Study On Low-Temperature Oxidation Mechanism Of Coal Based On Macroscopic Behaviour And Microscopic Characteristic And Its Application

Coal spontaneous combustion caused by low-temperature oxidation is a key safety issue in the mining, storage and transportation of coal, which has been and continues to severely restrict the development of coal industry. Systematic research on low-temperature oxidation mechanism of coal is of the important theory significance and application value in prediction and suppression of coal spontaneous combustion.Three types of coal, i.e. Ximeng lignite (XM), Shendong sub-bituminous coal (SD), and Zaozhuang bituminous coal (ZZ), were used in this study. With the help of kinetics and intermediate complex theories, various research techniques and methods were employed to systematically study the macro behavior and micro characteristic during low-temperature oxidation of these three types of coal. On the basis of these results a more general mechanism of low temperature oxidation of different types of coal was explored and some applications in prediction and suppression of coal spontaneous combustion were also discussed.Main conclusions drawn from this study are summarized as follows:1) CO and CO2 production in low-temperature oxidation of coal is resulted from two parallel processes:thermal decomposition of oxygenated groups at coal macromolecule backbone and chemisorption of atmospheric oxygen. For XM coal, most of the CO2 formed stems from the inherent oxygen of the coal macromolecule, whereas most of the CO2 for ZZ coal are from the interaction between coal and atmospheric oxygen. For XM coal, most of the CO is produced from the interaction between coal and atmospheric oxygen; whereas most of the CO produced for ZZ coal is mainly from the inherent oxygen of the coal macromolecule. The precursors released CO2 are the carboxylic acid oxides and the activation energy of its release is independent of coal characteristics, whereas the activation energy for the release of CO is dependent on coal characteristics. The precursors of CO released from the low-temperature oxidation of XM coal is mainly the ketone oxides, and for ZZ coal it is the quinones oxides.2) The subtractive spectra of TG (Thermogravimetry) and DSC (Differential Scanning Calorimetry) can not only describe the intrinsic reaction between coal and oxygen, but also can reduce the error brought by experiment conditions such as heating rate and mass of coal sample. The evolutions of coal mass (m) and system heat (q), caused by oxidation reaction between coal and oxygen, are intrinsically relevant. The ratio of q/m obtained from the simultaneous thermal analysis of TGA-DSC based on a single heating rate test may be used as a parameter to evaluate coal spontaneous combustion tendency.3) The distinctions of existing forms and contents of aliphatic C-H component and C=O containing species are the major reasons for the different behaviors of low-temperature oxidation among different types of coal. Self-oxidation of the coal can be initiated by the oxidation reaction between oxygen and the -CH2- groups in the a position to aromatic rings. The reaction can take different paths according to the different chemical functionalities attached to the -CH2- in the a position. These differences cause diverse reaction pathways of low-temperature oxidation among different types of coal. The transformation of methyl and methylene during the oxidation process can be described by the second order reaction model. The transformation ways for different types of C=O containing species are different and dependent on coal characteristics.4) The formation of activated oxygen-containing complexes and their subsequent thermal decomposition play an important role in low-temperature oxidation of coal. In this thesis, the complex macromolecular matrix of coal was divided into elements C, O, H, S, and N, and these elements were involved in the oxidation reaction. The kinetic and thermodynamic characteristics reflect the nature of coal self-heating during low-temperature oxidation of coal. The low rate constant and frequency factor values found in all cases represent the considerably low levels of oxidation that occurred, and reflect that the reaction was kinetically very slow. A kinetic compensation effect between K and Ea was also observed during the changes in elements occurrence. The activation energy values for C, H, and N were positive. The activation energy associated with the release of H was comparatively lower than that of C and N. A negative activation energy has been observed for the changes in O occurrence. Negative △H values observed for the changes in O occurrence indicate that the O incorporated in coal matrix produces heat, while the release of C, H, and N was found to be endothermic. The release of C and H are closely related to the decomposition products of their intermediate complexes. The negative entropy values (AS) for the changes in elements occurrence indicate a decrease in the degree of freedom of the systems. The positive values of △G represent a non-spontaneous nature for the changes in elements occurrence during low-temperature oxidation of coal.5) Based on the kinetic and thermodynamic characteristics, a dynamic model was proposed:This model can be used to estimate the heat evolution over a period of time during the process of coal oxidation.6) Transferable hydrogen is an important index in assessment of coal reactivity. An experimental relation expression has been put forward to estimate the amount of transferable hydrogen in coal: Hr=△m×w1%/14+△m×w2%/15.5. This is to provide a basis for the reactivity prediction of coal oxidation.7) The kinetic characteristics obtained by various research techniques showed three stages of subsection characteristic of low-temperature oxidation of coal, i.e.,30-80℃,80-150℃ and 150-200℃. For the same coal, the reactivity difference between methyl and methylene is mainly reflected in the first and the second stages. For different types of coal, the difference in reaction activation between coal and oxygen is also mainly reflected in the first and second stages. Therefore, the activation energy in the first and second stages can be used as an index for coal self-ignition evaluation.8) Based on the difference in reactivity, there exist the self-oxidation behaviors among the same type of active functional groups and different types of reactive functional group in the same reaction sequence during coal oxidation at low temperature. The former finding is termed as "self oxidation behavior among the same type of reactive functional groups" and the latter observation is defined as "self oxidation behavior among different types and same sequences of active functional groups". The co-existence and interaction of these two kinds of reaction behaviors contribute the development of coal spontaneous combustion.