The effect of solvents and processing conditions on the solvent extraction of coal

Solvent extraction of two bituminous coals to produce low-ash extract material was studied using solvents dimethylformamide (DMF), dimethylacetamide (DMAC) and N-methylpyrrolidone (NMP). The extraction efficiency was seen to be NMP > DMAC > DMF for the reflux extraction at the normal boiling point of the solvent. While the reflux-extraction of raw coal gave less than 40% yield, NMP-extraction at 350°C and elevated pressure gave higher yield (∼68%). Hydrogenated coal gave very high extraction yield (>70%) even at room temperature. The performance of DMAC was comparable to that of NMP for the extraction of hydrogenated coal. The reflux extraction showed mass balance error in the range of +/-5%. The positive mass balance was attributed to solvent retention and the tendency of retention was found to be much higher for NMP than for DMF and DMAC. The ash content and coking value were seen to be independent of the extraction solvent. The extract showed ash content <0.5%.