STEREOLOGICAL ANALYSIS OF THE MACROPORE STRUCTURE OF CHARS PRODUCED UNDER VARIOUS PYROLYSIS CONDITIONS AND THE INFLUENCE OF MACROPORE STRUCTURE ON CHAR GASIFICATION RATES IN THE PRESENCE OF STRONG DIFFUSIONAL LIMITATIONS

The effects of pyrolysis heating rates, particle size and ash upon the macropore structure of coal-derived chars were investigated for an Illinois #6 and a lignite coal. A new direct method was used to characterize the macropore structure of the chars. Polished cross-sections of char particles were analyzed with an integrated video microscopy/digital image processing system. Pore size analysis combined a stereological model and digital image processing techniques to overcome the limitations of indirect methods. Obtained results were used to investigate the effects of the macropore structure on global gasification rates under conditions of strong intraparticle diffusional limitations. Simulations employed a two-dimensional discrete model and image processing techniques to treat the reaction of the complex pore structures observed.; The coals were pyrolyzed in nitrogen to 900{dollar}spcirc{dollar}C with a microcomputer- controlled, heated wire-mesh reactor at heating rates between 0.1{dollar}spcirc{dollar}C/s and 1000{dollar}spcirc{dollar}C/s. The Illinois #6 char showed an increasingly complex macropore structure as the heating rate was increased. Measurements revealed a definite shift toward the formation of cellular structures with thin-walled large and small cavities. Pyrolysis heating rates had virtually no effect on the macropore structure of lignite chars. Three fractions of Illinois #6 coal with different particle sizes were also pyrolyzed at 10{dollar}spcirc{dollar}C/s. The char from the smallest size fraction exhibited a few large primary cavities but very little secondary pore formation was present. In contrast, the chars from the larger size fraction contained a large amount of secondary pores. The presence of ash during the pyrolysis process appeared to have very little effect on the development of the macropore structure.; Discrete modeling of char gasification with strong diffusional limitations showed that the wide variation of macropore structure exhibited by the Illinois #6 chars had a marked effect on the global gasification rates. An increase in the amount of secondary pores formed at the higher heating rates and larger particle sizes produced much more pronounced maxima in the rate vs conversion curves.