Process for the removal of nitrogen oxides from off-gases

The present invention relates to a process for the purification of off-gases containing nitrogen oxides. An off-gas is passed through a reactor in admixture with ammonia or an ammonia precursor over a plurality of spaced catalyst beds under conditions to reduce the nitrogen oxides to nitrogen gas while heat is liberated. After the gas stream has obtained a desired temperature, the gas flow is reversed.

This application is a continuation of copending International Application 
PCT/SU90/00212, filed on Aug. 31, 1990, now abandoned. 
1. Field of Art 
The present invention relates to processes for purification of off-gases 
containing nitrogen oxides so as to deal with the problems of atmospheric 
pollution from toxic contamination by nitrogen oxides (NO.sub.x). The 
present invention is applicable to the neutralization of nitrogen oxides 
in the off-gases resulting from various production processes. 
2. State of Art 
A number of processes for purification of off-gases to remove nitrogen 
oxides are known in the art. These processes are based either on 
absorption of nitrogen oxides by liquid absorbents, or on their conversion 
(reduction) into harmless compounds (water vapor and nitrogen). The most 
widely employed processes for reduction of nitrogen oxides use ammonia 
(so-called selective catalytical reduction - SCR). The SCR processes make 
it possible to carry out the reduction of nitrogen oxides to elemental 
nitrogen and water vapor in the presence of oxygen contained in the 
off-gases. The SCR processes are carried out either in a bed of granular 
catalyst, or with the use of honeycomb block-type catalysts. The reactions 
for the reduction of nitrogen oxides are conducted at a temperature above 
the temperature at the beginning of the reaction so as to exclude the 
formation of ammonium salts. The accumulation of salts in considerable 
amounts can result, due to their subsequent decomposition, in 
deterioration of the catalyst and reduction of its service life. These 
constraints make it necessary to maintain the gas temperature at the inlet 
of the catalyst bed equal to at least 180.degree. C. Ammonia is introduced 
into the gases being treated, the gases having been heated to a 
temperature within the range of from 180.degree. C. to 320.degree. C., 
immediately before passing the mixture to the catalyst bed (Handbook of 
Nitrogen Industry Worker, Zhavoronkov N. I. et al., 1986, "Khimiya" 
Publishers, Moscow, vol. 1, p. 220-221). 
Frequently, the temperature of the off-gases containing nitrogen oxides 
does not exceed 40.degree.-50.degree. C. This takes place after an 
absorption purification of the gases from nitrogen oxide, or after a wet 
sulphur purification of flue gases. To carry out a SCR process, the 
off-gases must be heated to the reaction initiation temperature. The gases 
being purified can be heated by means of mixing with hot flue gases, or by 
the recovery of the heat of the purified off-gases in heat-exchanging 
apparatus. 
The heated gas being purified is mixed with ammonia, whereafter the mixture 
is passed through a catalyst bed, wherein a further elevation of 
temperature occurs as a result of the heat of the reaction between 
nitrogen oxides and ammonia. Then the hot purified gas is passed through a 
heat-exchanger wherein it gives its heat to the initial gas being 
purified. The cooled purified gas leaving the heat-exchanger is vented to 
the atmosphere (Chemie-Ingienier-Technik, 1987, Bd. 59, N8, D. Deggin, J. 
Poller, K. Weinzierl. Die Bedeitung der Chemietecnik fur fortgeschritene 
Technologien zur Stromerzeugung, ss. 629-636). 
The realization of this process necessitates the availability of 
sophisticated and cumbersome heat-exchanging means and considerable rates 
of consumption of an additional fuel for heating of the feed being 
purified. Furthermore, in carrying out this process difficulties appear in 
maintaining high degrees of purification when concentrations of nitrogen 
oxides in the gases being neutralized vary. An increase in the volume of 
the off-gases can result in cooling of the regions of the catalyst bed 
adjacent to the gas inlet area and deposition of ammonium salts thereon 
which consequently reduces catalyst service life to 12-16 months. As a 
result of a low concentration of nitrogen oxides (NO.sub.x) the content of 
the residual ammonia in the purified gas might increase due to decreasing 
temperature and a shift of the stoichiometric ratio between nitrogen oxide 
and ammonia. In SCR processes carried out on a commercial scale, 
accumulation of ammonium salts due to the reaction between the residual 
nitrogen oxides and ammonia in the regions of the gas ducts with a low 
temperature is observed. To avoid accumulation of ammonium salts in 
considerable amounts, additional washing of the gas ducts is usually 
performed. 
