The Development of Macromolecular Models for Lignite and Their Interaction with Inorganic Species for Gasification and Sulphur Retention
To develop molecular model structures of low rank coals for catalytic gasification of brown coals and in situ sulphur retention. Models based on (1) transformations of lignin into lignite and (2) elemental composition, functional group analysis, and other known properties of brown coals have been optimised using molecular mechanics and semi-empirical quantum mechanics. MM/QM studies include the interaction of Na, Mg, Ca, Fe, and Ni aqua species within these coal models. Effects of inorganics on pyrolysis and low temperature gasification are being studied.
Principal InvestigatorGeorge DomazetisDepartment of ChemistrySchool of Molecular SciencesLatrobe University |
Projectf07, f16
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Co-InvestigatorsBruce D JamesDepartment of ChemistrySchool of Molecular SciencesLatrobe UniversityJohn LiesegangDepartment of PhysicsSchool of EngineeringLatrobe University |
RFCD Codes250106, 250201, 250599, 250601
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Models of low rank coals with inorganic species have been optimised using CAChe and MOPAC2002. Results show coal models containing hydrated ionic species [Na(H2O)6]+, [Mg(H2O)6]2+, [Ca(H2O)6]2+, and transition coordination complexes [Fe(H2O)6]3+ and [Ni(H2O)6]2+ are stabilised by hydrogen bonding and electrostatic effects, and destabilised through steric interactions. Polynuclear hydroxy-bridging complexes within the coal model are energetically favoured. When metal species are added to coal, solvated water molecules become incorporated in the coal by forming hydrogen bonds. The change in the overall stability of the coal model with added inorganic, relative to the stability of the initial coal model, is provided with MM3, and MOZYME-PM5 QM calculations also provide the partial charges and bond orders for main group species. Extended Hückel calculations provide partial charges for transition metal complexes added to the coal models.
All the model structures containing Na+, Mg2+ or Ca2+ and ‘solvation’ water molecules are less energetically favoured than the coal model to which the same number of water molecules have been added. Hydrogen bonds and electrostatic factors enhance stability, while steric interactions decrease the stability of these molecules. The coal model structure containing sodium has the nearest distance for Na+….‾OOC-R, at 3.63 Å, while other oxygen neighbour atoms are at 4.09 Å (HOOC-coal), 4.24 Å (OH-Ph) 4.45 Å (Me-O-Ph), and 5.31Å (OH-Ph). The nearest water molecule is 7.44 Å from the sodium cation. Calcium is 3.3Å and 6.0Å from the ionised carboxyl groups. Closest oxygen groups are 3.88 Å and 3.84 Å (OH-Ph), 4.63 Å (HOOC-R) and the nearest water molecules are at 3.72 Å and 4.31 Å. Magnesium is 3.35 Å and 3.09 Å from carboxyl anions, three water molecules remain near the cation, located at 2.76 Å, 2.82 Å and 3.49 Å, while the sixth oxygen is located at 3.52 Å from a (OH-Ph) group. The partial charge, relative to the hydrated species, is reduced on Na+ and Ca2+, but not on Mg2+.
Calculated bond lengths and angles for solution species of Fe(III) and Ni(II) are comparable to those measured from single crystal structure studies of similar structures. Complexes of these metals in coal models are studied with monodentate and bidentate carboxyl structures. Those with bidentate carboxyl group are less stable due to steric constraints. However, stable octahedral complexes may form at sites in the model in which carboxyl and phenoxy ligands are advantageously situated for octahedral coordination. Singly charged Fe(III) complexes with two monodentate carboxyl groups, phenoxy coordination bonds, and a carboxyl anion at some distance from the complex, are also shown to be stable. Polymeric complexes are favoured to form throughout the coal model with lower steric constraints than observed for single metal species. EH calculations show that a reduction in the partial charges on the metal centres may take place, relative to that of the solution complex, indicative of increasing electron density on the metal centre from the coal matrix.
The role of metal complexes in the pyrolysis of brown coals has been examined via a mechanism developed by Domazetis that predicts a weight loss of 58% of the coal mass. This may be compared to the measured weight losses of 36-50%. The features of this mechanism for pyrolysis, interpreted using a coal model with multi-iron hydroxy complexes (Composition C91 H87 Fe5 N O14) are: (i) FeIII-oxy molecules with loss of coordinated water through carboxyl coordination are in close proximity to various coal oxygen functional groups, (ii) carboxyl groups participate in the formation of iron carbonate complexes, followed by decomposition into CO2 and iron-oxy intermediates, (iii) cleavage of oxygen bonds in the coal model to provide CO and a char-like fragment. The iron complex participates in the breakdown of five oxygen functional groups, with the formation of CO2, CO, a char and a hydrocarbon fragment.
This mechanism is speculative at this stage and future work will involve modelling of intermediate coal/inorganic complexes that may be involved in pyrolysis and gasification, to elucidate the role multi-iron complexes may play in metal enhanced pyrolysis and gasification.
Work will continue with computational models representing pyrolysis of brown coal, the fate of sulphur and oxygen functional groups when coal is heated, and studies aimed at sulphur capture by calcium added to brown coal.
MOPAC2002 output files provide a detailed listing of all partial charges and bond orders for models studied.
The models were initially optimised with the Fujitsu CAChe 5.04 suite of programs. Optimisation of the structure of large molecules was performed using MOPAC2002 at the Australian Partnership for Advanced Computing National Facility (APAC NF). The MOPAC input file is created with CAChe MOPAC-PM5 after the structure has been optimised using MM. The input file is edited and transferred to the APAC National Facility as an input data file for MOPAC 2002. Large structures are optimised with the MOZYME algorithm in MOPAC 2002 - this uses localized molecular orbitals to lower the time-dependency to N1.4 from the N3 required for diagonalization and for construction of the density matrix. Only closed shell RHF calculations are allowed in MOZYME.
The APAC National Facility has enabled us to study large model molecules with main group and transition metal complexes. The models are of sufficient size to provide useful information on these systems. These studies, to our knowledge, are unique in that semi-empirical QM results have been obtained on the electron distribution of metals in brown coal models. Such studies could not be carried out without the APAC National Facility.
External Funding and Awards
None.
Publications
George Domazetis, Bruce D James, and John Liesegang, “Studies of Inorganics added to Low-Rank Coals for Catalytic Gasification.” Fuel Processing Technol., accepted for publication, 2004.
George Domazetis and Bruce D. James, “Molecular Models of Low Rank Coals Incorporating Metal Containing Species.” Org Geochem., submitted for publication, 2004.