A specific uncertainty of sampling polydisperse particulate matter in gravimetric dust concentration measurements in conduits

摘要

A study of a simulation-calculation character has been undertaken in order to determine one of the uncertainty components of the gravimetric measurement of the mass concentration of total particulate matter in flue gas flowing through a conduit. This uncertainty component is attributed to the degree of representativeness of the isokinetic or anisokinetic sampling of polydisperse particulate matter in the situation when the granulometric composition and density of dust are not known in detail, which is common in routine dust emission field tests. In such cases. the quantitative description of this representativeness, being part of global measurement accuracy analysis, is either ignored or overestimated in practice. Well estimated, the uncertainty component in question is therefore needed for practical purposes. In the study reported here, many dusts of diverse particle size distributions have been simulated representing dusts actually occurring in typical places of dust removal plants, i.e. before and after the dust collectors. Definite aspiration characteristics, pertaining to a really existing and used sampling nozzle, have been applied for the calculations. Typical dust densities and isokinetic sampling rates have been adopted. Using the above, the total dust concentration error recorded for the simulated sample taken has been calculated. Based on the distributions of the obtained errors, the uncertainty (along with a necessary correction factor) of the measured concentration has been established as being dependent on the general type of dust, the dust density range, isokinetic rate, and nozzle size. The discussed measurement uncertainty component is estimated to be up to 12% and should be appropriately taken into account in measurement results of total dust concentration in ducts and stacks.