Characterization of tuyere-level core-drill coke samples from blast furnace operation

A suite of tuyere-level coke samples have been withdrawn from a working blast furnace during coal injection, using the core-drilling technique. The samples have been characterized by size exclusion chromatography (SEC), Fourier transform Raman spectroscopy (FT-RS), and X-ray powder diffraction (XRD) spectroscopy. The 1-methyl-2-pyrrolidinone (NMP) extracts of the cokes sampled from the "bosh", the rear of the "bird's nest", and the "dead man" zones were found by SEC to contain heavy soot-like materials (ca. 10(7)-10(8) apparent mass units). In contrast, NMP extracts of cokes taken from the raceway and the front of the "bird's nest" only contained a small amount of material of relatively lower apparent molecular mass (up to ca. 10(5) u). Since the feed coke contained no materials extractable by the present method, the soot-like materials are thought to have formed during the reactions of volatile matter released from the injectant coal, probably via dehydrogenation and repolymerization of the tars. The Raman spectra of the NMP-extracted core-drilled coke samples showed variations reflecting their temperature histories. Area ratios of D-band to G-band decreased as the exposure temperature increased, while intensity ratios of D to G band and those of 2D to G bands increased with temperature. The graphitic (G), defect (D), and random (R) fractions of the carbon structure of the cokes were also derived from the Raman spectra. The R fractions decreased with increasing temperature, whereas G fractions increased, while the D fractions showed a more complex variation with temperature. These data appear to give clues regarding the graphitization mechanism of tuyere-level cokes in the blast furnace. These results from Raman spectroscopy were validated by XRD analyses of the demineralized and NMP-extracted cokes. The average lattice interlayer spacing d(002), Stacking height L, average crystallite diameter L,,, and average number of lattice layers N-c have been determined from the XRD patterns of the cokes. The raceway coke, which had been exposed to the highest temperature, was observed to possess the largest crystallite dimensions, possibly catalyzed by contact with iron. In contrast, the "dead man" coke had the largest interlayer spacing and smallest crystallite dimensions. The cokes were examined. for the presence of alkalis. None were found in the raceway coke while the highest concentration was encountered in the "dead man" coke. These alkalis seemed to interact with carbon by diffusion/adsorption rather than through intercalation: the averaged number of lattice layers of "dead man" coke, with the highest alkali concentration, was still quite large. The TGA reactivity of the "dead man" coke was highest among the five samples, consistent with the degree of disordering of its structure and probably due also to the catalytic effect of the alkalis present. The combination of SEC, Raman spectrometry, and XRD was shown to provide considerable insight into coke structures present in a blast furnace and to give information on conditions experienced by coke in different zones. This combination of techniques provides an improved way of characterizing cokes in different regions of blast furnaces, as a potential aid to furnace operation under coal injection conditions.