Measuring ammonia slip is an important task for any power station with a NOx reduction system. We talk to Rhys Jenkins, market sector manager at Servomex Group, about the challenges of traditional extractive and infrared monitoring techniques.
What is NOx and how important is it to reduce emissions?
Rhys Jenkins: NOx is the generic term used to describe for nitrogen oxide and nitrogen dioxide, which are produced in the reaction of excess nitrogen and oxygen during combustion, especially at high temperatures. I think the effective reduction of NOxemissions in that process is one of the key objectives for companies looking to meet environmental legislation. But it's about more than that. NOx is certainly subject to strong legislative requirements on environmental grounds, but effective management also has important benefits for the effectiveness and efficiency of a combustion process.
How is NOxreduction usually achieved in power plants?
There are two ways. In power plants and many other combustion processes, waste NOx reduction, which is referred to as deNOx is typically reduced by selective catalytic reduction (SCR) or selective non-catalytic reduction (SNCR). In the SCR process, ammonia (NH3) is injected into the gas flow from the combustion process, which reacts with NOx in the flue gas and, in the presence of a catalyst, forms 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 between 800-950°C. Temperature control is important because low temperatures can cause a surplus of unreacted NH3, commonly referred to as ammonia slip.
How important is it to monitor ammonia slip?
I think the accurate measurement of ammonia slip is a big 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 important impacts on deposition and plugging of the catalyst, as well as potential corrosion of the air pre-heater 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 concrete.
There are, however, some more positive benefits to monitoring ammonia slip. The levels of slip can help indicate the proper function of the SCR/SNCR system and the NOx reduction process, which enables operations personnel to accurately predict when the catalysthas to be replaced.
It's difficult to control ammonia slip without direct measurement, because many parameters influence the slip such as inlet NOx, fuel composition, and catalyst performance, while a carefully controlled ammonia slip of less than 2ppm is required to optimise the process. But, 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.
How difficult is it to measure ammonia slip?
NH3 is relatively reactive, which means 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
The measurement is difficult with traditional extractive and infrared techniques but, by using advanced tuneable diode laser (TDL) technology such as that offered by the Servomex SERVOTOUGH LaserSP 2900 gas monitor, a complete and effective solution for accurate Ammonia slip control can be achieved.
Consisting of a tuneable diode laser light source, transmitting optics, an optically accessible absorbing medium, receiving optics and a detector, TDL technologies are particularly suitable for in-situ cross-stack measurements, with a typical system consisting of the laser emitter module and receiver mounted across the process pipe line or flue stack.
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 then used to determine the gas concentration: being a spectroscopic absorption measurement technique, TDL effectively counts molecules - or number density of molecules - that fall within the beam.
With Servomex TDL technology installed in-situ across the system, NH3 is easily identified enabling precise monitoring of and subsequent control of NH3 concentration below the requisite 2ppm.
