FLASHCHAIN THEORY FOR RAPID COAL DEVOLATILIZATION KINETICS .7. PREDICTING THE RELEASE OF OXYGEN SPECIES FROM VARIOUS COALS

The release of oxygen from any coal primarily involves only three mech anisms: the shuttling of oxygen as an element in tar molecules; the si multaneous release of CO2, H2O, and small amounts of CO when labile br idges are converted into char links; and the release of CO from the re sidual oxygen in nascent char links at high temperatures. This modelin g study characterizes these processes for heating rates from 0.5 to 10 (4) K/s, temperatures to 1550 K, and pressures from 0.1 to 1 MPa. Eval uations against a database compiled from the behavior of 27 coal sampl es representing ranks from lignite to anthracite demonstrate that CO2, H2O, and CO yields plus the oxygen contents of tar and char can be pr edicted within useful tolerances throughout this domain. With Flashcha in, no additional parameters or rate expressions are required to quant itatively predict the contributions from tar shuttling from any coal a t any operating conditions. And the release rates of CO2, H2O, and low -temperature CO release are set equal to the previously evaluated form ation rates of char links. Only one reaction rate expression (but no h ypothetical ultimate yield parameter) is required to predict the yield s and evolution rates of CO at high temperatures. This modeling approa ch departs from multiple, independent conversion channels for each gas product and instead recognizes the conversion of labile bridges into refractory char links as the fundamental process underlying the releas e of most of the oxygen-bearing noncondensible gases. As consequences of this premise, (a) oxygen is shuttled away in tars and simultaneousl y released as CO2 and H2O at all heating rates; (b) oxygen gas yields, especially H2O and CO yields, diminish in tandem with enhanced tar yi elds for faster heating rates or lower pressures; and (c) the onset of oxygen gas release shifts to higher temperatures for coals of progres sively higher rank, particularly for low volatility coals. Production rates of high-temperature CO accelerate rapidly after the end of tar e volution, overtaking the production of CO2 and H2O from all coal types . CO is released over narrower temperature ranges for coals of progres sively higher rank.