DE3308888A1 - Electrochemical gas analyser for the SO2 content of gases, in particular flue gases - Google Patents

Abstract

In an electrochemical gas analyser for SO2 in (flue) gases comprising a measuring cell (2) through which the electrolyte containing the test gas circulates, a high response speed accompanied by an acceptable measurement sensitivity with a linear dependence of the measurement value on the SO2 content of the gas over a wide range is achieved by continuously and rapidly renewing the electrolyte with a 0.5- to 5-fold volume exchange per minute. The electrolyte draining from the cell is passed through an active carbon cartridge (5) having an air inlet so that the active carbon is at least partly wetted only by liquid, is then fed into a stock vessel (7) and is circulated from that point back to the cell by means of a pump (9) and a feed control device (11). Preferably, an identical second cell through which SO2-free gas is passed is provided as a reference cell (3) in the compensation circuit. <IMAGE>

Description

Jülich nuclear research facility
Company with limited liability

Electrochemical gas analyzer for the SO ~ content in gases, especially smoke gases

The invention relates to an electrochemical gas analyzer for the SO ₂ content in gases, in particular flue gases, with a measuring cell with a measuring electrode indicating the depolarization currents of the gases to be determined and a non-polarizable counter-electrode in the same electrolyte, -—- "- by the Measurement gas inlet is driven to a circulation flow which drives the electrolyte containing dissolved gas through the space between the measurement and counter electrode.

The control of the SO ₂ content in exhaust gases is currently a particularly important metrological task. All known measuring devices available up to now which are based on different principles (infrared, thermal conductivity, UV etc.) _y are either too expensive or can be used in very limited measuring ranges.

The applicant has therefore already produced an electrochemical gas analyzer of the type mentioned above Art developed (DE-OS 31 08 889 A1), the inexpensive and over a wide concentration range is applicable. With this analyzer, electrolyte is already dripped into the measuring cell and used up Electrolyte continued. Dilute sulfuric acid copper sulfate solution is used as the electrolyte.

It has now been shown, surprisingly, that COPY

The linearity of the dependence of the measured values for the SO ₂ content and the signal stability can be improved and the response speed can be increased if a very lively electrolyte exchange is ensured in the cell, in which at least about 50% of the electrolyte volume in the cell is exchanged per minute . Since the signal level decreases with increasing electrolyte exchange, the electrolyte exchange should also be limited upwards. At the same time, the lively electrolyte exchange makes recycling necessary.

The gas analyzer according to the invention is accordingly essentially characterized by a circuit for the continuous renewal of the electrolyte in the cell with a 0.5 to 5-fold, in particular 1 to 2.5-fold, volume exchange per minute with a pump, a feed control element at the entrance of the cell and a SO_ filter at the exit of the same.

The SO_ filter is in particular through a Activated charcoal cartridge provided with air access is formed during the electrolyte inlet and outlet are arranged in such a way that the coal is at least partially only wetted by liquid, so that there is a relatively large surface area where carbon, electrolyte and oxygen interact can come into contact.

This requirement is met in particular by a vertically arranged one that is open at the upper end Cartridge formed with a lower spout containing about 50 to 100 ml of granulated activated carbon with a Contains grain diameter of at least about 1 mm,

ORIGINAL INSPECTED ^

the electrolyte is added dropwise at the upper end.

The measuring electrode is preferably made of carbon and is in particular made of a graphite rod while the counter electrode is made of copper. Appropriately, both electrodes be separated from each other by a diaphragm, which then essentially controls the electrolyte flow limited.

For manufacturing reasons, a cell is particularly suitable in which the electrode arrangement is provided in a vertical bore of a block, in which the gas inlet is used Branch pipe is formed by an inclined bore opening at the lower end of the vertical bore, both bores in the upper area via a connecting line (especially with a minor Inclination towards the vertical bore), the extension of which as a gas

and electrolyte outlet from the cell. You can also provide both holes at an angle.

The electrolyte inlet flows into this connecting line, which in particular has a level vessel arranged above it with an outlet capillary (on the bottom).

A larger storage vessel is preferably located in the return circuit for the electrolyte (of about 5 1 capacity), its level by means of a level-regulated water inlet is controlled. In this way evaporation losses are compensated and the electrolyte concentration is compensated held essentially constant.

This storage vessel is conveniently located

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at a level below the cell, so that the electrolyte flowing out of the cell can flow into the storage vessel after passing the So_ filter and preferably a simple mechanical filter connected downstream (e.g. paper or fabric filter or glass frit). A pump then connects to this vessel, which pumps the _{electrolyte freed from So 2} from the supply into the element for electrolyte supply control (in particular formed by a level vessel provided with an outlet capillary).

