Measuring NH₃ Slip

With the very low NO_x and ammonia emissions, it becomes a challenge to accurately measure emissions. Measuring ammonia slip is particularly challenging. To meet this challenge several companies are supplying analyzers based on tunable diode laser (TDL) spectroscopy to measure ammonia slip. Yokogawa has a second generation TDL (TruePeak TDL 200). Claimed features are

In-Situ Analysis
Fast Response (2-20 seconds)
Interference Free for most applications
TruePeak measurement capable of measuring under changing pressure, temperature and background
Process Pressures up to 20 Bar
Process Temperature up to 1500ºC
Optical Measurement, no sensor contact with process
Low maintenance (no moving parts)
Flexible Installation Options
On Board Diagnostics

The advantage of TDL and in situ measurement over indirect NO_x measuring methods such as chemiluminesce detection and the ion electrode method is the faster response and elimination of sample line equipment. Sample lines require heated pipes to prevent NH₃ adhesion.

Sick Maihak offers the In-Situ TDLS NH₃ monitor with a minimum range of 0-10 ppm NH₃. It also has an extractive TDLS NH₃ analyzer also with a minimum range of 0-10 ppm.

The potential of the TDL is being recognized. The Electric Power Research Institute (EPRI) is evaluating the accuracy and response of commercially available monitors. The objective of the current project is to establish the response of vendor-provided tunable diode laser monitors in EPRI’s test lab under highly controlled operating conditions to changes in concentration, moisture level, and temperature, and to establish their accuracy and detection limit characteristics. Additional field tests will be conducted to establish monitor operability, reliability, and maintainability characteristics. This project is scheduled to be completed in December, 2013.

The advantages of accurate slip monitoring were documented by Jim Staudt more than a decade ago.

The cost of periodic air heater cleaning, which is unlikely to go away completely with an ammonia monitor, is likely to be reduced with the help of ammonia monitoring technology. If, on average, cleaning is reduced from, say, two times per year to one time per year. This is worth in the range of $20,000 to $40,000 per year (assuming $20,000 per cleaning).

http://www.andovertechnology.com/DOCUMENTS/ICAC2000_Paper.pdf

The chemiluminescent approach used by Thermo Fisher has the advantage of measuring NO_x as well as NH₃. The Thermo Scientific Model 17i Ammonia Analyzer uses the light producing reaction of nitric oxide (NO) with ozone (O₃) as its basic principle. The instrument has three modes of operation, NO, NO_x and Nt. The software subtracts NO from NO_x and NO_x from Nt and provides outputs of NO₂ and NH₃ respectively. The Model 17i analyzer can output NH₃ along with NO, NO₂, NO_x, and Nt to the display or electronic outputs.

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

Applied Analytics supplies a UV based extractive system (OMA 300).

Siemens uses a TDL and cites the low maintenance, fast response time and ability to measure as low as 1-2 ppm.

http://www.industry.usa.siemens.com/automation/us/en/process-instrumentation-and-analytics/process-analytics/pa-case-studies/Documents/P019DeanFinal.pdf

McIlvaine conducted a three-hour webinar on ammonia slip and related issues in October 2009. Experts provided insights on measurement of low levels of NO_x, NH₃ and HCl in the “CEMS for Measurement of Ammonia, SO₃ and Low Level NO_x”

CEMS for Measurement of Ammonia, SO3 and Low Level NOx (Part 1)
105 minutes Password: hth541

CEMS for Measurement of Ammonia, SO3 and Low Level NOx (Part 2)
27 minutes Password: hth541

The bios, abstracts and photos for our “CEMS for Measurement of Ammonia, SO₃ and Low Level NO_x” Thursday webinar can be viewed as follows: October 22, 2009 - Bios, Photos, Abstracts.htm

The individual slides are located in our NO_x Decision Tree as follows:

Richard Himes – EPRI

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Gas Monitoring and Analysis

NO_x Continuing Decision Process For: Gas Monitoring and Analysis