ReaxFF simulations of petroleum coke sulfur removal mechanisms during pyrolysis and combustion

Green petroleum coke (petcoke) is used as a feedstock for raw carbon material or as a fuel. Petcoke with high sulfur (S) content (>4 wt%) is typically restricted to fuel use unless extensive S removal is successful. Here, the S removal mechanisms during both pyrolysis and combustion were explored using the Reactive Force Field (ReaxFF) MD approach. A structural representation (C1648H77O59N24S47) of a green Qingdao petcoke was generated coupling high-resolution transmission electron microscopy lattice fringe image analysis and analytical data. This structure was consistent with elemental, aromaticity (FT-IR), the pair correlation function (XRD), and functional group (S, O, and N from XPS) data. The ReaxFF pyrolysis simulation produced gas and tar yields of 44.7 and 11.0 wt% at 3000 K after 250 ps of simulation. The combustion simulation on the same initial structure was performed in an O-2 environment. During the pyrolysis simulation, the first-step for S-removal was thiophenic sulfur conversion to C1-4S (mostly C2S), COS, or CNS. The heteroatom pyrolysis overlapped, for this structure, at these conditions. However, for the combustion simulation earlier conversion of thiophenic sulfur to COS was observed. No N-S containing structures occurred in this O-rich environment, as pyrrolic and pyridinic N quickly oxidized into CON or NO compounds. The S transformation during combustion can be summarized by COS -> CO2S -> CO3S -> CO4S. The H atoms reacted with S-containing gases like COS/CO2S/CNS producing HS and H2S rather than with the coke-S. (C) 2018 The Combustion Institute. Published by Elsevier Inc. All rights reserved.