Depolymerization Model for Coal Devolatilization: Bridges and Side Chains as the Reaction Centers

A phenomenological extension is developed
for coal devolatilization,
based on FLASHCHAIN’s mechanism for tar and lumped noncondensables.
It segregates the reaction center population further into bridges
and side chains with dissimilar reactivities that vary with rank.
At low and medium temperatures, most bridges dissociate to produce
tar precursors and noncondensables, whereas only a minor portion of
side chains decomposes into gases; a fraction of both types of reaction
centers may also be shuttled away as an element in tar molecules.
When devolatilization continues to sufficiently high temperatures,
bridges are depleted and side chain decomposition takes over to dominate
gas formation. In the evaluations against atmospheric reaction dynamics
of 16 coals from lignite to anthracite, the extension predicts with
accuracy the disparate time scales for the evolution of tar and noncondensables,
as well as the rank-dependent devolatilization rates. Also, except
for the earlier evolution history at low heating rates, the extension
demonstrates extrapolations within experimental uncertainty over a
broad domain of operating conditions (heating rates from 1 to 104 K/s and pressures from vacuum to 9 MPa) for coals across
the entire rank spectrum.