It is particularly useful to double the cell (with a branch pipe for introducing gas for the circulation flow), with both cells being accommodated in particular as bores within a single block. The second cell then serves as a reference cell through which air is simply passed, and the difference between the currents of the two cells is determined as the measured value. In this way, the otherwise necessary thermostatting is avoided, which would be necessary in particular when measuring the SO ₂ content in chimneys, in the vicinity of which different temperatures can prevail.

The magnitude of the polarographic current that flows through the cell at a given voltage is taken as a measure of the S0 ₂ concentration. This current results from depolarization effects on the anode and consists of two components:

a) Kinetically determined basic current, which is very temperature unstable and

ORIGINAL INSPECTED

b) Measurement current caused by diffusion, that of the SO concentration is proportional. Because it is an electrochemical diffusion current acts, it also increases with a rise in temperature. On the other hand will this increase in current due to the temperature-related reduction in the SO solubility again compensated so that this partial flow is practically not changed by temperature changes will.

If the entire measuring current is measured in just one cell, it changes under the influence of temperature the kinetic component and consequently the total flow. To this one To suppress effect, two cells are taken. In the first (reference cell) is only air or SO "-free exhaust gas (e.g. appropriately cleaned) is supplied to the electrolyte, so that this cell practically measures the basic kinetic current. In the second (measuring cell) the gas containing SO_ to be monitored is routed. The whole thing flows through this cell current composed of both components.

If the difference between these two currents is measured, only the pure diffusion-related component is obtained as the measuring signal, which _{is proportional to the SO 2} concentration and remains temperature-stable.

The invention is described in more detail below with reference to the accompanying drawings; show it (in the scheme):

Figure 1 shows an electrolyte circuit for a

SO analyzer with double cell;

-3.

FIG. 2 an SO filter;

Figure 3 shows the structure of a single cell with manostat

pipe;

Figure 4 is a simplified single cell with a gas flow controller in the

Gas supply line is operated; FIG. 5 shows the cross section of a double cell arrangement

with a single cell type according to FIG. 4; Figure 6 shows a measuring arrangement for determining the TO differential current with a double cell · and

Figure. 7 a calibration curve.

Figure 1 shows a double cell arrangement 1 with a measuring cell 2 and a reference cell 3, the outgoing electrolyte passes via a common line 4 to an SO ₂ filter 5, from which after further mechanical filtering at 6, the electrolyte freed from SO and coarse impurities in a storage vessel 7 flows with a level control.

The level is kept constant via a water inlet 8, as a result of which evaporation losses compensated and the concentration of the electrolyte kept essentially constant will.

A pump 9 conveys electrolyte via the line to a level vessel 11, from which the electrolyte Via two capillaries 11 'into cells 2 and 3 (or their connecting line to the Branch pipes serving gas inlet) flows.

Figure 2 shows a SO ^ filter in section: the at 12 dropping electrolyte from above wets the activated carbon filling 13 and so comes with the

COPY

at the same time in contact with 14 incoming air. Electrolyte freed from SO ₂ flows off at 15 at the lower end.

FIG. 3 shows the structure of a single measuring cell with a manostat tube 16: A measuring electrode 18 in the form of a carbon rod is arranged in the electrolysis cell, on which SO ₂ contained in the electrolyte is oxidized. A copper tube 19, which is separated from the carbon rod 18 by a frit 20, serves as the counter electrode. A branch vessel 21 is connected to the electrolytic cell 17, at the lower end of which gas to be measured is introduced via the pipe 22. In this way, a continuous electrolyte circulation is created in the manner indicated by the arrows (according to the mammoth pump principle).

Fresh electrolyte enters at 23, and at 24 gas and excess electrolyte escape the cell.

A measuring and recording device for the polarographic current is indicated by 25.

A cell according to FIG This cell, which is operated with a flow regulator 26 in the measuring gas feed line, is that Branch vessel provided as an inclined bore in the common measuring cell block.

