Stable Oxides on Chars and Impact of Reactor Materials at High Temperatures

This paper reports our first study on the deactivation of young chars in flame conditions. The quantity and strength of surface oxides on young chars are monitored in situ by temperature-programmed desorption (TPD) up to 1700 °C. Young chars contain more abundant surface oxides than old chars over a wide range of temperature. Lignite chars possess more oxides than chars derived from a bituminous coal. Chars oxidized at 629 °C show desoprtion products at three distinct temperatures: 725, 1430, and 1700 °C. The TPD peaks around 725 °C correspond to activation energies in the range of 107−170 kJ/mol and have been well-documented in the literature. CO desorbed at around 1430 °C corresponds to activation energies over 300 kJ/mol, signifying the possible roles of strongly bound oxides on the basal planes of carbon. Search of the oxygen source for the huge amount of CO production at 1700 °C reveals that commonly adopted alumina tubes and support materials decompose to Al2O(g) and emit a notable amount of O2 at temperatures above 1300 °C. Moreover, alumina tube and support materials react with CO and form CO2; they also react with carbon and form CO and aluminum oxycarbides. SiC tube, on the other hand, is oxidized by O2, CO2, and H2O and forms SiO(g), SiO2(s), Si(OH)4(g), and CO above 650 °C. Moreover, Si can also form through a secondary reaction of SiC and SiO2. Thus, alumina appears suitable for the oxidation part of the experiments, where up to 120 ppm of O2 emission is acceptable at a temperature of 1700 °C. SiC appears acceptable for TPD, though a small amount of SiC may be oxidized by the TPD product, CO2, at temperatures above 900 °C. Oxidation of SiC prior to TPD should be avoided.