Mechanism and Thermochemistry of Coal Char Oxidation and Desorption of Surface Oxides

Abstract

The present study investigates the coal char combustion by a combination of thermochemical and X-ray photoemission spectroscopy (XPS) analyses. Thermoanalytical methods (differential thermogravimetry, differential scanning calorimetry, and temperature-programmed desorption) are used to identify the key reactive steps that occur upon oxidation and heating of coal char (chemisorption, structural rearrangement and switchover of surface oxides, and desorption) and their energetics. XPS is used to reveal the chemical nature of the surface oxides that populate the char surface and to monitor their evolution throughout thermochemical processing. XPS spectra show the presence on the carbon surface of three main components. It is shown that the most abundant oxygen functionality in the raw char is epoxy. It decreases with preoxidation at 300 °C and even more at 500 °C, where carboxyl and ether oxygen functionalities prevail. The rearrangement of epoxy during preoxidation goes together with activation of the more stable and less reactive carbon sites. Results are in good agreement with semi-lumped kinetic models of carbon oxidation, which include (1) formation of “metastable” surface oxides, (2) complex switchover, and (3) desorption into CO and CO2.

Combustion of Biomass in Fluidized Beds: A Review of Key Phenomena and Future Perspectives

“…demonstrated that a further increase in the oxidation temperature results in the creation of other surface oxides, including epoxy, ether, and carbonyl–carboxyl groups. The same group of researchers analyzed preoxidized samples, noting that epoxy structures are metastable but, at high temperatures, transform into the “edge” groups, namely, the carbonyl–carboxyl and ether groups . When those preoxidized samples were heated in N 2 , the oxide species desorbed as CO and CO 2 .…”

“…The same group of researchers analyzed preoxidized samples, noting that epoxy structures are metastable but, at high temperatures, transform into the "edge" groups, namely, the carbonyl−carboxyl and ether groups. 296 When those preoxidized samples were heated in N 2 , the oxide species desorbed as CO and CO 2 . 150 The CO/CO 2 ratio increased with the temperature of preoxidation, correlating with a higher ratio of edge to epoxy groups.…”

Thermicity of the Decomposition of Oxygen Functional Groups on Cellulose-Derived Chars

“…Notably this technique may underestimate the total amount of OFGs as only acidic and basic groups are accounted for, whereas the neutral ether and carbonyl groups cannot be detected . More recently, indeed, X-ray photoelectron spectroscopy (XPS) and temperature-programmed desorption (TPD) have been used to investigate OFGs present on carbon materials. , Recently, some experimental works have tried to combine the analysis of the carbon surface by XPS with thermoanalytical techniques (TPD and differential scanning calorimetry (DSC)) in order to investigate the thermochemistry of the main steps of the carbon–oxygen reaction − in combustion processes. Levi et al and Cerciello et al within the theoretical frame of the three-step semilumped kinetic model of coal combustion proposed by Hurt and Niksa , suggested that the overall thermicity of a carbon–oxygen reaction results from the sum of exothermic (chemisorption and surface oxide rearrangement) and endothermic (CO x abstraction) steps .…”

“…More recently, indeed, X-ray photoelectron spectroscopy (XPS) and temperature-programmed desorption (TPD) have been used to investigate OFGs present on carbon materials. , Recently, some experimental works have tried to combine the analysis of the carbon surface by XPS with thermoanalytical techniques (TPD and differential scanning calorimetry (DSC)) in order to investigate the thermochemistry of the main steps of the carbon–oxygen reaction − in combustion processes. Levi et al and Cerciello et al within the theoretical frame of the three-step semilumped kinetic model of coal combustion proposed by Hurt and Niksa , suggested that the overall thermicity of a carbon–oxygen reaction results from the sum of exothermic (chemisorption and surface oxide rearrangement) and endothermic (CO x abstraction) steps . Accordingly, while the overall combustion heat would be always exothermic, the heat registered in a TPD step could be either endo- or exothermic depending on the operating temperature, as it results from the balance of the exothermic stabilization of metastable oxides and the endothermic desorption of CO x .…”

“…As noted, the above works referred to lumped reaction steps, developed in the context of coal combustion science, and were not able to resolve the heat effects originating from individual OFGs due to the high heating rates applied 22,25 or because these were hidden by the overall oxidation reaction. 23,25 Furthermore, these studies using natural chars may be strongly affected by contained minerals which are known to exhibit pronounced catalytic effects on carbon surface reactions. 23,28 Ab initio methods provided by computational chemistry can also be very useful to investigate the thermicity of generation, transformation, and decomposition of OFGs.…”