ZIROX SGM7, Руководство пользователя

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Oxygen Monitor SGM7 10 Appendix
HB_SGM7_eng.docx
32
In combustion processes with air deficiency, sufficiently high temperature
and, if necessary, with catalysts for the establishment of complete gas
equilibrium, a mixture of nitrogen, hydrogen, water vapor, carbon monoxide
and carbon dioxide (the so-called “water gas” which can be produced from
coal and water) develops from all organic substances. The reaction formula
for air-deficient combustion cannot be described merely by the parameters
λ , n and m. Rather, the following equation applies :
C
n
H
m
+ [(1-a/2) ⋅ n + (1-b) ⋅ m/4] O
2
→ (1-a) ⋅ n CO
2
+ a ⋅ n CO + (1-b) ⋅ m/2
H
2
O + b ⋅ m/2 H
2
,
where a and b are quantities determined by λ and the position of the
temperature-dependent water gas equilibrium CO + H
2
O = CO
2
+ H
2
.
Gas potentiometry with solid electrolyte cells only produces the oxygen
concentration
ϕ
(O
2
) in the respective measuring gas. However, in many
cases the determination of λ is desired. For the calculation of lambda, the
following equations can be derived :
λ
ϕ
ϕ
ϕ
l
O
V
O
O
Luft
=
+
+
−
1
2
1 2
1
2
2
( )
( )
( )
λ
ϕ ϕ
r
V
V
O
KC
O
KH
= −
+
+
+
+














1
1
1 2
1
2
1
1
2
( ) ( )
.
These equations, valid for single hydrocarbons for λ > 1 (lean burn) and λ <
1 (rich burn), contain the carbon/hydrogen-ratio in the hydrocarbon V = 2 n/m
and the thermodynamic equilibrium constants of the reactions
CO
2
= CO + 1/2 O
2
lg K
C
= 4.505 - 14700 K / T ,
H
2
O = H
2
+ 1/2 O
2
lg K
H
= 2.947 - 13008 K / T .
In practice, however, usually compounds of different hydrocarbons are
present. Fuel gases can also contain hydrogen, carbon monoxide, and
nitrogen. Apart from that, the supplied air with its humidity and carbon-dioxide
content also contributes to the gas equilibrium. In correspondingly modified
equations, average values of V must be used. Thinning with nitrogen has
some influence on
λ in the lean burn range but not in the rich burn range
because the equilibrium between the water gas components does not
depend on pressure, and thus does not depend on the water gas
concentration.
The change in the kind of equation that is needed for the calculation of λ , if
the conditions change from excess of oxygen to lack of oxygen, constitutes
a special problem. In that case for each measurement the concentrations of
the water gas components have to be calculated. Depending on whether
ϕ (CO) + ϕ (H
2
) is higher or lower than 2 ϕ (O
2
), one has to use either the one
or the other equation (DE 43 23 879). The software developed for this
purpose by the company
GO MESSTECHNIK Greifswald produces the
results virtually without time-delay.