Sensor for detecting the instantaneous concentrations of a...

Details Sensor for detecting the instantaneous concentrations of a... Sensor for detecting the instantaneous concentrations of a...

1999-10-06

2003-07-15

Warden, Jill (Department: 1743)

Chemical apparatus and process disinfecting, deodorizing, preser

Analyzer, structured indicator, or manipulative laboratory...

Means for analyzing gas sample

C422S083000, C422S094000, C422S098000

Reexamination Certificate

active

06592823

ABSTRACT:

The invention relates to a sensor for detecting the instantaneous concentrations of a plurality of gas constituents in a gas, in particular for exhaust-gas measurement, and combustion exhaust gases from incineration plants for fossil or biological fuels or waste or from combustion engines. For example, automobile engines operating on the diesel principle contain harmful substances which represent environmental pollution. In particular, nitrogen oxides, as toxic and environment-endangering substances, are at the center of public interest and should be removed as completely as possible from the combustion exhaust gases in order to avoid environmental pollution. Examples of such nitrogen oxides are NO, NO
2
, NO
3
, N
2
O
3
, N
2
O
4
and N
2
O
5
. These are known from the textbook literature, but NO and NO
2
are of particular importance.
A diesel exhaust gas can, according to the reference Kraftfahrtechnisches Taschenbuch, Robert Bosch GmbH, 1991, p. 513, have the following compositions:
NO
x
 50-2500 ppm
HC
 50-500 ppm C
1
CO
100-2000 ppm
soot
 20-200 mg/m
3
H
2
O (steam)
 2-11 vol %
O
2
 2-18 vol %
CO
2
 0-16 vol %
N
2
remainder
HC denotes hydrocarbons, H
2
O is in the form of steam, and the concentrations are based on volumes. The temperature is 100-750° C., and the pressure, which is not quoted, is assumed to be from 1 to 1.05 bar. SO
2
may additionally be present.
The nitrogen oxides NO
x
are frequently reduced using ammonia or ammonia-releasing substances, such as urea, ammonium carbonate, ammonium hydrogen-carbonate, ammonium cyanate and others. Urea can be fed in, for example, in a 30% strength aqueous solution, and NH
3
in the form of a gas. The reduction proceeds in accordance with the following reaction equations:
4 NO+4 NH
3
+O
2
→4 N
2
+6 H
2
O
NO+NO
2
+2NH
3
→2 N
2
+3 H
2
O
2 NO
2
+4 NH
3
+O
2
→3 N
2
+6 H
2
O
In order to convert the nitrogen oxides as completely as possible, an equimolar or higher proportion of ammonia is advantageously added. The molecular weights are 17 for NH
3
, 46 for NO
2
, 62 for NO
3
and 54 for (N
2
O
5
)/2. For a stoichiometric reaction, about three times the proportion by weight or a single proportion by volume of ammonia, based on NO
x
, is needed. If a 3% excess of ammonia is assumed with the abovementioned composition with on average 2000 ppm of NO
x
, 60 ppm of ammonia remain in the exhaust gas. If this is not to be exceeded, the measurement must still be able to detect 60 ppm with a reliability of 10 ppm. The measurement must still function reliably at exhaust gas temperatures of from 100 to 750° C. Furthermore, soot or ammonium salt dusts must not interfere with the measurements, and the sensor must not be affected by corrosion caused, for example, by sulfur oxides.
Incorrect measurements entail the risk of a relatively large excess of one of the environmental pollutants ammonia or nitrogen oxide being present in the exhaust gases. Both are very undesired in the environment. Attempts are being made to reduce the concentration of nitrogen oxides to below the legally permissible level and at the same time not to add any excess ammonia.
In DE-A-3 721 572, the nitrogen oxide (NO
x
) concentration in the exhaust gases of an engine are measured for selective catalytic reduction of NO
x
and, depending on the measured NO
x
concentration, fed to a catalyst NH
3
for conversion. At least 75% of the regulation of the amount of NH
3
is effected by the engine load data, while the remainder is regulated depending on the NO
x
concentration measured in the exhaust gases. It is hoped that this will improve the sluggish regulation of the NH
3
supply known hitherto by measuring the NO
x
concentration after the catalyst in non-steady-state operation, where the power and speed of the engine and thus the NO
