What is Ammonia Slip?

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What is Ammonia Slip?

Ammonia slip is a term used to describe unreacted ammonia that was intended to react with Nitrous Oxide to scrub out of combustion-based exhaust gases. The ammonia that slips or leaves the scrubbing system is both a pollutant and a financial loss.



Nitrous Oxides (NOx) is a common air pollutant that includes nitrogen oxide (NO) and nitrogen dioxide (NO2). NOx is formed through the combustion of air which contains nitrogen and fuel containing nitrogen. The nitrogen liberated in combustion oxidizes (forms with oxygen) and creates NO2. Stationary sources of NOx include: incineration plants, power plants, industrial boilers, rotary kilns, and turbines.

NO2 is recognized both by the EPA and the Canadian government as leading to respiratory system irritation, acid rain, and damage to vegetation.



To reduce the effect of NOx and NO2 on the environment many combustion sources that produce NOx employ a system to scrub these pollutants out of the exhaust or flue gas. One common method of reducing or scrubbing NOx emissions is Selective Catalytic Reduction (SCR). An SCR utilizes a chemical reaction to break NOx down into harmless byproducts, such as Nitrogen (N2) and Water (H2O).  Ammonia (NH3) is introduced to the NOx gas through a chemical reaction, Nitrogen from the ammonia joins with the Nitrogen from the NOx, and Oxygen from the NOx joins with the Hydrogen from the Ammonia leaving N2 and H2O as byproducts.

Modern SCRs often called De-NOx systems, achieve high efficiency with conversation rates around 95% or higher, but the ammonia that is injected into the reaction and not used in the reaction, “slips” through the system and can be emitted into the atmosphere. This creates both another pollutant and results in product giveaway, increasing plant operating costs.

The best prevention of ammonia slip is to monitor the gas components after the SCR. By monitoring for ammonia and NOx the performance of the SCR can be monitored and adjusted to maximize scrubbing and minimize ammonia losses.

Several analyzer technologies are used to monitor post-SCR performance, these include Tunable Diode Laser and extractive UV Spectroscopy.


Tunable Diode Laser Analyzers (TDL)

Tunable Diode Laser analyzers (TDL), use an absorption spectroscopy technique. A laser is tuned to an absorption band of NH3 and the device analyzes the energy lost from the laser passing through the gas and correlates this to a concentration of ammonia. A TDL system can be mounted directly in the path of the exhaust gasses, called in-situ, or can extract a sample for analysis, called ex-situ or extractive.

In-situ applications create a fast response but since the optical path of the analyzer is in the gas stream it is susceptible to vibration, particulate and dust, fluctuations in pressure and temperature, and misalignment or shifts in the optical path as the duct or stack warms and cools. Some systems will look to use a particulate or dust shield/filter around the optical path to minimize some of these concerns; however, the filter has a scrubbing effect on NH3 as it accumulates particulate.

A further noticeable concern with TDL systems is the inability to perform a validation or calibration. Reference gases cannot be properly applied to the optical path to examine consistent readings across the measuring range and a proper zero reference. This can create unreliable readings from the system and surprises when the system is audited.


Extractive and UV Spectroscopy

To alleviate the concerns around TDL some facilities, employ an extractive system. Extractive systems minimize the gas stream variable because the sample is pulled from the process and passed through a close optical path. However, many systems have long extraction paths slowing the response time and improper temperature control or design can create cold spots along the sample path, which could cause salts or acids to form in the sampling system and damaging the analyzer.

Extractive systems for SCR systems will often favor UV Spectroscopy.  This is a form of absorption spectroscopy in the ultraviolet (UV) spectrum. UV is used instead of Infrared (IR) as it allows for lower detection limits of NO, NO2, NH3, due to less spectral cross interferants, such as H2O.

Unlike a TDL analyzer where a specific frequency for a specific gas is used, a UV analyzer uses a broadband source. This approach allows for multiple gases to be measured including NO, NO2, NH3, and SO2

The removal of the optical path from the gas stream also allows the use of calibration gases to be used to check and adjust the performance of the analyzer, it also allows for tight control of temperature and pressure ensuring consistent and accurate readings.

Our Preferred Solution

At CEM Specialties we provide a variety of analytical solutions including both TDL and UV analyzers. When asked, our preference for monitoring De-NOx applications is a close-coupled hot/wet UV analyzer system.

Utilizing a UV analyzer in a hot/wet configuration like the Fodisch UVA17HW allows for low detection of multiple gases. Multiple gas analysis allows for a more complete picture of the performance of the SCR, including monitoring NO and NO2. Monitoring for NOx in addition to NH3 after the scrubbing provides a picture of scrubbing performance and catalyst health as these values can trend higher if there is poisoning of the catalyst in the reaction.

The UVA17 also has the advantage of long-lasting UV sources averaging 2 – 3 times longer than other UV analyzers, minimizing maintenance and part costs.

A close-coupled hot/wet extractive system means the analyzer is mounted in close proximity to the extraction probe on the stack or duct. This configuration provides quick response time, and the short sample line is easily and completely heated removing cold spots in the sampling system. For high particulate applications, CEMSI utilizes a blowback probe so dust and particulate can be automatically blown off the filter and probe tube.

This configuration is chosen because it minimizes problems experienced by customers and leverages the already discussed advantages.

Multiple gases detected at low levels

Quick response

Reduces dust and particulate clogging and scrubbing

Consistent performance regardless of temperature and pressure shifts

Easy Maintenance with no moving parts since the analyzer uses an eductor instead of a pump

Calibration gas ready

A closed coupled hot/wet extractive system provides low maintenance, quick response and is cost effective, making it our recommendation for post SCR monitoring.

A note on Chemiluminescence:

Chemiluminescence, is used by some facilities but mainly is used by third parties including stack testers to check the performance of an SCR and the SCR monitoring system.

A chemiluminescence analyzer measure the light emitted from a chemical reaction, for Nitric Oxide (NO) light is emitted when NO reacts with Ozone (O3).  Ammonia (NH3) does not have a direct luminescent response, so it is converted to NO using a high-temperature (800 0C) thermal converter before reacting with Ozone.

Ammonia content in the gas stream is calculated by comparing the difference between the total Nitride measurement including the converted ammonia and the measurement with just NO and NO2 and not converting the ammonia in the gas stream.  This is done by either two analyzers or two gas paths in a single analyzer.

The concern with chemiluminescence in this application is quenching. Products of combustion including H2O, CO, CO2 as well as the NH3 in the stream can quench the chemical response making the reading unreliable.