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 colourless as well as odourless gas, and nitrogen dioxide (NO₂), a reddish-brown gas with an irritating odour. 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 significant 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.
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-fuelled power plants are responsible for the highest quantity of the total NOx emissions from stationary sources. 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 and other emissions significantly.
NOx is formed by three different mechanisms during combustion: “Thermal”, “Prompt” and “Fuel-bound”. A major step forward was made by industrial boiler operators in changing from coal or oil boiler fuels to natural gas. Natural gas-fired boiler burners can achieve significantly lower emissions; however, the problem is not entirely solved. Although 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: reducing the peak temperature, or the residence time or the air content, especially in the combustion zone with the highest temperatures. Recent developments of specific combustion systems that reduce NOx formation include low-NOx burner systems with flame temperature adjustment and flue gas recirculation (FGR). The combination of the two techniques can help to reduce NOx emissions of natural gas-fired boiler burners from 60-90%.
Ultra-low NOx solutions
Although modern systems emit less than one-third of the NOx produced by older units, a clean-burning traditional gas burner still produces significant amounts of nitrogen oxides. In some areas, air quality compliance demands boiler emissions to meet Ultra-Low NOx (ULN) standards, typically less than 10-12ppm of NOx in the flue gas. The use of post-combustion control technologies, such as selective-catalytic-reduction (SCR) or selective noncatalytic-reduction (SNCR) is required. Both techniques are based on the injection of NH3 or urea into the flue gas stream, reducing NOx to N2 and lowering NOx-emissions down to 1ppm.
Future Low-NOx Energy Source
The Aachen University of Applied Sciences conducts pioneering studies with hydrogen as a viable alternative gas turbine fuel. The recent study ‘CFD based exploration of the dry-low-NOx hydrogen micromix combustion’ aims at analysing the influence of different geometry parameter variations on the flame structure and the NOx-emissions. The comprehensive goal is to make it applicable for gas turbines. For the determination of NOx-levels, a CLD700ELht was installed. The instrument was directly connected to the hot exhaust gas sample. Internal hot tubing and particle filters in the device allow analyses without pre-processing of the gas sample and prevent water condensation. The cross-sensitivity to the remaining water vapour in the sample is below 0.5% of the measured value. The measurement accuracy is ±0.1ppm (applied measuring range 0-10ppm).
A task for a new generation of chemiluminescence detectors
To ensure compliance with continuously enhancing standards, along with NOx-fee-savings, the use of precise standard-reference chemiluminescence detectors (ref. EN14792) is the key factor. Based upon the experience with widely applied analysers in continuous emission control, low-NOx burner research and SCR-catalyst testing, Eco Physics developed the neoCLD-Series. The two-channel NOx-analysers of the nCLD800-Series detect NO, NO2 and NOx at concentrations ranging from 5ppm to 5000ppm with minimal detection limits averaging from 0.05ppb to 0.5ppm. To ensure accordance with the sample conditions, the nCLD800-Series is modularly designed with “intelligent” components, providing tailor-made solutions. Furthermore, the nCLD822CMhr with its catalytic converter allows a specific assessment of the ammonia slip in SCR-systems.
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