ZIROX SGM7.2, Руководство пользователя

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Oxygen Monitor SGM7.2 10 Attachment
10 Anhang
10.1 Fundamentals of the use of potentiometric ZrO2 solid electrolyte
sensors for the optimal guidance of combustion processes
In many technological processes (e.g. production of glass or ceramics fibres,
firing of porcelain, and generation of energy or crude gas from solid or liquid fuels
etc.), the optimisation and reproducible guidance of combustion processes is a
prerequisite for a constantly good product quality and exploitation of resources.
Quality safeguarding standards like ISO 9000 stipulate the collection and
documentation of process-relevant data in order to guarantee the product quality.
As controlling quantities for the monitoring and control of such installations, in a
wide range of gas compositions signals are required, which are recorded
preferably in real-time and can clearly be attributed to completely established gas
equilibrium.
Nowadays, such signals are generally obtained by using potentiometric ZrO
2
solid electrolyte sensors. The ZIROX company in Greifswald has developed both
short and very long probes, which are equipped with sensors (either unheated or
electrically heated), for operating in situ in various kinds of combustion
installations, technical furnaces, and flames, producing the required signals.
Apart from that, ZIROX also produces devices with electrically heated sensors for
the analysis of externally pre-mixed fuel-air mixtures or of siphoned flue gases.
The chemical, thermodynamic, and electrochemical fundamentals, on which the
application of potentiometric solid electrolyte sensors (i.e. galvanic solid
electrolyte cells) in the guidance of combustion processes is based, are
described in the following.
Oxygen concentration and air factor lambda
The best way of describing the conversion of gaseous, liquid or solid fuels with air
is by using the air factor lambda. This quantity represents the ratio of the amount
of air which is fed to the combustion process and the amount of air that is needed
for a stoichiometric conversion of the supplied fuel. The amount of air can be
indicated in volume, mass, or amount of substance (quantities that according to
the ideal gas law are proportional to each other; units like m
3
, kg or mol shorten
itself at forming the ratio). If volume is the quantity that is used, lambda is given
by the following equation :
λ
= v(air volume fed) / v(air volume needed for stoichiometric combustion) .
If too much air is fed to the process (excess air) then
λ
> 1, if too little air is fed to
the process (air deficiency) then
λ
< 1. In the case of exactly stoichiometric
combustion
λ
= 1.
(Only in motor vehicle technology a different definition is used, because in motor
test beds the amount of fuel that is used is weighed, and the supplied air volume
is converted into mass. In the division of the air mass by the fuel mass, e.g. for
pure octane a value of 15.3 is obtained for an exactly stoichiometric conversion.)
For the combustion of a hydrocarbon (in motor fuel, natural gas, liquid gas) with
the gross formula C
n
H
m
, the following reaction formula is obtained for
λ
in case of
complete combustion in excess air:
C
n
H
m
+
λ ⋅
(n + m/4) O
2
→
n CO
2
+ m/2 H
2
+ (
λ
- 1)
⋅
(n + m/4) O
2
.
In combustion processes with air deficiency, at sufficiently high temperature and,
if necessary, in the presence of catalysts to ensure the establishment of complete
gas equilibrium, for all organic substances essentially a mixture of nitrogen,
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