Hydrothermal carbonization of peels of Carya cathayensis sarg (PCCS) by subcritical water or acetone- and ethanol-modified subcritical water was carried out at the temperature from 280 to 360°C and coal-like hydrochar was obtained. The hydrochar yield decreased with increasing of treatment temperature, but the higher heating value (HHV) and carbon content of hydrochar were promoted at higher treatment temperature. The HHV of hydrochar was in range of 30–46 MJ/kg with an increase from 52.4 to 127.6%, compared to the original PCCS. The HHVs of hydrochars obtained at 360°C could be comparable with those of heavy fuel oil (42.9 MJ/kg) and diesel oil (45.7 MJ/kg). The O/C and H/C values of hydrochar were similar to those of lignite and subbituminous, except the ash content. Especially, the O/C and H/C values of hydrochars prepared at 360°C could be compatible with those of bituminous. When treated with acetone- or ethanol-modified subcritical water, a synergistic effect of acetone–water or ethanol–water for hydrothermal carbonization was observed; the liquefaction rate with acetone–water or ethanol–water mixture was larger than that of water or acetone or ethanol only. There was no significant change on the O/C and H/C values and HHV on the hydrochar, although more PCCS, was decomposed when using acetone–water or ethanol–water mixture.

Determination of mercury species over solid waste from natural gas processing plants is a prerequisite for solid waste decontamination and mercury recycling. The temperature-programmed decomposition and desorption method (TPDD) is a promising technique to determine the mercury species over solid waste. Mercury species over solid wastes with complex components and similar desorption temperature, however, cannot always be determined using only the TPDD method. In the present study, the EPA 3200 method was applied to optimize the TPDD process for speciation of the mercury compound over solid wastes collected from a natural gas processing plant, including four spent adsorbents and three sludges. The mercury species with similar desorption temperatures over solid wastes were completely separated by the EPA 3200 method optimized TPDD process. The mercury species over spent adsorbents and sludges could be determined based on the fingerprints of the mercury compounds. The mercury compounds over spent adsorbents were specified to be Hg⁰ and/or M–Hg amalgam, HgS(black) and HgO(red), of which HgS(black) was the primary mercury compound. However, the mercury species over sludge were analyzed to be Hg⁰ and/or M–Hg amalgam, HgCl₂(minor), HgS(black), HgO(yellow) and HgS(red). The mercury recovery rates of solid wastes ranged from 73% to 120%. With the above results, it was considered that the EPA 3200 method optimized TPDD process was a promising method to specify and semi-quantify the mercury compounds in solid waste.