ZIROX SGM7, Руководство пользователя

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Oxygen Monitor SGM7 10 Appendix
HB_SGM7_eng.docx
31
10 Appendix
10.1 Fundamentals of the use of potentiometric ZrO2 solid electrolyte
sensors for the optimal running of combustion processes
In various 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 optimization and repeatable running of combustion
processes ensure a consistently good product quality and exploitation of
resources. Quality standards like ISO 9000 stipulate the collection and
documentation of process-relevant data in order to guarantee the product
quality. For the monitoring and controlling of such installations measuring
values are required as controlled quantity. These values are recorded in a
wide range of gas compounds, preferably in real-time , and can be clearly
assigned to completely established gas equilibrium.
Nowadays, those 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 produces devices with electrically
heated sensors for the analysis of externally pre-mixed fuel-air compounds
or of siphoned flue gases.
The chemical, thermodynamic, and electrochemical fundamentals, which the
application of potentiometric solid electrolyte sensors (i.e. galvanic solid
electrolyte cells) in the running of combustion processes is based on, are
described in the following.
Oxygen concentration and air factor lambda
The best way to describe 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
(which are proportional to each other according to the ideal gas law; units
like m
3
, kg or mol shorten themselves when 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 case of exact stoichiometric
combustion λ = 1.
(Only in automotive engineering a different definition applies. In engine test
benches the amount of consumed fuel is weighed, and the supplied air
volume is converted into mass. When the air mass is divided by the fuel
mass, e.g. for pure octane a value of 15.3 is obtained in an exact
stoichiometric conversion.)
For the combustion of hydrocarbon (in motor fuel, natural gas, liquid gas)
with the gross formula C
n
H
m
, the following reaction equation 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