Structural Characteristics Of Low-middle Rank Coals And Generation Mechanisms Of Methane And Hydrogen During Pyrolysis

Coal structure and composition are not only the important content in coal chemistry research, but also the basis of coal utilization and development with high efficiency. In view of chemical composition, coal is composed of large amount of molecules with different molecular weight; in view of petrology, coal is made up of different macerals; in view of structure, coal is an amorphous material with short range order but long range disorder; in view of the origin, coal has phased evolution characteristics, that is, coal is evolved through lignite to anthracite, the physical and chemical characteristics changed gradually. The above mentioned showed that coal structure is complex. So, different physical and chemical methods were used to study the coal structure. The results not only provide theory basis for the development of coal science, but also the basis for coal conversion.In this paper, different ranks of coals were used as the objects: WNT, SH, WLTG, HLH, YM, TF, PS, ML, XL, HDT. FTIR and TG-MS analysis were used in this paper, different FTIR spectrum in different coals were discussed to indicate geochemical mechanism during coalification. The evolution characteristics and dynamic mechanisms of methane and hydrogen were analyzed through TG-MS experiment.FTIR analysis showed that with the increase of coal rank, the aliphatic content decreased and aromatization increased. The higher amount of aliphatic hydrocarbon and the longer aliphatic chain are existed in HLH and YM coals, in which the aromatic/aliphatic is lower. The lower amount of aliphatic hydrocarbon and shorter aliphatic chain and higher aromaticity existed in higher rank coals. The aromaticity increased linear with the increase of coal rank; in lower rank coals, there are much oxygen-containing groups, which exist as side chains, with the increase of coal rank, the oxygen-containing group broke and exist mainly as oxygen-bridge in higher rank coals.The TG-MS analysis showed that the initial evolution temperature of methane is low and the initial evolution temperature of hydrogen is high, the evolution ranges of methane and hydrogen are broad; the total weight loss is related to the volatile matter in coal, the characteristic temperatures increased with the increase of coal rank. The amount of hydrogen evolved is related with the content of oxygen.From the FTIR spectra and dynamic analysis, combined with the evolution curves of methane and hydrogen, methane generation consist of four reactions : (a) reactions involving the cleavage of thermally unstable C-O and C-S bonds, firstly generating methyl, then methane was produced; (b) cracking of short aliphatic chain; (c) secondary cracking of long chain aliphatic; (d) polymerisation and condensation reactions, the generation of hydrogen is composed of five single reaction:(a)at the low temperature, dehydrogenation of hydro aromatics structures; (b) dehydrogenation of alicyclic structures; (c) cycloalkylation of aliphatic chain (d)aromatization of cyclanes ; (e) polymerisation and condensation of the aromatics nuclei.In the four reactions of methane, the first reaction has low activation energy, the second and third reactions have high activation energy and are the main reactions for methane evolution, the activation energy of the fourth reaction is also low but the value is higher than that of the first reaction. The different reaction has different activation energy. The evolution characteristics of methyl has close relation with that of methane, so the evolution of methyl and methane is closely related.During the process of coal evolution, the structural parameters are closely related with the individual activation energy of methane and hydrogen; with the decrease of H/C and O/C, the individual energy of methane increased and the individual energy of hydrogen increased first and then decreased. The dynamic analysis of hydrogen evolution showed that hydrogen generation has two different processes. The first stage is from 430 to 600℃and it's a slow generation stage. In this stage, the structure of coal macromolecule has severe cracking and pyrolysis, hydrogen generation may caused by condensation of free radical generated in the first stagethe second stage is from 600 to 1000℃, most hydrogen was generated in this stage, the main reaction in this stage is condensation.