SCR processes for the removal of nitrogen oxides from off-gases result in 
an insufficiently high degree of purification (usually not over 90-95%). 
The reasons for such low degrees of purification are an insufficient 
intermixing of the introduced reducing agent (ammonia) with the off-gas 
and insufficiently high temperatures within the catalyst bed, wherein 
nitrogen oxides enter into reaction with ammonia. 
Therefore, the removal of nitrogen oxide from off-gases by known 
catalytical processes does not at the present time, facilitate the 
achievement of high technical and economic parameters of the process due 
to the high power consumption for carrying out these processes, a reduced 
service life of the catalyst and insufficient purification. 
DISCLOSURE OF THE INVENTION 
The present invention is directed to a process for the removal of nitrogen 
oxides from off-gases which would ensure a high degree of purification on 
the order of 99% irrespective of variations in the concentration of 
nitrogen oxides in the off-gas and at minimum power consumption, a long 
service life of the catalyst, and the substantial absence of ammonia in 
the purified gases. 
This object is accomplished by passing the off-gases through a reactor in 
the presence of ammonia or a precursor thereof at a temperature within the 
range of from 180.degree. C. to 600.degree. C. to give a purified gas 
wherein, according to the present invention, the reactor comprises at 
least two adjacent spaced catalyst beds with an intervening space free of 
catalyst between the adjacent beds and ammonia or a precursor thereof is 
introduced in the space between the adjacent beds and admixed with the 
off-gas after the off-gas has passed through at least one of said beds in 
the reactor, said admixture then passing through at least the next 
adjacent bed, and when the gas mixture at a point which is at a distance 
from the point of introduction of the off-gas into the reactor equal to 
10-90% of the total length of the reactor, the gas mixture has attained a 
temperature in the range of 30.degree. C. to 400.degree. C., the direction 
of flow is reversed. 
Nitrogen oxides are formed in the production of nitric acid, combustion of 
different kinds of fuel and in some other industrial processes. Their 
content in the off-gases ranges from -10 g/m.sup.3 (after absorption in 
the production of nitric acid) to 0.6-1.5 g/m.sup.3 (in combustion of a 
fuel). 
Water vapor is also present in the off-gases in addition to nitrogen 
oxides. In certain cases the content of water vapor corresponds to the 
pressure of a saturated water vapor at a temperature of 20.degree. C. to 
40.degree. C. Due to the interaction of nitrogen oxides, ammonia and water 
vapor at a temperature of less than 180.degree. C. ammonium salts 
(ammonium nitrites and nitrates) can form and accumulate. The subsequent 
decomposition of ammonium salts frequently is extensive and can have a 
detrimental effect on the performance of the catalyst. For this reason, 
the process of purification of the off-gases by removing nitrogen oxides 
should be conducted at a temperature of not lower than 180.degree. C. 
In addition to the reaction of NO.sub.x with ammonia (reduction of nitrogen 
oxides) on catalysts suitable for carrying out SCR processes, oxidation of 
ammonia by oxygen contained in the off-gases also occurs. At a temperature 
below 350.degree.-500.degree. C. the rate of the reaction of oxidation of 
ammonia is considerably less than the rate of its interaction with 
NO.sub.x. At a temperature of above 600.degree. C. a greater portion of 
ammonia will be oxidized by oxygen and will not react with nitrogen oxides 
which could result in a reduction in the degree of purification. 
Therefore, the temperature range of 180.degree. C. to 600.degree. C. of 
the catalytical process for the removal of nitrogen oxides from off-gases 
using ammonia is optimal for nitrogen oxide-containing gases formed in a 
number of different production processes. 
Examples of catalysts useful for purification of the off-gases by reduction 
of nitrogen oxides are those based on precious metals, e.g. palladium; or 
oxide catalysts having as their active component oxides of vanadium, iron, 
copper, zinc and other metals or mixtures thereof. 
