When predicting the variation of pore structure during CO2 gasification of coal chars using the random pore model (RPM), it is impossible to calculate exactly the ψ parameter from the pore characteristics, which were obtained by means of N2 adsorption, such as BET surface area (denoted as N2 pore characteristics), of the char prior to gasification. The values of ψ, which were calculated from the pore characteristics of chars at various carbon conversions, should be fundamentally constant, unaffected by the conversion of the char. However, this investigation exhibited a drastic decrease of ψ at the initial stage of the gasification reaction. This phenomenon is the result of a significant increase of N2 pore characteristics, of which the starting chars are extremely small. This increase might be explained by the widening of submicropores which are undetectable through the N2 adsorption method or by the reopening of closed pores inaccessible even to helium molecules, followed by the formation of new micropores exceeding the detection limit of N2. Consequently, this study introduced the volume of submicropores and closed pores into the ψ equation as correction terms. The value of ψ at the reaction starting point was close to that at the intermediate stage of reaction, indicating that the accuracy for ψ estimation was elevated and that the submicropores and closed pores should be taken into account when using RPM.

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A maximum oil yield in the NEDOL coal liquefaction process was obtained with coal containing 76-78% carbon, while gas and water yields decreased with an increase in the carbon content. On the other hand, residue yield increased with an increase in the carbon content and this was attributed to an increase in asphaltene [(hexane insoluble)-(toluene soluble)] in the residue. From the experimental results, it was inferred that decomposition of the asphaltene is inevitable for improvement of the oil yield and that prolonged residence time in the liquefaction reactors is the most effective and practical measure for decomposing the asphaltene under the operating conditions of the NEDOL process. The average structure of the hexane soluble, asphaltene and preasphaltene [(toluene insoluble)-(THF soluble)] in the liquefaction residue exhibited the similar structural parameters in coals tested in the NEDOL coal liquefaction plants. The asphaltene fraction was then extracted from the residue by commercially applicable extraction conditions following which the extract was hydrotreated. About 35% of the extract was recovered as oil, which is lighter than 811K boiling point, by hydrotreatment. Furthermore, coal liquefaction experiments that used the extract as a part of solvent were conducted and an increase in total liquefaction oil yield was confirmed comparing with a coal liquefaction experiment without the extract.

To improve the product quality in the NEDOL coal liquefaction process (which was developed by the New Energy and Industrial Technology Development Organization, NEDO), improvement of the NEDOL process was proposed and tested by a continuous plant, which can process 1 ton of coal per day. In the improved process, the products and recycle solvent are simultaneously hydrogenated in the solvent hydrogenation section. Although the recycle solvent of the improved process became lighter and degradation of the recycle solvent quality was supposed, via moresevere hydrogenation conditions in the solvent hydrogenation section, the hydrogen-donating capability of the recycle solvent was maintained and degradation was not recognized. On the other hand, the hydrodynamics of the coal liquefaction reactors was studied and mean residence time of the liquid phase (MRT) in the coal liquefaction reactors was measured using neutrons. An increase in the MRT was observed, and this increase seemed to be caused by vaporization of the increased lighter fraction in the recycle solvent. As a result, the product oil yield was increased by 2-3 wt %, compared to the original NEDOL process, and remarkable reduction of the heteroatoms in the product oils was confirmed. The nitrogen content in the products was reduced from several thousands parts per million (ppm) in the original NEDOL process to <1000 ppm. Furthermore, through the experiment in the plant, stable continuous operation for >50 days in the improved process, which adopts simultaneous hydrogenation of the products and recycle solvent, was achieved, and the reliability of this process was verified.