Effects of pressure on rapid pyrolysis and resultant char structure of a caking coal.

摘要

An Illinois No. 6 coal was pyrolyzed in both high-pressure and atmospheric-pressure entrained-flow reactors (;Chars generated at 309 psig are more aromatic than those produced at 100 psig, as indicated by the diffuse reflectance infrared Fourier-transform (DRIFT) spectroscopy results and by their higher C/H ratios. Except at 1.7 s, tars generated at higher pressures are more aromatic as indicated by the Fourier-transform infrared (FTIR) spectroscopy measurements and by their higher C/H ratios. The change in composition of the gaseous pyrolysis products, measured by GC, suggests that more secondary reactions of the volatiles were occurring at 309 psig than at 100 psig, except at 1.7 s when most volatiles have left the particles.;Scanning electron microscopy shows that chars generated at different pressures have significantly different surface morphologies. Two different volatile transfer mechanisms, bubble transport and diffusion, appear to be dominating at elevated and at atmospheric pressures, respectively. The size and shape of the extremely brittle individual char particles were measured directly by the image analysis technique. The results show that this coal swelled most strongly when pyrolyzed at 100 psig than at other pressures. The observed swelling behavior was caused by the completing combined effects of an improvement in fluidity and resistance to swelling by the applied pressure. This hypothesis is supported by a high-pressure microdilatometry study. The apparent first-order rate constants for devolatilization and for swelling of this coal were calculated. The observed varying influence of pressure on devolatilization rate and on swelling rate support the idea that, depending on applied pressure conditions, two different mass transfer mechanisms were dominant during pyrolysis of this coal.;The changes in internal structure of this thermoplastic coal during pyrolysis, determined by CO