On the Effects of High Pressure and Heating Rate during Coal Pyrolysis on Char Gasification Reactivity

摘要

Effects of pyrolysis pressure on char reactivity remain a poorly understood aspect of the coal gasification process. In an attempt to address this problem, effects of pyrolysis pressure on char structure and reactivity are being investigated. In this paper, chemical reactivities to O_2, CO_2, and H_2O of chars made from three Australian black coals were measured in a pressurized thermogravimetric analyzer, under conditions where chemical processes alone controlled reaction rates. These chars were prepared at a range of pyrolysis pressures, using pressurized flow reactors and an atmospheric pressure tube furnace, to ascertain any effects of devolatilization pressure and heating rate on the chemical reactivity of the resultant coal chars. It was found that while the apparent (as measured) reaction rate can be affected by pyrolysis pressure, the rate normalized to the char surface area (intrinsic rate) is much less affected, because of large effects of pyrolysis pressure on char micropore surface area. This finding was supported by measurements of char carbon crystallite dimensions that were unaffected by pyrolysis pressure increases. These results indicate that effects of pyrolysis pressure and heating rate on char gasification rates are more likely to be due to effects of structure and surface area and (depending on reaction conditions) the consequent effects on diffusion of reactants to the char surface, rather than on the intrinsic reactivity of the coal chars.