M&C TechGroup CG-2H-W_-2MH-W Operator's manual - Manual (Page 10)

10
Gas sampling and conditioning technology 9-0.2-ME
The reaction equilibrium is shifted at room temperature onto the side of the original material NO
2
. A
shift of the equilibrium towards the products and the resultant high product yield can only be achieved
as subject to a high energy input, i.e. increase of temperature.
A nearly 100% conversion of NO
2
and therefore a complete conversion to NO is achieved with a car-
tridge filled with a metal catalyst within a temperature range of 620°C to 680°C.
The energy input can be reduced explicitly by using a carbon-molybdenum based catalyst. With this
type of catalyst a nearly complete conversion of NO
2
to NO is achieved within a temperature range of
320°C to 380°C.
The catalyst is supplied as a ready-to-use formatted cartridge.
N O T E !
Dust-loaded gases may choke up the catalyst filling. For this reason, an ap-
propriate gas conditioning system is to be mounted upstream of the convert-
ers CG... in order to separate out suspended particles !
N O T E !
For converters with unheated sample gas in- and outlet at dew points above
10°C a sample gas drying/cooling shall be provided!
To minimize the loss of NO
x
in the sample gas conditioning, according to DIN EN 14792:2005 (de-
scription oft he NO
x
reference measuring method) the usage of sample gas coolers is not allowed in
case the NO
2
fraction is higher than 10% of the NO
x
content (note: because of the high solubility of
NO
2
). If the dew point simultaneously is higher than 10°C, a converter with heated in- and outlet has to
be used (“hot gas converter”).
8.1 SELECTION OF THE APPROPRIATE CATALYST MATERIAL
For the conversion, as noted preliminary, two catalyst materials are available.
As a standard material a carbon-molybdenum mixture type C is used as catalyst. The carbon support-
ing material guarantees optimal contact between the gas to be converted and the surface of the cata-
lyst combined with a simultaneously low flow resistance.
The catalyst type C, operated within a flow range of 30
… 60 Nl/h, is able to convert NO
2
concentra-
tions up to 1000 ppm to NO, achieving conversion ratios of >95%. If necessary, the operating tem-
perature of the converter has to be raised within the admissible range.
N O T E !
For catalyst type C no oxygen in the sample gas as well as in the test gas is
necessary. A calibration with test gas NO
2
and N
2
is possible.
Alternatively to type C a cartridge with metal filling type SS is available. It is possible to convert NO
2
concentrations of up to 2000 ppm into NO. A temperature adaption is possibly also necessary.