I would like to provide more information on the new audit program study and a potential issue for DRE sampling in relation to compound with high boiling points.
One of the most asked questions still concerns the audits, or the lack thereof, and when that situation may change. There are still some delays in accreditation and minor criteria before the program can be initiated. Once the program is begun, there will still be some additional time before the audits would be available. .
To provide new data on the Method 25 audit pass/fail criteria, we have donated a significant amount of time, equipment and effort into a round of audit collection and analyses. We donated the equipment and support for the collection of 11 audits in duplicate and the analysis of a total of 44 audit samples. The audit cylinders were provided by Liquid Technology Corp. specifically for this new round of testing. One cylinder provided was unable to be used for the project due to loss of pressure.
The cylinders contained concentrations of carbon dioxide and two separate hydrocarbon compounds in a range of concentrations from 61 ppmC to 2882 ppmC. The reproducibility of both sets of duplicates was good, but there was some variation between the two sets for the same audit cylinder. This was mainly in the lower concentrations, which was not that unexpected. There is to be a presentation on this at the next SES conference, so I am not going to go into much detail on the results at this time. The only possible concern for the study was the audit concentrations reported for the specific cylinders did not match the records, but that appeared to only be a clerical error by the provider in the recording of the initial cylinder concentrations. The concentration levels recorded did match those reported even if the associated cylinder identifications did not match.
The new audit interfaces worked smoothly in the collection of the audits, although the flow rate was not as expected for the initial pressure set point. The flow could easily be increased by increasing the pressure regulator output and the waste seemed to be minimal. I had specified a flow rate of 200 cc/min at 30 PSI inlet for the critical orifice, but in the constructing the interface the 1/4" OD lines were replaced with 1/8" OD lines and the pressure/flow relationship was changed significantly. It is still very easy to maintain a flow >100 cc/min with a small adjustment to the inlet pressure regulator. With a flow of 60 cc/min and a waste flow of 40 cc/min, this setting uses very little of the audit gas pressure per sample, which may be even more important once the audits must be privately purchased for use. The only concern with the operation would be the effect of the initial sample flow being taken from the interface. The waste outlet flow can drop to no flow, which could cause some ambient air to be drawn in if the sample flow is the full 60 cc/min. To combat this we suggest setting the flow very low before turning the valve to the “sample” setting and then adjusting the flow. This eliminates the initial “shock” to the sample flow through the audit interface. Once the flow is set, it is not hard to maintain with regular monitoring, which is a standard requirement.
A potential flaw in the basic methodology for Method 25 came to light in a recent DRE project. The reported Method 25 concentrations of the inlet samples did not match the simultaneous Method 25A FIA analyses by a very significant amount. The reported results for all methods were consistent between themselves and were taken by an experienced team, which is probably why they were so consistent. The Method 25 inlet concentrations were just too close to the reported outlet concentration to show any real efficiency for the control device. The reported Method 25 inlet concentrations were also much lower than the concentrations measured using Method 25A. Differences between the FIA and Method 25 are not uncommon, but the Method 25 is generally not lower and not by orders of magnitude. The differential reminded me of the project where we discovered the effect the different filter specifications had. In that case, the filter with the same micron rating as the Method 5 train had similarly low Method 25 and 25A results when compared to the filter with a larger micron rating.
However, in this case there was no data to compare between using filters and not using filters with the same instrumentation as was the case with the initial sampling of the aforementioned filter project. This project’s inlet stream had a small amount of the normal BETX type solvent, but the majority was a naphtha-based solvent. All of the major components listed on the MSDS sheets had boiling points (BP) well over the temperature of both the heated probe and heated filter of the required sample train. One of those compounds had a BP approaching that used for the recovery of the traps, which is over twice the temperature of the heated probe.
There was no indication during the recovery of any significant amount of higher BP compounds, which would normally extend the recovery time due to the slower volatilization. This lack of higher BP compounds in the trap and the lack of reported concentrations was very similar to what the increased filtration level showed in the previous project. The heated probe and filter holder have a higher temperature on the metal surfaces in order to initially heat the sample stream to the 121o C and 129o C required by the method. However, the glass fiber filter will not acquire this increased level of heat even though it is in contact with the filter housing. That means the filter was much cooler than the BP for the majority of the compounds in the solvents, which could easily allow those compounds to stay on that filter instead of passing through.
In a situation such as this, the heated probe and filter has the potential to create a negative bias for the inlet. As the differential between particulate and NMOC emissions is not a concern in the inlet, this would be a situation where the older practice of using the long arm of the trap as a probe with a minimal in-stack filter would be the more technically correct methodology. This would not be a problem seen in every DRE project, but when it does show up it can be a significant problem for all involved.
I intend to suggest a change to the method to allow the use of an in-stack filter on the trap arm specifically for inlets in DRE projects. We have used similar procedures with success for DRE projects for landfill gas, which is allowed because the hazardous nature of the inlet gas stream precludes the use of a heated probe or filter. There needs to be some allowance for inlet monitoring for non-hazardous gas streams which may be subjected to significant biases with the heated probe and filter. The outlet monitoring where the data on particulate and NMOC concentrations may at some time need to be accurately compared, would require the use of the heated probe and filter.
Triangle Environmental Services, Inc.