Direct numerical simulation (DNS) of alkali metals released during char combustion

Abstract

Alkali metals in pulverized coal easily released in gas phase, cause significant fouling, atmospheric pollution and etc. during combustion and gasification. A TURE DNS method, based on fictitious domain strategy with finite volume (FV) scheme, was developed to describe the behavior of alkali metals release and transport in reacting fluid–solid flow at sub-particle level. Taking water-soluble sodium chloride (NaCl) of high harmfulness as an example, alkali metals release and transport in physical field of char combustion, as the main stage of coal combustion, was simulated. A 50 × 10−6 m single particle, and 150 1 × 10−3 m closely spaced particles combustion at 1273 K air environment were respectively calculated. Computational results revealed that continuous release of alkali metals depends heavily on the local high temperature from the stable combustion stage (approximately 85% of the whole combustion stage) which produces stable heat. And complicated movement of fluid medium has a great effect on the transport behavior of alkali metals in gas phase, especially the entrainment of vortexes, which results in the local enrichment of alkali metals in gas phase.