Ammonia (NH₃) Slip Measurement with a Portable Gas Analyzer

Ammonia (NH₃) emissions are an important contributor to fine particulate matter (PM) formation. Consequently, increased attention is being paid to accurate quantification and characterization of NH₃ emissions, especially where SCR’s and SNCR’s are utilized.

NH₃ can be a difficult gas to measure in exhaust ducts and stacks.  It is a “sticky” gas that is both highly soluble in water and easily adsorbed on the inner walls of sampling lines.  There are several technologies available for measuring ammonia on a continuous basis which offer plenty of their own challenges.  When it comes to measuring ammonia with a portable gas analyzer, there are fewer ways to do it, especially with a demand for precision and accuracy.

There are three (3) common ways to measure ammonia on a continuous basis that are used for both process monitoring and EPA compliance monitoring:

1. Calculated NH₃ slip using SCR Inlet and Outlet NOx values: This is probably the most used method to measure NH3 slip in the power plant industry.  Simply put, NH₃ is not measured directly!  A NOx CEMS is put at the SCR inlet and used in conjunction with the NOx CEMS at the stack (outlet).  Using the inlet NOx value and the outlet NOx value, the NH₃ slip can be directly calculated.
2. Direct NH₃ measurement at the stack using a “converter” method: In States where the calculated method above is not allowed and NH₃ must be measured directly, it’s common to use a method where the NH₃ is converted at the stack to NO so that it easily travels through a standard CEMS Umbilical and can be measured by a standard CEMS Analyzer.
3. Direct NH₃ measurement using Tunable Diode Laser (TDL) technology: This is a relatively newer method for measuring NH₃ that has addressed some of the major issues with other NH₃ approaches.  TDL uses a near-IR tunable diode laser absorption spectrometer to measure NH₃ in a process gas. 

When it comes to Portable Gas Analyzers the first two methods above won’t work .  You could try and measure inlet and outlet at the same time and do the calculations, but this gets onerous as it requires two portable analyzers and two people.  A stack Converter needs to run at close to 1,000 degrees F and would be dangerous to handle and difficult to power.  For portable measurement there are two main option:

1. Electro-chemical cells: this is a common and low-cost technology found in portable gas analyzers.  Unfortunately, they have many drawbacks.  Typically, these instruments are not capable of low range measurement (their lowest range may be 0-500 or even 0-1,000 ppm) which makes them impractical for accurately measuring NH₃ slip in single-digit concentrations.  They also have low accuracy, often ±10%.  The nature of these sensors is also problematic as high sensitivity means a short operating life due to evaporation of the moisture in the sensor through their porous membrane.  The cells also change properties with exposure to the target gas making them more inaccurate the more you use them unless you are performing constant re-calibrations. 
2.   Tunable Diode Laser (TDL): like continuous stack monitoring, this is the best technology for accurate and precise measurement of NH₃.  Utilizing a portable gas analyzer with TDL measurement, a user can expect to get meaningful results, with longer sampling periods, even at very low levels.

The ETG 6900 P Portable Gas Analyzer for NH₃  

In the ETG 6900 P analyzer, the hot-wet gas is drawn from a stack or duct and transported through a short, heated umbilical which keeps the sample from condensing, and the NH₃ from dropping out of the sample.

For measurement, a diode laser emits a beam of 0.1 nm near-infrared light, which passes through the process gas and is then reflected into the detector by an optical device inside the measurement bench. The wavelength of the laser diode output is tuned to a gas specific absorption line. The laser continuously scans the absorption lines with a very high spectral resolution. For analysis, absorption, strength and line shape of the return signal is used. The influence of cross interferences from background gases is negligible since the wavelength specific laser light is absorbed very selectively by only one specific molecule.

Electronic lock-in technology allows separating the gas absorption information from electro-optical system information, leading to a detection method eliminating the need for a physical reference channel and offering continuous sensor status monitoring.

The best part of this type of NH3 portable is the completeness of the stand-alone plug-n-play package and the ease of how it’s used.  The system comes with a heated probe and umbilical and an internal pump to draw the sample.  The stack temperature can be as high as 1,000 degrees F.

Warm-up time is about 1 hour.  Once it’s warmed up, insert the probe in the process and start measuring.  The 6900 P has a low range of 0-15 ppm for true accurate measurement of ammonia slip (with an upper range of 0-50 ppm).  A built-in touch screen display allows for intuitive operation and a long list of outputs are available including Ethernet, USB Remoting, MODBUS and Profibus.

Maintenance could not be simpler with zero calibrations required monthly and span calibrations required every 6 months.  A monthly inline filter check completes the maintenance requirements.  Cost of ownership is low with a 10+ year laser lifetime.

Monitoring Solutions can help users find the best methods and instruments for a wide range of gas monitoring.  If you need to measure NH₃ on a temporary, regular, or sporadic basis, contact us for more information and a quote for the ETG 6900 P analyzer.  We’ll help you meet your needs with a cost-effective, reliable, and easy to use solution.

Contact us for all your Continuous & Portable Emissions Monitoring needs at sales@monsol.com or 908-500-4010.   Or visit us at www.monsol.com