Investigation of particle–wall interaction relevant to entrained-flow slagging gasifiers

Understanding the phenomenology and proper design of slagging entrained-flow gasifiers requires the assessment of the fate of char particles as they impinge on the wall slag layer. The relative importance of coal conversion associated with the entrained-flow of carbon particles in a lean-dispersed phase and the segregated flow of char particles in a near-wall dense-dispersed phase have been recently studied. In a previous study, Montagnaro and Salatino developed a phenomenological model that considers the establishment of a particle segregated phase in the near-wall region of the gasifier. It was highlighted that char particles impinging on the wall slag layer can either be entrapped inside the melt (progress of combustion/gasification is hindered), or adhere onto the slag layer’s surface (progress of combustion/gasification is still possible). In the latter case, and if the slag layer is extensively covered by char particles, a particle segregated phase may establish in the close proximity of the wall ash layer, where the excess impinging char particles that cannot be accommodated on the slag surface accumulate. This annular phase is slower than the lean particle-laden gas phase, so that the segregated char particles residence times are longer than the average gas space-time, with a positive impact on carbon burn-off. Further studies confirmed the soundness of this phenomenological framework. Particle–wall interaction occurs according to different micromechanical patterns, which depend on parameters such as particle and wall temperatures, solid/molten status of the particles and wall layer, char conversion degree, surface tension of the slag layer, particle effective stiffness and char/slag interfacial tension. Char–slag interaction patterns are hereby classified on the basis of the stickiness degree of the wall layer and of the char particle:  _the material laying on the wall (prevailingly, inorganic ash) is sticky when the wall temperature is high enough to ensure an ash molten status, generating a liquid slag layer. An additional condition for the slag layer to be sticky is that it must not be extensively covered by non sticky char particles;  _the char particle is sticky when its temperature is beyond the ash melting point, and its carbon conversion degree is beyond a given threshold value, as the plastic behaviour is emphasized when the carbon content is reduced. On the basis of this classification, four interaction scenarios can be considered, namely: (i) non sticky char/ash particles impinging on a molten-slag-covered sticky wall (NSP–SW); (ii) non sticky char/ash particles impinging on a non sticky wall (NSP–NSW); (iii) molten, i.e. sticky, ash particles impinging on a non sticky wall (SP–NSW); (iv) molten sticky ash particles impinging on a sticky wall (SP–SW). This study aims at investigating near-wall particle segregation by using a lab-scale cold entrained-flow reactor. The cold flow model reactor ensures the formation of a dispersed phase and a near-wall layer to reproduce and characterize the four micromechanical interaction patterns.