Numerical analysis on non-uniform gas distribution induced conversion of carbon particles during entrained-flow gasification

Abstract

During the entrained-flow gasification process, the relative positions of the dilute particle group restrict the diffusion of gaseous reactants between the particles, leading to variations in reaction rates. In this study, the conversion of the double coal char particles moving in hot O₂/CO₂ environments is numerically investigated with pseudo-steady-state (PSS) approach. The double particles are placed parallel to the direction of the gas velocity in laminar flow. The Navier-Stokes equations are applied with energy and species transport model, as well as homogeneous and heterogeneous reaction mechanisms. The influences of the particle distance, particle size and bulk flow velocity corresponding to entrained-flow gasification conditions on reaction characteristics are considered. The results indicate that the space between two particles plays a significant role in restricting gas diffusion. This restriction leads to variations in gas velocity between the windward side and the leeward side of the double-particle system. Stefan flow generated by chemical reactions on the particle surface is initially perpendicular to the particles, and eventually aligns with the bulk flow field in the gasifier, contributing to the overall gas dynamics. When small-size particles (with a radius of less than 0.3 mm) are more widely dispersed, the flame sheet exhibits greater expansion. The overall carbon consumption rates of the double particles rise as the distance between particles and their size increase, with the carbon consumption rate ratio consistently exceeding 1. Higher bulk flow velocity accelerates convective mass transport, resulting in a thinner flame layer on the windward side. Additionally, the increased partial pressure of gaseous reactants elevates the reaction rate, and the higher pressure at elevated Reynolds numbers further enhances particle conversion efficiency.