SOURCE TESTING ASSOCIATION I Annual Guide 2017
Options for calibrating stack fl ow to EN ISO 16911-2
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Operators of combustion plant need to know the fl ue gas fl ow rate in order to calculate the mass release of pollutant emissions. The fl ue gas fl ow rate (m3
e.g., NOx
/s) is multiplied by the concentration (mg/m3 , to give the mass release rate in mg/s.
A new standard on fl ue gas fl ow rate measurement was published in 2013: EN ISO 16911 ‘Stationary Source Emissions – Manual and automatic determination of velocity and volume fl ow rate in ducts.
The standard is divided into two parts. Part 1 defi nes manual Standard Reference Methods (SRM) to be used for the calibration of continuous stack fl ow monitors and for other compliance purposes, such as periodic testing. Part 2 of the standard applies to continuous monitoring and specifi es the requirements for quality assurance. Operators of large combustion plant are required to have stack fl ow monitors calibrated or fl ow calculations from fuel consumption verifi ed based on EN 14181 and EN ISO 16911-2.
For a QAL2 at least 15 parallel fl ow measurements are required (this can be reduced to 9 if a pre-investigation survey is performed to characterise the fl ow profi le). The data should capture minimum and maximum load. Unlike EN 14181 there is no minimum time frame for completing the 15 tests.
When calibrating stack fl ow monitors or verifying calculated stack fl ow the standard allows point velocity measurements or tracer techniques.
Pitot tubes are used to obtain a representative fl ow measurement by traversing the duct at up to 20 positions, divided into centres of equal area. This method is time consuming - taking around 90 minutes on large ducts where a minimum of 20 points is required. The accuracy of the volumetric fl ow is also dependent on an accurate measurement of the stack Cross Sectional Area (CSA). To confuse matters further, some monitoring locations are not ideal and may not allow a full traverse.
“S” type pitot’s are the most popular style of pitot. Swirl (non-axial fl ow) can be identifi ed but it is not automatically compensated for. They often over-estimate velocity and fl ow rate since the velocity profi le close to the stack wall is too fl at. The measured stack fl ow can then be much higher than the stack fl ow calculated from fuel consumption.
The tracer dilution system directly determines the fl ue gas fl ow rate and does not, therefore, require the CSA to be known. A tracer is injected into the fl ue gas, for a short period of time, upstream of the fl ue, such that the tracer is well mixed with the fl ue gas. The concentration of tracer in the fl ue gas is then measured. A one-off EN 15259 concentration traverse must be performed to demonstrate that the tracer is well mixed for the given injection confi guration. Simple dilution relationships are then used to calculate the fl ue gas fl ow rate from the tracer injection fl ow rate and concentration.
Uniper Technologies offer an ISO 17025 accredited tracer dilution method for calibrating fl ow monitors or verifying fl ow calculations. The technique uses an inert tracer gas that is not affected by the process and can pass through combustion and abatement systems without losses. The diluted tracer gas is dried and analysed using a mass spectrometer. A tracer gas injection of around 1-2 minutes is required to obtain a spot volumetric fl ow measurement. Trials and commercial projects have shown the technique is very accurate giving results closest to the calculated fl ow rate, with a typical uncertainty of <2% at 95% confi dence.
16 Figure 1. Tracer dilution schematic
A recent comparison was made verifying a fl ow rate calculation on the same stack using the tracer dilution system and an “S” type pitot. Figure 2 shows a comparison of the calibration determined using the tracer dilution and “S” type pitot vs calculated fl ow. The tracer dilution technique gave better agreement with the calculated fl ow and was carried out in a matter of hours rather than days. The tracer dilution system was also benefi cial to the process operator as the low load period was only required for around 15 minutes. For comparison the “S” type pitot needed low load for a minimum of 3 hours to complete 2 traverses.
) of pollutant,
Figure 2. Comparison of tracer dilution and S type pitot vs calculated fl ow
Process operators have a choice of approaches to select from when calibrating fl ow monitors or verifying fl ow rate calculations. Pitot tube traverses are widely available and will be offered by most test laboratories. A typical QAL2 will take around 3 days and may over- estimate the fl ow rate. Tracer techniques are quicker, typically taking ½ a day to complete a calibration. They have proven to be more accurate than traditional Pitot velocity techniques giving good agreement with fl ow rate calculations based on fuel consumption.
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