Evolution and Distribution of Surface Voids Affecting Char Burning Rates at Diffusion-Limited Conditions


Research has indicated that the accurate calculation of char burning rates at diffusion-limited conditions requires consideration of the effects of voids (gaps in the continuous surface of burnable material that are deeper than their surface radius) on the particle burning area. Experimentally, void area distributions as a function of void size can be quantified using scanning electron microscopy and digital processing of quenched char particle images. Modeling void area distributions is problematic, even assuming voids behave as pores, because of initial surface imperfections and effects of combustion on surface morphology. This work examines the effects of factors that affect general pore formation for their effect on voids larger than 1 µm, such as heterogeneous combustion, devolatilization, and particle size and how these factors influence the calculation of diffusion limited burning rate from temperature and image data. Void distributions were predicted using an established pore distribution model, with and without the effects of pore combination. Results indicate that void distributions roughly follow expected pore behavior through devolatilization, but not after ignition, with void area fraction increasing with increasing coal volatile content and particle heating rate, but not significantly affected by oxygen concentration or initial particle size. Further, analytical pore models provide reasonable approximations for void distributions in the calculation of particle burning rates from temperature and image data.


Mechanical and Aerospace Engineering

Keywords and Phrases

Coal; Diffusion; Morphology; Particle Size Analysis; Porosity; Quenching; Scanning Electron Microscopy; Temperature; Vaporization; Char Burning Rates; Diffusion Limited Conditions; Oxygen Concentration; Surface Voids; Combustion

International Standard Serial Number (ISSN)

0010-2202; 1563-521X

Document Type

Article - Journal

Document Version


File Type





© 2000 Gordon & Breach Science Publishers Inc, All rights reserved.

Publication Date

01 Jan 2000