Coal samples of European and Australian origin have been oxidized in air at 300, 400, and 500Β°C for different lengths of time. The combustion causes formation and opening of pores in the remainder of the coal. Products have been examined by a gravimetric gas sorption technique. Results are presented for changes in surface area and porosity with carbon burnoff and discussed in relation to kinetics of the oxidation.
There is considerable development of surface and porosity as the oxidation proceeds, especially during the first half of the burnoff. However, oxidation of the coal is only slightly accelerated, since most of the new surface is located in micro-or mesopores where access to atmospheric oxygen is restricted by slow diffusion.
When samples of Polish Weglokoks coal are coked in flowing nitrogen at looo"C, most of the volatile matter is lost within 1 h and there is some increase in surface area to give a coke of about 8 m2 g -'.
On further coking up to 24 h, the surface area decreases to about 4 m' g-', becoming comparable with commercial metallurgical cokes (up to 4 m2 8-l). Oxidation of the Polish coke occurs in carbon dioxide at 1000Β°C by the Boudouard reaction (e.g., C + CO, + 2CO), and the surface area of the residue increases from 8 up to about 80 rn? g-' at about one-third burnoff. These increases are not as large as those found for the rapid conbustion of the coal in air at lower temperatures (400 or 500Β°C) of up to 320 mz g-' at just over half burnoff, when it is accompanied by loss of volatile matter. However, the tenfold increase in surface area for the coke is similar to those found previously for the commercial cokes and the earlier stages of the burnoffs also give approximately linear kinetics.