FLASHCHAIN Theory for Rapid Coal Devolatilization Kinetics. 9. Decomposition Mechanism for Tars from Various Coals

摘要

This paper extends FLASHCHAIN theory with a mechanism for tar decomposition with any coal at any temperature under inert gases at atmospheric pressure, including (i) continuous elimination of heteroatoms as noncondensables, which transforms primary tar into polynuclear aromatic hydrocarbons at moderate temperatures; (ii) disintegration of tar monomers with attached hydrogen sources into oils and additional noncondensables; and (iii) nucleation and addition of oils and tars to a nascent soot phase at elevated temperatures. The aromatic nuclei, labile bridges, char links, and peripheral groups in FLASHCHAIN describe tar decomposition without modification, and 7 of 11 proposed reactions were also transferred from FLASHCHAIN, albeit with markedly different kinetic parameters. Only the channel for oil production and the three for soot production are new and distinctive. All stoichiometric coefficients can be evaluated from a primary tar composition from FLASHCHAIN, and the associated variations in these coefficients are the distinguishing factors in the distinctive behavior of individual primary tar samples. Conversely, kinetics for tar decomposition are far less sensitive to coal quality than primary devolatilization kinetics, and the thermal response is much narrower. The mechanism accurately interprets the dynamics and product distributions throughout a validation database representing heating rates from 60 to roughly 50,000 degrees C/s; temperatures from 500 to 1300 degrees C; tar contact times from 40 ms through 14 s; and coal ranks from brown coal through anthracite (without subbituminous samples). The validation work also demonstrates that the kinetics for two reaction processes, bimolecular recombination of tar molecules and addition of oils and tars to soot, must explicitly depend on coal loading and, by inference, pressure in the subject tests or applications under consideration.