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Application Bulletin: Ammonia Slip Monitoring

Application

Monitoring ammonia slip after a selective catalytic reduction (SCR) system Process Description

Nitrogen oxides (NOx) are a significant byproduct of the combustion process in boilers, turbines, combustion engines, crackers, glass furnaces, waste incinerators, and other locations. Because NOx is a precursor of acid rain and contributes to smog formation, controlling these emissions is a key consideration for power plant boilers, refinery heaters, ethylene crackers, cement kilns, waste incinerators, and in many other areas.

To reduce NOx emissions, engineers working for Englehard Industries developed the first Selective Catalytic Reduction (SCR) system in 1957 in a nitric acid plant. The reactor converts NOx species into compounds that are not harmful. Since then, SCRs have become the most prevalent method of nitrogen oxides removal worldwide, used in the majority of installations with NOx emissions regulations. Many countries have enacted federal legislation regulating NOx emission levels in recent years.

SCRs use a catalyst combined with a reductant such as ammonia (NH3) or urea that is injected into the flue gas. The NOx compounds in the flue gas react with the reductant to form nitrogen gas and water vapor, with anywhere from sixty percent to over ninety percent efficiency. The chemical reaction is as follows (with an ammonia reductant):

4 NO + 4 NH3 + O2 -> 4 N2 + 6 H2O




Figure 1: Block Diagram of an SCR


Process Application Overview

Any unreacted ammonia in the flue gas downstream of the SCR is referred to as ammonia slip. It is vital ammonia slip levels are as low as possible, preferably no more than a few parts per million (ppm). Unreacted ammonia downstream can form ammonium sulfate ((NH4)2SO4) or ammonium bisulfate (NH4HSO4), both of which can corrode downstream equipment and cause line plugging. An additional consideration is the overuse of ammonia, which contributes to a potentially large additional expense that is unnecessary. Continuously monitoring for trace ammonia levels after the SCR is needed to address these concerns.

Problem

Extracting a sample downstream of the SCR provides a number of challenges. The first is simply finding a technology capable of detecting less than ten parts per million of ammonia in flue gas. Ammonia’s properties make it difficult to detect continuously. Secondly, the sample temperature of an extracted sample will be dropped from several hundred degrees to ambient. This will cause water vapor in the sample to condense, which would remove the ammonia due to its high solubility. Thirdly, response time is a big factor when a sample is extracted and transported some distance. This is made worse by ammonia’s properties, which make it tend to stick to stainless steel. Finally, the sample gas will be toxic to humans, creating a safety issue with transporting it to an analyzer.

An in-situ method of detection provides its own hurdles. The flue gas after the SCR is at a temperature of several hundred degrees and contains many corrosive compounds as well as water vapor.

Solution

Teledyne Analytical Instruments (TAI) Model LGA-4000 Tunable Diode Laser Absorption Spectroscopy (TDLAS) Analyzer uses light absorption technology to detect trace levels of ammonia in flue gas down to a 0-10 ppm range. The LGA-4000 consists of a transmitter unit and receiver unit that are mounted on either side of the duct. A laser beam from the transmitter unit travels across the duct where it is measured by the receiving unit. Any ammonia present will absorb the laser’s specific light wavelength, which the receiving unit will detect and translate to a volumetric ammonia level in the flue gas.



Figure 2: LGA-4000 Installation


Features/Benefits
  • Rugged in-situ optical design with high-sensitivity tunable diode laser
  • Narrow-band detection eliminates cross-interferences
  • Rated to 932°F (500°C) for ammonia detection
  • Continuous monitoring with response time of less than one second
  • Purge connections on the window, eliminating fouling of windows and associated maintenance
  • Excellent stability – recommended calibration interval of six months
  • Pressure and temperature transmitter inputs to ensure stability over a wide range of operating conditions


Specifications (for ammonia detection)
Ranges: 0-10ppm to 0-1% by volume
Accuracy: ±1% of full-scale at constant temperature
Response Time: less than 1 second
Zero Drift: negligible
Span Drift: ±1% of full-scale
Sample Temperature: up to 932°F (500°C)
Ambient Temperature: -4°F to 122°F (-20°C to 50°C)
Voltage: 110 VAC or 220 VAC, 50 or 60 Hz
Power: 30W max
Outputs: 4-20mA isolated, RS-232
Ingress Protection: IP-65
Note: central processing unit is an optional feature.


References
  1. Armendariz, Al (February 11 2008). The Costs and Benefits of Selective Catalytic Reduction on Cement Kilns for Multi-Pollutant Control. Retrieved August 2008 from www.4cleanair.org
  2. Becker, E. Robert, Daniel W. Ott, David P. Quinn (May 17 2000). The Case for Low Ammonia Slip. Retrieved August 2008 from www.netl.doe.gov
  3. U.S. Department of Energy, Southern Company Services (July 1997). Control of Nitrogen Oxide Emissions: Selective Catalytic Reduction. Retrieved August 2008 from www.fe.doe.gov
  4. Selective Catalytic Reduction. Retrieved August 2008 from www.wikipedia.org


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