Effects of Different Gaseous Plasmas and Plasma Treatment Durations on Coal Structure

Coal samples were treated with air, N₂, and O₂ plasmas for varying times to study plasma type and duration effects on coal structural characteristics. FTIR, Raman, and XRD analyses systematically evaluated the chemical structural changes. Results showed plasma type and treatment time significantly impacted coal's aromatic structures, aliphatic configurations, oxygen-containing groups, graphitization degree, aliphatic chain length, and aromatic stacking layers. FTIR parameters (<i>f</i> _a, <i>I</i>, DOC, <i>O</i> _c) indicated short-term air, N₂, and O₂ plasma treatments reduced aromaticity while increasing aliphatic groups and oxygen-containing moieties. Short-term plasma reduced crystallographic ordering with air plasma, causing the most pronounced structural changes. XRD revealed plasma-modified aromatic lamella densification via <i>d</i> ₀₀₂ spacing variations. Stacking height (<i>L</i> _c) and lateral dimension (<i>L</i> _a) contracted initially and then expanded with longer treatment for all plasma types. Aliphatic chain development (evidenced by an increased <i>L</i> _γ) influenced coal reactivity. Rising <i>N</i> and <i>P</i> values showed an increased graphitization with treatment time. The plasma-induced mechanistic changes in coal macromolecules were elucidated based on these structural alterations. This study first reveals dual-threshold effects of time-dependent plasma treatment on coal graphitization and reactivity.