Preparation of High-Grade Carbonaceous Materials Having Similar Chemical and Physical Properties from Various Low-Rank Coals by Degradative Solvent Extraction

Abstract

Eight kinds of low-rank coals including lignites and sub-bituminous coals were subjected to a degradative solvent extraction method that treats carbonaceous resources in a non-hydrogen donor at around 350 °C. The low-rank coals were separated into the residue which cannot be extracted by solvent at 350 °C (termed residue), the fraction which can be extracted at 350 °C but precipitates from solvent at room temperature (deposit), the fraction which is solvent soluble even at room temperature (soluble), and liquid fraction mainly consisting of water and gaseous products mainly consisting of CO2. The soluble fraction was finally recovered as solid by removing solvent. The moisture of the coals was completely removed without phase change, and the ash was almost completely concentrated in the residue. The carbon based yields of the three solid fractions were 19.4–31.2% as solubles, 4.2–16.8% as deposits, and 54.7–69.2% as residues when 1-methylnaphthalene was used as the non-hydrogen donor solvent. Overall, more than 94.4% of carbon was recovered as solid fractions. Meanwhile 30.5–54.9% of oxygen was removed as either H2O or CO2. The interesting findings were that the solubles and deposits obtained from all of the coals were respectively very close to each other in elemental composition, chemical structure, molecular weight distribution, thermal decomposition behavior, and thermoplastic behavior. Elemental compositions of solubles were C = 81.8–84.8 wt %, H = 7.5–8.1 wt %, and O = 6.5–10.2 wt %, which were rather close to the elemental composition of bituminous coal. Thus, the degradative solvent extraction method was found to be effective in converting various types of low-rank coals into residues and compounds having very similar chemical and physical properties without losing heating values. Detailed characterization of the solid fractions showed potential utility of the fractions as solid fuel or precursors of chemicals and carbon materials.