Effect of coal pre-oxidation on the optical texture and porosity of pyrolysis chars

The preparation of activated carbons from bituminous coals usually involves three steps: oxidation, pyrolysis and activation. The porous texture of the final products is conditioned by the previous treatments. In this work, we study the effect of the variables used in the oxidation and pyrolysis steps on the optical texture and the porous structure of the materials obtained. Coals with thermoplastic properties must be oxidised in order to reduce or eliminate plastic behaviour and so ensure a char with a suitable porous network development during the carbonisation step.A high-rank coal was ground and sieved to 1–3 and 0.125–0.425 mm particle size, and then oxidised in an oven with a forced circulation of air, at 270 °C, for periods between 1 and 7 days. The pyrolysis of fresh and oxidised coal samples was performed under nitrogen up to 850 °C at a heating rate of 60 °C min−1.Optical and scanning electron microscopy techniques show the progression of oxidation in the char particles following the shrinking-core model. As the pre-oxidation time increases, the chars lose any vestige of plasticity and the materials show adequate properties as precursors of activated carbons.