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

Abstract

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 Al 2 O (g) and emit a notable amount of O 2 at temperatures above 1300°C. Moreover, alumina tube and support materials react with CO and form CO 2 ; they also react with carbon and form CO and aluminum oxycarbides. SiC tube, on the other hand, is oxidized by O 2 , CO 2 , and H 2 O and forms SiO (g) , SiO 2(s) , Si-(OH) 4(g) , and CO above 650°C. Moreover, Si can also form through a secondary reaction of SiC and SiO 2 .Thus, alumina appears suitable for the oxidation part of the experiments, where up to 120 ppm of O 2 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, CO 2 , at temperatures above 900°C. Oxidation of SiC prior to TPD should be avoided.

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

“…On the other hand, the roles of possible strong surface oxides that desorb only above 1100 °C, as suggested by studies of graphite about 16 years ago, have not been receiving adequate attentions for char oxidation. Furthermore, the presence of stable oxides from young coal-derived chars was recently revealed in our study, suggesting their possible roles in the flame . Their structures appear more complex than those on graphite.…”

“…To obtain “super-cleaned” chars, that is, chars without adsorbed species on their surfaces, chars have traditionally been pyrolyzed in inert gas at 1000∼1100 °C for several hours before reactivity studies. Evidences suggest that this level of thermal treatment destroys many important chemical and physical characteristics of young chars, leading to underestimations of the complexity of char oxidation and erroneous estimation of the char’s reactivity in flame, where char has a residence time on the order of seconds. − It is generally believed that young char undergoes at least four simultaneous classes of processes in the flame: devolatilization, reactions of adsorbed oxygen on char surface, thermal annealing of carbon, and reactions of mineral constituents. They induce a complex interrelated network of rapid chemical and physical changes to the char structure and therefore to its reactivity in flame.…”

“…To accomplish the objective stated above, we have successfully minimized the interferences of reactor wall that were reported in a previous paper of ours so that the CO productions from TPD can be properly interpreted and correlated to oxygen from different sources: gaseous, organics, and minerals. At the outset of this project, we assessed the thermodynamics properties of four commonly used tube materials in combustion laboratories: SiO 2 , Al 2 O 3 , fully yttria-stabilized ZrO 2 , and SiC.…”

Photochemical and acoustic interactions of biochar with CO2 and H2O: Applications in power generation and CO2 capture

“…During pyrolysis, the sample was swept by He at 400 mL/min flow rate. The ultrahigh purity He was purified by copper turnings at 500°C in a furnace to remove any trace oxidants before entering the pyrolysis reactor . The copper turnings were periodically regenerated by 30% CO balanced with He at 250°C for 20 min.…”

Phase Transformation of Alumina, Silica and Iron Oxide during Carbothermic Reduction of Fly Ash for Ceramics Production

“…The condensate was composed of SiC, AlN and C, as shown in Figure 8a. At high temperatures SiO 2 was reduced by carbon to produce gaseous SiO [46], and the generated SiO continued to react with carbon to produce SiC by reaction (10). Al 2 O 3 was unavailable to react with C by the reaction (12) due to the excessive initial reaction temperature of 2353K.…”