Oxygen Monitor SGM7.2 10 Attachment
hydrogen, water vapour, carbon-monoxide and carbon dioxide (the so-called
”water gas”, which can be produced from coal and water) is obtained. The
reaction formula for air-deficient combustion can not 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
,
in which a and b are quantities that are 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 at first 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 deduced :
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, that are 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 the practice however, usually mixtures 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 oxygen-deficiency constitutes a
special problem. In that case for each measurement the concentrations of the
water gas components have to be calculated and depending on whether
ϕ
(CO) +
ϕ
(H
2
) is higher than or lower than 2
ϕ
(O
2
), one has to use either the one or the
other equation (DE 43 23 879). The software that has been developed for this
purpose by the company
GO MESSTECHNIK Greifswald
produces the results
practically without time-delay.
Gas potentiometry using solid electrolyte sensors
Crystals of mixed oxides from ZrO
2
and CaO or Y
2
O
3
have vacancies in the
oxygen ion sublattice, through which oxygen ions can migrate at high
temperatures. Thus, they are solid electrolytes (i.e. solid ion conductors). At
HB_SGM72_Rehm_eng.DOC
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