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FLASHCHAIN Theory for Rapid Coal Devolatilization Kinetics. 10. Extents of Conversion for Hydropyrolysis and Hydrogasification of Any Coal

Niksa, Stephen
Energy & fuels 2018 v.32 no.1 pp. 384-395
carbon dioxide, databases, gasification, hydrogen, hydrogenation, lignite, methane, methane production, steam, temperature, uncertainty, weight loss
This paper extends FLASHCHAIN theory with mechanisms for (i) hydrogenation of labile bridges in the condensed coal phase which enhance tar yields during primary devolatilization under some, but not all, conditions and (ii) heterogeneous hydrogasification of char into CH₄. Under the H₂ pressures associated with entrained coal gasification technology, coal devolatilization at heating rates faster than 10³ °C/s does not provide sufficient time for appreciable bridge hydrogenation so tar yields are not enhanced. Conversely, tar yields under slow heating conditions are strongly enhanced. The mechanisms proposed for bridge hydrogenation and its associated impact on fragment recombination in FLASHCHAIN accurately depict the joint impact of heating rate and H₂ pressure on tar yields. Whereas tar yields from rapid devolatilization diminish for progressively higher pressures, total weight loss remains steady or passes through a minimum at some H₂ pressure around 1 MPa, because char hydrogasification counteracts the lower weight loss associated with diminished tar yields at progressively higher pressures. A single, half-order methanation reaction within the CBK/G framework gave results within the measurement uncertainties through 15 MPa H₂. In combination, the mechanisms for hydropyrolysis and hydrogasification accurately interpreted a database representing coals of rank from lignite to medium volatile bituminous; heating rates from 1 to 10³ °C/s; temperatures from 550 to 1150 °C; reaction times to 180 s; and particle diameters to 1 mm. The assigned hydrogasification reactivities are far less variable than those for char gasification by steam and CO₂, and show no consistent trend with rank.