Removing ammonia from an ammonia-containing gas mixture

A process of removing ammonia from an ammonia-containing gas mixture including introducing a precursor of nitrogen oxides into the gas mixture at a temperature above the decomposition temperature of the precursor and subsequently contacting the gas steam with a catalyst composition which catalyses the reduction of nitrogen oxides in the presence of ammonia to obtain a gas mixture having a reduced ammonia content.

FIELD OF THE INVENTION 
The invention relates to a process of removing ammonia from an 
ammonia-containing gas mixture by means of oxidation in the presence of a 
catalyst composition which catalyses the reduction of nitrogen oxides in 
the presence of ammonia. The amount of ammonia in such a gas mixture is in 
the range of from 1 ppmv (part per million by volume) to 90% v (per cent 
by volume), and typically the amount of ammonia is in the range of from 
100 ppmv to 2% v. 
BACKGROUND OF THE INVENTION 
European patent application publication No. 393 917 discloses a process of 
removing excess ammonia by oxidizing the ammonia with molecular oxygen in 
the presence of a metal promoted zeolite. According to this publication, 
the oxidation of ammonia with molecular oxygen takes place in the presence 
of an iron promoted zeolite at temperatures above 500.degree. C. However, 
in the presence of the same catalyst the catalytic reduction of nitrogen 
oxides in the presence of ammonia takes place at about 350.degree. C., 
which is a much lower temperature. 
It is an object of the present invention to provide a process of removing 
ammonia from an ammonia-containing gas mixture by means of oxidation which 
can be carried out at substantially the same temperature as catalytic 
reduction of nitrogen oxides. 
SUMMARY OF THE INVENTION 
To this end the process of removing ammonia from an ammonia-containing gas 
mixture according to the present invention comprises introducing a 
precursor of nitrogen oxides into the gas mixture at a temperature above 
the decomposition temperature of the precursor and subsequently contacting 
the gas mixture in a reactor with a catalyst composition which catalyses 
the reduction of nitrogen oxides in the presence of ammonia to obtain a 
gas mixture having a reduced ammonia content.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In the specification and in the claims the expression `precursor of 
nitrogen oxides` is used to refer to a compound from which nitrogen oxides 
are formed by decomposition, wherein nitrogen oxides are suitable selected 
from the group containing NO and NO.sub.2. 
A suitable precursor of nitrogen oxides is nitric acid. A further suitable 
precursor is an aqueous solution of nitric acid, the concentration of 
nitric acid in water is not critical, suitably the concentration of nitric 
acid is between 20 and 60 per cent by mass. On heating at atmospheric 
pressure to a temperature above about 50.degree. C. nitric acid will 
decompose into water, nitrogen dioxide and molecular oxygen. Above about 
150.degree. C. nitrogen monoxide is formed as well. 
Another suitable precursor is a nitrate or a nitrite, or an aqueous 
solution of a nitrate or a nitrite. Suitable salts are those that are 
stable below about 50.degree. C. and that decompose below about 
500.degree. C. and preferably below 200.degree. C.; examples of suitable 
cations of such salts are cobalt, iron, lead, magnesium and nickel. A 
further suitable precursor is nitrous acid or an aqueous solution of 
nitrous acid, which nitrous acid decomposes into nitric acid, nitrogen 
monoxide and water. 
Unless otherwise stated, the temperature at which the nitrogen oxide 
precursor decomposes is the decomposition temperature at the pressure of 
the ammonia-containing gas mixture. To achieve decomposition of the 
precursor dispersed in the ammonia-containing gas mixture, the temperature 
of the dispersed precursor should be above its decomposition temperature. 
This can be achieved by heating the precursor to above its decomposition 
temperature before introducing it in the gas mixture. On the condition 
that the temperature of the ammonia-containing gas mixture is sufficiently 
high, this can also be achieved by allowing the gas mixture to heat the 
precursor to above its decomposition temperature. 
Suitably a catalyst composition is used that catalyses the reduction of 
nitrogen oxides in the presence of ammonia, for example a catalyst 
composition containing a noble metal (platinum, rhodium, ruthenium, 
palladium) or a metal of the iron group (iron, cobalt, nickel), or a 
catalyst composition containing oxides of vanadium, titanium, tungsten or 
molybdenum. The catalyst composition can include a support, for example 
silica or alumina, or mixtures. 
