Wall effects in entrained particle-laden flows: The role of particle stickiness on solid segregation and build-up of wall deposits

Particle–wall interaction is relevant to the performance of entrained-flow slagging gasifiers. Different micromechanical char–slag interaction patterns may establish, depending on the stickiness of the wall layer and of the impinging char particle. The main goal of this study is to improve the mechanistic understanding of particle–wall interactions, by using the tool of physical modeling. The idea behind this research campaign is to use molten wax as a surrogate of fuel ash. The wax had rheological/mechanical properties resembling those of a typical coal slag. Experiments have been carried out in a 0.10 m-ID lab-scale cold entrained-flow reactor, optically accessible, and equipped with a nozzle whence molten wax atomized into a mainstream of air. Reactor lengths in the range 0.1–0.6 m were investigated, while the wax was atomized at a temperature of 100–110 °C. Two interaction regimes were investigated: the “sticky wall–sticky particle” regime was simulated by setting the air mainstream and the wall temperatures at values beyond the wax melting range (160 °C and 140 °C, respectively); the “nonsticky wall–nonsticky particle” regime was simulated by setting both temperatures at 30 °C, i.e. well below the wax softening range. Assessment of the flow and segregation patterns was based on direct visual observation by means of a progressive scan CCD video camera, while the partitioning of the wax droplets into the different phases was characterized by their selective collection at the reactor exhaust. The micromechanics of particle–wall interactions in the “nonsticky–nonsticky” regime was analyzed on the basis of particle impact and of hydrodynamics of gas mainstream and jet flows. Threshold gas velocities for particle detachment were evaluated for the characterization of particle resuspension phenomena.