DE8307214U1 - Electrochemical gas analyzer for SO ↓ 2 ↓ content in gases, especially flue gases - Google Patents

Description

Jülich nuclear research facility
Limited liability company

Electrochemical gas analyzer for the SO ₂ content in gases, especially flue gases

The invention relates to an electrochemical gas analyzer for the SO "content in gases, in particular flue gases, with a measuring cell with one of the depolarization currents to be determined Gas-indicating measuring electrode and a non-polarizable counter electrode in the same electrolyte, which is driven by the measuring gas inlet to a circulation flow, the dissolved Gas-containing electrolyte drives through the space between the measuring 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, ÜV etc. ) 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 surprisingly been shown that no / ha

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 S-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 ₂ ~ filter at the output 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,

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 5, while the counter electrode is made of copper

consists. The two electrodes can expediently be separated from one another by a diaphragm, which then essentially limits the electrolyte flow.
10

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 15 branch pipe through one at the lower end of the vertical I opening inclined bore is formed,

I with both holes in the upper area above

I a connection line (especially with low

I with a docile inclination towards the vertical bore) in

I 20 are connected, the extension of which as gas

I and electrolyte outlet from the cell is used. One can

? also provide both holes at an angle.

"The electrolyte-

i 25 allow, in particular a

_; "Level vessel with outlet capillary (on the bottom).

'; Located in the return circuit of the electrolyte

j preferably a larger storage vessel

) 30 (of about 5 1 capacity), its level

is controlled by means of a level-regulated water inlet. In this way, evaporation losses are compensated and the electrolyte concentration is kept essentially constant. ] 35 This storage vessel is conveniently located

■ a ■

cc * «

at a level below the cell, so that the electrolyte draining from the cell after Passing the So ^ filter and preferably a downstream simple mechanical filter (e.g. paper or fabric filter or glass frit) can flow into the storage vessel. To this A pump is then connected to the vessel, which removes the electrolyte from the reservoir in the element for controlling the electrolyte supply (in particular formed by an outlet capillary provided level vessel) promotes.

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

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 is again due to the temperature-related reduction in SO_ solubility compensated so that this partial flow is practically unchanged by temperature changes is changed.

If the entire measuring current is measured in just one cell, the kinetic component changes under the influence of temperature and, as a result, the total current. To suppress this effect, two cells are used. In the first (reference cell) only air or S0 ₂ -free exhaust gas (eg appropriately cleaned) is supplied to the electrolyte, so that this cell practically measures the basic kinetic current. The SO 2 -containing gas to be monitored is fed into the second (measuring cell). The entire current composed of both components flows through this cell.

If the difference between these two currents is measured, only the pure one is obtained as the measurement signal diffusion-related component, that of the SO concentration is proportional and remains temperature stable.

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

FIG. 1 shows an electrolyte circuit for an SO_ analyzer with a double cell;

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 ο differential current with a double cell; and

FIG. 7 a calibration curve.

Figure 1 shows a double cell arrangement 1 with a measuring cell 2 and a reference cell 3, whose outgoing electrolyte reaches a SO »filter 5 via a common line 4, from after further mechanical filtering at 6 the freed from SO “and coarse impurities Electrolyte flows into a storage vessel 7 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.

FIG. 2 shows an SO ₂ filter in section: the electrolyte dripping in from above at 12 wets the activated carbon filling 13 and thus comes with the

«T · · ι ■

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

FIG. 3 shows the structure of a single measuring cell with a manostat tube 16: In the electrolytic cell a measuring electrode 18 is arranged in the form of a carbon rod / on the one contained in the electrolyte S0_ is oxidized. A copper tube 19, which is passed through a frit 20

is separated from the carbon rod 18. With the electrolytic cell 17 is a branch vessel 21 in connection ^ at its lower end via the pipe 22 measuring gas is initiated. In this way, in the manner indicated by arrows, a continuous one is created Electrolyte circulation (according to the mammoth pump principle).

\ Fresh electrolyte comes in at 23, and at

; '; 20 24 get out of gas and excess electrolyte

the cell.

