Coal Characterization and Extraction Techniques

Coal characterisation and extraction techniques form the foundation for efficient utilisation of this globally abundant fossil resource. Characterisation typically involves proximate and ultimate analyses to quantify moisture, volatile matter, fixed carbon and ash, alongside microscopic examination of maceral composition to reveal the distribution of vitrinite, liptinite and inertinite phases. Advanced spectroscopic methods such as Fourier-transform infrared spectroscopy and nuclear magnetic resonance provide insight into functional groups and molecular connectivity, while thermal methods including thermogravimetric analysis and calorimetry elucidate combustion and pyrolysis behaviour. Extraction techniques range from conventional physical cleaning by flotation and dense-medium separation to chemical routes such as solvent extraction, oxidative depolymerisation and catalytic ethanolysis. Thermal dissolution and ionic-liquid pretreatments weaken cross-linking in low-rank coals, enhancing yields of benzene carboxylic acids or other value-added chemicals. Pyrolytic processes convert coal into tars, gases and char for downstream applications, whereas gasification under CO₂ or steam produces synthesis gas for chemicals or power. The interplay between detailed structural characterisation and tailored extraction methods underpins advances in clean-coal technologies, carbon capture and conversion of coal into speciality chemicals.

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One recent investigation employed Fourier-transform infrared spectroscopy and peak deconvolution to compare residues from single-stage solvent extractions with acetone, carbon disulfide and mixed solvents. Results demonstrated that mixed solvents preferentially extract phenolic compounds, whereas pure acetone yields longer straight aliphatic chains. Alterations in aromatic ring substitution patterns were quantified, informing strategies for selective removal of coal tar components in chemical refining.

A study based on thermogravimetric analysis and the Coats-Redfern kinetic approach examined sequential extract residues obtained by petroleum ether, carbon disulfide, methanol and acetone. Activation energies and pre-exponential factors varied markedly with residue composition, and the medium-temperature stage (350–550 °C) was identified as critical for depolymerisation and decomposition. These insights aid in the design of thermal conversion reactors for maximising volatile yields.

Another work explored catalytic ethanolysis of lignite using nanostructured Fe₂(MoO₄)₃ catalysts with rod-, sheet- and flower-like morphologies. Nanorods exhibited the highest ethanol-soluble fraction yield, boosting extraction from 29 % to 48 %. Enhanced production of ethers, esters and phenols was attributed to effective C–O bond scission on exposed (100) facets. This demonstrates the potential of tailored catalysts in mild-condition coal depolymerisation for high-value chemicals.

Coal Characterization and Extraction Techniques publication trend

The graph below shows the total number of articles in coal characterization and extraction techniques across all publications each year (not limited to Nature Index journals).

Technical terms

Maceral composition: The proportion of organic constituents (vitrinite, liptinite, inertinite) in coal, determined by microscopy.

Proximate analysis: A procedure to measure moisture, volatile matter, fixed carbon and ash content, used to assess coal quality.

Solvent extraction: A chemical method in which organic solvents dissolve specific coal components to produce extracts and residual solids.

Pyrolysis: Thermal decomposition of coal in an inert atmosphere, yielding tars, gases and char.

Ethanolysis: Depolymerisation of coal macromolecules using ethanol, often catalysed, to generate oxygenated compounds.

Activation energy: The minimum energy barrier that must be overcome for thermal decomposition reactions to proceed.

References

1. FTIR Analysis of the Functional Group Composition of Coal Tar Residue Extracts and Extractive Residues. Applied Sciences (2023).
2. Pyrolysis Kinetic Analysis of Sequential Extract Residues from Hefeng Subbituminous Coal Based on the Coats-Redfern Method. ACS Omega (2022).
3. Insights into the Relationship between the Microstructure and the Catalytic Behavior of Fe2(MoO4)3 during the Ethanolysis of Naomaohu Coal. Molecules (2023).