With the current review of some of the issues with Method 25-A being undertaken I had the opportunity to have a brief discussion concerning the field portable Method 25 analyzer and how it would differ from the current types of analyzers. This brought about the question of whether the true mass of the compounds of interest could be determined for both the inlet and outlet of a control device using either method.
The short answer is no. Based on the MSDS and the concentration determined by the analyzer as methane or propane equivalent, for example, the inlet mass can be determined to a possibly acceptable extent from the known compounds in the inlet stream. The basic math problem is not applicable for the outlet, however, because what goes into a control device may not be what exits that same device. There are too many combinations of types of control devices and gas streams to be controlled to make any hard and fast claims because there just is not readily available data.
To illustrate this point, I will use the biggest data base I know on how the gas stream from various sources can be converted by a catalytic oxidizer - my experience with the Method 25 trap fraction recovery system. In order to provide a more complete understanding of the situation, I have to point out some of the improvements we have made to the recovery process. The requirements in Method 25 call for a 3/8” OD tube packed with 6” of chromium on alumina and with a maximum flow rate of 250 cc/min going into that oxidation catalyst. This is assumed to be sufficient given the initial performance tests and the daily performance test to convert 1% CH4 to CO2.
I know from prior experience these requirements are not sufficient for all sources. This could be seen even when the analysis of the trap fraction was for only the CO2 content, produced from the conversion of organic compounds in the trap, as specified in the method. This was because CO and CH4 elute before CO2 and it was clear when those compounds appeared in the trap fraction, but not the tank fraction, that they were products of the conversion process. This indication of a probable incomplete conversion of the trap fraction to CO2 during the recovery process led to engineering a more robust system.
Given the possible IDs for the 3/8” OD tubing, as specified in the method, the 6” of catalyst material would give a maximum catalyst volume of between 13.5 cc and 23.5 cc. This volume would give a residence time in the catalyst material itself of between 3 to 7 seconds with the maximum flows through the system. We enlarged the catalyst to a volume of 60 cc of commercially produced catalyst material and reduced the maximum flow through that catalyst material to give a residence time of ~24 seconds to further improve the conversion of the organic compounds.
We also extended the analysis of the trap fraction to include any NMOC which may not be converted. Given the over engineering of this system, I expected there would never be any unconverted organic compound in any trap fraction analyses if the initial catalyst in the method was expected to work for the majority of the sources. I was wrong.
Over the years this full analysis of the trap fraction has caught not only CO and CH4 from incomplete conversion but NMOC compounds in the trap fraction. Since there is no way to determine what the compounds are during the analysis, there are still a lot of unknowns, but two things which seem to be connected are specific compounds and concentrations over a certain level. It seems that the larger the molecule, the higher the potential for cracking rather than complete conversion. This appears to be how we have both CO and CH4 in trap fractions where there was neither in the sample tank for that sample stream. The conversion process must be the source of both of these compounds, as well as the unconverted NMOC in the trap fraction, which is the larger portion of the molecule being cracked. This has been documented in the sample recovery system, which deals with the organic compounds from only a minimum volume of a 3.6 L sample of the gas. If this cracking is observed in the sample from an outlet stream of a control device there is a concern that the recovery system is not completely converting the organic compounds. A larger concern would be that the control device is also not succeeding in complete conversion either.
In either case, there are some sources where this cracking will occur due to catalytic or even thermal processes. This means the mass of the outlet stream may not be the same as the inlet. It also means the response of the same analyzer to each stream may be different. If the process oxygenates a percentage of the compounds to be destroyed, there can be a reduced or even eliminated response from a straight FID. My catalytic FID system should hold the same response in most cases, but the mass would still only be based on the carbon in the NMOC molecules. A carbon mass can be determined for the inlet and outlet and a total mass may be determined for the inlet stream based on the MSDS, but the outlet is always going to be a problem. The attempt to identify the compounds in the outlet stream through some type of analysis for identification of the various species will narrow the unknowns, but the cost to benefit ratio will dictate that some may never be determined.
This indicates that merely using the same method for both the inlet and outlet analyses of a control device may not give directly comparable results, although certain methods will have a greater chance than others.
To clarify any possible confusion, we do report any of the additional carbon from CO, CH4 or NMOC in the trap fraction analysis as the trap fraction concentration to prevent any negative bias. We also do not track the source information, in fact we often have little information on the source so there are no preconceived expectations for the samples. The tracking of sources, compounds, control device types, efficiency of the devices, efficiency of our recovery for any of the combinations, or the concentrations is not done in order to protect the data owned by our clients and their clients. These data would be great to track for research, but obtaining the clearance is something we rarely do and only on a specific case by case basis.
Triangle Environmental Services, Inc.
P.O. Box 13294 122 US Hwy 70 E
Research Triangle Park, NC 27709 Hillsborough, NC 27278
(919) 361-2890 (800) 367-4862 Fax: (919) 361-3474