Nitrogen Oxides (NOx) are a by-product of nearly every combustion process. The term "Nitrogen Oxides" is usually used to describe two gases: nitric oxide (NO), a colorless, odorless gas and nitrogen dioxide (NO₂), a reddish-brown gas with irritating odor. They react with stratospheric Ozone molecules and play a significant role in the formation of ground level Ozone. The ground level ozone synthesis leads to photochemically induced smog. Nitrogen Oxides are also a major contributor to the greenhouse effect and also induce acid deposition and eutrophication. A precise control and reduction of NOx-emissions is essential (U.S. Environmental Protection Agency, 1998). Although some NOx-emissions have natural causes, the man-made anthropogenic input is enormous. The main contributors in the EU are the transport sector and the industrial sector, where fossil-fueled power-plants produce the largest quantity of the total NOx-emissions.
NOx is formed by three different mechanisms during combustion: Thermal (Zeldovich-mechanism), Prompt and Fuel-bound (European Commission 2013). A major step forward towards significant NOx-emission-reduction was made by changing from coal-/oil-supplied to natural-gas boilers. While fuel-bound NOx is of no further concern with natural gas, the air used for the combustion process does not cease being a NOx-source, because of the prevalent mechanism that supports the formation of thermal NOx.
There are various approaches to decrease the formation of thermal and prompt NOx: Reduction of peak temperature, residence time, or air content (Spliethoff et al.1996). Low-NOx-burners operate with flame temperature adjustment and flue gas recirculation (FGR). These burners reduce NOx-emissions by lowering the flame peak temperature and slowing down the combustion process. The combination of both techniques helps to reduce NOx-emissions of natural-gas fired boilers and burners by 60-90% (U.S. EPA,1998). In some areas, air quality compliance demands boiler emissions to be further reduced or meet “Ultra-Low NOx” (ULN) standards, which are typically less than 10-12ppm of NOx in the flue gas. In this case, the use of post-combustion control technologies, such as SCR (Selective catalytic reduction) or SNCR (selective non-catalytic reduction), is required. Both techniques are based on the injection of a reducing agent such as NH3 or urea into the flue gas stream, lowering NOx emissions down to only 1ppm (European Commission 2013). To ensure and anticipate current and future norm compliances, the use of precise and reliable chemiluminescence detectors is inevitable. Standards and emission trading are based on a standard reference method, which, for NOx determination from stationary sources, is the chemluminescence principle (EN 14792). Based on the experience in continuous emissions monitoring, but also in research applications, like the Low-NOx-burner-design or the catalyst-testing for SCR, ECO PHYSICS has developed the neoCLD Series. The two-channel, chemiluminescence-based NOx-analyzers detect NO, NO2 and NOx simultaneously from 5 to 5000ppm with minimal detection limits averaging from 0.05 to 0.5ppm. To ensure accordance to the sample conditions, the nCLD800-series is modularly designed. It can be equipped individually, for instance with a pressure regulation, a heated sample line, or an additional catalytic converter, that allows a specific assessment of the ammonia slip in SCR systems (nCLD822CMhr).
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