Method of removing oxides of nitrogen from flue gas

A flue gas stream having passed a flue gas desulfurization plant 2 enters a reactor 6 flowing through an activated carbon bed. The flue gas entering the reactor continuously is discontinuously blended with ammonia, thereby achieving a low ammonia content of the flue gas leaving the reactor, while maintaining a high efficiency of nitrogen oxide removal. The duration of ammonia addition and the rate of flow at which ammonia is added to the flue gas stream are controlled as a function of the desired efficiency of nitrogen oxide removal and the ammonia concentration measured in the reactor.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention concerns the removal of oxides of nitrogen (herein 
referred to as "NO.sub.x ") from flue gases and, more particularly, the 
removal of NO.sub.x from a continuous stream of flue gas by reacting said 
flue gas in a bed, a layer or a structure of adsorption means having 
catalytic effects with ammonia (herein referred to as "NH.sub.3 ") as a 
reducing agent. 
2. Prior Art 
West German patent application No. 2 911 712 divulges a two-stage method of 
pollutant removal providing for a first-stage moving bed for the removal 
of a substantial part of oxides of sulfur contained in flue gas and a 
second-stage moving bed containing a granular carbon-containing adsorption 
agent for the catalytic reduction of NO.sub.x to molecular nitrogen 
following the controlled addition of gaseous NH.sub.3 and for the removal 
of further oxides of sulfur by a method of the nature mentioned 
hereinabove. However, if activated carbon is used as a catalyst for such a 
method of NO.sub.x control using NH.sub.3 as a reducing agent, large 
quantities of NH.sub.3 must always be present on and retained by the 
surfaces of said activated carbon, thereby necessitating a high NH.sub.3 
partial pressure implying a high NH.sub.3 concentration in said flue gas 
exceeding the stoichiometric concentration necessary for the conversion of 
the NO.sub.x contained in said flue gas. As soon as the NH.sub.3 
concentration exceeds the activated carbon equilibrium load, substantial 
quantities of NH.sub.3 will therefore pass through said activated carbon 
and will be contained in the flue gas downstream of said activated carbon 
bed. 
All known methods of the nature described provide for a continuous addition 
of the reducing NH.sub.3 to such flue gas upstream of said activated 
carbon bed at a rate depending upon the desired NO.sub.x removal 
efficiency. If conventional methods are used, a high NH.sub.3 content of 
the flue gas downstream of said activated carbon bed is inevitable, if a 
high NO.sub.x removal efficiency such as the removal of more than 70% of 
the NO.sub.x originally contained in such flue gas is desired. 
According to the state of the art hitherto known, if both oxides of sulfur 
and NO.sub.x are removed from flue gas, excess ammonia is retained in a 
separate unit through which activated carbon from the sulfur dioxide 
removal unit is continuously cycled. Said activated carbon is loaded with 
acidic compounds from said sulfur dioxide removal unit and thence 
substantially decreases the NH.sub.3 content of said flue gas. However, if 
activated carbon is exclusively used for NO.sub.x control, the quantity of 
activated carbon loaded with acidic compounds is insufficient for 
effective NH.sub.3 control. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to eliminate the disadvantages 
associated with conventional methods of NO.sub.x removal as described 
hereinabove and to minimize at reasonable expense the NH.sub.3 
concentration downstream of a reactor containing adsorption means even if 
the NO.sub.x removal efficiency is high and, more particularly, in excess 
of 70%. 
According to the teachings of the present invention concerning a method of 
removing NO.sub.x from a continuous stream of flue gas, said flue gas 
preferably having passed a flue gas desulfurization plant is blended with 
NH.sub.3 and thereupon flows through at least one bed or one structure 
consisting of adsorption means having catalytic effects, NH.sub.3 blending 
with said flue gas which flows continuously being discontinuous. Said 
discontinuous blending operation consists of a cycle of two phases, the 
rate at which NH.sub.3 is added to said flue gas changing substantially as 
a first phase of said cycle is followed by a second phase and said second 
phase is followed by a new first phase. Whilst, according to the present 
invention, the rate at which NH.sub.3 is so added is relatively high 
during each such first phase, it is relatively low or even zero during 
each such second phase. 
