Portable Biogas analyser series GAS 3200 BIOGAS
Operation manual – Issue EN2014V0
Page
10
of
45
4.2.2
H
2
S measurement with 3-electrodes Electrochemical Cell
The 3-electrodes substance-specific electrochemical H
2
S sensor used in the GAS 3200 Biogas analyser has been designed for biogas
applications with high H
2
S concentrations in continuous mode and offers the following key features:
The working electrode
(also called the sensing electrode) is
designed to optimize the oxidation or reduction of the toxic gas to
be measured. This electrode allows the gas to come in contact
with both electro-catalyst and electrolyte to create a three-phase
interface of gas, liquid and solid. The other two electrodes in the
cell, the counter electrode and the reference electrode usually
have a similar chemical composition to the working electrode
.
The cell electrolyte
provides ionic electrical contact between the
electrodes.
The working electrode is the surface where the electrochemical oxidation (H
2
S) occurs. A high surface area catalyst is used to
optimize the sensor performance, resulting in a high sensor capacitance leading to susceptibility to electromagnetic interference.
This electrode is exposed to the outside air and hence is directly exposed to all gases in the air including the gas to be measured.
Therefore this electrode may be poisoned if exposed to certain gases that either adsorbs onto the catalyst (e.g. acetylene onto CO
sensors), or react, creating by-products which inhibit the catalyst (e.g. NO
2
or aromatics onto H
2
S sensors). This is the same problem
as poisoning in fuel cells and car catalytic converters.
The counter electrode
balances the reaction of the working electrode – if the working electrode oxidizes the gas, then the counter
electrode must reduce some other molecule, generally O
2
, to generate an equivalent current, in the opposite sense.
The reference electrode
anchors the working electrode potential to ensure that it is always working in the correct region of the
current-voltage curve – see figure 2 below. It is important that the reference electrode has a stable potential, keeping the working
electrode at the right electrochemical potential to maintain a constant sensitivity, good linearity and minimum sensitivity to
interfering gases. The potentiostat circuit ensures that the counter electrode is provided with as much current as it needs, also
maintaining the working electrode at a fixed potential, irrespective of how hard it is working.
Technical specifications of H2S sensor
Low cost
Compact
Several measuring range are available: 50/200/500ppm/1000/2000/5000 or 9999 ppm
Reliable measure with < ±3% FS accuracy
Fast response time T90 (< 30 Sec)
No interference from CO
2
and CH
4
No maintenance
1 year expected lifetime when discontinuously exposed to H
2
S
Immunity to vibrations
No warm-up time
4.3
N
2
calculation (option)
N
2
value is an optional parameter on GAS 3240 BIOGAS analyser and is obtained by calculation according to the formula:
% N
2
= 100% - (%CH
4
+ %CO
2
+ %O
2
+ %H
2
S)
4.4
Temperature measure
The analyser is delivered with a PT100 temperature probe for the measure of ambient or biogas temperature in the range of -10°C
to +110°C
