High Rate PCI and the Interaction with Coke Quality

This substantially experimental study was aimed at investigating the interaction of coke with pulverised coal, under conditions configured to simulate coke at the back of the raceway of an ironmaking blast furnace under a regime of high rate pulverised coal injection. In the test rig used for these tests, pulverised coal was injected into a 1200 oC blast upstream of a restriction imposed by a tuyere (70 mm) and the combusting plume of pulverised coal subsequently underwent a relatively free expansion on passing through the combustion test section of nominally 300 mm diameter. In this way, it is hoped to provide a more realistic simulation of the actual processes taking place in injection into a blast furnace raceway.

The study considered five cokes (three pilot oven cokes and a production coke with DI 150/15's of greater than 84 and CSR values covering the range 34.9 to 72.4, and a blast furnace bosh coke essentially matched to the production coke sample) exposed to pulverised coal flames developed with eight coals ranging in volatile matter from 11.2 %(db) to 36.4%(db). The coals were selected to be typical of the coals commonly deployed in blast furnace PCI installations.

The analysis has been restricted to test conditions approximating equivalent injection rates of 150 kg/tHM at 3 per cent blast oxygen enrichment. The limited number of coke samples exposed to flames developed at equivalent injection rates of approximately 200 kg/tHM at 5% blast oxygen enrichment have not been analysed.

Attempts to view the interaction process with the fibre-optic viewing probe were unsuccessful.

In both the SEM and optical microscopic inspections of the coke cylinders exposed to the pulverised coal flames there was no evidence of any accumulation of pulverised coal at the reacted surface of the coke cylinders as:

• physical accumulations of char developed over the reacted surface,
  
• accumulations trapped within the pore volume of pores open to the surface of the coke cylinder, or
  
• coal char wetted by the substantially fused mineral matter developed at the surface of the coke cylinder.

The microscopic investigations, and the optical microscopic inspections in particular, indicate that the reaction between the coke cylinder and the constituents of the pulverised coal flame is confined to the surface of the coke, or to the porosity that is likely to have an opening to the surface of the coke cylinder.

There was some degradation of the microtextural components throughout the entire volume of the coke cylinders. Patchy degradation of porous IMDC grains was common in all cokes exposed to the pulverised coal flames, and in all cases there was a very close association to mineral matter originally contained within the pores of the porous inerts. There was also some degradation associated with selected RMDC microtextural components in each series of cokes. Such degradation is described as "onset pervasive" and was identified by the development of reacted material around mineral matter encapsulated by the RMDC components and over much larger domain sizes (100 - 300 ?m), a subtle decrease in the range of the anisotropic colours and the reflectance.

In ordering the exposures of each series of cokes in respect of decreasing volatile matter of the injected coals, the presence of such "onset pervasive" degradation within the RMDC components generally increased in the direction of decreasing coal volatile matter. The most extensive development was in all cases in cokes exposed to coals of the lowest volatile matter, including the matched blast furnace bosh coke.

The mineral matter developed at the surface of the cokes exposed to the pulverised coal flames, as well as the cokes heated under an atmosphere of flowing argon, was in all cases substantially quenched from a fused condition, while the mineral matter contained within the body of the coke cylinders was, from the SEM inspections, in a substantially unfused condition. Mineral matter in the near surface region of the coke cylinders (depth of 50 to 100 ?m below the surface) consisted of both fused and unfused entities.

Energy dispersive X-ray microanalysis (EDS) of mineral matter developed at the surface of the coke cylinders commonly shows significant differences in respect of the mineral matter contained within the body of the coke cylinders. This is interpreted as suggesting interaction between the fused coke mineral matter developed at the surface of the coke cylinder and the mineral matter of the coal. Loss of silica would appear to be common, and the most prominent development was again with flames developed with coals of the lowest volatile matter studied. It is suggested that this, and the development of degradation in the cokes, is affected by the temperatures attained. The temperatures attained by the coke cylinders on exposure to the pulverised coal flames was beyond the thermocouple measurements used in this study.