AIR POLLUTION
MONITORING
& SAMPLING NEWSLETTER                 December  2001  No. 266

Title Automatic vs. Manual HCl Measurement  )

The Environment Agency of England and Wales is involved in independent monitoring programs for various industrial processes where permits require measurement of hydrogen chloride (HCl) in stack gas releases. These programs have discovered a number of difficulties associated with currently used HCl monitoring techniques. In addition, significant differences have been observed between results obtained using manual methods and those obtained using CEMS, according to Maciek Lewandowski at CEM 2001. Therefore, the Agency commissioned a project to compare and assess various measurement techniques under controlled and repeatable laboratory conditions. The work was performed by the U.K.'s National Physical Laboratory (NPL) and consisted of a series of tests involving CEMS and Standard Reference Methods (SRMs) using a special test chamber.

The instrumental methods tested included gas filter correlation (GFC), differential optical absorption spectroscopy (DOAS), tunable diode laser (TDL) spectroscopy, Fourier transform infrared spectroscopy (FTIR)- medium and low resolution, and ion mobility (mass) spectrometry (IMS). The two manual methods tested were SRMs as specified by EN1911 (U.K.) and U.S. EPA Method 26A. According to Lewandowski, the tests demonstrated that, in general, the instrumental techniques perform well and produce much more repeatable results than SRMs. No major interferent effects were observed across all techniques. The SRMs were not challenged with gaseous chlorides which would have caused much greater discrepancies between the automatic techniques and the manual ones. However, in Lewandowski's opinion, the performance of SRMs puts a question mark over their use as a principal means of CEMS calibration, especially in view of the possible interfering effect of gaseous chlorides.

While, overall, the instrumental techniques performed well, there were some problems with each one. For example, even though TDLs have been promoted as being much superior to other techniques for difficult to measure pollutants such as HCl, problems were experienced with the TDL analyzer during the NPL tests. They included: a decrease in transmission due to ambient vibrations; spectral broadening of the sample gas absorption line when exposed to interferents; misidentification of HCl when exposed to methane; and a loss of sensitivity in the presence of water vapor. In addition, the TDL technique, in common with other purely spectroscopic techniques, was incapable of monitoring molecules bound in complex compounds.

Lewandowski also reported that the TDL analyzer was found to be very sensitive to the alignment of the optical beams and this caused the instrument to be sensitive to vibration. If the analyzer were to be subjected to typical vibrations on a stack, Lewandowski thought it possible that the transmission, and hence the sensitivity, might be affected. He stated that this is a potential problem with cross stack instruments in general.

s stated above, the manual methods performed less consistently than the CEMS. Their linearity was acceptable but not as good as most of the CEMS. However, they showed no consistent systematic effect due to the presence of water vapor or NO₂. On the other hand, the manual methods are known to be sensitive to total chlorides rather than HCl specifically. This response, which is an inherent part of the methods, was not challenged in these tests. Lewandowski noted that, under field conditions, chloride ions might well be present in a number of forms other than HCl. For the details on this intercomparison, see MO 011203.

[Editor’s Note: In EPA’s proposed NESHAP for HCl production (September 18, 2001 — see MO 011121) the agency proposed initial compliance tests using Method 26A to directly measure HCl outlet concentrations for process vents, storage tanks and transfer operations. EPA rejected FTIR CEMS due to cost.]