LAWR

Source TeSTing ASSociATion | Annual Guide 2014 4 O perators of combustion plant need to know the flue gas flow rate in order to calculate the mass release of pollutant emissions. e flue gas flow rate (m 3 /s) is multiplied by the concentration (mg/m 3 ) of pollutant, e.g., NO x , to give the mass release rate in mg/s. is information may be required for emissions trading, compliance or in ventory reporting, or for air quality modelling purposes. A new standard on flue gas flow rate measurement was published in 2013: EN ISO 16911 'Stationary Source Emissions – Manual and automatic determination of velocity and volume flow rate in ducts'. e scope of the standard, based on the original mandate from the European Union, is linked to the requirements of European Direc tives, including the Industrial Emissions Directive (IED) and the EU Emissions Trading System (EU ETS) which allows this alternative 'measurement' approach for CO 2 and requires it for emissions of N 2 O and CH 4 from other sectors, all subject to defined uncertainty requirements. European Directives require the use of CEN standards when available. e standard is divided into two parts. Part 1 defines manual Standard Reference Methods (SRM) to be used for the calibration of continuous stack flow monitors and for other compliance purposes, such as periodic testing. Part 2 of the standard applies to continuous monitoring and speci fies the requirements for the certification, calibration and ongoing control of continuous flow monitors. Part 1: Manual Reference Method Part 1 of the standard is performance based, that is, a number of different techniques may be used as the manual reference method provided that the specified performance requirements are satisfied. e alternative techniques in clude: velocity traverses with Pitot probes (various designs) or vane anemometers; tracer (dilution) and tracer (timeof flight) methods. Under certain circumstances, flow calcula tion from fuel consumption can be used to perform compli ance checks and a mandatory calculation approach is also provided in Part 1 (Annex E). Table 1 summarises the ap plicability of the different techniques. Point velocity measurements are, evidently, required when measuring the velocity profile in order to determine if a given measurement plane is suitable for the installation of a flow monitor, for example. Any type of Pitot tube or vane anemometer, with a traceable calibration can be used for this purpose, provided that the level of swirl is low (nomi nally less than 15° swirl angle at all traverse points). If the level of swirl is significant, then the traverse must be con ducted using a 3D or 2D Pitot, noting that a conventional Stype Pitot can be operated as a 2D Pitot with measure ment of the swirl angle. e 3D approach, as the name suggests, measures all three velocity components, including the axial velocity that is required for an unbiased flow rate determination. e spherical (5hole) Pitot, shown in Figure 1a, is an example of a 3D device. is is inserted into the flow and turned until one of the ∆P measurements is nulled. Wind tunnel calibration relationships are then used to calculate all three velocity components from the various measured ∆Ps. e operation of 3D Pitots is described in detail in US EPA Method 2F. e Stype Pitot, shown in Figure 1b, is commonly used The new flue gas flow rate standard EN ISO 16911 Measurement Objective Applicable techniques Velocity profile Point velocity measurement: - Pitot tubes (∆P measurement) - vane anemometer Swirl angle Point swirl angle measurement: - S-type Pitot tubes - 3D or 2D Pitot tubes Periodic measurement of average - Pitot tube traverse (∆P) (averaged) velocity (flow rate) - vane anemometer traverse (averaged) - tracer dilution technique - tracer transit time technique - calculation from fuel consumption Calibration of flow monitors for - Pitot tube traverse (∆P) (averaged) average velocity (flow rate) - vane anemometer traverse (averaged) - tracer dilution technique - tracer transit time technique Figure 1b: S type Pitot head Figure 1a: Spherical (5-hole) Pitot head Table 1: Applicability of manual reference techniques STA Article 2 WP.indd 4 05/02/2014 19:46 5 Annual Guide 2014 | SOURCE TESTING ASSOCIATION to establish iso-kinetic sampling conditions when measur- ing dust concentrations. is is normally inserted into the ow so that the 'impact' ori ce faces into the ow and the 'wake' ori ce is then positioned at 180° to this. Operation as a 2D Pitot is described in detail in US EPA Method 2G. Note that, if a Pitot tube is used in a con guration with a closely coupled gas-sampling probe, then the device shall be calibrated in this con guration. For determining the average velocity, the traverse points are located at centres of equal area so that a simple average of the point readings gives an area weighted