The effects of acid and hydrothermal pretreatments and the addition of polar compounds on the production of ashless-coal (HyperCoal) from subbituminous coals using cost-effective industrial solvents were investigated. The extraction yield of Wyodak subbituminous coal (C%, 75.0%) using crude methylnaphthalene oil (CMNO) at 360 °C was increased significantly by 19% following acid pretreatment; it was 41.3% for the raw coal and 60.5% for the acid-treated coal. The addition of strongly polar compounds, such as N-methyl-2-pyrrolidinone (NMP), also increased the extraction yields. For Pasir subbituminous coal (C%, 73.0%) the yield increased by 10% from 54.3% for the raw coal to 64.2% when 20% NMP was added to CMNO. The highest extraction yield of 72.2% was obtained for acid-treated Wyodak coal using CMNO with 20% NMP added. The ash content in HyperCoal tended to decrease following acid pretreatment and was less than 200 ppm in some coals. Hydrothermal pretreatment had a negative effect on the thermal extraction at 360 °C, but increased the yield at extraction temperatures below 200 °C.

The use of "HyperCoal" (ash-free coal) as feedstock for gas turbines results in higher net power output with lower CO 2 emissions. HyperCoal can be produced by thermal extraction from lowrank coals with industrial organic solvents in an inert atmosphere, providing raw materials. The pretreatment of low-rank coals with carbonic acid (CO 2 dissolved in water-CO 2 /H 2 O) produced a strong increase in HyperCoal yields at relatively lower CO 2 pressures of 0.1-0.5 MPa; the thermal extraction yields at 360°C increased by 7%-15% with extraction yields of 52% and 45% obtained for Wyodak sub-bituminous coal and Beulah-Zap lignite, respectively. In the range of 320-360°C, crude methylnaphthalene oil (CMNO) extraction yields of pretreated Wyodak coal increased significantly (by 4%-11%) over those of raw coal. The enhanced extraction yields of these low-rank coals are attributed to disruption of cation-bridging crosslinks on acid pretreatment, and the release of the hydrogen bonds by CMNO extraction.

Generating power from HyperCoal is a high-efficiency process in which the organic portion of coal is extracted with industrial solvents at a temperature around 360°C and fed to a gas turbine directly. This study sought to establish a selection index for identifying subbituminous coals that give high extraction yields. Subbituminous coals were extracted at 360°C with flowing industrial solvents, and we investigated the relationship between the extraction yield and the quantity of oxygen-containing functional groups in the coal. The extraction yield with a polar solvent, crude methylnaphthalene oil (CMNO), increased with the quantity of carboxylate groups bridged by metal cations, such as Ca 2+ and Mg 2+ (COOM). The correlation coefficient between the extraction yield and the quantity was 0.82. Acid treatment of coal before extraction released COOM cross-links, increasing the extraction yield. These results suggest that the thermal extraction of lowrank coals strongly depends on the cross-links rather than the hydrogen bonds. Therefore, the thermal extraction yields of low-rank coals can be estimated from the quantity of COOM in the original coals. The intercept of the regression line between the quantity of COOM and the extraction yield with CMNO was 57.8%. This value is the average extraction yield for low-rank coals with free COOM.

To maximize the conversion of low-quality coal into good coke, we investigated the thermoplasticity of various binary blends of caking coals with slightly or noncaking coals using a dynamic viscoelastic technique with a temperature-variable rheometer. Coal blend samples were prepared by mixing two coals (1:1 by weight), which were heated from room temperature to 600°C at a rate of 3-80°C/min. At the slow rate of 3°C/min, the blends had a tan δ that was generally lower than the calculated value, showing that a negative interaction caused a loss of thermoplasticity. In contrast, at the rapid heating rate of 80°C/min, the tan δ of some blends was higher than the calculated value, indicating a positive interaction that enhanced the thermoplasticity. With rapid heating, the thermoplasticity of each coal itself increased, and their thermoplastic temperature ranges widened with rapid heating. Therefore, rapid heating was effective at converting these coal blends into good cokes. Moreover, even with slow heating, when a combination of coals (Gregory:Enshu, 1:1) showing some thermoplasticity in nearly the same temperature range was blended, a desirable synergistic effect of the blend was obtained. This suggests that blending coal with an overlapping thermoplastic temperature range is important for the synergistic effect, regardless of the heating rate.

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