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The liquid product was richer in heterocompounds including polar ones compared to
that predicted from simple additive effects. Kim and Mitchell (2012, pers. com.) examined the decomposition of coal–biomass mixture. The results show that at 647.3 K and
220.9 atm pressure, small polar and nonpolar organic compounds released from the mixture were completely miscible with SCW. The hydrolysis of large organic molecules in
SCW resulted in high concentrations of H 2 , CO, CO 2 , and low-molecular-weight hydrocarbons with very little tar, soot, and PAH (polyaromatic hydrocarbons) formation.
Sulfur, nitrogen, and many trace elements in coals were oxidized to form insoluble salts
in SCW. There were no gaseous emissions, and all products were dissolved in the SCW.
The salts can be precipitated from fluid mixture and removed along with ash. Matsumara
et al. [90] examined co-liquefaction of coal and cellulose in SCW at 673 K and 25 MPa.
The coal used was Ishikari coal. Unlike the results of synergy reported by Veski et al.
[98], in this study no synergy between coal and cellulose conversion was found. Simple
additive method for each compound product distribution worked well for this system.
More discussion on synergistic effects in mixture decomposition has been recently discussed by Lee and Shah [117].
Sunphorka et al. [96] examined co-liquefaction of coal and plastic mixtures
containing high-density polyethylene, low-density polyethylene, polystyrene, and
polypropylene. The experiments were performed in the temperature range of
450°C–480°C, 40–70 wt% plastic mixtures, and a water/feedstock ratio of 2 to 10.
During co-liquefaction, all experimental variables had effects on liquid yield, but
temperature did not have a significant effect on the conversion. Long residue in the
oil product decreased with increasing temperature while it increased with increasing
water/feedstock ratio. For the plastic mixture alone, only temperature had a significant
effect on the oil yield. Maximum conversion and liquid yield of 99% and 66%, respectively, were obtained. Onsari et al. [95] examined co-liquefaction of lignite coal and
rubber tires in the temperature range of 380°C–440°C and water/feedstock ratio of
4/1 to 10/1 by weight. Variable tire concentration was examined. The maximum conversion and oil yield were obtained at 400°C, 1 min residence time, water/feedstock
ratio of 10% and 80% tire concentration. The co-liquefaction of coal and tire yielded a
synergistically increased level of oil production. Moreover, the total conversion level
with co-liquefaction was almost equal to that obtained in the presence of either Fe 2 O 3
or Ni/Mo catalysts under the same conditions. The study concluded that SCW is a
good medium for the dissolution of the volatile matter from a coal and used tire matrix.
Mitsubishi Materials Corp. [87] with the project support of Petroleum Energy
Center, Japan, developed a thermal process that used SCW to crack vacuum distillation
residue (VR) oil into clean lighter oil products. The final volume of solid waste generated was below 5%. The process was carried out in two stages in the same reactor. At
the bottom of the reactor, heavy VR components (pitch) are decomposed into lighter
components using 5% SCW at temperatures 400°C–450°C and pressures 200–250 atm.
In the upper part of the reactor, lighter components are cracked at a slightly higher
temperature with SCW and hydrogen to form lighter products. Untreated pitch was
withdrawn at the bottom and sent to a reformer where it is partially oxidized at SCW at
1000°C to form hydrogen gas and soot. This hydrogen stream is passed onto the upper
section of the cracking reactor. Overall, the process converted 70% of VR into lighter
products, which included 15% gas, 7% liquefied petroleum gas (LPG), 11% naphtha,