Near-wall phenomena in entrained-flow slagging gasifiers

The slagging conditions occurring during combustion/gasification of solid fuels play a key role in the design of modern entrained-flow reactors, aimed at increasing the overall process efficiency. 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. 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. 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. Experiments were 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. Assessment of the flow and segregation patterns for different particle–wall interaction regimes 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 lean-dispersed phase and into the wall layer was characterized by their selective collection at the reactor exhaust. In particular, the slag layer formation and the interaction of non sticky particles with the sticky wall were investigated. 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, 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 on both the local hydrodynamics and the near-wall micromechanical conditions