Abstract
Four bituminous coals of the same rank but with different thermoplastic properties were oxidized at 40 and 50°C for one month. The chemical changes due to oxidation were studied by applying Gieseler plastometry, the free-swelling index (FSI), thermogravimetric analysis and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). In general, oxidation at 50°C produced a greater impairment of coking properties, the different responses to oxidation depending on the parent coal. The main effects caused by oxidation were a loss of thermoplastic properties, a decrease in aliphatic hydrogen and a slight increase in groups with oxygen functionalities.Both the fresh and oxidized coals were carbonized in a movable-wall oven of 17kg capacity and the quality of the resulting cokes was tested by means of the JIS and ASTM standard methods. The results showed that, in certain cases, mild oxidation produced an improvement in coke quality. Oxidation also affected the porosity of the cokes, which may be related to the coke quality. It was found that the higher quality cokes had less macroporosity, whereas the lower quality cokes had a higher pore volume.
Highlights
The natural oxidation of coal occurs when the coal is still in the seam and during its transport and storage
The results showed that, in certain cases, mild oxidation produced an improvement in coke quality
All the coals are of the same rank, with a volatile matter content of around 31 wt. %, and sulphur and ash contents of less than 0.93 and 10.5 wt. %, respectively, which is characteristic of coals used for metallurgical coke production [19]
Summary
The natural oxidation of coal (weathering) occurs when the coal is still in the seam (outcropping) and during its transport and storage. It is well known that oxidation leads to a gradual impairment of the fluid properties of coal [3,4,5,6], which becomes more inert during the coking process with a consequent decrease in coke quality and an increase in reactivity [7,8,9]. Because of this and because oxidation causes a reduction in coke yield, carbonization rate and coke productivity [4,7,10], the oxidation of coal is a cause of great concern for the cokemaking industry. Slight weathering improves the quality of cokes from high volatile coals and of those from medium and low volatile coals [13,14,15]
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