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.  In a typical Continuous Emissions Monitoring Systems, a hot-wet gas is pulled from a source (stack or duct), carried in a heated CEMS Umbilical (sample line) to prevent condensation, then run through a sample conditioning system (or sample dryer) to cool the sample and remove the moisture.  From there it’s on to the CEMS Analyzers for measurement.

Many issues will hinder ammonia’s ability to ever make it from a stack down to a CEMS Analyzer:

CEMS Umbilical: NH₃ is very difficult to transport.  NH₃ will adsorb in the sample line (CEMS Umbilical) walls never making it all the way down.  Using a typical Extraction CEMS umbilical will normally result in little to no NH₃ reaching the CEMS Analyzer for detection. A user can pretty much expect close to a zero (0) reading without any special treatment of the sample or CEMS Umbilical. 

Sample Conditioner: to prevent condensation of water vapor inside a CEMS Analyzer, it is necessary to remove the water vapor from the sample.  Cooling the sample far below its dew point removes water vapor but would also remove a large fraction of the NH₃ leaving only a small fraction of the original NH₃ concentration behind.  Cooling the sample gas can also lead to the formation of ammonia salts which can precipitate inside the sampling system and the CEMS Analyzers which can cause plugging and other problems.

CEMS Analyzer: if one were to get the gas transported without adsorption loses (very expensive, specially coated umbilical, heated filters and more) there’s really only one CEMS Analyzer that can measure it directly and that’s a FTIR analyzer (Fourier-Transform Infrared).  FTIR analyzers measure gases that are hot and wet (unconditioned) are very expensive ($80K plus), difficult to maintain, expensive to repair, and require a lot of expensive sampling modifications.  They are simply too expensive to be a viable option.

So how does a plant that’s required to report NH₃ slip accomplish this given the inherent nature of the gas?  There are three ways that are typically used:

1. Calculated NH₃ slip using SCR Inlet and Outlet NOx values
2. Direct NH₃ measurement at the stack using a “converter” method
3. Direct NH₃ measurement using Tunable Diode Laser (TDL) technology

Let’s examine the 3 methods and their advantages and disadvantages.

1. Calculated Slip with Inlet and Outlet NOx

This is probably the most commonly 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 Using a Stack-Converter

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 something else that easily travels through a standard CEMS Umbilical and can be measured by a standard CEMS Analyzer.

That “something else” that NH₃ is converted to is NO (Nitric Oxide) – the largest part of what constitutes NOx. 

By heating a stack sample with NH₃ in it to over 1100 degrees F and running it through a stainless-steel converter with an oxide layer, the NH₃ reacts with the oxide layer and is converted to NO & water per the following formula:

        NH₃ + O₂ = NO + H₂O - Where 1 ppm NH₃ converts to 1 ppm NO.

Using two CEMS Umbilicals, the converted NH3 is brought down one, and the stack sample down the other.  Two NOx Analyzers measure NO in both respectively.  The stack CEMS Umbilical represents the amount of NO measured in just the stack.  The converted NH₃ Umbilical represents the stack NO “plus” the converted ammonia NO (called the N_total).  If the stack NO is subtracted from the N_total, it is easily determined how much of the total NO is comprised of converted NH₃.

3. Direct Measurement with Tunable Diode Laser

There’s 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. 

This is an in-situ measuring device meaning it measures directly in the stack or duct.  Two different versions exist: a “two-flange” version where a combination transmitter/receiver sends a beam across the stack where it then bounces off a retroreflector and returns to be measured in the same unit the beam originated from (alternately the receiver and transmitter might be separate units – one on each side of the stack).  

Another version (one-flange) has a combined transmitter and receiver in a single probe that extends into the stack approximately a meter.  The probe is designed with the retroreflector at the end of it where the beam hits and reflects back.

For measurement, a diode laser emits a beam of near-infrared light, which passes through the process gas and is then reflected back into the detector by an optical device that is situated on the end of the probe or across the stack. 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. The minimum detectable limit, the accuracy and the resolution is dependent on the probe length (optical path length), the process temperature, and pressure.

Monitoring Solutions can help users determine the best method for their application and review the pros and cons of different solutions.  We can also offer proposals to evaluate the different costs of each method – both short-term and long-term.  Our experiences engineers can find the most cost-effective solution for your application and our professional service group can insure a successful installation and start-up.

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