Modelling entrained-flow slagging gasification of solid fuels with near-wall particle segregation

A compartmental model of entrained-flow slagging gasifiers of solid fuels is hereby presented. The novel feature of the model is a comprehensive "particle segregation" submodel, based on the near-wall phenomena associated with bulk-to-wall particle transfer. Ash deposition onto the reactor walls, adhesion and inelastic rebound of char particles upon collision with the wall, confinement and dissipation of kinetic energy in the near-wall region promote the formation of a dense-dispersed particle phase where an extended inventory of char particles accumulates. The fate of char particles is analyzed by considering migration toward the wall, interaction with the dry or the slag-covered wall, deposition onto the slag layer, accumulation in the near-wall region and heterogeneous gasification reactions. The core of the model consists of mass balance equations on solids and gases in lean (bulk), dense (near-wall) and slag phases. The constitutive equations and parameters of the particle-wall interaction submodel were partly determined by validation against experimental results, partly treated as model parameters. Although the model cannot be used as a predictive tool at its present stage, due to the uncertainties associated with the determination of the model parameters, it provides a powerful tool to assess the relevance of particle segregation to the gasifier performance and to point at the research priorities that could improve its predictive ability. Application of the model is demonstrated with reference to a downdraft entrained-flow slagging gasifier. Model results are discussed in terms of partitioning of char/ash particles among lean, dense and slag phases, carbon gasification degree, syngas composition and lower heating value. A sensitivity analysis with reference to selected model parameters is carried out to evaluate their relevance to the gasifier performance prediction.