Reaction Mechanisms And Characteristics Of Methane Generation During Pyrolysis Of Different Rank Coals

Methane generation during coal pyrolysis is not only an important process and reaction of coal pyrolysis, but also is main source of coal-generated gas and gaseous hydrocarbons, therefore, understanding of its formation characteristics and reaction mechanism is very important for the evaluation and exploration of gases accumulation. But because of complexity of coal structure, methane generation process may be the result of synthesis of various chemical reactions, identifying of these chemical reactions is the fundamental for the deeply comprehension of coalification chemical process and the reaction mechanism of methanegeneration. In this paper, the structure of 11 different rank coals was studied by FT-IR. Thermogravimetry-mass spectrometry (TG/MS) was used to investigate the pyrolysis behavior of coals, the instantaneous evolution of the methane and the effect of minerals on coal pyrolysis and methane evolution. On the basis of the investigation of methane evolution of different rank coals and combining with their structural feature, the characteristics and reaction mechanism of methane generation were studied. Following convulsions have been obtained.(1) Coal pyrolysis characteristics: During coal pyrolysis, with increasing of coal rank, the total weight loss reduces and the typical pyrolysis temperatures increase. Because more alkyl functional group and bridge bonds exist in medium rank coals, their DTG curves have the heaviest weight loss peaks. After demineralized by HCl-HF acids, total weight loss and the heaviest weight loss velocity increase markedly, the typical pyrolysis temperatures reduce for low rank coals, but for more rank coals this effect does not exist, which indicate that demineralization by HCl-HF can promote the pyrolysis of low rank coals.(2)The generation characteristics of methane during coal pyrolysis: The evolution of methane includes two stages, first stage is the temperature range from 100 ~ 345 ℃, second stage is 345 ~ 900 ℃ . Methyl form 342 ℃ to 900 ℃ . The second stage methane formation closely relates with the evolution of methyl. The first stage methane comes form desorption of adsorption methane in coal and not pyrolysis products. The evolution of methane and methyl during coal pyrolysis can be divided into four stages by rate curve-fitting, which are not simple parallel and series reactions. The analysis of their kinetics indicates that they are from four different reactions. The activation energies of the first and fourth constitute reaction are lower than the second and t hird c onstitute r eaction w hich r eflects t he d ifferent c hemical reaction mechanism.(3)Reaction mechanism of methane formation during coal pyrolysis: On basis on analysis of coal structure, the evolution and kinetics characteristics of methane generation, four constitute reactions are responsible for the thermal methane generation from different rank coals. Reaction I generates methane and releasemethyl groups by breaking bonds between C-0 and C-S bonds which only exists in low and moderate rank coals. The activation energy of this reaction has close relation with oxygen content in coal. With the O/C decreases, the activation energy increases. Reaction II is related to the release of methyl groups by secondary cracking of long chain hydrocarbons and cleavage of short aliphatic hydrocarbons. The former, which contributes the major part of total methane potential, mainly exists in low and moderate rank coals, especially in bituminous coal. The latter mainly exists in high rank coals and the greater activation energy are demanded. Reaction III : The origin of the methane's carbons are from methyl groups, which link to aromatic and naphthenic structures. This reaction is mainly responsible for methane generation in high rank coals. Reaction IV are polymerization and condensation reactions. Its activation energy is lower than reaction III.