The present invention teaches therefore that reducing NH.sub.3 is not added 
continuously to the flue gas to be treated as has hitherto been 
conventional practice, but is blended discontinuously with said flue gas 
being emitted continuously prior to the entry of said flue gas into the 
reactor containing adsorption means, said discontinuous blending operation 
being part of a regular cycle comprising two phases as described 
hereinbefore. During the NH.sub.3 blending phase of each such sycle, the 
rate at which NH.sub.3 is added to said flue gas may be higher than in the 
case of conventional continuous NH.sub.3 blending techniques, thereby 
allowing an increase in both the NO.sub.x removal efficiency and the 
NH.sub.3 load of said adsorption means which may be activated carbon over 
the NO.sub.x removal efficiency and the NH.sub.3 load in conventional 
processes, whilst said relatively high rate of NH.sub.3 addition does not 
result in an excessive NH.sub.3 concentration of said flue gas downstream 
of said adsorption means. 
The duration of each such phase of ammonia addition and the rate at which 
ammonia is so added to said flue gas are determined by the desired 
NO.sub.x removal efficiency as well as by the NH.sub.3 concentration 
measured at an appropriate point in the bed of said adsorption means 
inside the reactor containing said adsorption means. 
According to a further aspect of the present invention, the NO.sub.x 
concentration is thence measured in a downstream part, in the direction of 
flue gas flow, of the reactor containing said adsorption means and the 
rate at which NH.sub.3 is added to said flue gas and/or the duration of 
each NH.sub.3 addition phase are controlled as a function of the NH.sub.3 
concentration so measured. The desired NO.sub.x removal efficiency may be 
inputted, as a setpoint, into a device for the control of the rate and/or 
the duration of NH.sub.3 addition. 
It is apparent that it is also possible to control the rate of NH.sub.3 
addition or the duration of each NH.sub.3 addition phase as a function of 
the NH.sub.3 concentration measured as described hereinabove and to preset 
the other variable not so controlled in accordance with the desired 
NO.sub.x removal efficiency. 
During each phase of NH.sub.3 addition to the flue gas to be treated at a 
high rate, said NH.sub.3 rate should at least be twice as high as during 
the second phase of the cycle of NH.sub.3 addition. In view of the 
relatively long duration of flue gas treatment by the method proposed by 
the present invention, the speed of the transition from a relatively high 
rate of NH.sub.3 addition to a relatively low or a zero rate of NH.sub.3 
addition and from a relatively low or a zero rate of NH.sub.3 addition to 
a relatively high rate of NH.sub.3 addition is, for the purposes of the 
present invention, not a critical factor, allowing the use of relatively 
slow actuators and control systems. 
The duration of each phase of NH.sub.3 addition to the flue gas to be 
treated at a high rate is preferably shorter or not longer than the 
duration of each phase of low or no NH.sub.3 addition. The ratio of the 
durations of said two phases may be set between 1:60 and 1:1. 
The method proposed by the present invention allows the use of a fixed bed 
or a moving bed consisting of a granular carbon-containing adsorption 
means. 
The advantages of a discontinuous addition of NH.sub.3 to a flue gas to be 
treated as described herein may be achieved using any adsorption means 
having catalytic effects, if said adsorption means retains substantial 
quantities of the components involved in the NO.sub.x removal reaction 
(NH.sub.3, NO.sub.x and oxygen) by its relatively large surface. Apart 
from a granular carbon-containing adsorption means as mentioned 
hereinabove, zeolite catalysts proposed for selective adsorption for the 
removal of NO.sub.x from flue gas for example by U.S. Pat. No. 4,046,888 
and a paper published in Energie, 37th year, No. 6, June 1985, may for 
instance be used for the application of the method proposed by the present 
invention. Further, molecular sieve catalysts have become increasingly 
important for the selective catalytic reduction of NO.sub.x which is the 
field of the present invention. The advantages of such molecular sieve 
catalysts include high space velocities, high reduction efficiencies and 
high resistance to mechanical wear. The high selectivity of molecular 
sieves attributable to the defined pore sizes of their lattice structures 
is another advantage of such molecular sieves the catalytic effect whereof 
is, as is generally known, explained by their crystal lattice structures. 
According to another aspect of the present invention, a molecular sieve is 
used for the reaction whereby NO.sub.x is removed from a flue gas.

