THE ROLE OF IRON CENTERS IN COAL OXIDATION: A SYSTEMATIC REVIEW AND QUANTITATIVE ESTIMATION OF SURFACE ACTIVE SITE DENSITY

Olga N. Shagarova 

Iron in coals occurs in various forms (pyrite, ultradispersed particles, ions in the carbon matrix) and plays a key role in oxidation processes, spontaneous combustion, liquefaction, and gasification. Although extensive experimental data have accumulated over recent decades, a systematic generalization linking quantitative estimates of the surface density of catalytically active Fe centers to coal rank has been lacking.

This study provides a systematic analysis of literature data on the content, occurrence forms, and catalytic activity of Fe centers in coals from different deposits and rank stages. Data from Russian (Agroskin, Epshtein, Larionov), German (Kreysa), American (Kaiser, Silbernagel), and Chinese (Chen) research groups over more than 40 years are analyzed and compared.

A quantitative estimate of the surface density of active sites n_act (m⁻²) is derived from three independent experimental approaches: (1) kinetics of coal interaction with ozone; (2) influence of Fe additives on the activation energy of oxidation; (3) electrochemical oxidation data with Fe centers as mediators. Conversion of active site concentration (mol/g) to surface density (m⁻²) uses data on specific surface area accessible to water and dissolved reagents, with sensitivity analysis for this parameter.

Based on the Epshtein classification, three coal groups are distinguished: Group 1 (minimal oxidation propensity, anthracites, lean coals) with n_act < 10¹⁴ m⁻²; Group 2 (pronounced oxidation propensity, gas-fat, fat, coking coals) with n_act = 10¹⁵–10¹⁶ m⁻²; Group 3 (high oxidation propensity, brown and long-flame coals) with a wider scatter (10¹⁵–10¹⁷ m⁻²) and lower center stability.

The estimate for Group 2 coals is supported by all three methods: from ozonation data (n_act = 4×10¹⁵–1.7×10¹⁶ m⁻²); from Fe additive data (n_0 = 10¹⁴–10¹⁶ m⁻²); and from electrochemical data (n_eff = 10¹⁴–10¹⁶ m⁻² after accounting for surface accessibility). Sensitivity analysis shows that variation of specific surface area in the realistic range (3·10⁵ – 3·10⁶ m²/m³) changes the estimate by no more than a factor of 3, preserving the order of magnitude.

Factors affecting center accessibility under in situ conditions (pore blocking by water, poisoning by sulfur, competition with methane, thermal deactivation) are analyzed, giving a conservative estimate for effective in situ density of n_eff ≈ (2·10¹⁴ – 5·10¹⁵) m⁻².

The quantitative estimates obtained may be useful for parameterizing theoretical models of catalytic processes in coals, including resonant oxygen activation methods for methane mitigation in coal seams.