Development of a hybrid shrinking‐core shrinking‐particle model for entrained‐flow gasifiers

Abstract

in Wiley Online Library (wileyonlinelibrary.com) Slagging entrained-flow gasifiers operate above the melting temperature of the ash. As slag is highly nonwetting on the surface of char (carbon) particles, it is likely that it will agglomerate into one or several slag droplets and some of these droplets can detach from the char particles. If the slag exists in the form of droplets on the char surface rather than as a solid shell around the unreacted char particle, a shrinking particle model would be more physically realistic representation in comparison to the widely used shrinking core model (SCM). In the early section of the gasifier, the temperature remains below the ash melting temperature and, therefore, the SCM is more appropriate in this region. With this motivation, a novel hybrid shrinking-core shrinking-particle model has been developed. The model provides spatial profile of a number of important variables that are not available from the traditional SCM.

Dynamic Model of a Slagging Entrained-Flow Gasifier Including Models of Slag Transport, Deposition, and Slag Layer

“…Due to the exothermic homogeneous combustion reactions that immediately follow the devolatilization reactions, there is a sharp temperature gradient at the entrance of the gasifier. A high temperature is attained very quickly, and the heterogeneous reactions readily take place resulting in slag formation . Since the char temperature remains above the ash melting temperature throughout the gasifier, it is likely that the ash present in the char particles melts forming slag droplets.…”

“…Therefore, the traditional shrinking core model used in the literature, where the ash is considered to form a shell around the char particle, does not seem to be a good characterization. The authors recently presented a new hybrid shrinking-core shrinking-particle model, where a shrinking core model is applied until the solid temperature exceeds the melting point of the ash after which a shrinking particle model is applied. In the shrinking particle model, it was assumed that the slag droplets keep growing in size as char reacts.…”

“…However, deposition of slag due to char impaction may not be the only mechanism by which slag gets added to the flowing slag layer on the wall. As the operating temperature of a slagging gasifier is generally much higher than the ash melting temperature, it is likely that molten ash may be liberated into the gasifier bulk. − The authors of this paper recently developed a shrinking particle model (SPM) that captures the formation of slag droplets due to reaction of char particles . A fraction of these slag droplets can also get deposited on the wall in addition to the char particles.…”

“…In this work, slag deposition and slag layer models have been developed and integrated into the novel shrinking-core shrinking-particle model of a downward-firing, entrained-flow, slurry-fed, oxygen-blown gasifier, recently presented by the authors . Slag deposition, due to char impaction and slag droplet impaction, have been considered.…”

Structural evolution and reactivity of coal chars during combustion in oxyfuel atmosphere

“…Therefore, these structural variations, chemical reaction, pore diffusion, film mass transfer as well as heat transfer should be considered in the modelling of coal char combustion simultaneously. Several gas–solid reaction models, which are commonly used to depict the combustion characteristics of coal chars, should be taken for being appropriate for application. But relatively little attention has been paid to the mathematical model of structural evolution of the char during reaction, while the random pore model (RPM) developed in terms of a pore size distribution can be used to study pore structural variation of char reaction .…”