Evolution of the Morphology of Chars From Pyrolysis of Sugarcane Bagasse

Evolution of the Morphology of Chars from Pyrolysis of Bagasse

Pradeep K. Agrawal¹, Carsten Sievers¹, John D. Muzzy¹, Pranjal Kalita¹, Gautami M. Newalkar¹, Steven Lien², Scott A. Sinquefield², John P. Henley³, Derrick W. Flick³, and Brien A. Stears³

¹School of Chemical & Biomolecular Engineering,²Institute of Paper Science & Technology, Georgia Institute of Technology, Atlanta, Georgia 30332; ³Dow Chemical Company, Midland, Michigan

A major advantage of biomass gasification in thermochemical platform for biomass utilization is that any type of biomass can be converted to syngas, making it attractive for unmerchantable wood, harvest residue, and waste from forest and paper industry. The mechanism of biomass gasification is complicated and can be considered as two processes in series – biomass pyrolysis and char gasification. The morphology of char from biomass pyrolysis is affected by a number of variables, e.g., ash content and composition, pressure, temperature, and heating rates. This, in turn, would affect the char gasification kinetics. A combination of two complementary techniques is being utilized in this study: (i) pressurized entrained flow reactor (PEFR) provides high heating rates (≥ 10^{3 o}C/sec) and short residence times (1-20 sec), and (ii) thermo-gravimetric analyzer (TGA) provides lower heating rates (0.1-1 ^oC/sec), but larger residence times (5-100 min).

We present here the results on char morphology obtained during the pyrolysis of sugarcane bagasse at several pressures and temperatures. The ash content of bagasse stalk differs greatly from the bagasse leaves, which affects the char morphology and subsequent gasification rates. Pyrolysis studies were carried out at pressures between 1-20 bars and at temperatures between 600-1000 ^oC. High ash content, particularly silica, appears to impact on the char morphology. Nitrogen physisorption studies show nearly two orders of magnitude increase in the char surface area, indicating a significant role of pyrolysis temperature and pressure on the surface area at high heating rates, such as those inherent in an entrained flow reactor. CO₂ adsorption studies at 0 ^oC, on the other hand, present a very different trend for the same chars.

Lower heating rates (0.1-1 ^oC/sec) as typical of TGA pyrolysis yield different char morphology which is not representative of what one would expect in a commercial pyrolysis/gasification set-up. Approaches to obtain industrially meaningful kinetic rate data are discussed.