Selective non-catalytic reduction

The selective non-catalytic reduction (short SNCR , from English selective non-catalytic reduction ) is a secondary process for flue gas denitrification . By thermolysis is ammonia (NH ₃ ) or urea with the gaseous oxides of nitrogen (NO _x ) to water vapor and nitrogen implemented.

Procedure

Ammonia (NH ₃ ) is injected as an aqueous solution at temperatures between 900 and 1000 ° C in several levels directly into the combustion chamber . There it reacts with nitrogen monoxide (NO) to form nitrogen and water vapor.

${\displaystyle \mathrm {4\ NO+4\ NH_{3}+O_{2}\longrightarrow 4\ N_{2}+6\ H_{2}O} }$

Instead of ammonia, urea (NH ₂ CONH ₂ ) can also be injected. Urea solution is easier and safer to handle because it is not corrosive and no ammonia is released under ambient conditions . When used, urea reacts like ammonia. In addition, carbon dioxide is released.

${\displaystyle \mathrm {NH_{2}CONH_{2}+H_{2}O\longrightarrow 2\ NH_{3}+CO_{2}} }$

In use, ammonia slip occurs in the exhaust gas, which can be kept low by appropriate process management, but cannot be prevented.

Process reactions

At high temperatures, ammonia forms NH ₂ radicals. They come from the reaction of ammonia with hydroxyl and oxygen radicals , which are present from other reactions at normal temperatures in the furnace.

${\displaystyle \mathrm {NH_{3}+O\cdot \longrightarrow NH_{2}+OH\cdot } }$

${\displaystyle \mathrm {NH_{3}+OH\cdot \longrightarrow NH_{2}+H_{2}O} }$

The NH ₂ radicals reduce nitric oxide to nitrogen N ₂ :

${\displaystyle \mathrm {NH_{2}+NO\cdot \longrightarrow N_{2}+H_{2}O} }$

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

${\displaystyle \mathrm {4\ NO+4\ NH_{3}+O_{2}\longrightarrow 4\ N_{2}+6\ H_{2}O} }$

If urea is used, it is also split into NH ₂ radicals. The resulting carbon monoxide can be oxidized by oxygen.

${\displaystyle \mathrm {NH_{2}CONH_{2}\longrightarrow 2\ NH_{2}+CO} }$

${\displaystyle \mathrm {2\ CO+O_{2}\longrightarrow 2\ CO_{2}} }$

The same reactions follow as when using ammonia.

Ammonia slip and nitrous oxide emission

The NO _x reduction with the help of ammonia or urea is based on many partial reactions, the equilibrium of which depends on the reaction temperature and the initial concentration of the compounds involved. Even with a stoichiometric ratio of NH ₃ to NO _x , nitrogen monoxide cannot therefore be completely removed. Part of the reducing agent also emerges from the reaction as ammonia. So even at the maximum NO reduction rate of around 950 ° C there is an ammonia slip. This increases with lower temperatures. At higher temperatures, nitrogen monoxide increasingly emerges from the overall reaction. At temperatures above around 1700 ° C, a reaction to the greenhouse gas nitrous oxide also takes place.

${\displaystyle \mathrm {NH_{2}+NO\cdot \longrightarrow N_{2}O+H_{2}} }$

A temperature window must be maintained for a maximum NO degradation rate and low NH ₃ or N ₂ O emissions. For this purpose, nozzles are often installed in several levels in the furnace wall. Using temperature measurements (e.g. acoustic gas temperature measurement ), those nozzles are opened that are closest to the level with the optimal reaction temperature . Individual nozzle control, which is based on a temperature calculation with high spatial and temporal resolution (e.g. online CFD), is more complex. Furthermore, the ammonia slip only remains minimal if there is a sufficient residence time at these temperatures.

At the concentrations that occur, the slip is primarily undesirable because of the low odor threshold of NH ₃ and the hazard to water. NH ₃ is also converted to nitric oxide in the atmosphere.

There are additives that can be used to minimize slippage.

Similar procedures

Ammonia is also required for selective catalytic reduction (SCR). The reaction takes place over a catalyst at lower temperatures.

Individual evidence

1. ^ W. Duo, K. Dam-Johansen, K. Østergaard: Kinetics of the gas-phase reaction between nitric oxide, ammonia and oxygen. In: Canadian Journal of Chemical Engineering . 70 (5), 1992, pp. 1014-1020, doi : 10.1002 / cjce.5450700525 .
2. ↑ Product information ERC GmbH: Additives in Carbamine 5722 avoid side reactions, such as NH3-slip ( Memento of the original from March 5, 2016 in the Internet Archive ) Info: The archive link was automatically inserted and not yet checked. Please check the original and archive link according to the instructions and then remove this notice. , accessed April 13, 2015.