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 been investigated in an electrodynamic chamber (EDC). The main reasons for using the EDC for studying high temperature kinetics of single particles are to (1) sustain the particle without moving at all times at a known point, (2) eliminate heat and mass transfer limitations, (3) observe particle-to-particle differences, (4) fully characterize the particle prior to combustion, and (5) monitor the important properties of the single particle through its entire combustion history. In this device the particle is heated radiatively by a focused laser (CO₂) beam to 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 comparable to the particle diameter. A strong proportional-integral-derivative (PID) position controller was developed to maintain the particle at the center of the EDC, with position stability better than 0.6% of its diameter. Further development of the EDC included (1) real time measurement of the particle shape and diameter with a temporal resolution of 0.1 ms and (2) infrared optical pyrometry with wide spectral bands to determine the particle temperature to within ± 10 K. Oxidation of synthetic char particles (Spherocarb) was studied in the EDC at temperatures around 900 K. Transients of the particle weight, size, shape, temperature, and position of the particle were measured in real-time. Using the present EDC two new phenomena were observed when highly porous chars were heated (in the range 800-900 K): (1) Prior to conversion 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 combustion-the initially spherical particles were consumed nonuniformly in all the numerous experiments. Eventually the spherical particle became a disk. Quantitative results are presented for both phenomena.