x
concentration in the raw emission from the engine change rapidly. This reaction time is given as about 1 minute, while the addition of ammonia remains constant for a different amount of exhaust gas and a different exhaust-gas composition, since the sluggish sensor has not yet detected the change.
EP-B-0 447 537 furthermore discloses that, for operation of an oxidation catalyst, the NH
3
portion must be as low as possible in order to achieve the catalytic action for destruction of dioxins.
The journal “Sensors and Actuators”, B4, 1991, page 530, gives a response time of 30 s for the commercial SOLIDOX-NH
3
system. Here too, the measurement time is too long for adequate regulation of a non-steady-state operating mode.
U.S. Pat. No. 2,310,472 discloses analyzing automobile exhaust gases by burning them catalytically and measuring the temperature increase as an increase in the resistance of the combustion catalyst. This is carried out by means of a Wheatstone bridge. The catalyst used is a cerium oxide-coated platinum wire or a filament of catalytic material, such as platinum. The addition of ammonia for the catalytic NO
x
reduction of the automobile exhaust gas is not mentioned.
U.S. Pat. No. 2,583,930 discloses analyzing combustible gases or vapors by burning them catalytically. The catalyst proposed is a platinum wire or a platinum/rhodium wire whose change in resistance is measured, again using a Wheatstone bridge. The particular advantage here is the avoidance of drift.
EP-A-0 591 240 discloses a sensor for measuring ammonia. The sensor is sensitive to one gas constituent through a thin layer of platinum or palladium applied to an oxide surface. A combustion reaction, for example, is catalyzed, and the primary signal is the electrical resistance of a semiconductor. At least two sets of electrodes attached to the semiconductor at different distances enable an impairment in the function to be recognized.
U.S. Pat. No. 3,586,486 discloses an analysis of an automobile exhaust gas in which the catalyst employed for the combustion is platinum in the form of a platinum black film in a thickness of 0.0508 mm (0.002 inch). The resistance of the catalytic resistance element depends on the temperature, and the measurement is carried out using a Wheatstone bridge.
The fact that the above-described sensor does not work satisfactorily is noted by U.S. Pat. No. 4,197,089, which proposes a WO
3
film as sensor. This is sensitized for NH
3
by a small amount of a platinum catalyst. The platinum catalyst is in the form of a thin layer under the WO
3
film. The change in resistance is measured by means of a Wheatstone bridge, with the resistance of the WO
3
film dropping owing to the reducing agent H
2
S or NH
3
.
DE-A-4 117 143 discloses analyzing the proportion of NH
3
in automobile exhaust gases from diesel engines. NH
3
is oxidized catalytically, with the increase in the temperature of the gas caused by the evolution of heat being taken as a measure of the NH
3
concentration. The catalyst is located within the honeycomb channel, or alternatively a gas sub-stream can be taken. Furthermore, stoichiometric feed of NH
3
at constant full-load operation is disclosed, but otherwise cycled, super-stoichiometric addition is the subject matter of this publication.
DE-C-3 543 818 describes how an electrochemical ZrO
2
cell functions on this basis for measuring the oxygen concentration in gases. Another, likewise known process for measuring the oxygen concentration is disclosed in an information sheet from Dittrich Elektronik, Bahnhofstrasse 67, 76532 Baden-Baden, which is commercially available together with the oxygen measurement cell.
The slowness of the sensors is a recurring problem since too few measurement data or an excessively long measurement time make regulation of the non-steady-state operating mode more difficult.
Significantly shorter reaction times are obtained with a sensor with which the instantaneous concentrations of oxygen, ammonia and nitrogen oxide in combustion exhaust gases are determined simultaneously. In the sensor, three different zones are provided, in one of which the oxygen concentration is measured and in the other two the concentrat

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