# Clean Air Engineering , U.S. Opsis AB, Sweden

COMMENTS TO DRAFT PERFORMANCE SPECIFICATION 18
, PERFORMANCE SPECIFICATIONS AND TEST PROCEDURES FOR HCl CONTINUOUS EMISSION MONITORING SYSTEMS IN STATIONARY SOURCES
, DRAFT VERSION 9

- 19

- 2012
Submitted to USEPA November 9, 2012 by James E. Wright , Clean Air Engineering

General: This current draft contains a new requirement, compared to previous drafts, rega rding calibration of integrated path CEMS. The gas audit cell has to be maintained in

- line of the optical measurement path in the stack, meaning t hat all calibration will be off set by the n ative stack concentration of HCl . There has been an assumption that results can simply be corrected with a native stack concentration recorded some time before or after the calibration.
It is clear that this condition generates a level of uncertainty that will be a significant disadvantage for integrated path CEM S technol ogy.
There is a fundamental difference between an extrac tive type of system and a n integrated path CEMS . The integrated path CEMS reads a concentration of the gas directly in the stack, whereas in an extractive system the sampling system, including probe, filter, sample line and conditioning unit will by a mix of effects which cause a slower, smoother signal. The timing parameters

s uggested in the draft seem to be based on the latter case.
To perform tests of an in tegrated path CEMS as suggested will require significant attention by the operator o n the behavior of the native HCl stack concentration. The calibration error check as def ined in 11.4 is possible, provided that some flexibility is allowed to the test sequence. To run such tests for the seven

- day drift test as defined in 11.5, in an automatic, unattended mode, will not be possible considering the narrow error margins allowed

. The layout of the calibration error check has to be more flexible if this is being performed with a variable native stack HC l concentration as background. It must explicitly allow the zero to be measured before and also after the HC l span gas measurem ent . An average native stack HCl background would then be calculated. It must be allowed to perform this at each calibration gas point. Flexibility in the timing requirements for the measurements must also be provided to allow compression of the total calibra tion test sequence.Comments to Dr aft Performance Specification 18 Clean Air Engineering November 9, 2012 Page 2

Specific comments follow below and also appear in the MS Word document as reviewer
comments.

Section 3.12 : The one

- minute requirement could be overly restrictive for in

- situ CEMS. A typical cross

- path TDL system operates on an entirely different time scale than that of extractive systems, producing several independent and direct measurements of fresh stack gas per minute

– latency from a sampling system is eliminated.
This may become important if in

- situ systems are required to compensa te for changes in native gas concentration during calibrations

– the duration of the entire calibration cycle must be minimized to be successful if native concentrations vary significantly.

Section 11.0 : Add Light Attenuation Test for Path CEMS. A light at tenuation test establishes that the HCL span response, within a defined range, is independent of the actual intensity of light received. Test details and specifications may be manufacturer

- specific, but a 3% of span tolerance would be reasonable. Test ran ge should include points between 10

- 100% of light intensity. Tests can be performed simply by using perforated screens with different patterns. This could be an in

- house test like interference and LOD.
From an operations perspective this test is important if there is a variation in stack dust level and if the IP path CEMS is affected by alignment issues caused by movement in the stack.
This test could also be used to demonstrate that the IP path CEMS can use only a portion of the light signal, provided tha t the same source and detector system is used, to perform a calibration measurement on a path that excludes the native stack HCL concentration.

Section 11.4.2.1 : T he measurements of the upscale standards (not just zero, as implied) also include the native stack concentration.

Section 11.4.2.5 : The sequencing here is ambiguous and possibly too prescriptive. By definition in Section 3.12, each independent measurement consists of a one

- minute average reading of the upscale gas followed by a cell flush. So, does this paragraph imply that this has to be done three times with a two

- minute flush between each upscale measurement? If so, this process could take ten minutes or longer to complete.
If the native concentration varies significantly during the time period of the calibrations, flexibility must be allowed to compress the calibration period as much as possible.

Section 11.4.2.6 : Can we assume that zero responses bracketing each upscale measurement (i.e., before and after) can be averaged?