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, regarding 

calibration of integrated path CEMS. The gas audit cell has to be maintained in-line of the 

optical measurement path in the stack, meaning that all calibration will be offset by the native 

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 CEMS technology.

There is a fundamental difference between an extractive type of system and an 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 

suggested in the draft seem to be based on the latter case.

To perform tests of an integrated path CEMS as suggested will require significant attention by 

the operator on the behavior of the native HCl stack concentration. The calibration error check as 

defined 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 HCl concentration as background. It must explicitly allow the zero to be 

measured before and also after the HCl span gas measurement. 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 calibration test sequence.
Comments to Draft 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 compensate 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 attenuation 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 range 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 that 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: The 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?