Interactions of Illinois No. 6 bituminous coal with solvents: A review of solvent swelling and extraction literature

There has been copious research exploring the interactions between coal and various solvents either for coal structural investigations using coal swelling or industrial consideration exploring extraction yields. Here, the literature is reviewed for Illinois No. 6 coal. This is the most well-studied coal in existence and therefore offers an opportunity to reveal a wealth of information with a wide variety of solvents and extractions. This coal swells extensively (raw) in a good solvent such as pyridine (Q-factors of 2.2) and to lesser degrees in benzene (Q-factor of 1.1). However, if first extracted, the extent of swelling can be greater. The swelling process is typically slow, sometimes taking days with anisotropic swelling being evident on the first exposure but absent from subsequent exposures. Overshoot phenomena, similar to polymer swelling, is also observed for some particles. These behaviors are related to the complex interactions of solvent diffusion and coal "relaxation" that is related to the nature of the solvent and its interactions with coal. Extraction yields are similarly varied with values ∼30 wt % (dry ash-free (daf) basis) with pyridine although greater values are obtained with two-stage, higher temperature, and exhaustive extractions. With CS₂/NMP/H₂O, a yield as high as 70% wt % (daf basis) have been reported at 600 K. The use of light cycle oil has yields between 20 wt % and 70 wt %, depending on the conditions at 633 K. The swelling evidence supports a cross-linked structure and solvent extraction supports a considerable amount of extractable material. These statements are not necessarily in conflict.