System for removing NOx from exhaust gases

Details System for removing NOx from exhaust gases System for removing NOx from exhaust gases

1999-05-26

2002-09-24

Johnson, Jerry D. (Department: 1764)

Chemical apparatus and process disinfecting, deodorizing, preser

Control element responsive to a sensed operating condition

Control element responds proportionally to a variable signal...

C422S105000, C422S107000, C422S108000, C422S168000, C422S180000, C422S181000

Reexamination Certificate

active

06455009

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a system for and a method of removing NOx from exhaust gases emitted from a combustion apparatus such as an internal combustion engine or the like by reacting NOx with NH
3
to produce N
2
and H
2
O.
2. Description of the Related Art
As shown in
FIG. 23
of the accompanying drawings, one general NOx removal system for removing NOx from exhaust gases includes an NOx remover
202
, which comprises a catalytic converter, for reacting NOx emitted from a combustion apparatus
200
such as an internal combustion engine or the like with NH
3
to produce N
2
and H
2
O. The NOx removal system also has an NH
3
supply controller
208
for introducing NH
3
at a preset rate depending on a load
204
on the combustion apparatus
200
or an output power thereof into a gas passage
206
from the combustion apparatus
200
, and an NOx analyzer
210
connected to a discharge passage of the NOx remover
202
through a sampling line L. The NOx analyzer
210
determines the difference between the concentration of NOx in exhaust gases emitted from the NOx remover
202
and a predetermined reference concentration, and the NH
3
supply controller
208
controls the rate of NH
3
introduced into the gas passage
206
based on the determined difference. For details, reference should be made to Japanese laid-open patent publication No. 64-83816.
According to other conventional NOx removal processes, the concentration of NOx in exhaust gases emitted from the combustion apparatus
200
is sampled upstream of the NOx remover
202
, and NH
3
is introduced upstream of remover
202
based on the sampled concentration of NOx, or combustion apparatus
200
is measured downstream of the NOx remover
202
for the control of rate of NH
3
to be introduced. The NOx removal system shown in
FIG. 23
is more advantageous than those conventional NOx removal processes in that since the concentration of NOx is sampled downstream of the NOx remover
202
, the NOx removal system requires no precision measuring instrument, and any discharge of NH
3
from the NOx remover
202
is minimized.
However, the NOx removal system shown in
FIG. 23
suffers the following problems:
While the concentration of NOx in emitted exhaust gases is sampled by the NOx analyzer
210
, NH
3
contained in the emitted exhaust gases is not measured. Therefore, an excessive introduction of NH
3
cannot be detected, and may be discharged into the atmosphere.
Therefore, the NOx removal system shown in
FIG. 23
is premised not to increase the rate of NH
3
to be introduced when the concentration of NOx in the emitted exhaust gases is lower than a certain value, e.g., 80% of a reference value according to the air pollution regulations. If the rate of NH
3
to be introduced were increased to further reduce the concentration of NOx in the emitted exhaust gases, then an increased amount of NH
3
would be discharged into the atmosphere.
The NOx removal system shown in
FIG. 23
may not necessarily minimize the amount of both NOx and NH
3
discharged into the atmosphere.
Another drawback of the NOx removal system is that when the purifying efficiency of the NOx remover
202
is lowered to the point where the amount of emitted NOx exceeds a predetermined level, the rate of introduced NH
3
is automatically increased even though it is already sufficient, and hence the amount of NH
3
emitted into the atmosphere is increased.
The above drawback may be eliminated if an NH
3
analyzer is added downstream the NOx remover
202
for monitoring emitted NH
3
so that the NH
3
supply controller
208
controls the rate of introduced NH
3
in a manner to keep the concentration of NH
3
at a low level and lower the amount of emitted NOx. However, adding the NH
3
analyzer will increase the size of the NOx removal system and require the NH
3
supply controller
208
to have a more complex control circuit. Even if two analyzers, i.e., an NOx analyzer and an NH
3
analyzer, are added, since they usually have difference responses, it is difficult for the NH
3
supply controller
208
to control the NOx and NH
3
concentrations highly accurately.
NOx analyzers are usually CLD or NDIR analyzers that are highly expensive and have slow responses which do not allow the rate of introduced NH
3
to be highly accurately controlled.
Another conventional NOx removal system has an NOx catalyst and an oxidizing catalyst which are disposed in the exhaust system of a diesel engine, and an apparatus for introducing urea into the exhaust system upstream of the NOx catalyst. The introduced urea produces NH
3
in the NOx catalyst.
In this NOx removal system, the rate of urea to be introduced is controlled according to mapping information (representative of the relationship between engine operating conditions and NOx concentrations) of NOx which is stored in a microprocessor, and the temperature of the NOx catalyst. However, the NOx concentration or the amount of NOx determined from the mapping information may deviate from an actually emitted amount, with the result the NOx removal system may fail to remove NOx efficiently from the exhaust gases, and the introduced urea or NH
3
decomposed therefrom may be emitted from the exhaust system.
Japanese laid-open patent publications Nos. 4-358716 and 7-127503 disclose NOx removal systems in which HC is introduced instead of NH
3
, an NOx sensor is attached downstream of an NOx catalyst, and the rate of HC to be introduced is controlled by a signal from the NOx sensor.
These systems have a poor NOx reduction efficiency because HC is used as a reducing agent. Even if an efficient catalyst temperature range is selected, the NOx reduction efficiency ranges from 40% to 60% at most, and almost half of the introduced amount of HC is discharged. Furthermore, the efficient catalyst temperature range is very limited, i.e., it is 400° C.±50° C. Consequently, these systems may possibly frequently cause HC to be discharged beyond its emission control level in applications where the temperature of exhaust systems vary widely, e.g., automobile engines. To avoid such a possibility, it is necessary to add catalyst cooling and heating devices to the system.
Japanese laid-open patent publication No. 5-113116 reveals a system similar to the above systems except that a basic HC rate determined in advance depending engine operating conditions is corrected on the basis of a signal from the NOx sensor. This system, however, also suffer the above shortcomings.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a system for and a method of removing NOx from exhaust gases emitted from a combustion apparatus by accurately controlling the rate of a reducing agent such as NH
3
and/or urea to be introduced through a simple arrangement and minimizing the emission of NH
3
and NOx.
The above and other objects, features, and advantages of the present invention will become apparent from the following description when taken in conjunction with the accompanying drawings which illustrate preferred embodiments of the present invention by way of example.


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