(9e) Stable Surface Oxides On Young Chars Derived From Coals

This study is aimed at elucidating the amount and strength of stable surface oxides from young chars in the very stage of the coal combustion in flames, i.e., at temperatures within 1100 and 1650 ?aC and with a residence times less than 1 s. Desorption of surface oxides at temperatures below 1100 ?aC has long been considered the rate controlling steps of char oxidation. A two-staged experimental apparatus has been uniquely designed and fabricated to produce young char and to conduct temperature-programmed desorption (TPD) in-situ up to 1650 ?aC. For the production of young, oxidized or pyrolysis chars, coal particles pass through the high purity Al2O3 tube in the upper furnace at a specific temperature with a controlled residence time and O2 concentration in He base, and are trapped in the middle of a SiC tube in a lower furnace by SiC particles and a SiC foam supported by a SiC rod at room temperature. TPD is conducted after a desirable amount of char sample is collected in the SiC tube. An Agilent model 6890N gas chromatograph, an Agilent model 5973 mass spectrometer, and a HP PC with spectra libraries are online with the reactor. Carbon burnout is controlled by O2 concentration and gas velocity. The use of reactor tubes of two different materials in series uniquely takes into considerations of the detrimental reactor wall interferences that became known in the combustion works only recently (Energy and Fuels, 21(2), 778-792 (2007)), but have previously prohibited proper studies of char reactions at high temperatures. Young chars oxidized at 900 ?aC show desorption products at three distinct temperatures: 730, 1280, and 1560 ?aC. The CO productions from the oxidized chars at about 730 ?aC are similar to those from the pyrolysis chars, suggesting the incomplete devolatilization of coals' oxygen functional groups during the 0.43 to 0.94 s residence times. CO desorbed from the two high temperatures, 1280, and 1560 ?aC, suggest that the existence of stable surface oxides on young chars. Nevertheless, they are much more complex than the single-peak TPD profiles of oxidized graphite reported earlier. There are higher amounts of CO desorbed from lignite chars than those desorbed from chars of a bituminous coal. From the elemental analyses of the oxidized and pyrolysis chars, we observed very little apparent oxygen gains of lignite chars during oxidation; even the young lignite chars have notably higher gasification rates and higher CO emissions from TPD. Thus, it appears that there is a vigorous oxygen turnover on the lignite char surface that replaces almost all oxygen functional groups within the first second in flame. The oxygen functional groups in coals seem to play a very important role during the early stage of char combustion. It is possible that CO2 from mineral decompositions oxidizes the carbon and/or the SiC tube materials and contributes the peaks at about 1560 ?aC. To investigate the roles of minerals decomposition products, coals were burned in a Bunsen burner below 1000 ?aC, and their ashes were heated in the SiC tube with a heating ramp identical to that for obtaining the TPD's for chars. Comparison of the TPD profiles for the oxidized chars, pyrolysis char, and ash from a bituminous coal suggests that there are high populations of stable oxides contributed by the carbon oxidation by gaseous O2. There are some oxygen functional groups that retained in the char and decompose at 730, but little of them decompose at 1560 ?aC. Moreover, O2 in the gas phase seems to promote the mineral decomposition and its subsequent oxidation of carbon at lower temperatures. TPD profiles from lignite products are more complex than those from the bituminous coal. Comparison of the TPD profiles for the oxidized chars, pyrolysis char, and ash from lignite also suggests that both O2 in the gas phase and the oxygen in the organic portion of the char promote the mineral decomposition and the subsequent oxidation of carbon at lower temperatures. Most interestingly, there are only very low populations of stable oxides contributed by the carbon oxidation by gaseous O2. This observation seems suggest that oxygen from various sources have saturated the active sites on carbon surface. In conclusion, we study reveals that oxygen of various sources profoundly affects the char oxidation mechanisms in the early stage of char oxidation. Oxygen not only forms surface oxides, but also mitigates thermal annealing of carbon and enhances mineral decomposition at lower temperatures. At flame temperatures, oxygen originating from different organic functional groups of the coal, mineral constituents, and oxidants in the gaseous phase reacts with carbon through a complex reaction network.