Catalysts for reduction of nitrogen oxides with ammonia are, as a rule, 
formed of granules of different shapes (sphere, cylinder, ring) or of 
blocks with a honeycomb structure. The use of honeycomb-structure blocks 
considerably lowers the hydraulic resistance of the catalyst bed (by 
10-100 times). 
Reducing agents for nitrogen oxides can include gaseous ammonia or a 
derivative of ammonia, for example, ammonium water or an aqueous solution 
of urea. 
As mentioned hereinbefore the reactor is divided into at least two adjacent 
spaced beds with an intervening space substantially free of catalyst 
between the adjacent beds and ammonia or a precursor thereof is introduced 
in the space between the adjacent beds, and is mixed with the off-gases 
which have first passed through at least one of the catalyst beds, said 
admixture then passing through at least the next adjacent bed. The 
reaction of ammonia with nitrogen oxides on the catalyst results in the 
evolution of some heat. As a result, a temperature profile is formed, 
which moves through the catalyst bed in the direction of flow of the gas 
being purified. The propagation of the liberated heat along the catalyst 
bed results in heating of the regions of the catalyst bed adjacent to the 
outlet of the purified gas. If a gas mixture is passed for quite a long 
time in one direction, the catalyst bed will liberate the heat accumulated 
in it and a temperature equal to the inlet temperature will prevail along 
the entire length of the catalyst bed with the result that the degree of 
removal of nitrogen oxides will become substantially equal to zero. In 
order to retain the heat released during the reduction reaction the 
direction of flow of the gas being purified through the catalyst bed is 
reversed. The reversal of the direction of flow of the gas being purified 
is generally carried out when the gas mixture reaches a temperature in the 
range of about 30.degree.-400.degree. C. at a specified distance from the 
point of inlet of the off-gas into the reactor within the range of 10 to 
90% of the total length of the reactor. 
After the reversal in the direction of flow of the gas being purified, the 
heat accumulated in the catalyst bed regions adjacent the outlet (formerly 
the inlet) is employed for heating the incoming off-gas. After the heating 
of the gas by contact with the hot catalyst, a reducing agent (ammonia or 
a precursor thereof) is introduced. The gas mixture is then passed through 
the subsequent parts of the reactor and heat liberated by the reduction 
reaction accumulates in the previously cooled regions of the reactor. In 
this manner, by periodically reversing the direction of flow of the 
off-gas in the reactor, it is possible to maintain, at a relatively low 
inlet temperature of the off-gas (e.g. of 20.degree.-50.degree. C.) and an 
outlet temperature of the purified gas (usually 30.degree.-400.degree. 
C.), a required temperature in the central portion of the reactor for an 
effective performance of the SCR process (i.e. 180.degree.-600.degree. 
C.). 
It is the most advantageous to introduce the ammonia in a zone of maximum 
temperature. This makes it possible to substantially eliminate the 
formation of ammonium salts. For this purpose, the reactor should be 
preferably divided into two equal parts or in close volume proportions and 
ammonia introduced between these parts. 
One of the reasons for a reduced service life of the catalyst is variation 
in temperature in some regions of the reactor which can result in a local 
deposition of ammonium salts. It has been experimentally found that upon a 
periodic reversal of the direction of flow of the off-gas, the time of the 
formation of such regions is equal to several hours. During the time the 
gas being purified flows in one direction (usually not longer than 60 
minutes), the temperature drops in the regions of the reactor that have 
not had time to reach their maximum temperature with a corresponding 
reduction in purification in those regions. The intermixing of the flowing 
gases containing nitrogen oxides with ammonia in the space between the 
catalyst beds generates heat and eliminates the temperature drop in 
different points of the catalyst bed cross-section which were formed in 
the previous direction of flow of the gas being purified. 
From a processibility standpoint, it is preferable to divide the reactor 
into three or four parts. However, as the number of parts of the reactor 
is increased, the hydraulic pressure is increased, the circuit of 
introduction of ammonia becomes more complicated and, the aerodynamic 
conditions are impaired since the length of individual catalyst beds 
becomes small and it is very difficult to ensure uniformity of the gas 
flow in such catalyst beds. If these process difficulties could be 
eliminated, the number of catalyst beds would be generally unrestricted. 
