Study On The Efficient And Clean Utilization Of Low Concentration Coal Mine Methane By Solid Oxide Fuel Cell

Coal mine methane extraction and utilization is the foundamental method to control the coal mine gas disaster,which can also turn the harm into the benefits to reduce the greenhouse gas emission and increase the clean energy amount.However,the utilization ratio of low-concentration coal mine methane?LC-CMM?is very low?<40%?,leading to the severe energy loss and environmental pollution.The low utilization ratio can be mainly attributed to the low and fluctuant methane concentration((8_CH4<30%)and the explosion risk of LC-CMM,which greatly increases the utilization difficulty.The existing internal combustion engine using LC-CMM has some serious defects such as low power generation efficiency?²5%?,high NO_x pollution output and large noise.Therefore,this thesis proposes the solid oxide fuel cell?SOFC?using LC-CMM,which has the distinct advantages such as high generating efficiency,low pollution output?no NO_x and noise output?,and all solid state without corrosion liquid leakage.The technology is promising to provide the new practical approach for the safe,efficient and clean utilization of LC-CMM,which can promote the safe production,energy conservation and emission reduction of coal mine.The composition of LC-CMM is complicated,which is rarely studied as the unconventional fuel of SOFC by now.The thesis studied the key technical problems of LC-CMM fueled SOFC,which used the multiple research methods including experiment,theoretical analysis,numerical simulation and engineering analysis.The main research results of this thesis are as follows:?1?In order to make the composition of LC-CMM meet the safe SOFC utilization requirement which prevents the gas explosion risk in the high-temprature environment of SOFC and avoids both carbon deposition and oxidiation of SOFC anode using LC-CMM,the deoxygenation and methane purification of LC-CMM was proposed to be realized through the minor-pressure vacuum pressure swing adsorption based on the adsorption kinetic selectivity of carbon molecular sieve?CMS?.The influence rule of technological parameters on deoxygenation and methane purification effect was discussed,and the safety of gas separation process was analyzed.The research results show the technique can decrease the oxygen concentration of LC-CMM from 19%to1.8%,improve the methane concentration from 3.5%to 8.6%and achieve the methane recovery ratio of 79.5%.The produced gas can meet the requirement of SOFC.Decreasing the desorption pressure and inlet gas amount can reduce the oxygen concentration and improve the methane concentration of the product gas,and increasing gas source pressure can improve the methane recovery ratio.The gas mixture in the adsorption column has no risk of explosion.?2?The oxygen to methane ratio R_mix was set as the key variable which represents the LC-CMM composition,and the influence mechanisms of R_mix on all kinds of SOFC performances were studied.The effect mechanism of O₂ in the LC-CMM on anode kinetics reaction was revealed and the H₂S and higher alkane tolerance of SOFC using LC-CMM was analyzed.The research results show the feasibility of LC-CMM fueled SOFC and provide the important reference for the optimization of operating conditions.The experiment result shows that SOFC can accommodate the composition change of LC-CMM(0?_mix?1.1),and maintains the good electrochemical performance and long-term discharging stability.Besides,the increase of R_mix can lead to the decrease of open circuit voltage?OCV?,peak power density?PPD?under high temperatures and internal reforming efficiency,and the increase of concentration polarization loss and oxidation risk,but can also reduce the carbon deposition,sulfur poisoning and activation polarization loss.The optimal interval of Rmix is 0.25?_mix?0.8,which can optimize the comprehensive performance of SOFC.The oxidation reaction of LC-CMM firstly improves but then decreases with the R_mix;O₂ in the LC-CMM can improve the anode tolerance for H₂S and higher alkane.In addition,the increase of R_mixix is beneficial to the anodic surface mass transfer and charge transfer,but can hinder the production and diffusion of electro-active H₂ and CO.?3?The multiphysics-coupled heterogeneous reaction kinetics and charge-transfer model was established to study the reaction mechanism of coal mine methane in SOFC anode.The calculated results show that oxygen-bearing LC-CMM mainly occurs CH₄partial/full oxidation reaction at anode inlet,but the steam reforming and water-gas shift reactions gradually improve along with the gas flow direction.O₂ can adsorb and dissociate on Ni metal surface to promote CH_x conversion and carbon removal.The theoretical boundarty conditions of carbon deposition and nickle oxidation of anode were also studied,showing the condition of 0.25?_mix?0.8 can prevent the carbon deposition and oxidation of anode.Besides,the theoretical results illustrate that the increase of coal mine gas flow rate and anodic pore diameter,thickness and specific area can improve the electrochemical performance of SOFC.?4?In order to solve the coking problem of LC-CMM fueled SOFC anode,the novel anti-carbon reforming layer Mo doped NiTiO₃?Mo-NiTiO₃?over anode was proposed,which can be decomposed into compound material of Mo-TiO_2-?and Ni metal with nano-network structure.Due to the strong water-adsorbing ability of Mo-NiTiO₃,high catalysis activity of Ni metal and the numerous reaction interface in the nano-scale structure,the novel material of reforming layer can greatly promote the wet reforming of CH₄ and water-gas shift reaction to prevent the carbon deposition on anode and produce large amounts of electro-active H₂ and CO.The nickle particles on the TiO_2-?substrate was used in the carbon deposition test and synchrotron radiation-based X-ray adsorption spectra test,further validating the good carbon-resisting performance and the carbon-resisting mechanism of the anti-carbon material.In addition,experiment result showed that the electrochemical performance,anode internal reforming efficiency,anti-carbon performance and long-term discharging stability of the SOFC with the reforming layer using LC-CMM were much superior to those of the SOFC without the reforming layer.?5?The high-power SOFC stack used in the engineering was studied to evaluate the electricity-generating performance and its impact factors,and the engineering design of large LC-CMM fueled SOFC system was conducted based on the performance paramenters obtained in the experiments.The above research is significant to guide the enginnering practice and optimization of the technique.The research results showed that the SOFC stack using LC-CMM((8_CH4=13%)had relatively high electrochemical performance.The peak power density and electricity-generating efficiency respectively reached 150 mW/cm² and 38.23%,and the fuel utilization ratio under the maximum efficiency condition was 72.2%.The temperature difference of different areas of SOFC stack is relatively small,and the thermal runaway cannot happen in the SOFC stack since the temperature rise caused by the LC-CMM oxidation reaction is not large.The SOFC stack using LC-CMM can maintain the long-term discharging stability.The above results demonstrated that the high effeciency and stability of SOFC stack using LC-CMM in the engineering application.Besides,the minor decrease of CH₄ concentration cannot cause the large decrease of SOFC stack performance,the decrease of N₂ content and LC-CMM gas flow can improve the stack power density,and the stack performance can significantly decrease when the operating temperature is lower than 730 ^oC.Finally,the 50 kW LC-CMM fueled SOFC system was designed,which gave the significant engineering parameters and the cost and income of the system.The benefit analysis shows the good market application prospect of the power system.The above research results provide the elementary theoretical,experimental and practice foundation for high temperature SOFC using LC-CMM,which is also great significance to optimize and improve the technology in the future.This thesis has 168 figures,16 tables and 159 references.