Study On Microwave Heating Characteristics Of Coal Gangue And The Process For The Deoxygenation Of Coal Mine Methane

To the best of our knowledge, coal mine methane(CMM) and coal gangue are two kinds of wastes released in coal mining. CMM mainly consists of methane, which is considered to be a unconventional natural gas resource. Because air was mixed during underground extraction operation, direct utiliztion easily explode, and in order to prevent exploding, it is necessary to remove oxygen. Coal gangue, as a solid waste, possesses the characteristics of low calorific value, high ash and is difficult to burn. The huge stockpile of coal gangue has occupied amounts of land and resulted in severe environmental pollution. In previous studies, we come up with a deoxygenation technology that heating coal gangue removes oxygen in CMM and simultaneously to consume carbon in coal gangue effectively, realizing resource utilization of those two wastes ultimately. Nevertheless, conventional electric furnace fails to apply in large-scale industry because of the long heating time and the low energy efficiency. Microwave irradiation, as a new-type of fast and efficient heating technology, has been successfully used in mineral processing and metallurgical industry. So the present work focuses on CMM deoxygenation process by microwave heating coal gangue, involving that the microwave heating characteristic of coal gangue and the influence of different technology parameters on deoxygenation are investigated in detail, the mechnisms of these phenomenon are discussed, and the opitimum technological conditons are obtained in the end. The main contents of this research are as follows:(1) The rapid heating of coal gangue can take place when using microwave heating compared with conventional heating. After studing the heating characteristic of different components in coal gangue, it was found that the un-carbonaceous mineral compositions can hardly absorb microwave so that it rarely contributes to the action of rising temperature, while it is the carbonaceous compositions that play an important role, whose graphitizition futher improve the microwave absorbtion ability in turn.(2) Comprehensive characterizations including TEM, XRD and Raman are employed to explain the phenomenon of graphization. The results show that with the increase of temperature, the structure of coal gangue goes from random to gradual integrity, and the degree of graphization enhances, resulting in the improvement of the microwave heating character. This process is likely to be that the microwave heating make carbon migration more orderly, so that the graphization of carbonaceous compositions can happen under the lower temperature, which can be verified by the behavior of coke heated by microwave.(3) The influence of the textural properties of coal gangue before and after microwave heating on deoxygenization is discussed, and the result shows that the specific surface area of coal gangue becomes larger when treated by microwave, which facilitates its contact to CMM, and benefits removing the oxygen.(4) In view of the methane loss in deoxygenation, extensive discussions have been made based on the combination of experimetal results and relevant references. It is shown that the pyrolysis of methane is the main reason of methane loss. Specifically speaking, the increment of reaction temperature or the carbonaceous compositions will accelerate this process.(5) In this paper, the influences of several CMM deoxygenation process conditions, such as the temperature of microwave heating, the type, usage amount, particle size of coal gangue as well as the the oxygen content of CMM, are detailedly investigated. The results show that the temperature of microwave heating poses an effect on the deoxygenation by changing the oxidation rate and the amount of the methane pyrolysis. Different kinds of coal gangue have different amount of carbonaceous compositions, leading to different deoxygenation effect; The amount of coal gangue affects the contact area and contact time with CMM, meanwhile, the particle size affects the stacking porosity, which finally affects deoxygenation effect; The higher the oxygen content is, the more favorable the methane pyrolysis becomes. The optimal technological parameters were obtained, that is, microwave heating temperature of 650 °C, coal gangue particle size of 124 μm, coal gangue mass of 100 g, CMM flow rate of 300 m L/min. For CMM with O2 concentration less than 16.8%, the appreciable deoxidization effect can be achieved with the oxygen content in CMM is less than 1%, and the loss rate of methane is lower than 2.2%.