The contribution of small leaks in a baghouse filter to dust emission in the PM2.5 range—A system approach

Abstract

The contribution of leakage in a baghouse filter (defined as a short circuit between the upstream and downstream sides of the filter) to the emission of fine particles is quantified in comparison to other dust emission sources, and the influence of key operating variables on overall system response is analyzed. The study was conducted on a well-maintained pilot-scale filter unit (9 bags of 500g/m2 calendered polyester needle felt; total surface area 4.2m2) operated in Δp-controlled mode over a range of pulsing intensities, with two types of test dust (one free-flowing and the other cohesive) at inlet concentrations of 10 and 30g/m3. Leaks included single holes between 0.5 and 4mm diameter, intentionally placed in either the plenum plate or one of the filter bags, as well as seamlines from bag confectioning. Emissions were separated by source into a transient contribution due to dust penetration through the filter bags after each cleaning pulse, and a continuous contribution from leaks. This separation was based on a novel method of data processing that relies on time-resolved concentration measurements with a specially calibrated optical particle counter. Tiny leaks on the order of 1mm generated the same emission level as all the bags combined, and dominated continuous emissions. The equivalent leak cross section (leakage=media emission) was about 1ppm of the total installed filter surface, independent of upstream dust concentration. Leakage through open seamlines amounted to 75% of media emissions in case of free-flowing test dust. Leakage was restricted to aerodynamic diameters less than ∼5μm (roughly the PM2.5 mass fraction). For comparison, time-averaged mass penetration through conventional needle-felt media ranged from about 10−5 to 10−6, depending on cohesiveness of the particle material and pulse cleaning intensity, giving emission levels between about 0.02 and 0.2mg/m3 at the reference concentration of 10g/m2.