Test & measurement
The Standard does not specify whether the analyser should be heated or unheated, or whether the detector should be vacuum based or atmospheric. In addition, it does not specify
whether the NO2 component should be converted to NO prior to any water in the sample being condensed for removal. The EN standard reference method EN 14792 is intended for stack testing laboratories, to verify the installation of a Continuous Emissions Monitor on an industrial chimney stack.
desigNiNg a true refereNce aNalyser - key cONsideratiONs
1
chemiluminescent photon detector
The chemiluminescence reaction of Ozone and Nitric Oxide causes the oxidation of Nitric Oxide (NO) to
waters. Globally, emission limits for NOx are becoming more stringent as governments seek to address these concerns.
NOx measuremeNt iN prOduct develOpmeNt The designers of engines, boilers, turbines and incinerators seek to optimise combustion efficiency whilst minimising the harmful emissions of gases such as NOx. In many cases, this means that products have to be designed to comply with the local emission regulations, wherever they are sold. Organisations conducting combustion
research have a different monitoring requirement to plant operators and stack testers. This is because stack testers simply require analysers that comply with certain performance requirements, whereas those in R&D require highly accurate, definitive
measurements of NO, NO2 and NOx in order to fine-tune their designs and optimise the performance of their products. In addition, it will frequently be necessary for them to provide defensible measurement data in order to secure formal approvals in local markets. This is extremely important because the untimely discovery of any NOx measurement errors, interferences or excessive levels of uncertainty could prove costly.
refereNce methOd – chemilumiNesceNce (cld) The European Standard EN 14792 specifies chemiluminescence as the standard reference method (SRM) for the measurement of NOx in stationary source emissions. However, the method cites several alternative CLD configurations but it only stipulates Chemiluminescence as the method of detection.
Instrumentation Monthly February 2021
Nitrogen Dioxide (NO2) which generates photons that are measured using a photomultiplier tube. To obtain maximum sensitivity from the photomultiplier tube it must be cooled so that its ‘dark current’ can be reduced (dark current is a small electric current that flows even when no photons are entering the device). It would be possible to increase the
sample flow to the detector to increase its sensitivity, but it is necessary to keep the sample flow as low as practicable to achieve the quenching target. Typically, at Signal Group, the flows of Ozone and sample are set in a ratio of 10:1. Signal’s CLD instruments employ two detectors for continuous measurements of NO
and NO2. The sample is passed directly to one of the detectors, and via the
NO2 converter for the other. The NO2 value is derived by continuously subtracting the NO-only detector signal from the other detector’s signal. The reaction chamber design is particularly important with a vacuum based analyser because the reaction gases pass very quickly to the vacuum pump.
2 advantages of a vacuum cld
Water vapour and Carbon Dioxide within a sample will reduce, or quench, the reading of NO, and combustion exhausts usually contain high levels of both. Signal Group has therefore developed a CLD with a vacuum in the reaction chamber, which eliminates the quenching effect, and increases the signal to noise ratio considerably. An additional advantage of the vacuum is that it allows for the delivery of a hot wet sample gas to the reaction chamber without the threat of condensation.
3 Ozonator design
Ozone is necessary for the oxidation of NO, but a number of potential pitfalls exist in the choice of ozone generation technology. For example, high-voltage corona discharge lamps would oxidise any Nitrogen in the air
feed to NO and NO2; artificially raising the readings and presenting a threat of corrosive Nitric Acid formation. This could be resolved by using pure Oxygen as the feed gas, but that would be prohibitively expensive. Signal Group therefore employs a soft-discharge neon lamp in its ozonator, which does not oxidise Nitrogen.
4 NO2 converter
The choice of NO2 converter material is an important consideration because at high
temperatures stainless steel would oxidise any Ammonia in the sample to NO. Furthermore, if there is insufficient oxygen in
the sample, it will be ‘robbed’ from NO2, so an oxygen bleed to the sample would be necessary. To avoid this complication, the Signal Group CLD analysers use a vitrified type of carbon in a heated quartz vessel which has none of these characteristics, and lasts for at least two years of operation.
5 vacuum pump
Traditional oil filled rotary vane pumps are prone to problems caused by Ozone contaminating the oil. Signal Group therefore employs membrane pumps with a catalyst for removing Ozone.
With over 30 years of experience in the design
and manufacture of reference CLD analysers, Signal Group has continually refined its product offering to ensure the highest levels of accuracy, sensitivity and repeatability. Recognising Signal’s expertise in NOx analysis, MD James Clements is now a member of the SAE E-31 Aircraft Exhaust Emissions Measurement committee. In addition to the advantages of Signal’s core
CLD technology, recent developments have included functionality enhancements such as compatibility with 3G, 4G, GPRS, Bluetooth, Wifi and satellite communications. Each instrument now has its own IP address and runs on Windows software; providing users with simple, secure access to their analysers at any time, from anywhere. The most recent development is an option for
a detachable screen. Customers will find this useful when it is necessary to install the analyser in a location with difficult access – such as: in an ATEX enclosure; in vehicle exhaust gas test cells; with a raised gantry on a stack; in a combustion test rig, or on any site where the location of analysis is not an ideal or safe working space.
Signal Group
www.signal-group.com 53
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74
