Monitoring & metering Managing and measuring NOx gas emissions


Gases formed as part of combustion, such as in vehicle engines or industrial processes, have both health and environmental effects. These gases, collectively known as NOx, are, therefore, tightly regulated. In this article, James Clements, managing director of UK analyser manufacturer Signal Group, explains


the role of monitors in managing NOx gas emissions N


itrogen and oxygen are the two main components of atmospheric air, but they do not react at ambient


temperature. However, in the heat of combustion, such as in a vehicle engine or within an industrial process or furnace, the two gases react to form nitrogen oxide (NO)


and nitrogen dioxide (NO2). This is an important consideration for the developers and manufacturers of combustion equipment because emissions of these gases (collectively known as NOx) have both health and environmental effects, and are therefore tightly regulated. Nitrogen dioxide gas is a major ambient air


pollutant, responsible for large numbers of premature deaths, particularly in urban locations


where vehicular emissions accumulate. NO2 also contributes to global warming and in certain circumstances can cause acid rain. A wide range of regulations therefore exist to limit NOx emissions from combustion sources ranging from domestic wood burners to cars, and from industrial furnaces and generators to power stations. The developers of engines and furnaces therefore pay great attention to the NOx emissions of their designs, and the operators of this equipment are generally required to undertake emissions monitoring to demonstrate regulatory compliance.


The rOle Of mONITOrING IN NOx redUCTION NOx emissions can be lowered by reducing residence time and peak temperature, by chemical reduction and by lower nitrogen in feedstocks. These primary NOx reduction methods often involve extra cost or lower combustion efficiency, so NOx measurements are essential if engine/boiler efficiency is to be optimised. Secondary NOx reduction measures are possible by either chemical reduction or sorption/neutralisation. These measures also require accurate monitoring and control.


ChOOSING aN aNalySer In practice, the main methods employed for the measurement of NOx are infrared, chemiluminescence and electrochemical. However, emissions monitoring standards are generally performance based. Infrared analysers measure the absorption of an


emitted infrared light source through a gas sample. In Signal’s PULSAR range, Gas Filter Correlation technology enables the measurement of just the gas or gases of interest, with negligible


42


interference from other gases and water vapour. Electrochemical sensors are low cost and


generally offer lower levels of performance. Users should also be aware of potential cross- sensitivities, as well as calibration requirements and limited sensor longevity. The chemiluminescence detector (CLD)


method of measuring NO is based on the use of a


controlled amount of Ozone (O3) coming into contact with the sample containing NO inside a light sealed chamber. This chamber has a photomultiplier fitted so that it measures the photons given off by the reaction that takes place


when NO contacts O3. NO is oxidised by the O3 and becomes NO2 and photons are released as a part of the reaction. This chemiluminescence only occurs


with NO, so in order to measure NO2 it is necessary to first convert this to NO. The NO2 value is added to the NO reading and this


equates to NOx. For regulatory monitoring, NO2 is


generally the required measurement parameter, but for combustion research and development NOx is the common measurand. Consequently, CLD is the preferred measurement method for development engineers at manufacturer laboratories working on new technologies to reduce NOx emissions in the combustion of fossil fuels. For regulatory compliance monitoring, NDIR (Non-Dispersive Infrared) is more common. Typical applications for CLD analysers


therefore include the development and manufacture of gas turbines, large stationary diesel engines, large combustion plant process boilers, domestic gas water heaters and gas fired factory space heaters, as well as combustion


research, catalyst efficiency, NOx reduction, bus engine retrofits, truck NOx selective catalytic reduction development and any other manufactured product which burns fossil fuels. These applications require better accuracy


than regulatory compliance because savings in the choice of analyser are negligible in comparison with the market benefits of developing engines and furnaces with better emissions and superior efficiency. Signal’s new QUASAR Series IV gas analysers


employ CLD for the continuous measurement of NOx, Nitric Oxide, Nitrogen Dioxide or Ammonia in applications such as engine emissions, combustion studies, process monitoring, CEMS and gas production. The QUASAR instruments exploit the advantages of heated vacuum chemiluminescence, offering higher sensitivity with minimal quenching effects, and a heated reaction chamber that facilitates the processing of hot, wet sample gases without condensation. Signal’s vacuum technology improves the signal to noise ratio, and a fast response time makes it ideal for real-time reporting applications. However, a non- vacuum version is available for trace NOx measurements such as RDE (Real-world Driving Emissions) on-board vehicle testing, for which a 24VDC version is available. A key feature of these latest instruments is the


communications flexibility – all of the new Series IV instruments are compatible with 3G, 4G, GPRS, Bluetooth, WiFi and satellite communications; each instrument has its own IP address and runs on Windows software. This provides users with simple, secure access to their analysers at any time, from anywhere.


Signal Group www.signal-group.com August 2019 Instrumentation Monthly


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