Solution for ammonia monitoring in CEMS and DeNOx applications

The SERVOTOUGH Laser 3 Plus Environmental delivers Tunable Diode Laser sensing precision to ammonia monitoring applications in a low-maintenance, cost-effective platform.

Rhys Jenkins

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Article Summary

DeNOx processes and the importance of measuring ammonia slip. Reducing the NOx emissions created during the combustion processes is a key objective for meeting environmental legislation, and an area of growing importance for power generation and hydrocarbon processing markets.

NOx is a generic term for the nitric oxide (NO) and nitrogen dioxide (NO2) produced when excess nitrogen and oxygen react during combustion, especially at high temperatures.

It is important to effectively control NOx to meet legislative requirements and avoid fines; in addition, effective management has important benefits for the effectiveness and efficiency of a combustion process.

In power plants and various other combustion processes, the removal of waste NOx — referred to as a DeNOx process — is typically achieved by selective catalytic reduction (SCR) or selective non-catalytic reduction (SNCR).

In the SCR process, ammonia (NH3) or NH3 in the form of urea, is injected into the flue gas flow from the combustion process. This reacts with NOx in the flue gas on a catalyst surface to form water (H2O) and nitrogen (N2).

The SNCR process requires a hotter environment and longer pathway for the NH3 or urea to react with the NOx in the flue gas, with temperatures optimised to between 1472-1742°F/800-950°C. Temperature control is important in the process, as low temperatures can cause a surplus of unreacted NH3, commonly referred to as ammonia slip.

Typically measured downstream from the SCR catalyst unit, or downstream of the NH3 injection point for SNCR, the accurate measurement of ammonia slip is of concern for the operation of DeNOx equipment for several reasons.

Firstly, high levels of unreacted NH3 are both wasteful and costly for the plant operator. Ammonia slip may also have a significant impact on deposition and catalyst plugging, as well as potential corrosion of downstream equipment such as the air pre-heater used in power generation. The excessive slip in a coal- fired power plant can create fly ash with high levels of NH3, rendering it unfit for sale as a mineral filler in asphalt or cement plants.

The level of ammonia slip also provides diagnostic information on the proper function of the SCR/SNCR system and the NOx reduction process, enabling operations personnel to accurately predict when the catalyst must be replaced.

Ammonia slip is generally held to < 2 parts per million (ppm) to optimise the process, and this makes it difficult to control without direct measurement, because many parameters influence the slip, such as inlet NOx concentration, fuel composition, catalyst performance, and nozzle plugging.

However, despite the important operational, cost and safety benefits to measuring ammonia slip, the measurement has been difficult to achieve with extractive techniques.

The relatively reactive nature of NH3 means wetted components play a key part in the analyser measurements and must be kept hot and be non-adsorbing. They also suffer from a range of interferents, including the presence of sulfur dioxide (SO2) or water (H2O) formed by the process and high levels of dust.

While extractive systems have traditionally measured ammonia slip, the uncertainty of the measurement due mainly to adsorption of NH3 upon the wetted materials, combined with the growing demand for ultra- low measurements requiring higher sensitivity and a faster response, meant these systems were increasingly unable to meet the demands of the process

Tdl sensing: a superior solution for nh3 measurement
Advanced Tunable Diode Laser (TDL) technology presents a complete and effective solution for accurate ammonia slip control.
Consisting of a tunable diode laser light source, transmitting optics, receiving optics and detector, TDL technology is suitable for in-situ, cross-duct measurements, with a typical system consisting of the laser transmitting module and receiver module mounted across the process duct or flue stack.

The gas concentration information is obtained from the gas absorption line shape, collected by scanning the laser wavelength over the specific NH3 absorption line. If NH3 is detected in the beam path it causes a reduction in the measured signal intensity, detected by a photodiode and then converted to the NH3 gas concentration. Being a spectroscopic absorption measurement technique, TDL effectively counts molecules — or number density of molecules — that fall within the beam.

Because the analyser is installed across a duct or stack, it produces an average concentration reading rather than a point reading that is obtained from an extractive sample. This means that NH3 concentration levels can be measured with much greater certainty in the TDL measurement. The NH3 sample also never leaves the process, so the complications experienced with sample transports in extractive methods are avoided, as well as contaminant problems posed by the dust, heat, and water.

This means not only is the measurement considerably more accurate, but plant operators avoid much of the downtime required for cleaning of extractive systems.

Servomex’s next-generation solution for nh3 measurements
Using TDL absorption spectroscopy analysis, the SERVOTOUGH Laser 3 Plus Environmental is designed to help operators take control of combustion emissions by delivering accurate NH3 measurements in DeNOx processes.

A highly compact gas monitor that can be installed directly into process ducts, and is ideal for monitoring ammonia slip during NOx reduction processes such as Selective Catalytic Reduction (SCR) or Selective Non-Catalytic Reduction (SNCR), controlling the level of ammonia slip to between 2-3 parts per million.

Taking an average measurement of NH3 across the duct, it avoids stratification issues and returns precise readings, even during inconsistent flow conditions. With auto-validation software available to ensure ongoing QA/QC requirements and accuracy, the Laser 3 Plus Environmental provides industry-leading installation flexibility with cost and performance benefits.

In addition to the immediate advantages of TDL technology — a non-depleting measurement which requires no sample conditioning system — the Laser 3 Plus Environmental’s compact design greatly reduces installation time, with intuitive alignment for easy set-up and maintenance, reducing costs at all stages. It also has full ethernet communications for commissioning, diagnostics, and troubleshooting.

The hardware advances of the analyser are matched using advanced Wavelength Signal Modulated Spectroscopy processing software, and eliminating drift over extended operational periods using line-lock technology that automatically tracks the NH3 absorption line from a sealed reference cuvette.

This robust tracking mechanism ensures a highly reliable and accurate operation over long periods, with calibration frequencies extended beyond twelve months for many applications.

The result is a sturdy, general-purpose design, optimised for hazardous areas, which ensures the Laser 3 Plus Environmental can handle the most challenging of locations, including particulate-rich environments. It meets ATEX, IECEx and North American hazardous area approvals, and is IP66 rated.

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