High temperature kinetics of pulverized-carbon combustion

Bartholomew Joseph Waters, Purdue University

Abstract

Simultaneous temperature, size, and velocity measurements of individual carbon particles (90 $<$ d$\sb{\rm p} < 170 \mu$m) burning in an entrained-flow reactor are used to calculate overall and intrinsic combustion rates and rate parameters at conditions typical of pulverized-fuel combustors (i.e. particle temperatures from 1300 to 2300 K). In addition, particles are collected and the carbon mass loss is determined at three residence times in the reactor for eight different sets of bulk O$\sb2$ mole fractions (from 0.12 to 0.36) and gas temperatures (from 1300 to 1800 K). The two model chars employed in the studies described here were chosen for their low ash and volatile matter contents to preclude the complicating effects of these components on the oxidation process. In one study, the measured carbon mass loss of spherical particles is compared to calculations of the particle-mass profile based on the overall combustion rate parameters obtained from the experimental particle temperatures. A second study employed the same experimental techniques to examine the effects of highly non-spherical carbon particles on the analysis of kinetic combustion data. Despite the uncertainty in the particle temperatures caused by the irregular shape of the particles, the kinetic parameters are found to be relatively insensitive to the simplifying assumption of a spherical particle. In a third study, the morphological development and intrinsic reactivity of the two microporous carbon chars are investigated. The partially oxidized samples (0 to 65% mass loss), which were collected from the entrained-flow reactor, are experimentally characterized by their total surface area, particle diameter, bulk density, pore volume distribution, and macroporous surface area. SEM photomicrographs of the external particle surface and reflected-light photomicrographs of the internal porosity of the particles are also presented. The Thiele model of diffusion and reaction in a porous particle is used to calculate intrinsic combustion rates from the measured overall rate data for both chars. The high temperature (1300 to 2300 K) intrinsic combustion rates calculated here show good agreement with extrapolated intrinsic rates measured in a TGA apparatus at temperatures below 850 K. The ramifications of this agreement on the current mechanistic understanding for the carbon-oxygen reaction are discussed. (Abstract shortened with permission of author.)

Degree

Ph.D.

Advisors

Squires, Purdue University.

Subject Area

Chemical engineering

COinS