On the combustion kinetics of heterogeneous char particle populations

Abstract

A laboratory-scale laminar flow reactor is used to measure char combustion rates under conditions simulating those found in pulverized coal-fired boilers. In the present article, a unique optical technique is used to measure, in situ , the temperatures, sizes, and velocities of individual burning char particles from Pocahontas #3 low-volatile bituminous coal. These optical data are used in conjunction with a model of surface reaction and gas-particle transport processes to compute single-particle burning rates and global, n th -order rate coefficients. The optical technique records a broad distribution of char particle temperatures at any given location in the laminar flow combustor. When single-particle rate coefficients are derived from these optical data, characteristic features are observed on Arrhenius plots, consisting of distinct, linear groupings of single-particle rate coefficients. These linear features are shown to be the outlines of characteristics curves representing the equations governing gas-particle transport processes. The linear groupings of rate coefficients, like the theoretical characteristic curves they follow, exhibit pseudo activation energies that are independent of the actual global kinetic activation energy. The characteristic curve concept is used to discuss the features of single-particle combustion. Particle-to-particle variations in reactivity and physical properties are identified as the origin of the particle temperature distributions and of the linear features on Arrhenius plots. It is shown that no singl,, simple rate expression is capable of describing the unique combustion behavior of each particle. This fact notwithstanding, many applications require simple rate expressions that approximately describe the combustion behavior of the marcoscopic sample. Techniques for determining such rate expressions for heterogeneous particle populations are evaluated.