ECO PHYSICS’ NOX NOX


DETECTORS / IN THE FIELD OF BURNERS AND BOILERS


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 as well as odorless gas, and nitrogen dioxide (NO2


), a reddish-brown gas with an irritating odor. Both gaseous pollutants have


diverse negative effects on the environment and human health. They react with ozone molecules in the stratosphere, accounting for the destruction of the ozone layer. They also play a signifi cant role in the formation of “unwanted ozone” (or ground level ozone) that contributes to the creation of photochemically induced smog. They are also a major contributor to the greenhouse effect and are involved in acid deposition and eutrophication. Therefore it is of vital interest to precisely control and reduce NOx


emissions (U.S. Environmental Protection Agency, 1998). NOx Sources Although some of the NOx emissions are of natural origin, a big part is generated by anthropogenic activities. The main contributors to the NOx emissions in the EU are the transport


sector and the industrial sector, of which fossil-fueled power- plants are responsible for the highest quantity of the total NOx


and signifi cant efforts were undertaken to control and reduce NOx , NOx and other emissions signifi cantly.


emissions from stationary sources. In recent years, widespread emissions. Most environmental


protection agencies around the world continue to set higher and stricter standards that require power and heat generating plants to reduce CO2


NOx NOx


Formation is formed by three different mechanisms during combustion: Thermal (Zeldovich mechanism),


Prompt and Fuel-bound (European Commission 2013). A major step forward was made by industrial boiler operators in changing from coal or oil boiler fuels to natural gas. Natural gas- fi red boiler burners can achieve signifi cantly lower emissions; however, the problem is not entirely solved. While fuel bound NOx


is of no further concern with natural gas, the air used for the


combustion process does not cease being a source of nitrogen oxides, because of the prevalent mechanism that supports the formation of thermal NOx


. Lowering NOx Emissions Several strategies can be followed to decrease the formation of thermal and prompt NOx formation include Low-NOx :


reducing the peak temperature, or the residence time or the air content, especially in the combustion zone with the highest temperatures (Spliethoff et al. 1996). Recent developments of specifi c combustion systems which reduce NOx


burner systems with


emissions signifi cantly by lowering the fl ame peak temperature and slowing down the combustion process. The combination of the two techniques can help to reduce NOx


fl ame temperature adjustment and fl ue gas recirculation (FGR). These burner systems reduce NOx


emissions of natural gas-fi red boiler burners from 60-90% (U.S. Environmental Protection Agency, 1998). AET October / November 2017 www.envirotech-online.com


Figure 1: European Union emission inventory report 1990–2011 under the UNECE Convention on Long-range Transboundary Air Pollution (LRTAP)


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