(373d) Stable Oxides on Chars and Impact of Reactor Materials at High Temperatures | AIChE

(373d) Stable Oxides on Chars and Impact of Reactor Materials at High Temperatures

Authors 

Chen, W. - Presenter, University of Mississippi
Shi, G. - Presenter, University of Mississippi
Wan, S. - Presenter, University of Mississippi

This paper reports our first study on the deactivation of young chars in flame conditions.  Young chars are oxidized, and the quantity and strength of their surface oxides are monitored in-situ by temperature-programmed desorption (TPD) up to 1700°C.  Young chars contain more abundant surface oxides than those on old char over a wide range of temperature.  Lignite chars possess more oxides than those on chars derived from bituminous coal.  Chars oxidized at 629°C show desoprtion products at three distinct temperatures: 725°C, 1430°C and 1700°C.  The TPD peaks around 725°C correspond to an activation energy in the range of 107 to 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 notable amount of O2 at temperature above 1300°C.  Moreover, alumina tube and alumina 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, Si(OH)4 (g) and CO above 650°C.  Thus, alumina appears suitable for oxidation part of the experiments where up to 120 ppm O2 emission is acceptable at temperature at 1700°C.  SiC is not suitable for oxidative environments, but appears an acceptable choice for TPD, though a small amount of SiC may be oxidized by the TPD product, CO2 at temperature above 900°C.  SiO2(s) on the oxidized SiC can be removed by repeated heating at 1650°C.


Figure 1. TPD spectra of chars pyrolyzed and oxidized at 629°C.  Young chars produce much more CO and CO2; lignite chars produce more desorption products than bituminous coal chars. CO emissions are highest at three temperatures ranges: 685-750°C, 1410-1455°C, and 1700°C.  Chars from the bituminous coal seem to have another small peak at 1100°C.  Notable CO2 emission takes place only at about 700°C.  The CO peaks produced in the temperature range of 1410-1455°C suggest the existence of stable surface oxides on the basal planes with activation energies above 300 kJ/mol.  The large emissions of CO at 1700°C become a focal point in the second part of the current study.


Figure 2. Alumina tube-involved reactions.  Trace I illustrates the CO emissions from reaction of 1 g of graphite and alumina U-tube, products may also include aluminum oxycarbides.  Trace II illustrates the CO2 production form reaction of 8000 ppm of CO and alumina tube, such as .  Trace III represents O2 production from an empty alumina tube, such as ; highly purified He was used as the carrier this experiment.

 

 

Figure 3.  Reactions of SiC tube with 300 ppm O2.  The oxidation of SiC begins at about 650°C.  Both CO2 and CO form initially, and CO is the only oxidation product above 1250°C.

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