Mass emissions and carbon trading: a critical review of available reference methods for industrial stack flow measurement

Narrow stack emissions: Errors in flow rate measurement due to disturbances and swirl

“…where homogeneous flow conditions and concentrations can be expected," which is stated to be generally fulfilled in a section of duct with no less than five hydraulic diameters of straight duct upstream of sampling plane, as well as two hydraulic diameters downstream, as long as this section of duct is of constant shape and does not include any additional flow disturbances. The field validation trials for the SRM were however carried out at plants with no significant swirl (Dimopoulos, Robinson, and Coleman 2017) or cyclonic flow, so that this part of the SRM requirements is poorly validated, even though the standard does provide ways to correct for swirl when the swirl angle exceeds 15°, after assessment of the non-axial flow in accordance with ISO 10,780 (ISO 1994). It is noteworthy that in the second part of EN ISO 16911 (CEN 2013b), which describes quality assurance methods for automated measuring systems, computational fluid dynamics (CFD) is considered as a method for pre-investigation of the flow profile in the selection of a suitable position of the automated measuring system.…”

Spectroscopic Techniques versus Pitot Tube for the Measurement of Flow Velocity in Narrow Ducts

“…For example, particulate matter, often present in high concentration in many plant exhausts, such as biomass boilers, can obstruct the orifices for measuring the differential pressure or clog the moving parts of an anemometer. A recent review paper [ 1 ] addresses the origin of random and systematic errors for in-stack velocity measurements using Pitot tubes, how these problems are treated in the pertaining international standards and what effect they may have on the uncertainty of the measurements of pollutant emissions. From this survey, it emerges that, in the presence of cyclonic flows, the use of S-type Pitot tubes can result in errors of up to 12%, due to non-axial flows.…”

Flow rate measurement in stacks with cyclonic flow – Error estimations using CFD modelling

“…This gives also the uncertainty limit for the flow rate measurement. In order to meet the uncertainty requirements of the EU's ETS CEN published EN ISO 16911-1 [3] superseding ISO 10780 [5], which was not able to meet such requirements [6]. One of the most common methods used as the standard reference method (SRM) for flow rate measurement in stacks is the measurement of speed of the gas with S-type Pitot tubes in a grid of points inside the stack [3][4][5].…”

“…The standards [5,9] describe how to identify and measure the tangential velocity component (swirl) in a stack with the Pitot tube and [3] then introduces a correction for the swirl that is applied if the swirl angle (yaw angle) exceeds 15°. However, the radial velocity components that also influence the error of the Pitot tubes are not considered by the standard, as neither are swirl angles below 15° (see [6] for a summary). Moreover, if an asymmetric velocity profile occurs, it can rotate with height in the stack due to the swirl and special attention must be paid to the orientation and height of the measurement ports when we want to minimize the flow measurement error.…”

“…Moreover, if an asymmetric velocity profile occurs, it can rotate with height in the stack due to the swirl and special attention must be paid to the orientation and height of the measurement ports when we want to minimize the flow measurement error. The standard [3] was validated via field trials [7], however, the trials were carried out at plants with no significant cyclonic flow so this part of the standard has not been validated [6], which gives a further motivation for investigating the effects of swirl.…”