N-Methyl-2-pyrrolidone: A Green Solvent for Coal Extraction

N-Methyl-2-pyrrolidone (NMP) is a versatile, green industrial solvent that can be used in a variety of industries and applications such as paint and coating removal, pharmaceutical solubilizer, engineering plastic coating, agrochemicals, electronic/industrial/household cleaning, separation of olefins and aromatics in petroleum and other industries, refined oils, etc. In addition, NMP is a polar aprotic selective coal solvent and a good H-bond acceptor with affinity for aromatic nuclei. Coal extraction using NMP under atmospheric reflux conditions has been reported with extraction yields ranging from 0.1% to 50.9%. The extraction yield is closely related to the grade of the coal. However, the extraction capacity of a single solvent for coal is quite limited.

Ethylenediamine (EDA) has been shown to be a powerful coal swelling solvent capable of extracting large amounts of coal. It is considered a special coal solvent because of its ability to disrupt specific interactions in coal, such as H-bonds. Therefore, a sequential extraction method using NMP and EDA in a stepwise manner may help to improve the extraction yield. Chinakuri coal, a low-rank bituminous coal, and Loiyo coal, a high-rank bituminous coal, were selected as candidate coals for the study. Different solvent sequences of NMP and EDA were selected on the premise that extraction with a weaker solvent would open up the structure of the stronger solvent. EDA was found to be the stronger solvent. Swelling studies using NMP, EDA, and a mixture of NMP+EDA (1:1, v/v) were also conducted. Interestingly, the maximum swelling of the coal occurred in the mixture, followed by EDA and then NMP. Since swelling is entirely dependent on the interaction forces between the solvent molecules and the polymer segments and is not affected by stirring or agitation, the above results indicate that the NMP+EDA solvent system has a synergistic effect. The NMP solvent system containing a small amount of EDA is called e,N.

For all the coals studied, the highest extraction yield was obtained in the e,N solvent system compared to NMP or EDA. Continuous extraction using the e,N solvent system can also be performed in a stepwise manner. SCC yields ranging from 45% to around 70% can be obtained by extracting coal in steps using a NMP solvent system containing a small amount of EDA.

In general, the presence of EDA is preferred during the extraction process, otherwise pretreatment of the coal with EDA and subsequent removal of the EDA can adversely affect the extractability of the coal in NMP. Using a NMP solvent system containing a small amount of EDA, approximately 45%-70% of the coal can be extracted in 4 hours by step extraction. Extraction of coal with NMP containing a small amount of EDA does not involve the breaking of chemical bonds. Therefore, the coal interacts with EDA and NMP only through physical interactions. Adding a small amount of EDA to NMP not only increases the extraction yield in NMP, but also reduces the extraction temperature of NMP extraction of coal under reflux conditions, thereby saving thermal energy.

References:

[1] SHAILAJA PANDE; D. K S. Ethylenediamine-Assisted Solvent Extraction of Coal in N-Methyl-2-pyrrolidone: Synergistic Effect of Ethylenediamine on Extraction of Coal in N-Methyl-2-pyrrolidone[J]. Energy & Fuels, 2001, 16 1: 194-204. DOI:10.1021/ef0001742.

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