SYNOPSIS: -Recent trends and future outlook of researches on chemical structure have been written since Krevelen researched. There are three major trends of researches, one is to show the average coal chemical structure, another is to illustrate the coal struc-ture, and the other is to consider coal as three dimensional structure. Finally future outlook of the researches is discussed.
-Iodine as an electron acceptor interacts with coal molecules producing charge transfer complex. The change of spin concentration of iodine doped coal, Ns reflects the π-π interaction in condensed aromatic rings in coal structure. The value of Ns in-crease with the increase of rank up to 90% of carbon (daf). On the other hand, the value of Ns for TCNQ doped coal increases with decrease of coal rank. Interpretation for the fact above is that TCNQ molecule goes to the sites associating with oxygen containing functional groups, which are able to form hydrogen bonding.
Coal structure is reviewed based on the results from solvent extrac-tions of coals, especially on those with Carbon disulfide-N-Methyl-2-pyrrolidinone mixed solvent extraction. It has been widely suggested that coals consist of covalently three-dimensional macromolecular networks and a small amount of relatively low molecu-lar-weight molecules. However, recently, several results which can not be explained by the above concept, have been reported. First, the very high extraction yields obtained for some bituminous coals suggest that a considerable amount of solvent soluble molecules exist ori-ginally in coals. Next, there are many evidences which indicate that noncovalent inter-molecular interactions such as hydrogen bondings, aromatic-aromatic, charge transfer, and dipole-dipole interactions play important roles for the formation of coal macromolecular structures. It is suggested that coals seem to be comprised of “aggregate” of coal molecules having a continuous molecular weight distribution.
Nowadays coals are believed to be complex macromolecular networks containing many covalent and noncovalent cross-links based on accumulated works utiliz-ing the solvent swelling technique. In this paper the importance of the noncovalent bondings in the coal structure was clarified by referring to the literature. Then a new and simple method which estimates the number of noncovalent bondings was introduced. This method utilizes the phenomena of the solvent-induced and the thermally induced swelling of coal, and analyzes the DSC profile measured while the solvent desorbs from the solvent swollen coal. Some noncovalent bondings were found to be broken thermally at 100 to 200°C. By uti-lizing this phenomenon flash pyrolysis of the coal preheated at 100 to 200°C was found to be effective to increase the coal conversion and the yield of liquid products.
The recent development of the study on the coal structure by means of the solvent swelling is reviewed forcusing on the swelling dynamics, which is able to provide informations concerning delicate bonding interactions in the macromolecular net-work of coal, such as hydrogen bonding, charge transfer bonding or π-π bonding.
-Phenols in coal-derived naphthas were selectively and efficiently separated by extraction with various kinds of sodium alkylbenzenesulfonate aqueous solu-tion. The partition coefficient (K) of phenols was exponentially increased with the increases in the concentration and alkyl carbon number of the surfactants, but was lowered by the presence of oxygen compounds other than phenols in naphtha. The effect of methanol addition was significant and the combined use of methanol and the surfactant showed synergistic effect for the extraction of phenols. The extraction sol-vent was repeatedly used without lowering the extraction efficiency. The transfer of hydro-carbon into water phase and the contamination of the surfactant into oil phase were very trace.
-Twenty six kinds of coals, which were chosen at random, were burnt to ashes by combustion either in air at 800°C or in an oxygen plasma at 185°C. The ashes were submitted to either X-ray fluorescence or ICP analysis and the content of the mineral elements such as Hg, As, Pb, Cu, Cd, Zn, Cr, Mn and Ni was determined. It was found that each element tends to decrease in its amount in the course of ashify-ing at 800°C and the tendency was marked for Hg, As and Zn. Appreciable differences were also observed between the Plasma-ICP and Carius-ICP procedures for the cases of Hg. Between X-ray fluoresence analyses and ICP analyses there was no obvious discrepancy excepting some cases of Pb, Mn and Ni.
Solid state 13C-NMR spectra of oxidized coals were measured in order to investigate changes of NMR behaviors of the coals through low temperature oxidation by air. By the oxidation for one week at 100-105°C, low rank coals showed increases in the signal intensity and spin relaxation time in the rotating frame (TiρH). On the other hand, decreases in the intensity and T1ρH were observed on high rank coals. Radical concentra-tion of the low rank coals decreased, but that of the high rank coals increased by the oxidation. Measurements of FT-IR spectra and pyrolysis reaction were also carried out for the oxidized coals, suggesting that various oxygen containing functional groups such as C=O and -OH are generated mainly by the oxidation of aliphatic carbons and that as a re-sults the value of the aromaticity (fa) increased during the oxidation. It was concluded from the results that the enhancement of T1ρH values of the low rank coals is due to the in-crease in fa values which results in the restriction of the molecule motion and to the de-crease in radical concentration. On the contray, the lowering of T1ρH of high rank coals is considered to be brought about predominantly by the increase in the radical concentration during the oxidation.
Permeability of a packed bed of coal and coke, which is very impor-tant property for the analysis of pyrolyzed gas flow behavior within the packed bed or porous media, is measured in the room temperature after the carbonization process. Below the softening temperature (Tsf), the packed bed of coals is heated up in the vessel which is made to be able to set directly in the measurement section. On the other hand, a semicoke which is produced by heating up to an arbitrary temperature over Tsf, in the vessel is cut off to a length of 2cm, its side is coated with a thermosetting resin and then the coke is fixed in the test section. Permeability for a packed bed of every coals is increased gradually with the carbonization temperature below Tsf and decreased drastically near Tsf being the minimum value. The tendency has no relation to the coal type and the heating rate. Over Tsf, permeability of Saraji coal (V.M.=18.6%) is once increased and then decreased with an in-crease of temperature having the maximum value at 930K. In case of Gregg River coal (23.8%) permeability is increasing up to 1100K.