FIG. 5 shows a section through a measuring block with a double cell according to the cell type according to FIG

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f. / M-

A suitable electrical circuit is shown in FIG. 6: The two cells are indicated schematically by 27 and 28. (Structure as in Figure 3 or 4). Both graphite electrodes 18 are connected to the common voltage source. The measurement and reference currents are then compared with one another at an operational amplifier 29 connected as a differential amplifier, and any difference that occurs is amplified accordingly. The operational amplifier 30 connected subsequently amplifies this signal to the voltage required for the display measuring device 31. The potentiometer 32 is used to set the zero point on the display while introducing the same gas, in particular SO ^ -free gas, into both cells. A calibration gas mixture is then passed through the measuring cell and the display on the measuring _{device 31 is set using the potentiometer 33 to match the S0 9} concentration of the calibration gas. During the actual measurement, the gas to be monitored is then introduced into the measuring cell under the same conditions and its SO 2 content compared

i ^ -

determined the gas passed through the reference cell, ι

I.

In the case of the circulating electrolyte, which is _{freed from SO 2} outside the cell, the change in Cu concentration that occurs during prolonged operation should be corrected If the measured value is only weak, in the preferred embodiment of the device as a twin cell, the measuring and reference cell can be exchanged periodically, with

ORIGINAL INSPECTED

in the reference cell, the copper deposited in the measuring cell goes back into solution through oxidation by the oxygen contained in the gas.
5

example

With an I-fold or 2-fold volume exchange of the electrolyte (0.5 m H SO., 0.2 m CuSO) in the ^{analyzer located at 20 0} C (cell with gas inlet branch pipe and connecting paths), through which a mixture of nitrogen with the specified SO_- Concentrations was passed at a rate of 380 ml gas / min, a calibration curve is obtained, as can be seen from FIG. The signal magnitude remains stable over time with a constant SO2 concentration, and the time (^ ₀ ) required for a 90% change in the signal height was around 110 s.

With a volume exchange below 0.5, Cg ₀ becomes significantly longer (up to 200 s and more). The signal stability is then low and the slope of the calibration curve is reduced.

ORIGINAL INSPECTED

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Claims (10)

Jülich nuclear research facility limited liability company Claims , '1. ) Electrochemical gas analyzer for the SO ₂ content in gases, especially flue gases, with a measuring cell with a measuring electrode indicating the depolarization currents of the gases to be determined and a non-polarizable counter-electrode in the same electrolyte, which is driven to a circulation flow by the measuring gas inlet, drives the electrolytes containing dissolved gas through the space between the measuring and counter electrodes, characterized by a circuit for the continuous renewal of the electrolyte in the cell (17 to 21) with a 0.5 to 5-fold volume exchange per minute with a pump (9), - ^ 15 a feed control element (11) at the entrance the cell and an SO ₃ filter (5) at the output • of the cell. 2. Gas analyzer according to claim 1, characterized characterized in that the S0 _o filter (5) is formed by an activated carbon cartridge provided with an air inlet (14) with such an arrangement of electrolyte inlet (12) and outlet (15), that the coal is at least partially wetted only by liquid speed. 3. gas analyzer 'according to claim 2-, characterized' marked ^; h '~ h e t that the active fifia charcoal cartridge with a vertical cartridge nope / ha COPY an activated carbon filling (13) leaving the upper end free and having a grain size of at least 1 mm, a lower outlet (15), an upper drip feed (12) and air inlet openings (14) is formed at the top. 4. Gas analyzer according to one of the preceding claims, characterized in that that the measuring electrode (18) is made of carbon and the counter electrode (19) is made of copper. 5. Gas analyzer according to one of the preceding claims, characterized in that that measuring and counter electrode (18, 19) are housed in a vertical bore of a measuring cell block, at the lower end of a Inclined bore for the gas inlet opens, the. at the upper end a connection line to the Has vertical bore into which connecting line the electrolyte supply line (11 ') opens (Figure 4). 6. Gas analyzer according to one of the preceding claims, characterized in that that the electrolyte supply control element through an outlet capillary (11 *) at the bottom having level vessel (11) above the connecting line is formed between the branch vessel (21) serving to introduce the gas and the cell (17) receiving the electrodes. 7. Gas analyzer according to one of the preceding claims, characterized by a storage vessel (7) in the electrolyte circuit with level control via a level-controlled one Water inlet (8). 33088 8. Gas analyzer according to one of the preceding claims, characterized by a diaphragm (20) between the measuring and counter electrodes (18, 19). 9. Gas analyzer according to one of the preceding claims, characterized by a doubling of the cell with air supply to the second cell, which is used as a reference cell for determination the difference between the currents of the two cells is used (FIGS. 5, 6). 10. Gas analyzer according to claim 9, characterized by a merging of the electrolyte flows at the output of the first and second cells (2, 3) in front of the SO ₂ filter (5) ORIOINAL INSPECTED