Modification of coking properties due to oxidation. Effect of relative humidity

This work is a systematic laboratory study of coal oxidation under well-controlled temperature and humidity conditions that attempts to simulate those found in the transport and storage of coking coals. Four bituminous coals of different rank were oxidised at 50 °C under different humidity conditions in order to establish the effect that the moisture of the air medium has on their coking properties. The effect of oxidation was examined by means of the free swelling index (FSI), the Gieseler test, thermogravimetry and coke microstrength measurements. The point of zero charge (pHPZC) was explored as a possible method for detecting coal oxidation. In general, the results obtained revealed a deterioration of the thermoplastic properties, a decrease in the point of zero charge and a diminution of the maximum rate of volatile matter evolution. It was observed that the highest humidity level (90% rh) retarded the oxidation of the lowest rank coal during the first few days of oxidation, whereas the higher rank coals were affected more by high humidity conditions. It was also found that low volatile coals become more dangerous after oxidation regardless of the humidity conditions. No significant variations were observed in the mechanical strength of the resultant cokes attributable to the humidity conditions.