Physical modelling of near-wall phenomena in entrained-flow coal gasifiers
Creators: name: Troiano, Maurizio
Publisher: name: Università degli Studi di Napoli Federico II
Year: 2015

description: Combustion and gasification under slagging conditions are key aspects of the design of modern entrained-flow reactors for thermal conversion of solid fuels, aimed at increasing the overall energy efficiency. In these systems, solid particles migrate toward the reactor walls, due to swirled/tangential flow induced in the reaction chamber and to turbophoresis, generating, thanks to the very high operating temperatures, a slag layer that flows along the reactor internal walls and is drained to the bottom of the reactor. The recent literature on entrained-flow gasification has addressed the fate of char particles as they impinge on the wall slag layer. Different micromechanical char–slag interaction patterns may establish, depending on the stickiness of the wall layer and of the impinging char particle (namely sticky wallsticky particle, non sticky wallnon sticky particle, non sticky wallsticky particle and sticky wallnon sticky particle regimes).
This study aims to contribute to the development of a phenomenological model of the fate of coal/ash particles which considers the establishment of particle segregated phases in the near-wall region of the gasifier. In particular, near-wall phenomena were investigated and mechanistic understanding of particle–wall interaction patterns in entrained-flow gasifiers was pursued using the tool of physical modeling. Montan wax was used to mimic, at atmospheric conditions, particle-wall interactions relevant in entrained-flow gasifiers. As a matter of fact, this wax had rheological/mechanical properties resembling under molten state, those of a typical coal slag and, under solid state, those of char particles. Experiments have been carried out in a lab-scale cold entrained-flow reactor, optically accessible, and equipped with a nozzle whence molten wax atomized into a mainstream of air to simulate the near-wall fate of char/ash particles in a real hot environment. Reactor lengths in the range 0.1–0.6 m were investigated, while the wax was atomized at a temperature of 100–110 °C. The four particle-wall interaction regimes were investigated. 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/particles into the different phases was characterized by their selective collection at the reactor exhaust. Results showed that the particle-wall interaction mechanisms and segregation patterns are deeply affected by the stickiness of both the wall layer and the impinging particle. In particular, the partitioning results of the wax into the lean-dispersed phase and the wall layer indicated that sticky particles mainly adhere on the wall surface, regardless the stickiness of the wall, whereas non sticky particles may rebound, deposit and be resuspended into the main flow upon the impact on a dry wall, depending also from the local hydrodynamic conditions. As regards the interaction of non sticky particles with a sticky wall, the partitioning results lie between those obtained for the other regimes. Moreover, from a phenomenological point of view, particles follow the gaseous streamlines at the center of the duct, as expected in dilute particle-laden flows. On the other hand, the particle flow pattern in the near-wall region is barely influenced by the gas flow, whereas it is strongly affected by particlewall micromechanics, which induces particle segregation and accumulation phenomena. It is possible to conclude that micromechanical interaction of a particle with a sticky wall enhances particle transport to the wall and the tendency to reach a segregation-coverage regime with the formation of a dense-dispersed phase in the near-wall of the reactor.
Another kind of experiments was also accomplished, in order to study the micromechanical particle-wall interaction. To achieve this objective, a proper lab-scale apparatus was designed and built up, in which high speed imaging and tracking of wax particles impacted onto a flat surface at near-ambient conditions were carried out. Particle–wall collision was described in terms of normal and lateral restitution coefficients and capture efficiency. The influence of the particle stickiness, impact velocity and angle, and surface properties and structure of the target on the rebound patterns was studied. Results indicated that the elastic–plastic adhesive model provides an adequate representation of the non sticky particlewall collisions. Moreover, the presence of a powder layer on the target favours energy dissipation and accumulation of particles close to the surface. This pattern promotes the establishment of a dense-dispersed phase in the near-wall zone of entrained-flow slagging gasifiers. Increasing the temperature, particles shift from the solid/plastic to the fluid state and the coefficient of restitution drops to vanishingly small values, confirming that deposition is the prevailing phenomenon during the collision of sticky particles on a wall.