The creation of optimal temperature conditions of operation of the reactor 
and prevention of overheating or cooling of the reactor with the flow rate 
and composition of the off-gas fluctuating with time is achieved by 
periodically reversing the direction of flow of off-gas when the gas 
mixture attains a predetermined temperature (30.degree.-400.degree. C.) at 
a fixed distance from the point of introduction of the off-gas into the 
reactor in the range of from 10 to 90% of the total length of the reactor. 
The lower limit of the above-specified temperature range of the gas mixture 
corresponds to the temperature of the off-gas close to its introduction 
into the reactor, that is after it passes through 10% of the total length 
of the reactor. The upper limit of the gas mixture temperature corresponds 
to the temperature it achieves close to its exit from the reactor, that is 
after it passes through 90% of the total length of the reactor. Where the 
temperature of the gas mixture exceeds 400.degree. C. because of the heat 
obtained from the reactor, the degree of purification by the reaction is 
lowered. 
To accumulate heat and create the best conditions for distribution of the 
gas mixture over the cross-section of the reactor, it is advisable that a 
gas-permeable mass of an inert material be positioned in the spaces 
between the catalyst beds. Such materials include, for example, ceramic or 
porcelain Raschig rings, or crushed quartz. The inert material also 
prevents the catalyst from cooling during the purification process, 
extending its service life. 
The off-gases from production processes have varying contents of nitrogen 
oxides and the heat evolved as a result of their interaction with ammonia 
may be insufficient for the creation, in the catalyst bed, of a 
temperature zone necessary for a significant conversion of nitrogen 
oxides. In this case it is advisable to pass the off-gases through the 
reactor in the presence of a hydrocarbon fuel employed in an amount of up 
to 5 kg per 1,000 m.sup.3 of the off-gas. The fuel components are oxidized 
on the catalyst, liberating heat, and ensuring the required level of 
temperature for the reduction reaction. A fuel consumption rate over 5 
kg/1,000 m.sup.3 of the off-gas is inexpedient, since it results in higher 
rates of power consumption and overheating of the catalyst. Such fuels can 
include, for example, vapor of solvents (acetone, xylene), combustible 
gases (propane, butane), or kerosene vapor. 
As compared to known catalytical processes, the process for removal of 
nitrogen oxides from off-gases according to the present invention makes it 
possible to increase the degree of purification of gases up to 98-99.9 
percent, to extend the life of the employed catalysts by 30-60 percent and 
to considerably lower the power consumption for the purification process. 
As compared to the known methods, the process according to the present 
invention is simple as regards the procedure and equipment employed and is 
applicable to purification of gases irrespective of the considerable 
variation in volumes of the gases or in the concentrations of nitrogen 
oxides therein. According to the process of this invention, the 
purification is carried out in a single apparatus without using any 
special heat exchange equipment, thus reducing metal-intensity of the 
production units by 2-5 times as compared to conventional processes. An 
effective use of the heat from chemical reactions in the process according 
to the present invention makes it possible to avoid fuel consumption for 
pre-heating of the off-gas with a temperature of 5.degree.-150.degree. C. 
The use of the process according to the present invention makes it possible 
to neutralize excess (as compared to the stoichiometrically required) 
ammonia which occurs as a result of decreasing the off-gas volume or 
lowering concentrations of nitrogen oxide in the off-gas. This virtually 
eliminates the appearance of ammonia at the outlet. The process according 
to the present invention ensures observance of the requirements of the 
modern sanitary norms regarding the degree of removal of nitrogen oxides 
from the off-gas and the content of ammonia in the purified gas. The 
process according to the present invention makes it possible to ensure the 
required degree of purification using various types of catalysts suitable 
for carrying out SCR processes. 
The process for the removal of nitrogen oxides from off-gases according to 
the present invention is simple and can be performed in the following 
manner. 
