Use of the SERVOTOUGH MiniLaser Ammonia can reduce NOx emissions in combustion power plants. Dr Stephen Firth and Rhys Jenkins, photometric product manager and market sector manager, respectively, at Servomex Group, discuss how the MiniLaser Ammonia not only ensures compliance with new legislation, but offers a range of cost, maintenance and by-product quality benefits
The effective reduction of NOx emissions in combustion processes is a key objective for meeting environmental legislations. A generic term for NO and NO2 produced from the reaction of excess nitrogen and oxygen during combustion, especially at high temperatures, NOx is not only subject to legislative requirements for effective control on environmental grounds, but effective management also has important benefits for the success and efficiency of a combustion process.
Ammonia slip control
In power plants and various other combustion processes, waste NOx reduction, referred to as deNOx, is typically achieved by selective catalytic reduction (SCR) or selective non-catalytic reduction (SNCR). In an SCR process, ammonia (NH3) is injected into the gas flow from the combustion process. This in turn reacts with NOx in the flue gas, in the presence of a catalyst, to form H2O and N2. The SNCR process requires the introduction and mix of ammonia or urea with flue gases in a hotter environment with a temperature optimised to 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 plants for a number of reasons. In the first instance, high levels of unreacted NH3 are wasteful and costly for the plant operator. In power generation plants, ammonia slip may also have huge impacts on deposition and plugging of the catalyst, as well as potential corrosion of the air preheater located downstream of the SCR or SNCR process. In coal-fired plants, excessive slip can impact NH3 absorption and create fly ash with high levels of NH3, which is then unsuitable for use as a mineral filler in asphalt control.
There are, however, some more positive benefits to monitoring ammonia slip. Fundamentally, the levels of slip help to indicate the proper function of the SCR/SNCR system in addition to the NOx reduction process, enabling operations personnel to accurately predict when the catalyst has to be replaced.
It is difficult to control ammonia slip without direct measurement. Many parameters influence the slip, such as inlet NOx, fuel composition and catalyst performance, while a carefully controlled ammonia slip of <2ppm is required to optimise the process. Yet despite the important operational, cost and safety benefits to measuring ammonia slip, the measurement has been difficult to achieve with traditional extractive and infrared analysis techniques.
The relative reactive nature of NH3 means analyser measurements are open to potential interference from a range of contaminants, including the presence of SO2 or H2O formed by the process and high levels of dust. While extractive systems traditionally measured ammonia slip, the uncertainty of the measurement, combined with growing demand for ultra-low measurements requiring higher sensitivity and a faster response, meant extractive systems were increasingly unable to meet the demands of the process.
Reduced size for improved performance
The measurement is difficult to achieve with traditional extractive and infrared techniques. However, by using advanced tunable diode laser (TDL) technology such as that offered by the new SERVOTOUGH MiniLaser Ammonia gas monitor, a complete and effective solution for accurate ammonia slip control is now possible.
Consisting of a TDL light source, an optically accessible absorbing medium, transmitting and receiving optics, and a detector, the TDL technology used in the MiniLaser Ammonia is suitable for in-situ cross-stack measurements.
The gas concentration information is held in the gas absorption line shape, which is obtained by scanning the laser wavelength over the specific absorption line. This causes a reduction of the measured signal intensity, which is detected by a photodiode and used to determine the gas concentration. As a spectroscopic absorption measurement technique, TDL effectively counts molecules - or number density of molecules - that fall within the beam.
With the MiniLaser's TDL technology installed in-situ across the system, the NH3-optimised measurement easily identifies ammonia, enabling precise monitoring and subsequent control of NH3 concentration below the requisite 2ppm. As the cross-duct installation gives an average reading rather than a point reading from an extractive sample, NH3 concentration levels can be measured with much greater certainty in the measurement. Furthermore, given the NH3 sample never needs to leave the process, complications experienced with sample transports used in extractive methods are avoided, as well as contaminant problems posed by dust, heat and water. This means that not only is the measurement considerably more accurate, but plant operators also avoid much of the downtime required for the calibration or cleaning of extractive systems.
Reaping the benefits: how to maximise power plant operations
The benefits of Servomex TDL technology make it a vital and beneficial measurement to supplement power plant operations, with more than 300 Servomex units installed for this application worldwide. In addition, the revolutionary design of the MiniLaser makes it 90% smaller and 80% lighter than many comparable TDL analysers.
The MiniLaser's revolutionary compact size and dramatically reduced footprint offers unparalleled installation flexibility, and a host of additional cost and performance benefits. Instead of the time-intensive installation associated with traditional, bulky TDL analysers, the MiniLaser can be installed quickly by just one person.
Simple installation is supported by a new built-in display that negates the need for using a laptop for configuration and diagnostics, while improved advanced optical performance achieves a wider acceptance angle.
To ensure precision alignment from the outset, Servomex has also included a new mounting assembly for multidirection adjustment. When combined with the new quick-release mechanism, fast and accurate reinstallation (without the need for realignment) is achieved every time.
The MiniLaser also includes Servomex's new purge design, which reduces nitrogen and air purge costs by an impressive 90% a year - a significant return on investment when calculating lifetime operational costs.