DETAILED DESCRIPTION OF THE DRAWING 
In the accompanying drawing, flue gas flows through a flue gas duct 1 to a 
flue gas desulfurization plant 2 where it is desulfurized and from where 
the flue gas so desulfurized passes through a second flue gas duct 3 to a 
heat exchanger means 4 wherein the temperature of said flue gas is raised 
to the temperature of reaction in a downstream reactor 6 connected with 
said heat exchanger means 4 by a third flue gas duct 5. Said reactor 6 
contains a fixed bed or a moving bed of granular carbon-containing 
adsorption means and preferably activated carbon. If said bed is a moving 
bed, said adsorption means enters said reactor 6 through a top entry 7 and 
is discharged from said reactor 6 through a bottom outlet 8. 
Ammonia (NH.sub.3) supplied through an NH.sub.3 line 9 is blended with said 
flue gas in flue gas 5 at an NH.sub.3 rate of flow controlled by valve 
means 10. Said valve means 10 is alternately opened and closed by a 
controlling device 12, NH.sub.3 addition to said flue gas flowing 
continuously through said duct 5 thereby being a discontinuous operation. 
In the embodiment of the present invention illustrated by FIG. 1 said 
controlling device 11 is a computer such as a microcomputer controlling 
the length of each phase of NH.sub.3 addition to said flue gas and the 
quantity of NH.sub.3 added to said flue gas during each such phase in 
response to a setpoint NO.sub.x removal efficiency inputted manually into 
said controlling device 11 via interface 12 and in response to the 
NH.sub.3 concentration obtained from an NH.sub.3 partial pressure pick-up 
device 13 inside reactor 6, said partial pressure pick-up device being 
arranged at an appropriate point in the downstream part of the bed in said 
reactor 6. The values measured by said pick-up device 13 are converted by 
a transducer means 14, which may be coupled with an analog-to-digital 
converter, into electrical signals proportionate to said measured values 
and transmitted by line 15 to said controlling device 11 for control 
action. 
It may for instrumentation reasons be useful not to measure the NH.sub.3 
concentration but the NO.sub.x concentration (expressed as NO.sub.2) at 
the point of pick-up 13 or downstream of reactor 6 and to provide for the 
control of the discontinuous addition of NH.sub.3 to the flue gas to be 
treated by the technique the principles whereof have been described 
hereinabove. 
As NH.sub.3 is added discontinuously to the flue gas to be treated in flue 
gas duct 5, the NH.sub.3 content of the flue gas leaving said reactor 6 
through a further flue gas duct 16 is very low even if the efficiency of 
NO.sub.x removal is very high and exceeds 70%. 
Typical data illustrating, by way of example, the application of the method 
proposed by the present invention are summarized in Table 1 below. 
TABLE 1 
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Typical Data 
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Rate of untreated flue gas flow 
1,000,000 m.sup.3 /hr. (normal 
conditions, dry) 
NO.sub.x content of untreated flue gas 
1,000 mg/m.sup.3 (normal con- 
ditions, dry), expressed 
as NO.sub.2 
SO.sub.2 content of untreated flue gas 
0 mg/m.sup.3 (normal conditions, 
dry) 
NO.sub.x content of treated flue gas 
200 mg/m.sup.3 (normal conditions 
dry), expressed as NO.sub.2 
NH.sub.3 content of treated flue gas 
35 mg/m.sup.3 (normal conditions, 
dry) 
NH.sub.3 consumption 
330.6 kg/hr. 
1. Example 1 
Total cycle length 
60 min. 
Length of phase 1 5 min. 
NH.sub.3 quantity added in phase 1 
330.6 kg 
Length of phase 2 55 min. 
NH.sub.3 quantity added in phase 2 
0 kg 
Phase 1 to phase 2 NH.sub.3 ratio 
330.6:0 = .infin. 
2. Example 2 
Total cycle length 
30 min. 
Length of phase 1 5 min. 
NH.sub.3 quantity added in phase 1 
110.2 kg 
Length of phase 2 25 min. 
NH.sub.3 quantity added in phase 2 
55.1 kg 
Phase 1 to phase 2 NH.sub.3 ratio 
110.2:55.1 = 2 
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Any person versed in the art will appreciate that the present invention is 
not limited to the aspects, embodiments and examples described hereinabove 
and that numerous other aspects and embodiments and a large variety of 
modifications and variations are within the scope of the novel method of 
NO.sub.x removal divulged herein. Said novel method is more particularly 
not restricted to the use of pure ammonia as a reducing agent and a number 
of substances from which ammonia may be obtained, such as aqueous ammonia 
solution, ammonia carbonate, aqueous ammonia carbonate solution or urea or 
the like solutions of ammonia carbonate, may be used for the application 
of the method proposed by the present invention, such substances producing 
ammonia when they evaporate.