Factors affecting the tensile strength of hot-gas filter dust cakes

Abstract

The University of North Dakota Energy and Environmental Research Center is determining some of the fuel-, sorbent- and operations-related conditions that lead to blinding or bridging of hot-gas particle filters in pressurized fluidized bed combustion systems. Several mechanisms contribute to the stickiness of dust particles within a filter cake: electrostatic and van der Waals attractions, and mechanical, liquid and solid bridging. The magnitudes of these mechanism impacts depend on the size and composition distributions of the dust particles, cake porosity, gas composition and system temperature. High-temperature tensile strength measurements of filter dust cakes have shown that only a small amount of liquid is necessary to double the tensile strength of a cake. The amount of strength increase is related more to the wettability of the dust than to the liquid's viscosity or air-liquid surface tension. Also, if the tensile strength of a filter cake is less than approximately 50 N/m 2, the cake re-entrains after being pulsed off and can lead to blinding of the filters. If the cake strength is over approximately 300 N/m 2, it can bridge between filters, in some cases forcing them apart and causing them to break. However, the factors affecting cake strength are not independent variables and laboratory measurements have shown that the tensile strength of filter dust does not adequately explain its propensity to bridge across filters in a hot-gas filter system. A better measurement is the tensile strength divided by the density of the cake, or its specific strength. The authors have defined the cake specific strength as its critical thickness index (CTI) because it indicates the maximum thickness for which a cake would build before failing or shedding under its own weight. Laboratory measurements show that the CTI is a better indicator of the observed propensity of a particular dust to form bridges between filters than its simple tensile strength.