Coalification and coal alteration under mild thermal conditions

Abstract

Coalification temperatures are often considered to be approximately 100-170°C for bituminous coal and 170-275°C for anthracite. However, our micropetrographic observations, solid state 27 Al magic-angle spinning nuclear magnetic resonance measurements, interpretation of d 13 C values for whewellite in pelosiderite concretions from Carboniferous sediments, and assessment of whewellite thermal stability show that coalification temperatures can be significantly lower. Also the temperatures of coal alteration may be substantially lower than is stated. Ordinarily, hightemperature alteration is reported, but microthermometric measurements of fluids temperatures and micropetrographic observations show that the coal alteration can take place at low temperatures. For this reason, coals from the Kladno-Rakovník Basin, part of Late Paleozoic continental basins of the Czech Republic, were analyzed. Regarding coalification, micropetrographic characterizations of unaltered coals, the presence of thermally unstable Al complexes in the coal organic mass documented using 27 Al MAS NMR method, and proven occurrence of whewellite in pelosiderite concretions suggest a lower coalification temperature, max.~70°C. Regarding coal alteration, micropetrographic observations revealed (a) the weaker intensity of fluorescence of liptinite, (b) mylonitic structures and microbreccia with carbonate fluid penetration, and (c) high oxygen content in coals (37-38 wt.%). These phenomena are typical for thermal and oxidative alteration of coal. As the temperature of carbonate fluids inferred from fluid inclusion analysis was evaluated as * 100-113°C, the temperature of coal alteration was suggested as * 113°C; the alteration was caused by hot hydrothermal fluids.

Prototaxites reinterpreted as mega-rhizomorphs, facilitating nutrient transport in early terrestrial ecosystems

“…In contrast, the Heider quarry fossil presents a clear separation between the G and D2 bands. The presence of both bands has previously been recognized as an indication that the fossil experienced temperatures high enough to partially transform the organic matter into anthracite (Straka and Sýkorová 2018), which agrees with the different thermal maturation temperatures calculated for each fossil of 128 ± 10 and 269 ± 14 • C, respectively. This could also explain the results obtained by FTIR for the Heider quarry fossil, which did not show any remains of aliphatic vibrations containing, e.g., carbonyl or alkene groups.…”

Evolution of Aromatic Clusters in Vitrinite-Rich Coal during Thermal Maturation by Using High-Resolution Transmission Electron Microscopy and Fourier Transform Infrared Measurements

“…Thermal simulation experiments are generally used to study structural changes during coalification. − One of the two major controlling factors, time and temperature, will compensate for the other one’s effects on coal maturity. − In other words, coal samples exposed to high temperature for a short time can have an equivalent maturity with samples exposed to lower temperature for a longer heating time. , Thermal simulation experiments can be divided into an open system, a closed system, and a semiclosed system . Among these systems, the closed system is a good method to simulate the coalification process at in situ stratigraphic status, if some water is added into the system .…”

A Probable Origin of Dibenzothiophenes in Coals and Oils

“…For the condition of the mild reaction temperature, it can be easily achieved by the geothermal energy that also provides heat for the coalification process [45][46][47]. What is more, the current investigation shows that the higher the temperature of the thermal degradation of thiophenolate is, the faster the generation of DBTs is.…”