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Last update: 21/07/2017

NOx reduction

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 Fig. 1a
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 Fig. 1b
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 Fig. 1c
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 Fig. 1d
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 Fig. 2

The limit values for NOx emissions continue to decrease as a result of statutory or regulatory requirements. At the same time, the operators of thermal waste treatment plants are increasingly being pressurized to reduce investment and operating costs.


To meet these requirements, MARTIN has developed various concepts in cooperation with partner companies, using which the NOx values downstream of the combustion system are already significantly reduced via primary measures. This is due to the fact that chemical reactions that convert the primarily formed NOx back to nitrogen are promoted as a result of the reduced excess air and consequently higher temperatures in the lower area of the furnace.

 

Fig. 1a shows the excess air for a conventional combustion system setting. The underfire air is set to be slightly overstoichiometric. An excess air rate of approx. 1.8 is achieved by supplying overfire air for flue gas burnout. The NOx content is typically at 400 mg/Nm3. The development in Fig. 1b shows the introduction of a part of the overfire air in the upper furnace area being moved. The NOx content can be reduced to approx. 300 mg/Nm3. This process option, known as LN (Low NOx), is extremely suitable for retrofitting existing plants. However, it can also be implemented in the design of new plants.

 

Fig. 1c shows the process known as VLN (Very Low NOx). During this process excess combustion gases above the rear grate area are extracted and overfire air pressures are reduced. Consequently excess oxygen in the lower part of the furnace is reduced and temperatures are increased. The extracted VLN gas is returned to the upper part of the furnace as mixing gas. This ensures optimal mixing of the flue gases.

 

It has been proven under continuous commercial conditions that NOx values below 250 mg/Nm3 are achieved with the VLN process. These values are reduced to less than 80 mg/Nm3 (all NOx values referred to 11 % O2) by injecting ammonia or urea. A further feature of this process is that a low NH3 slip is adhered to at the same time.

 

The VLN system shown in detail in Fig. 2 also has the following benefits: it reduces the excess air, which has a positive effect on the boiler efficiency, and makes it possible for the size of downstream components to be reduced.

 

Both LN and VLN processes have been tested on an industrial scale over longer periods and are offered by MARTIN for new plants, and also within the framework of plant retrofits.

 

Fig. 1d shows a further option, the process known as VLN-GM (Very Low NOx-Gasification Mode). This option was specially developed for customers preferring gasification to combustion. There is no injection of overfire air in the lower furnace area in this model. Significantly substoichiometric conditions (gasification) are created in the furnace by simultaneously extracting flue gas in the rear combustion chamber area. Mixing gas and overfire air are supplied to the upper furnace area to ensure complete oxidation of the flue gases.