Influences of igneous intrusions on coal rank, coal quality and adsorption capacity in Hongyang, Handan and Huaibei coalfields, North China

Localized igneous intrusions with varying types of intrusion patterns were found in the Pennsylvanian–Permian coals in north China. Five typical patterns, including dike cut-through (pattern-I), dike cut-in (pattern-II), floor intrusion by sill (pattern-III), roof intrusion by sill (pattern-IV) and dual intrusions of roof and floor by sills (pattern-V), were investigated at five different underground profiles. It was found that the influence of localized intrusions on rank, petrology and coal quality characteristics are mainly related to the emplacement temperature, the style of heat transfer (convection or conduction), the intrusion forms (dike or sill) and size, the distance from the contact and the thermal properties of the surrounding rocks at the contact of the intrusion. Among the five patterns, only pattern-V was found to have two distinct contact metamorphic aureoles within a distance of 1–2 times of the thickness of the intrusion, whereas patterns I through IV show wave-like profiles of vitrinite-reflectance, ash and volatile matter. This resulted from the typical characteristics of “multiphasic and superimposed thermal metamorphic evolution” of north China coals. Except for the heat conduction by intrusion contact, the hydrothermal convection and tectonic-heat played important roles in heat transfer away from dike/sill. Intrusion-induced coal changes including coal rank, organic/inorganic composition and pore properties work together to influence the adsorption capacity of coals. The effect of intrusion upon the adsorption capacity of altered coals is related to the values of their altered coal ranks. Adsorption capacity is elevated from the Langmuir volume (VL) of 7.6 (pre-intrusion) to 17.5 m3/t (post-intrusion) for altered bituminous coals and semi-anthracites with VRr < 2.1%. In contrast, the adsorption capacity is moderately reduced from background levels of about 27.2 m3/t to about 19.3 m3/t for altered semi-anthracites and anthracites with VRr of 2.1%–3.4%. Adsorption capacity is significantly reduced (< 5 m3/t) for altered anthracites and meta-anthracites with VRr of > 3.4%, because of the accumulation of coal basic structure unit and distinctly decreasing pores. The relationships between altered coal rank and adsorption capacity are different for different coals, which is mainly due to changes of pore characteristics and coal structures resulting from three important coalification jumps during contact and/or other heat metamorphism.

► We investigated five intrusions patterns (dike cut-through, dike cut-in, floor, roof and dual).
► The effect of the intrusion patterns on influencing coal rank and coal properties is compared.
► Contact-metamorphism increases adsorption capacity of bituminous coals (< 2.1% VRr).
► Contact-metamorphism moderately decreases adsorption capacity of anthracites (2.1–3.4% VRr).
► Contact-metamorphism significantly decreases adsorption capacity of meta-anthracites (> 3.4% VRr).