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

A cell according to FIG

\ of this cell, which is equipped with a flow regulator 26

is operated in the measuring gas supply line, that is

'"> 30 branch vessel as an inclined hole in the common

': Measuring cell block provided.

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

• 4 · r
»•••« II ··

In Figure 6 a suitable electrical circuit is shown
Shown: through 27 and 28 are the two cells
indicated schematically. (Structure as in figure
3 or 4). Both graphite electrodes 18 are located
at the common voltage source. The measuring

and the reference current are then sent to a
Differential amplifier connected operational amplifier 29 compared with each other and amplified an occurring difference accordingly. Of the
subsequently connected operational amplifier 30
amplifies this signal to the voltage required for the display measuring device 31. With the potentiometer 32 while introducing the same gas, | in particular from S0 _o -free gas, in both cells £

P the zero point set on the display. After that |

a calibration gas mixture is passed through the measuring cell I

and with the potentiometer 33 the display on the 1st

Measuring device 31 in accordance with the SO_-Konzen- I

calibration gas is set. The actual measurement is then carried out in the measuring cell

Gas to be monitored under the same conditions §

I initiated and its SO content compared to |

the gas conducted through the reference cell fj

middle. I.

ι

In the case of the circulating electrolyte, the \

is freed from SO- outside the cell, i should reduce the Cu concen- trations that occur during prolonged operation

tration changes are corrected. Since the }

Change in concentration when monitoring

Gases that contain only a little SO_, low and ■■

their influence on the measured value is only weak;

you can easily measure and in the preferred embodiment of the device as a twin cell

Periodically swap reference cell, where

in the reference cell that deposited in the measuring cell Copper is redissolved through oxidation by the oxygen contained in the gas goes "

example

With an I-fold or 2-fold volume exchange of the electrolyte (0.5 m H SO ₄ , 0.2 m CuSO ₄ ) in the ^{analyzer located at 20 0} C (cell with gas inlet branch pipe and connecting paths) through which a mixture of nitrogen and 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. 7. The signal magnitude remains stable over time with a constant SO_ concentration, and the time ( 'C.ag) required for a 90% change in the signal height was around 110 s.

With a volume exchange below 0.5, the C ₉₀ becomes significantly longer (up to 200 s and more). The signal =

Jr «

The stability is then low and the ^λ steepness of the calibration curve is reduced.

Claims (10)

Jülich nuclear research facility limited liability company claims 1. Electrochemical gas analyzer for the SO ^ 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 by the measuring gas inlet to a circulating flow, the dissolved gas containing electrolytes through the space between the measuring and counter electrodes, characterized by a circuit for the continuous renewal of the electrolyte in the cell (17 with 21) with a 0.5 to 5-fold volume exchange per minute with a pump (9), a feed control element (11 ) at the entrance of the cell and an SO ₂ filter (5) at the exit of the cell. 2. Gas analyzer according to claim 1, characterized characterized in that the SO ₂ filter (5) is formed by an activated carbon cartridge provided with air inlet (14) with such an arrangement of electrolyte inlet (12) and outlet (15) that the carbon is at least partially only wetted by liquid. 3. Gas analyzer according to claim 2, characterized in that the activated carbon cartridge with a vertical cartridge ■ · »J an activated carbon filling curve (13) leaving the upper end free and having a grain size of at least 1 rom, 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 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 which opens an inclined bore for the gas inlet, which at the upper end has a connecting line to the vertical bore, into which connecting line the electrolyte supply line (II ¹ ) opens (Figure 4). 6. Gas analyzer according to one of the preceding Anil sayings, characterized thereby net that the electrolyte supply control element "through an outlet capillary (11 *) at the bottom having level vessel (11) above the Ver connection line between the branch vessel (21) serving for gas inlet and the electrode receiving cell (17) is formed. 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 water inlet (8). • · * fc · "· · · · · Ct Cl flf 8. Gas analyzer according to one of the preceding claims, characterized by j 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 j second cell, which serves as a reference cell for determining [ the difference between the currents of the two cells
(Figures 5, 6). 10. Gas analyzer according to claim 9, characterized by merging the electrolyte flows at the outlet of the First and second cell (2, 3) in front of the SO ₃ filter (5).