Monte-Carlo modelling to demonstrate the influence of alternative flow reference techniques on annual mass emission uncertainty

Industrial emissions into the atmosphere are quantified by measuring the concentration and flow rate in a duct or stack prior to release. These measurements are combined to produce a mass emission which is reported to the European Pollutant Release and Transfer Register. These measurements have to be made according to standards to ensure that they are consistent and accurate. Uncertainty limits are set in relevant EU legislation which must be adhered to. The standard for flow measurement, EN ISO 16911, is in two parts covering manual reference methods and automated measuring systems (AMS). The former outlines six validated methods for calibrating an AMS, while the later outlines the quality control system to be followed to meet the requirements of the legislation. However the standard does not provide consistent information on the impact of the different calibration methods on the uncertainty of the reported emissions. Here we model a system monitoring stack flow and pollutant concentration, including quality control procedures in EN ISO 16911. Several alternative reference methods are modelled to compare the effects of using these different techniques for calibration on the uncertainty of reported annual mass emissions. In a low uncertainty regime the L- and S-type Pitot provide the best performance for both constant and variable flows, however in higher uncertainty regimes all the techniques led to a bias that would lead to misreporting of emissions. Uncertainty information on the techniques is not equally characterised, however, this information represents the best current knowledge of these uncertainties in the emissions community and so the comparisons reported here are all made with the best knowledge available.