A further suitable catalyst composition is a metal promoted zeolite such as 
described in European patent application publication No. 393 917. 
Another suitable catalyst composition includes titanium and vanadium 
compounds as described in European patent specification No. 217 446. 
The temperature at which the gas steam is contacted with the catalyst 
composition which catalyses the reduction of nitrogen oxides in the 
presence of the above catalysts is in the range of from 120.degree. to 
350.degree. C. 
DETAILED DESCRIPTION OF THE DRAWINGS 
The invention will now be described by way of example in more detail with 
reference to the accompanying drawing showing schematically a layout of 
the process of the invention. 
Through conduit 1 an ammonia-containing gas mixture at a temperature in the 
range of 200.degree. C. is passed towards reactor 3. A precursor of 
nitrogen oxides, in the form of aqueous nitric acid, is introduced through 
distributor 5 in the gas mixture passing through conduit 1. The precursor 
is dispersed in the ammonia-containing gas mixture as the gas mixture 
passes via mixing vessel 7 and conduit 8 to the reactor 3, and because the 
temperature of the ammonia-containing gas mixture is above the 
decomposition temperature of the precursor nitrogen oxides are formed. As 
a result a mixture of the ammonia-containing gas and nitrogen oxides is 
supplied to the reactor 3. 
Reactor 3 is filled with a catalyst composition which catalyses the 
reduction of nitrogen oxides in the presence of ammonia at a temperature 
in the range of from 120.degree. to 350.degree. C. From the reactor 3 a 
gas mixture having a reduced ammonia content is withdrawn, this gas 
mixture is supplied via conduit 9 to chimney 10, through which the gas 
mixture is vented. 
The amount of precursor is determined by the amount of ammonia that has to 
be removed from the ammonia-containing gas mixture. The system as shown in 
the drawing can be provided with means to control the amount of precursor 
supplied to the ammonia-containing gas mixture. For the sake of simplicity 
the system has not been shown in the drawing. 
When the temperature of the gas mixture is below the temperature at which 
the reduction of nitrogen oxides is catalysed, the ammonia-containing gas 
mixture has to be heated. Suitably a heater is installed upstream of the 
location where the precursor is introduced into the ammonia-containing gas 
mixture. 
When the temperature of the ammonia-containing gas mixture is below the 
decomposition temperature of the precursor of nitrogen oxides, the 
precursor has to be heated to above its decomposition temperature before 
it is introduced into the ammonia-containing gas mixture. 
The above described oxidation of ammonia with nitrogen oxides can precede 
the oxidation of ammonia with molecular oxygen, this two-stage process is 
suitably applied when the ammonia-containing gas contains already 
molecular oxygen. In this two-stage process both oxidation reactions take 
place in the same reactor. The oxidation of ammonia with nitrogen oxides 
starts at a lower temperature than the temperature at which the oxidation 
of ammonia with molecular oxygen starts. As the oxidation of ammonia is an 
exothermic reaction, the catalyst composition in the reactor is heated; 
the temperature above which the oxidation of ammonia with nitrogen oxides 
starts plus the adiabatic temperature increase is above the temperature at 
which the oxidation of ammonia with molecular oxygen is supported. The 
two-stage process of removing ammonia from an ammonia-containing gas 
mixture which ammonia-containing gas mixture further contains molecular 
oxygen then comprises introducing a precursor of nitrogen oxides into the 
gas mixture at a temperature above the decomposition temperature of the 
precursor and subsequently contacting the gas mixture in a reactor with a 
catalyst composition which catalyses the reduction of nitrogen oxides in 
the presence of ammonia to obtain a gas mixture having a reduced ammonia 
content, and then interrupting the introduction of precursor into the 
ammonia-containing gas mixture and allowing molecular oxygen to oxidize 
ammonia in the reactor. The application of this two-stage process has two 
advantages. As the temperature at which the oxidation of ammonia with 
nitrogen oxides starts is lower than the temperature at which the 
oxidation of ammonia with molecular oxygen starts, less heat has to be 
supplied to the ammonia-containing gas mixture which implies that a 
smaller heater can be used; and as the oxidation of ammonia is an 
exothermic reaction, the catalyst composition in the reactor is heated and 
with time the temperature in the reactor will rise to a level sufficient 
to support the oxidation with molecular oxygen. Interrupting the 
introduction of precursor is done when the temperature in the reactor has 
reached the level required to support the oxidation of ammonia with 
molecular oxygen.