Predicting ultimate soot yields from any coal

Abstract

This study presents a simple prediction method for soot yields from any coal, provided that the thermal processing is severe enough to completely incorporate all the aromatics in primary tars and oils into a so-called ultimate soot yield, and given the yield and elemental composition of primary tar. It also introduces a 3-step quasi-global reaction scheme to describe the dynamic approach to ultimate soot yields for deployment in CFD furnace simulations, where the rate parameters are evaluated to match the predicted dynamics from a comprehensive tar conversion mechanism. The predicted ultimate soot yields are validated with measurements for coal ranks from brown coals to anthracites, for heating rates of at least several thousand degrees per second; temperatures from 900 to 1300 °C; and tar contact times from 75 ms through 14 s. Primary tar, defined as the aggregate amount of all condensed aromatics from primary devolatilization, determines the ultimate soot yields from any coal at any operating conditions. Ultimate soot yields from any coal can be evaluated from the yield and FLASHCHAIN®-based elemental composition of primary tar as accurately as they have been measured. On average, only 18% of the primary tar mass is excluded from the ultimate soot product, which mostly reflects the exchange of C2H2 into soot for the CO that carries away tar-oxygen. This exchange nearly compensates for the much more extensive elimination of heteroatoms and aliphatics from tar during soot production. The proposed three-step global scheme accurately depicts the reaction dynamics for tar decomposition at elevated temperatures from the full tar decomposition mechanism, including the maximum and ultimate saturation value of secondary tar. Whereas the kinetic parameters for primary and secondary tar display the established tendencies for different operating conditions and coals, those for soot production are remarkably uniform across this domain.