Direct optical observation of coal particle fragmentation behavior in a drop-tube reactor

An experimental plant - called Single Particle Disintegrator (SPaltor) - is presented that allows an observation of coal particle fragmentation during the thermochemical conversion process. The SPaltor is a lab-scale drop-tube reactor which runs at atmospheric pressure and temperatures up to 1600 °C. A high-speed camera, a long-distance microscope, and a high-power LED outside the reactor in combination with two mirrors arranged inside the reactor in a 45°-position are used to monitor the inside of the reactor. In this setup, the particle behavior of three different feedstocks were recorded at nitrogen atmosphere. Two recordings are presented in this paper for a reactor temperature of 1400 °C. While a brown coal particle shows no fragmentation, but a bright gas tail due to devolatilisation, an anthracite particle disintegrates into smaller fragments in the first third of the heated zone. For detailed investigation of primary fragmentation, three non-swelling feedstocks were used: A brown coal from Germany, a high-volatile bituminous coal from South Africa, and an anthracite from Germany. Two key figures were used for the description of fragmentation behavior: Fragmentation probability and fragmentation number. The influences of temperature (800-1400 °C), particle size (0.8-3.15 mm), reactor height (0-90 cm), and calculated residence time (0.25-0.5 s) were investigated. Increasing these parameters led to an increase of fragmentation of the anthracite and the high-volatile bituminous coal. In contrast, fragmentation was never observed for the brown coal in any experiment. For the chosen particle sizes, the residence time in the heated zone is smaller than 0.5 s and comparable to free-fall conditions. The heating rate referring to the particle center varies heavily with particle size and coal type.