The purification is effected in a reactor containing at least two adjacent 
spaced catalyst beds. Before the beginning of purification, the reactor is 
heated to the temperature of initiation of the reaction between nitrogen 
oxide and ammonia or a derivative thereof. The off-gas is passed through 
the reactor at a speed sufficient to heat it as a result of contact with 
the catalyst. After passing through one or more catalyst beds of the 
reactor the gas being purified has been heated to a temperature at which 
the reaction of nitrogen oxides with ammonia occurs at a high speed. Then, 
ammonia is introduced into the space between the adjacent catalyst beds in 
a stoichiometrically required amount. The introduction of ammonia is 
effected, for example, by purging into a perforated tube or through a 
distribution tubular grate. After mixing of the off-gases with ammonia the 
mixture is passed through one or more subsequent catalyst beds. The 
gaseous mixture reacts on the catalyst, liberating heat. The liberated 
heat warms up the regions of the catalyst bed adjacent to the outlet of 
the purified gas from the reactor. The purified gas is vented to the 
atmosphere. 
The temperature of the off-gas changes as it is passed through the reactor. 
First it is heated and then reacted with ammonia, liberating heat, and 
finally it is cooled by transferring its heat to the catalyst. When the 
gas mixture reaches a temperature of 30.degree.-400.degree. C. at a 
distance of 10-90% of the total length of the reactor from the inlet point 
of the off-gas, the direction of flow of the gas being purified is 
reversed by a switching-over means or with the help of a system of gas 
valves. In so doing, the catalyst bed or beds formerly acting as outlet 
bed or beds become inlet parts. The heated parts of the reactor (one or 
more) serve to preheat the off-gas. After introduction of ammonia between 
the adjacent catalyst beds the mixture of ammonia with the off-gas is 
passed sequentially through one or more catalyst beds, wherein the gas is 
purified by removal of nitrogen oxides. Thereafter, the purified gas is 
vented to the atmosphere. The direction of flow of the off-gas is again 
reversed on attainment of a given temperature (30.degree.-400.degree. C.) 
by the gas mixture at a selected distance (10-90% of the total length into 
the reactor). Further purification steps are effected by regularly 
reversing the direction of flow of the gas being purified through the 
reactor. 
When using an inert material, such material is placed in the spaces between 
catalyst beds, i.e. on a fire grate. Ammonia is fed directly into the 
inert material and the process is further conducted as described 
hereinabove. 
At a concentration of nitrogen oxides of, for example, from 0.7 to 2.0 
g/m.sup.3 the purification of the off-gases is preferably conducted in the 
presence of a hydrocarbon fuel. The hydrocarbon fuel can be introduced 
either together with off-gases or between the adjacent catalyst beds.

For a better understanding of the present invention, some specific Examples 
are given hereinbelow. 
EXAMPLE 1 
An off-gas with a concentration of nitrogen oxides of 6 g/m.sup.3 was 
passed into a vertically disposed reactor loaded with a bed of granular 
vanadium oxide catalyst to a total depth of 1.0 m. The catalyst zone in 
the reactor consisted of two spaced beds of 0.5 depth each. The 
temperature of the off-gas was 10.degree. C., the linear speed at the 
inlet of the reactor was 0.6 m/s. The reactor was previously heated to a 
temperature of 220.degree. C. 
The gas containing nitrogen oxides was passed through the first catalyst 
bed. In the space between the beds gaseous ammonia was introduced by means 
of a system of distribution tubes in a stoichiometrically required amount. 
After mixing, the obtained gas mixture was passed through the second 
catalyst bed, wherein purification to remove nitrogen oxides was carried 
out. 
The purified gas was vented to the atmosphere. The purification of the gas 
was carried out at a temperature of 180.degree. C. to 400.degree. C. in 
the catalyst bed. When the gas mixture reached a temperature of 30.degree. 
C. at a distance of 0.1 m (10% of the total length of the reactor) from 
the point of introduction of the off-gas into the reactor, the direction 
of flow of the gas was reversed. The degree of purification of the off-gas 
was 99.5%. The residual content of ammonia in the purified gases was 10 
ppm. 
EXAMPLE 2 
An off-gas to be purified contained 1.5 g/m.sup.3 of nitrogen oxides. The 
catalyst comprising vanadium pentoxide, titanium dioxide and noble metals 
had a block-shape with through channels. The reactor consisted of three 
beds of 0.5 m depth with beds of an inert material (porcelain Rasebig 
rings of 15.times.15.times.3 mm size) positioned thereinbetween. The 
catalyst beds and beds of the inert material were preheated to a 
temperature of 300.degree. C. 
