Technical Market Support » Metallurgical Coal
The aim of this project was to assess the suitability of drop tube furnaces as a tool for assessing coal combustibility for use in pulverised coal injection (PCI) into blast furnaces. Drop tube furnaces (DTF) are far less expensive than pilot scale PCI rigs, they are reliable and available at many laboratories. The project established a recommended methodology for a DTF test. The coal's burnout produced in a drop tube furnace, operated at "standard" operating conditions (at high oxygen‐in‐oxidant/carbon-in‐coal ratio), was compared with the burnout produced by the same coal in the BlueScope Steel test program, which was previously completed on the BHP Billiton PCI rig located at BHP Billiton's Research Laboratory at Newcastle. The morphology of selected chars produced in the two rigs was compared.
An assessment of the data of utilisation and performance of PCI test rigs available in the open literature was also undertaken.
Drop tube furnace tests were found to provide a generally similar ranking for coals on the basis of combustibility (burnout) against the proximate analysis volatile matter content to that from PCI pilot‐scale tests. The results from both DTF and PCI rigs provide a trend line for coals from anthracite to high volatile bituminous coals for combustion with respect to coal volatile matter. Differences exist between the trends produced on the drop tube furnace and the PCI rig, with the PCI rig indicating better relative burnout performance for low volatile coals. The drop tube furnace also produced a wider range of burnout compared to the PCI rig which may be due to the differences in combustion conditions of the two techniques and the relative contribution of the component processes in combustion. Low volatile coals varied their position within the results of the two rigs, with one coal sitting well above the PCI trend line. This coal had good performance in DTF but not the outstanding performance seen in PCI. The results suggest volatile release has a significant impact on combustion performance in PCI and further work is required to prove this.
The DTF technique was validated by confirming that the chars produced in the two rigs are morphologically similar. The char formed in the PCI rig is intrinsically less reactive but has significantly larger surface area resulting in apparent reactivities that are slightly larger. The PCI chars have larger crystallite sizes, especially chars produced from low volatile coals, compared to DTF chars.
The detailed review of the open literature on PCI test rigs, undertaken in Stage 1 of the project, has revealed that significant differences exist between the four identified types of experimental rigs used to study coals for PCI applications. Both the tubular combustion test rigs with constant internal diameters and the two furnace "Aachen type" rigs operated with air based combustion gases produced results with low burnout when low volatile coals are tested. Comparison of PCI test rig results and DTF test results have not been completed prior to this study.