTO). For an RTO, as long as the exhaust CO ppm is relatively low, the outlet compounds are generally the same as the inlet. This is due to incomplete chamber flushing and/or valve leakage, since the low CO level indicates that those organic compounds exposed to the combustion chamber were destroyed. The valve switches show up on the FIA as a ppm spike in the exhaust VOC concentration with height of the spike dependent upon the completeness of the flushing of the prior inlet chamber. For RTO(s), only Method 25A was used when the between spike, average exhaust VOC concentration carbon was less than 50 ppm carbon and the ppm CO was likewise less than 50. For different reasons, a case could also be made for the use of Method 25A on other types of TO(s), provided the DE is relatively high, i.e., greater than 97%. The real time advantage of the FIA and its equivalent results, when coupled with the contamination problem with Method 25 cold traps, its increased sampling time, and the additional manpower requirements, all combine to make Method 25A the preferred test for TO DE testing.