BS/EN 14792-2017 pdf download.Stationary source emissions Determination of mass concentration of nitrogen oxides Standard reference method: Chemiluminescence.
Some of the NO2 created during the reaction of NO and 03 is in an excited state. When returning to the basic state, these NO2 molecules can radiate light, the intensity of which depends on the NO content and is influenced by the pressure and presence of other gases.
In a chemiluminescence analyser. gas is fed at a constant flow rate into the reaction chamber of the analyser, where It is mixed with an excess of ozone for the determination of nitrogen oxide only. The emitted radiation (chemiluminescence) is propartional to the amount of NO present In the sampled gas. The emitted radiation is filtered by means of a selective optical filter and converted into an electric signal by means of a photomultiplier tube.
For the determination of the amount of nitrogen dioxide, the sampled gas is fed through a converter where the nitrogen dioxide is reduced to nitrogen monoxide and analysed in the same way as described previously. The electric signal obtained from the photomultiplier tube is proportional to the sum of concentrations of nitrogen dioxides and nitrogen monoxides. The amount of nitrogen dioxide is calculated from the difference between this concentration and that obtained for nitrogen monoxide only (when the sampled gas has not passed through the converter).
When a dual type analyser is used, both NO and NO results are determined continuously. On the contrary, with a single type analyser, the reaction chamber is alternatively fed with gas having not passed the converter and with the gas having passed the converter that reduces NO2 to NO. Therefore, NO and NO are determined alternately.
Chemiluminescence analysers are associated to an extractive sampling system and a sample gas conditioning system. A sample of gas is taken from the stack with a sampling probe and conveyed to the analyser through a sample gas line and suitable gas conditioning system. The values from the analyser are recorded and/or stored by means of electronic data processing.
Interference due to CO2 in the sample gas is possible, particularly in the presence of water vapour, due to quenching of the chemiluminescence. The extent of the quenching depends on the CO2 and H20 concentrations and the type of analyser used. Any necessary corrections may be made to the analyser output to increase its accuracy for example by reference to correction curves supplied by the manufacturers or by calibrating with gases containing approximately the same concentration of CO2 as the sample gas.
NOTE I A vacuum chemiluminescent NO1 analyser reduces the CO and H20 quenching error.
NOTE 2 A correction of the NO1 concentration may be necessary if the NH3 concentration is higher than
20mg/rn3.
In flue gases from conventional combustion systems the nitrogen oxides consist of more than 95% NO.
The remaining oxide is predominantly NO2. These two oxides (NO + NO2) are designated as NO1.
It should be noted, that in other processes, the ratio of NO to NO1 may be different and other nitrogen
oxides may be present.
A filter removes the dust in the sampled volume before the sample is conditioned and passed to the analyser.
Different sampling and conditioning configurations are available in order to avoid water vapour condensation in the measuring system (see also Annex B).
Possible configurations are:
— Configuration 1: removal of water vapour by condensation using a cooling system;
— Configuration 2: removal of water vapour through elimination within a permeation drier;
— Configuration 3: dilution with dry, clean ambient air or nitrogen of the gas to be characterized;
— Configuration 4: heating of the complete sampling system from the inlet of the sample gas line to the heated analyser at a temperature above the dew point.
Configurations 1 to 3 require that the user shall check that the dew point temperature or the outlet temperature of the conditioning system is lower or equal to 4 °C at the inlet of the analyser.
For configuration 4 the user may correct the results for the remaining water content in order to report results on a dry basis (see Annex B in EN 14790:2017).
It is important that all parts of the sampling equipment upstream of the analyser are made of materials that do not react with or absorb NOR.
The temperature of all components of the sampling equipment coming into contact with the wet sample gas shall be maintained at a sufficiently high temperature to avoid any condensation.
Conditions and layout of the sampling equipment contribute to the combined uncertainty of the measurement. In order to minimize this contribution to the combined measurement uncertainty, performance criteria for the sampling equipment and sampling conditions are given in 6.2 and Clause 7.
Some other sample gas conditioning systems may exist and could be acceptable, provided they fulfil the requirements of this European Standard and have been validated with success during the certification process. For example, some systems put gas in depression using a simple Sonic nozzle in the collection probe in order to create a partial vacuum (between 50 hPa and 100 hPa absolute) so that the head of collection and the sample gas line does not need to be heated and water vapour condensation is avoided.
6.2 Sampling and sample gas conditioning system
6.2.1 Sampling probe
In order to reach the measurement points of the measurement plane, probes of different lengths and inner diameters may be used.BS/EN 14792-2017 pdf download.