Technical Market Support » Metallurgical Coal
As efforts to improve the efficiency and reduce costs of the blast furnace continue, increased effort to increase the amount of pulverised coal injection will occur. With increasing injection rates, the pressure to control coal character and the quality of other components increases. One of the issues that needs to be controlled is blockage of the PCI lance. Some operators select coals based on the coals swelling behavior, with high CSN values eliminating the coal from selection. Other operators assess coals based on flowability. This project aimed to assess coal swelling and flowability measures to see if they are suitable techniques to determine a coals potential for blocking the PCI lance.
Eight coal samples were used in this study, with a variety of crucible swell numbers (CSN), ranging from 0 to 6. Each coals propensity to block the lance was determined by passing the coal in a nitrogen gas stream through a lance-like tube heated to 1200°C in a vertical tube furnace. Pressure measurements at either end of the tube were used to measure the change in pressure drop across the tube as blockages developed, a reflection of the reduction in area available for gas flow. The rate of change of pressure drop across the tube with time was used as the coals blockage propensity.
Each coals blockage propensity was compared with their CSN, swelling and thermal conductivity and heat capacity measured by CATA, and flowability determined by Carr Indices and shear testing.
No correlation existed between swelling measured by CSN or CATA, and it can be stated that CSN should not be used to assess coals for PCI to eliminate coals that may block the lance.
A correlation was found between blockage propensity and Carr's flowability Index, a ranked score based on simple measurements including compressibility and angle of repose, though outliers occurred. Similarly, a correlation also existed between the blockage propensity and the shear tests yield strength of the compacted coal at 2.5 kPa shear stress, the force applied to the coal in the tube by the gas flow, but this too had outliers. The measurements included in Carr's flowability Index contain significant errors that eliminate the index as a useful tool for assessing coals if these errors cannot be reduced significantly. Measurements included in Carr's Flowability Index and the Shear Tests are completed on the feed coal, which is very different to the material that forms the blockages within the lance due to the changes that occur during heating. This makes Carr's Index unsuitable to assess coals for blockage potential. Unfortunately, these changes occur over such a limited time frame that shear testing under its current procedure could not be completed at elevated temperatures to attain a suitable result. A new technique should be developed based on the shear test's principles.
No link was found between the proportion of fines present in the feed coal and the blockage propensity. Fines were observed on the tube walls in one blockage but this was not present in the other blockages.
The blockage mechanism is associated with the formation of a liquid in the coal, producing sticky particles. This liquid forms the main structural component of the blockage and can be seen in the SEM images of the blockages as fused material. Unfused particles are included in some of the blockages, probably collected by the sticky fused particles. Two types of blockage were found to form; the first where the coal particles stuck to each other, forming a central column in the tube with connection to the wall at the blockage initiation point. In these coals the wettability of the fused particles with the tube wall would be low. Blockage propensity, the rate of pressure drop increase with time, of these coals all occurred around 11 kPa/min. The second type of blockage produced a tortuous gas path through a coal matrix where particles stuck to each other and the tube wall. These blockages produced a wide range of blockage propensities: between 2.8 and 22.65 kPa/min. One coal, a semi-anthracite, did not form a blockage.
Two coals formed their blockages further along the lance than the other coals suggesting that the time for the sticky material in the coal particles to form took longer to form in these coals, highlighting that the length of the lance in avoiding blockages may be just as important as the coals blockage propensity.