Selective non-catalytic reduction

The selective non-catalytic reduction ( SNCR short, of English selective non- catalytic reduction) is a secondary process for flue gas denitrification. Thermolysis of ammonia ( NH3) or urea with the gaseous nitrogen oxides ( NOx) is converted to water vapor and nitrogen.

Method

Ammonia ( NH3) is injected as an aqueous solution at temperatures between 900 and 1000 ° C in multiple planes directly into the combustion chamber. It reacts there with nitric oxide ( NO) to nitrogen and water vapor.

Instead of ammonia and urea ( NH2CONH2 ) can be injected. Urea solution is easier and less dangerous to handle, since it is non-corrosive and outgas ammonia under ambient conditions. Urea reacts in the application, such as ammonia. Additionally, carbon dioxide is released.

In the application occurs, an ammonia slip in the flue gas, which can be minimized by appropriate process control, but not prevented.

Process reactions

From ammonia NH 2 radicals are formed at high temperatures. They originate from the reaction of ammonia with hydroxyl and oxygen radicals, which are present at the usual temperatures in the combustion chamber from other reactions.

The NH2 radicals reduce nitric oxide to nitrogen N2:

In the overall reaction, the radical formation reactions occur twice and four times on the reduction reaction:

If urea is used, it is also split into NH2 radicals. The resulting carbon monoxide can be oxidized by oxygen.

Followed by the same reactions as in the case of the ammonia applied.

Ammonia slip and nitrous oxide emissions

NOx reduction using ammonia or urea based on many reaction steps, the equilibrium is dependent on reaction temperature and the initial concentration of the compounds involved. Even with over-stoichiometric ratio of NH3 to NOx can not be completely removed, therefore nitric oxide. Also, a part of the reducing agent is again produced as ammonia from the reaction. As an ammonia slip is also available at the maximum NO reduction rate to temperatures of about 950 ° C. This increases with lower temperatures. Is again increased at higher temperatures produce nitric oxide from the overall reaction. At temperatures above about 1700 ° C also a response to the greenhouse gas nitrous oxide takes place.

For a maximum NO reduction rate and low NH3 and N2O emissions a temperature window is observed. These nozzles are mounted at several levels in the combustion chamber wall often. Based on temperature measurements (e.g. acoustic gas temperature measuring ) those nozzles are open, on which the layer with the optimum reaction temperature in the next. More complex is a single nozzle control, ( eg online CFD) is based on a spatially and temporally highly resolved temperature calculation. In addition, the ammonia slip is minimal only when a sufficient residence time at these temperatures.

The slip is undesirable in the concentrations that occur primarily because of the low odor threshold of NH3 and the aquatic environmental hazards. NH3 is also converted in the atmosphere to form nitric oxide.

Similar procedures

Also, in the selective catalytic reduction (SCR ) of ammonia is necessary. However, the reaction takes place at lower temperatures over a catalyst.