DE3839199A1 - Process for decreasing the nitrogen oxide content of combustion gases - Google Patents

Abstract

Process for decreasing the nitrogen oxide content of combustion gases, in particular flue gases from power station furnaces, using NH3 or other nitrogen-hydrogen compounds. The NH3 or the other nitrogen/hydrogen compounds are first converted into NH2 or NH. These free radicals are introduced individually or in a mixture into the combustion gases where they react free of catalyst.

Description

The invention relates to a method for reducing the nitrogen oxide content of combustion gases, in particular flue gases from power plant boilers, using NH ₃ and / or other nitrogen / hydrogen compounds. - In practice, such processes are briefly referred to as denitrification processes.

Nitrogen oxides, especially nitrogen monoxide, are formed when air, gaseous, liquid and solid fuels are combusted with air. They also form during the combustion of nitrogenous compounds. Although the nitrogen oxide concentrations can be minimized by means of firing measures, concentrations are often still formed which are too high from an environmental point of view. For this reason, NO _x reduction methods have been developed and used for years.

So far, two different reduction methods have prevailed. On the one hand, the direct injection of NH ₃ into the combustion exhaust gases, and on the other hand, the guidance of the combustion gases together with NH ₃ over a catalyst. In both processes, the nitrogen oxides are converted into molecular nitrogen N ₂ . Both processes have some major disadvantages.

The injection of NH ₃ into the combustion gases is tied to a certain temperature range. At temperatures above 1095 ° C, more NO is formed from the injected NH ₃ than reduced. At temperatures below 705 ° C, however, the NO _x reduction reaction freezes. NH ₃ must first be activated. This converts NH ₃ to NH ₂ . This happens through OH radicals as follows:

NH ₃ + OH = NH ₂ + H ₂ O.

The natural concentration of OH radicals is strongly temperature-dependent. At temperatures below 700 ° C the concentration is so low that there is no activation of NH ₃ . An attempt was therefore also made to artificially increase the OH concentration by adding H ₂ O ₂ . H ₂ O ₂ thermally decomposes to OH at certain temperatures. This made it possible to extend the reaction temperature window up to 500 ° C. At temperatures below 500 ° C, this artificial increase in OH concentrations has remained unsuccessful.

For this reason, the combustion gases together with NH _{3 have been passed} over metal oxide catalysts at temperatures below 500 ° C., thus catalytically activating the reduction reaction. This process is particularly costly. All combustion gases have to be led over the catalytic converters. Certain dwell times must be observed. The amounts of catalyst used are therefore very large. In addition, the service lives of the catalysts are short, since they are poisoned by contaminants present in the combustion gases.

In contrast, the invention has for its object to perform the denitrification in such a way that both catalytic devices, over which the entire combustion gas flow would have to be passed, are no longer required, and the temperature constraints which are disruptive when NH _{3 is introduced} into the combustion gases no longer exist .

The solution to this task results from the characteristics of the To sayings 1 to 5, individually or in combination.

Technological explanations of the invention

The invention makes use of the fact that nitrogen oxide NO can be converted directly into nitrogen N ₂ by NH ₂ / NH radicals. There are reactions in which NH ₂ - / NH radicals and nitrogen oxide, NO, are converted directly into nitrogen, N ₂ :

NH ₂ + NO = N ₂ + H ₂ O,
NH + NO = N ₂ + OH.

These two homogeneous gas reactions are independent of temperature. It is therefore possible to reduce nitrogen oxide even in low temperature areas without the direct use of catalysts in the combustion gases. Since NH ₂ - / NH radicals are not stable, they are injected into the combustion gases in the shortest possible way after their formation from stable starting compounds.

The temperatures during the direct injection of NH ₃ into the combustion gases or the necessary high amounts of catalysts when simultaneously guiding NH ₃ and combustion gases over catalysts are avoided by the invention. According to the invention, the formation of the selectively acting reducing agents NH ₂ and NH and thus the activation of the starting materials is removed from the combustion gases and shifted into an upstream process. The raw materials are no longer activated in the combustion gases. This z. B. no more high OH concentrations in the combustion gas necessary or the combustion gases no longer need to be passed over the catalysts. Only the output connections need to be activated. As mentioned, not only NH ₃ , but also other nitrogen-hydrogen compounds can be used as starting compounds. For example, saturated acyclic azanes or unsaturated acyclic azenes can be used.

In the catalytic activation, metal and / or metal oxide catalysts are used. The catalysts are there arranged as close as possible to the outlet nozzles. At the photo chemical activation of the starting compounds, e.g. B. by the Reactions

NH ₃ + h _Y = NH ₂ + H or
NH ₃ + h _Y = NH + H ₂ ,

the radiation devices should also be arranged as close as possible to the outlet nozzles. In terms of quantity, one NH ₂ or one NH molecule is necessary for each NO molecule to be degraded. However, since the NH + NH = N ₂ + H ₂ reaction inevitably takes place during activation and there is no complete conversion of the starting materials into NH ₂ and NH, it is necessary to use a certain excess in relation to the number of NO Working molecules. Regulation of the ratio of NH ₂ or NH in the mixture to NO as a function of the NO concentration is advantageous in order to minimize the consumption of the starting compounds.

Since an optimal mixture of combustion gas and NH ₂ / NH mixture is important for the reduction reactions, the mixture can be applied not only directly but also by means of a carrier gas. Both air and recirculated flue gases can be used as the carrier gas.

When injecting NH ₂ / NH mixtures, the NO reduction is independent of temperature. It can therefore be injected at any point. However, in order to avoid new NO formation, the temperatures of the combustion gases at the injection point should be below 1100 ° C.

Claims (5)

1. A method for reducing the nitrogen oxide content of combustion gases, in particular flue gases from power plant furnaces, using NH ₃ or other nitrogen / hydrogen compounds, characterized in that first the NH ₃ and / or the other nitrogen / water compounds in NH ₂ or NH converted and these radicals are introduced individually or as a mixture into the combustion gases, where they react without a catalyst. 2. The method according to claim 1, characterized in that the radi kale or the mixtures of radicals immediately after formation be introduced into the combustion gases in the shortest possible way. 3. The method according to any one of claims 1 or 2, characterized net that the radicals are produced catalytically. 4. The method according to any one of claims 1 or 2, characterized in net that the radicals are produced photomechanically. 5. The method according to any one of claims 1 to 4, characterized in net that the radicals or the mixtures of the radicals with respect the denitrification reactions introduced in stoichiometric excess will.