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):
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.
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.
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
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