| Coal fire has been a severe disaster and challenge in China. It burns out a large number of coal resources and seriously threatens the local ecological environment and groundwater resources at the same time. Currently, coal fire detection and project management in China have made remarkable achievements, but lacking specialized researches on the mechanism of the development of coalfield fire area. Therefore, we cannot accurately explain the occurrence and disaster-causing mechanism of coal fire. Meanwhile, the coal fire prevention work is facing new problems which need to be solved urgently. They are: new fire area generating process is unclear; characteristics and mechanism of action of the coal fire’s rapid development are unknown; characteristics and indicators of fire district resurgence governance are blurring. Taking these problems into account, this paper conducted relevant experimental and chemical reactions calculation study on typical low rank coals in fire zone. The research mainly studied the phased development characteristics of their oxygen-lean combustion, transformation mechanism of their structure and functional groups and kinetic mechanism of reactions.Thermal analysis calculation was applied to test the thermogravimetric dynamic characteristics of typical low rank coal from coal fire area in pyrolysis and different oxygen-lean combustion; based on the structural transformation of coal, we comparatively analyzed the characteristic temperature and development process of the coal combustion and pyrolysis, and revealed the development process of coal oxygen-lean combustion in view of structural differences in the thermal and oxidative conversion. The 9 combustion processes of coal structure transformation in oxygen-lean combustion were obtained, and 6 development categories of coal combustion were determined. From burning mechanism changing, burning capability, combustion stability, the maximum combustion intensity and apparent activation energy in coal oxygen-lean combustion, the influence of the oxygen concentration on coal fire combustion was revealed, and the limited oxygen concentration for coal oxygen-lean combustion was ensured in range of 1-3%.The evaluation of burning characteristics, structural transformation in thermal and oxidation and apparent activation energy verified above limited oxygen concentration.A new method of coal groups’ distribution and quantitative analysis of the infrared spectrum has been proposed based on quantum chemical calculations. Group distribution and structural characteristics of 15 samples of coal from lignite to anthracite from 5 coal-accumulation area with different coal-forming period.The accuracy of the new method was verified by infrared accurate analysis of the structural parameters of aromatic linear growth relationship with rank and vitrinite reflectance, which provides technical support for the further analysis of coal structural transformation and group distribution。Using Fourier transform infrared spectroscopy in situ testing techniques, in-situ sample pool contained adapt coal high-temperature reaction(~650 ?C) was improved, and the real time changes of infrared structure in coal pyrolysis and oxygen-lean combustion were measured. Based on the effect of high temperature, light intensity fluctuations and differences of initial infrared light intensity, the baseline correction method for three-dimensional infrared spectra of coal pyrolysis and combustion was constructed. The real-time changes of the groups and effects of the oxygen concentration were analyzed accurately. It can be illuminated that the breakage of the coal ether bridges and other weak bond is a key factor which has an influence on the preliminary separate temperature of the volatile. The initial temperature of the aliphatic hydrocarbons’ rapid reduction and the sustaining decomposition of ether oxygen bond affects the ignition of coal. The conversion from ether oxygen, aliphatic and aromatic hydrocarbons to oxygen-containing carboxyl, aldehyde and ketone carbonyl group, and consumption of oxygen-containing groups determines the coal comprehensive combustion ability in oxygen-lean conditions. Simultaneously, the oxygen limited critical functional groups transformation influence on combustion process of coal oxygen-lean combustion was revealed. The influence of rank and oxidation time on the oxygen-lean limiting functional groups changing was ascertained.According to the thermogravimetry analyzes of the coal combustion and the in-situ infrared real-time change of aromatic hydrocarbon, the combustion remnant and reburn character were clarified, based on the new calculation method of the combustion remnant and reburn index. Low oxygen can aggravate the conversion of the aromatic skeleton to volatile matter, and it can reduce the oxygen consumption rate of the volatile matter, which leaded to an increase of volatile. Meanwhile, reburn index of coal combustion under the oxygen-lean conditions was presented: the more remain volatile matter, the smaller reburn value and the better resurgence.Coal tar and char are two important residual products of coal fire. Based on thermogravimetric analysis(TGA) and in-situ infrared testing method, this paper studied the reactivity and changing rules of functional groups during the process of oxygen-lean tar combustion. It also analyzed the characteristics and transformation mechanism of functional groups during the tar combustion with oxygen concentration decreasing. Based on controllable pore structure Char and HRTEM reduction technology, three-dimensional char structure controlled pore distribution was established. The molecular dynamics method was applied to exploring reactivity and structural characteristics of coal char during combustion transformation. The pore development process with char reactivity was obtained: new pore created and enlargement increase char reactivity in initial burning stage, then pore consolidation decrease char reactivity but create more oxygen channel.Elementary reaction quantum chemistry calculation method is based on the analysis of coal native and secondary active sites frontier orbital during oxidation. According to this method, this paper comprehensively build the basic structural units of coal active sites; calculated the reaction pathways and kinetic parameters of their continuous elementary reaction process; established their mutual transformation and chain cyclic reaction pathways according to the sequence and continuity principle; and put forward a chain reaction mechanism of the development of spontaneous coal combustion. This paper also established the hydroxyl radicals and carbon centered radicals as critical active groups during coal oxidation; proposed elementary reactions which are critically linked by the formation and decomposition of peroxide; and suggested aliphatic hydrocarbon and their free radicals and critical exothermic groups. This paper established four successive elementary chain reaction pathways that generate carbon-oxygen products, and two hydrocarbon gas generating pathways; and put forward a new chemical combination methods of coal fire’s initial stage, which is centered of aliphatic hydrocarbon and their free radicals that are resistance and high exothermic reactions.Based on coal components, functional groups distribution and structural parameters, a molecular structure of jet coal was built from Hatcher model. The Reax FF reaction force field was used to exploring detailed reaction pathway and kinetics mechanism of coal structure transformation in pyrolysis and combustion. A reaction mechanism model of coal structure pyrolysis process via cleavage of backbone link bridge bond, interaction of small molecule intermediates with pyrolysis structural fragments was proposed. The group conversion reaction kinetics mechanism through coal combustion under different oxygen concentration changed oxygen attacked coal structure active parts and the proportion of active bridge bond self-decomposition was clarified. |
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