Characterization of Early-stage Coal Oxidation by Temperature-programmed Desorption

Abstract

To obtain representative temperature-programmed desorption (TPD) profiles of young oxidized chars up to 1650 °C with minimal reactor wall interferences, the chemistry and physics of four ceramic materials has been critically reviewed. A two-staged experimental apparatus is then uniquely designed to produce chars in an Al2O3 flow reactor with 1−21% O2 followed by in situ TPD with a SiC tube. Comparison of TPD profiles of oxidized chars with those from pyrolyzed chars and ashes suggests early-stage char oxidation is profoundly influenced by oxygen from three sources: organics oxygen, mineral matters, and gas phase O2. Young chars oxidized at 1000 °C with less than 0.3 s residence time shows CO desorption peaks during TPD at three distinct temperatures: 730, 1280, and 1560 °C. The peaks at 730 °C are mainly caused by incomplete devolatilization. The peaks at 1280 °C mainly represent desorption of stable surface oxides and incomplete devolatilization. Increasing the gas phase oxidants notably increases the amount of stable surface oxides. The broad peaks between 1400 and 1650 °C are attributed to the reactions of oxidants decomposed from minerals and carbon in the char or SiC tube. Gas-phase oxygen shifts these reactions to lower temperatures. Detailed oxygen balance based on the CO and CO2 yields and elemental compositions of both pyrolysis and oxidized chars reveals that oxygen uptakes are very high, +0.056 mg O per mg of carbon, in chars derived from bituminous coal, whereas lignite chars show negative oxygen uptake, −0.020 mg O per mg of carbon, in char. Indeed, lignite char seems to possess little amount of stable surface oxides other than those contributed by the minerals. The extensive emissions of CO from lignite chars during TPD seem to suggest that either O2 or minerals promotes the oxygen transfer on char surface and subsequent carbon oxidation.