The off-gas temperature at the point of introduction to the reactor was 
50.degree. C., and the gas speed was 0.5 m/s. The off-gas was first mixed 
with a hydrocarbon fuel (propane) in the amount of 0.5 kg per 1,000 
m.sup.3 of the off-gas. The resulting mixture was passed through the first 
catalyst bed and through the bed of an inert material, whereinto by means 
of nozzles a 25% aqueous solution of urea was introduced in an amount 
equal to half of that stoichiometrically required. The resulting mixture 
was passed through the second catalyst bed, wherein a partial purification 
was effected to remove nitrogen oxides simultaneously with oxidation of a 
hydrocarbon fuel, and then through the second bed of an inert material, 
wherein there occurred intermixing with the remaining amount of the 
stoichiometrically required reducing agent (a 25% aqueoussolution of 
urea). 
While passing through the remaining catalyst bed, the off-gas was finally 
purified to remove nitrogen oxides and fuel components, whereafter it was 
vented to the atmosphere. The temperature at which the gas was purified to 
remove nitrogen oxides was 300.degree.-450.degree. C. The direction of 
flow was reversed when the gas mixture reached a temperature of 
300.degree. C. at the distance of 33% of the total length of the reactor 
from the point of introduction of the off-gas to the reactor. The degree 
of purification was 98%. The residual content of ammonia in the purified 
gas was 2-3 ppm. 
EXAMPLE 3 
An off-gas with a concentration of nitrogen oxides of 10 g/m.sup.3 was fed 
into a reactor loaded with a vanadium oxide catalyst divided into two beds 
of 0.4 m depth each, between which an inert material (ceramic Rasebig 
rings of 25.times.25.times.4 mm size) was placed. The off-gas temperature 
was equal to 20.degree. C., the linear speed at the point of introduction 
to the reactor was 0.7 m/s. The catalyst bed and the inert material bed 
were preheated to a temperature of 240.degree. C. The off-gas was passed 
through the first catalyst bed and fed onto the bed of an inert material, 
whereinto evaporated ammonia water was introduced through a perforated 
distribution plate in a stoichiometrically required amount. The resulting 
vapor gas mixture was passed through the second catalyst bed, wherein 
purification to remove nitrogen oxides was carried out. The temperature at 
which the purification of the gas occurred was 400.degree.-600.degree. C. 
The purified gas was vented to the atmosphere. The direction of flow of 
the off-gas was reversed when the gas mixture reached a temperature of 
400.degree. C. at the distance of 0.6 m (75% of the total length of the 
reactor) from the introduction of the off-gas into the reactor. The degree 
of purification was 99%. The residual content of ammonia in the purified 
gas was equal to 1-2 ppm. 
EXAMPLE 4 
An off-gas to be purified contained 0.6 g/m.sup.3 of nitrogen oxides and 
had a temperature of 30.degree. C. The off-gas was mixed with a 
hydrocarbon fuel (kerosene vapor) in the amount of 5 kg per 1,000 m.sup.3 
of the off-gas and fed at the linear speed of 1 m/s into a reactor loaded 
with granular iron-chromium catalyst in the shape of rings 
25.times.25.times.4 mm size. The reactor was separated into two equal beds 
of 0.8 m depth each and preheated to a temperature of 350.degree. C. The 
mixture of the off-gas with the hydrocarbon fuel was passed through the 
first catalyst bed, wherein the hydrocarbon fuel was oxidized, liberating 
heat. Then gaseous ammonia was introduced in a stoichiometrically required 
amount by means of a system of gas-distribution tubes. After mixing, the 
gas mixture was passed through the second catalyst bed, wherein it was 
purified to remove nitrogen oxides. The purified gas was vented to the 
atmosphere. The purification of the gas was effected at a temperature 
within the range of from 400.degree. C. to 550.degree. C. 
When the gas mixture reached a temperature of 400.degree. C. at a distance 
of 1.44 m (90% of the total length of the reactor) from the point of 
introduction of the off-gas into the reactor, the direction of flow of the 
off-gas was reversed. The degree of purification of the gas was 99.9% and 
the content of ammonia in the purified gas was 1 ppm. 
INDUSTRIAL APPLICABILITY 
The present invention can be useful in industry for neutralization of the 
nitrogen oxides in the off-gases resulting from various industries.