Partitioning of boron during the generation of ultraclean fuel (HyperCoal) by solvent extraction of coal

Extraction of coal by organic solvent is an effective strategy for producing clean fuel and thereby minimizing the ash-related problems and improving coal power generation efficiency. Up to date, most elements in coal extracts have been investigated, but little attention has been given to boron. In this paper, the partitioning behavior of boron has been investigated in one coal extraction process, named HyperCoal (HPC) generation, developed in Japan. Five coals were extracted by both 1-methynaphthalene and its mixture with 20 wt % polar indole for 1 h at 360 degrees C. Boron in coal extract (HPC), coal residues, and raw coals has been examined by inductively coupled plasma-optical emission spectroscopy and solid-state 1113 magic angle spinning-nuclear magnetic resonance spectroscopy. The results indicate that, regardless of the coal rank, boron has a lower concentration in HPC than in the corresponding raw coal and coal residue. Up to 20% of the original boron could be eluted. The eluted boron is an organoboron complex associated chemically within the coal matrix which has the following characteristics: resonates from -5.88 to -7.00 ppm in B-11 NMR spectra, completely decomposes when coal is ashed by plasma at 150-200 degrees C, and is insoluble in acetic acid while preferentially extracted by organic solvents. No boron-bearing mineral particles have been detected in HPCs. Regarding the elution amount of boron, it is a function of the fraction of organoboron in raw coal and HPC yield. For raw coal having more organoboron as well as a higher HPC yield, a larger fraction of its original boron could be extracted accordingly. The eluted boron could also in part undergo chemical reactions with coal radicals and/or polar indole to form new coordination states in HPCs.