average in a duct of circular cross-section. e procedures for deter- mining the required number and location of points are speci ed in EN15259, noting that the 'tangential method' is required by EN ISO 16911, i.e., the centre-line of the duct cannot be included. Twenty measurement points are normally su cient in large ducts. e eld trial validation indicated that lack of uniformity of the ow pro le (Figure 3) caused by a poor measurement location did not signi cantly a ect the average velocity deter mination. at is, a 20 point average form a poor ow pro le gave the same result as a 20 point average from a uniform ow pro le. Performance requirements and quality assurance require- ments are speci ed for each technique. For Pitot tubes, a pre-test leak check is required and, when using an elec tronic pressure reading device, a daily calibration check is required using a liquid manometer (temperature corrected) or a cali- brated pressure sensor with an uncertainty better than the test device. e repeatability also needs to be determined at a single measurement point (the standard deviation of ve consecutive 1 minute velocity readings). Each point velocity measurement must be obtained from a one minute average ∆P based on a continuous measurement or at least three separate readings. A velocity traverse to EN 15259 does not have su cient resolution to capture the very low velocity boundary layer at the duct wall. For a large duct, this can optionally be measured according to US EPA Method 2H. However, the correction is usually very small and it is normally su cient to multiply the measured average velocity by a Wall Adjust- ment Factor of 0.995 for a smooth duct or 0.99 for a rough (brick-lined) duct of circular cross-section. is is a require- ment when calibrating a ow monitor. Tracer transit time methods determine the bulk (average) velocity directly by recording the time taken for a tracer material to travel between two measurements stations (∆t). e distance between these two stations, situated in duct work of constant cross section, is divided by the measured time-of- ight to obtain the average velocity. e example in the standard is based on the injection of a radioactive tracer, upstream of the ue. Two sets of clamp-on detectors are then used to detect the arrival of the tracer at two di erent heights within the ue. e medians of the recorded tracer concentration peaks are extracted so that the shape of the detector response is taken into account to obtain an accurate ∆t. In order to obtain the volumetric ow rate, the average velocity must be multiplied by the duct cross-sectional area. EN ISO 16911 requires the Test Laboratory to measure the duct dimensions, across at least two axes, rather than simply relying on plant drawings. e tracer dilution method directly determines the ue gas ow rate and does not, therefore, require the cross- sectional area to be known. A tracer is injected into the ue gas, for a short period of time, well upstream of the ue, so that the tracer is intimately mixed with the ue gas. e concentration of tracer in the ue gas is then measured. A one-o EN 15259 concentration traverse must be per- formed to demonstrate that the tracer is well mixed for the given injection con guration. Simple dilution relationships are then used to calculate the ue gas ow rate from the tracer injection ow rate and concentration. If all of the above techniques are regarded as di erent implementations of the same method, the ensemble average uncertainty, based on validation eld trials, is estimated to be ± 5% at 95% con dence, assuming that the ow is non- swirling. However, it is anticipated that a lower un certainty can be obtained using a speci c technique in a given appli- cation. e Test Laboratory must calculate the uncertainty of the method, using the approaches described in the stand- ard, and ensure that this complies with the requirements of the Test Objective. Part 2: Automated Measuring Systems Part 2 of the standard is also performance based, that is, provided that the speci ed performance requirements are satis ed, any continuous monitoring technique can be employed, e.g., single point or averaging Pitot tubes, hot * © British Standards Institution (BSI – www.bsigroup.com). Extract reproduced with permission. Source: BS EN 15259:2007 Air quality. Measurement of stationary source emissions. Requirements for measurement sections and sites and for the measurement objective, plan and report. FD- points FD 25 20 15 10 5 -1.4 -1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 0 D C A FD+ points Velocity proles (20m) L1 & L2 U = 19.5 m/s AE-CG Traverses 5-6-9-10-12-13-16-17-19-20 Radial position (m) Velocity (m/s) F G E FD- H B Figure 3 Velocity proles from a validation eld trial 1 2 x d 3 4 i Figure 2: EN15259 Traverse Points* STA Article 2 WP.indd 5 05/02/2014 19:47

• Cover