Measurement of Gaseous Hydrogen Chloride Emissions at Portland Cement Kilns by Fourier Transform Infrared (FTIR) Spectroscopy – USEPA Method 321

USEPA Method 321 is designed for the application of Fourier Transform Infrared Spectroscopy (FTIR) in extractive measurements of gaseous HCl concentrations in portland cement kiln emissions.
 
This method applies to the measurement of hydrogen chloride (HCl) concentrations both before and after particulate matter control devices.  It also applies to either continuous flow through measurement (with isolated sample analysis) or grab sampling (batch analysis).  HCl is measured using the mid-infrared spectral region for analysis (about 400 to 4000 cm-1 or 25 to 2.5 µ m).
 
The analytical range is determined by the instrumental design and the composition of the gas stream.  For practical purposes there is no upper limit to the range because the pathlength may be reduced or the sample may be diluted.  The lower detection range depends on:

• the absorption coefficient of the compound in the analytical frequency region;
• the spectral resolution;
• the interferometer sampling time;
• the detector sensitivity and response;
• the absorption pathlength.

The practical lower quantification range is usually higher than the instrument sensitivity allows and is dependent upon:

• the presence of interfering species in the exhaust gas including water (H₂O), carbon dioxide (CO₂), and sulfur dioxide (SO₂);
• analyte losses in the sampling system;
• the optical alignment of the gas cell and transfer optics;
• the quality of the reflective surfaces in the cell (cell throughput).

Under typical test conditions (moisture content of up to 30% and CO₂ concentrations from 1 to 15 percent), a 22 meter path length cell with a suitable sampling system achieves a lower quantification range of from 1 to 5 ppm for HCl.
 
Sample transport presents the principal difficulty in directly measuring HCl emissions.  This identical problem is overcome by any extractive measurement method.  HCl is reactive and water soluble.  The sampling system is adequately designed to prevent sample condensation in the system.
 
Potential interferences include:

• analytical interferences
• background intereferences
• spectral interferences
• sampling system interferences.

This method will not measure HCl under conditions:

• where the sample gas stream condenses in the sampling system or the instrumentation;
• where a high moisture content sample relative to the analyte concentrations imparts spectral interference due to the water vapor absorbance bands.