OBSERVATION OF NONUNIFORM SHRINKAGE AND ACTIVATION OF HIGHLY POROUS CHARS DURING COMBUSTION IN AN IMPROVED ELECTRODYNAMIC CHAMBER

Combustion of single particles of highly porous synthetic char has bee n investigated in an electrodynamic chamber (EDC). The main reasons fo r using the EDC for studying high temperature kinetics of single parti cles are to (1) sustain the particle without moving at all times at a known point, (2) eliminate heat and mass transfer limitations, (3) obs erve particle-to-particle differences, (4) fully characterize the part icle prior to combustion, and (5) monitor the important properties of the single particle through its entire combustion history. In this dev ice the particle is heated radiatively by a focused laser (CO2) beam t o the desired temperature. During the heating the particle should not move by more than 1% of its diameter since the waist of the beam is co mparable to the particle diameter. A strong proportional-integral-deri vative (PID) position controller was developed to maintain the particl e at the center of the EDC, with position stability better than 0.6% o f its diameter. Further development of the EDC included (1) real time measurement of the particle shape and diameter with a temporal resolut ion of 0.1 ms and (2) infrared optical pyrometry with wide spectral ba nds to determine the particle temperature to within +/-10 K. Oxidation of synthetic char particles (Spherocarb) was studied in the EDC at te mperatures around 900 K. Transients of the particle weight, size, shap e, temperature, and position of the particle were measured in real-tim e. Using the present EDC two new phenomena were observed when highly p orous chars were heated (in the range 800-900 K): (1) Prior to convers ion there was a stage in which mass loss or size change did not occur. This is attributed to activation of the char and was found to depend on the particle temperature. (2) Nonuniform shrinkage during combustio n-the initially spherical particles were consumed nonuniformly in all the numerous experiments. Eventually the spherical particle became a d isk. Quantitative results are presented for both phenomena.