infoBits: Sampling Hydrogen Chloride (HCl) from Stacks
1. Hydrogen Chloride (HCl) is an acid gas classified as a Hazardous Air Pollutant (HAP), and is identified as harmful to the environment and human health. HCl is highly corrosive and can damage metal structures over time. It is also highly water soluble (known as Hydrochloric Acid in solution) and will affect the chemistry and ecology of bodies of water or certain types of soils.
3. US EPA Method 26. Originally promulgated in 1991, this test method extracts a sample from the stack or duct at a constant rate and passes the sample through a filter to remove particulate. After the filter, reagent-filled impingers capture chloride ions as the sample is bubbled through the sampling train. Since the method does not measure HCl directly, but rather the captured chloride ions, other chlorine compounds have the potential to generate a positive bias, if present. The chloride ion concentration is measured via ion chromatography.
4. US EPA Method 26A. Originally promulgated in 1994, this method employs the same underlying principles as Method 26 while conforming to most Method 5 specifications for sampling particulates. Specifically, Method 26A isokinetically extracts a sample from the stack or duct and passes it through a Method 5 filter to remove particulate. The impingers capture chloride ions just as in Method 26; however, to account for higher flow rates and higher sample moisture levels, the impingers hold larger volumes than in Method 26. The EPA created this method to address possible sampling biases in “wet” stacks, or stacks that are at or near the moisture saturation point and where entrained water droplets containing HCl may be present.
5. US EPA Method 320. Method 320 is a spectrographic method. Unlike the “wet” Methods 26 and 26A, this method measures the HCl in the gas phase without first absorbing it into a liquid reagent. A sample is extracted from the stack or duct at a constant rate and passed through an FTIR sample cell. The FTIR scans the volume of gas in the cell and measures HCl (as well as other compounds) in near real-time. American Society for Testing and Materials (ASTM) D6348 12e1 is an FTIR method like Method 320 that may also be used when FTIR is approved for use at a source. These methods directly measure HCl, not chloride ions and so are less susceptible to some biases.
6. Which method should be used for compliance? The Mercury and Air Toxics Standards (MATS) Rule lowered the HCl emissions limits for all existing power generating units, while giving sources different options for complying with the new limits. Certain sources may use sulfur dioxide (SO2) as a surrogate for HCl, certain low emitting sources (LEE) may be exempt from monitoring HCl, and other sources may choose to comply with a permanent HCl CEMS. For sources not qualifying for or opting not to use these compliance options, Methods 26, 26A, and 320 are the HCl measurement methods to be used for quarterly stack testing compliance demonstration. For sources with a wet stack, Method 26A must be used. For sources with a dry stack, Method 26 or Method 320 can also be used. Non-power generating units historically restricted methodology to Methods 26 and 26A for HCl compliance; however, FTIR has been approved for some sources in some states.
7. What about measurements that are not for compliance? Recently, source owners have installed dry sorbent injection (DSI) and other emission control devices to comply with lower HCl limits. During the performance guarantee for newly installed control devices, as well as subsequent tuning or diagnostic testing, source owners choose the measurement methodology that they believe best helps them meet their test program goals and quality objectives. There are no regulations that specify measurement methodology when the testing is not for compliance purposes.
8. Entrained water biases in Method 26. Method 26 uses small filters, small impingers, and extracts samples at a slow, constant rate. When sampling a wet stack, this method will not representatively sample entrained water droplets. Typically, the low flow rate and small sampling train makes it likely that smaller entrained water droplets and gas molecules will be overrepresented in the sampling train, while larger entrained water droplets will be underrepresented in the sampling train. This means that the HCl concentration will typically be biased low when Method 26 is used in the presence of entrained water droplets.
9. Chlorine biases. Chloride salts and elemental chlorine exist in the flue gas and cause molecular interactions to occur in the bulk flow. Conditions found in typical flue gases drive conversion of most chlorine content to gaseous HCl. However, any residual chlorine in the bulk flow could also convert to HCl in the sampling system if the sampling system is not carefully constructed and operated. This transport issue can be a problem with all of the methods referenced above.
10. Keep it hot, rinse a lot. Chloride ions deposit on the surfaces of unheated and chilled portions of the sampling train (the impingers), so the post-run collection and rinse methodology for wet methods becomes very important. Sample system temperature has a direct effect on the degree of chloride deposition. Sample system temperature must be kept as hot as possible during sampling at all points. Any cold spots, even small ones, can introduce a significant bias to the measurement. Also, laboratory studies show that a large volume of rinse water and thorough wetting of all surfaces of sampling equipment is necessary to fully remove all collected chloride. The important of thorough rinsing cannot be overstated – research shows that recovery and rinsing bias is tester-specific. Like the wet methods, Method 320 also requires transport of the sample from the source to the instrument. Adequate and consistent heating of the gas sample along the entire transport path is critical in achieving good performance with this method. M26 and M26A specify a temperature range of 248-273 °F, while M320 does not specify temperature. Temperatures must not exceed 400°F if Teflon components are used since Teflon is unstable above this temperature.
11. What are some alternatives? There have been mixed results with in-stack measurements using FTIR as well as tunable diode laser (TDL) technology. These approaches eliminate sample transport biases entirely, but are more amenable to long-term measurements and CEMS applications rather than short-term compliance tests. More recently, EPA has considered the use of sorbent traps (OTM-40) for HCl as Alternate Test Method 129 (ALT-129), which can eliminate some of the transport concerns.
Overall, HCl is a difficult compound to quantify, not because of a lack of analytical techniques that can measure HCl, but because of the complications of sample transport. CleanAir conducted a study, for the Electric Power Research Institute (EPRI) comparing wet methods to FTIR for halide measurements. Also, in the special case of a supersaturated stack, CleanAir performed diagnostic testing to help troubleshoot HCl issues with an isokinetic FTIR. If you have any questions concerning HCl measurement, we can help you with research, data, and testing.