Semiempirical and density functional theory molecular modeling of brown coal chars with iron species and H2, CO formation

This paper is an overview of the calculations performed on various molecular models of char containing iron species, developed from a brown coal model with iron complexes, to mimic pyrolysis over 200-700 degrees C. The semiempirical (SE) optimization of four char models and density functional theory (DFT) calculations on the fourth smaller model were assessed on the basis of calculated heats of formation, total energy, bond lengths and angles, and partial charges. The formation of hydrogen was examined via H abstraction from [O-H] and [C-H] groups by iron clusters in char to form the hydride, followed by the dihydride, and accompanied by [C-O-Fe] and [C-Fe] bond formation. Single-point self-consistent field DFT and SE calculations for relevant structures containing iron clusters indicated that H abstraction and hydrogen formation were energetically favored. Two routes were examined for CO formation: via the adsorption of FeO and Fe2O on graphite followed by a loss of CO and via decomposition of the newly formed [C-O-Fe] group. The latter was shown to be the likely route. A concerted reaction for hydride formation during CO formation has been discussed and a reaction scheme for H-2 and CO suggested; the chemistry for H2 and CO from catalytic steam gasification is briefly discussed.