Study On The Emission Of Methane-rich Gas During Deep Underground Coal Gasification

China’s deep coal reserves are abundant,with up to 2.9 trillion tons of coal over1000 m.With the reduction of shallow coal resources,the development and utilization of deep coal resources have become an inevitable trend.However,the traditional mining methods of deep coal resources are facing many challenges due to high stress,high temperature,high gas and other disaster issues.Underground coal gasification（UCG）is expected to become an important way of deep coal mining and conversion.Deep UCG is characterized by high reaction pressure and high methane content,which is expected to become an important supplement to coal-to-gas in China in the future.In this context,this thesis decomposes deep UCG into pressurized pyrolysis and pressurized gasification,and systematically investigates the characteristics and mechanism of methane-rich syngas generation,in order to provide a scientific basis for deep UCG to produce methane-rich syngas.Relying on the key project of Beijing science and technology plan,"Study on key technologies of low-cost hydrogen production by in-situ conversion of deep coal seam and equipment development",we carried out pressurized pyrolysis and pressurized gasification experiments with the dense coal core samples from the Ehebaolige coal field in Inner Mongolia as the research object.The release characteristics of methane-rich syngas during the pressurized pyrolysis of dense coal samples were studied.Combining FT-IR,XRD,SEM,¹³C NMR and pressurized TG-FTIR characterization methods,the evolution of functional groups,carbon crystallite structure and pore structure of char was investigated,and the mechanism of CH₄ generation during pressurized pyrolysis was revealed.Parameters such as carbon conversion（x_c）,gasification activity index（R_s）and converted hydrogen yield（η_c）were introduced to investigate the effects of key parameters such as pressure,temperature and vapor-to-oxygen ratio on the release characteristics of methane-rich syngas.The effect of the applied catalyst on the release profile characteristics of methane-rich syngas in the UCG processes was investigated;the mechanism of the role of dispersant in the catalytic pyrolysis process was revealed;an inexpensive liquid composite catalyst suitable for UCG processes was developed.Based on the characteristics of UCG with wide temperature pyrolysis,700°C was selected as the typical pyrolysis temperature and the pyrolysis pressure range from atmospheric pressure to 3 MPa,focusing on the influence law of pressurization.The main conclusions of this study are as follows.（1）The study of pressurized pyrolysis of dense coal samples showed that the CH₄yield increased significantly and the H₂ yield decreased with the increase of pyrolysis pressure.Compared with atmospheric pressure,when the pressure reached 3 MPa,the CH₄ yield increased from 32.7 L/kg to 77.3 L/kg,the H₂ yield decreased from 63.7 L/kg to 33.7 L/kg,and the pyrolysis gas yield increased by 47.2%,which was mainly due to the contribution of CH₄.Curve-fitted of FT-IR of coal char showed that the fat side chains of coal char became longer and the number of branched chains decreased with the breakage of chemical bonds such as bridge bonds during atmospheric pressure pyrolysis,but pressurized pyrolysis promoted the breakage of long fat chains.Carbon microcrystalline structure studies showed that when the pressure is greater than 1 MPa,the stacking heights（Lc）and lateral size（La）decreases with the increase of pressure,and the influence of pressure on the graphitization degree of pyrolysis char is not obvious.In addition,the pressure leads to increased deformation of the char surface,which promotes the generation of mesoporous structures around 10 nm and contributes to the generation of richer pore structures in coal char.The pressurized TG-FTIR study showed that the light aromatics in tar started to be generated at around 300°C during pressurized pyrolysis,and the generation rate was maximum at around 480°C,and the gas release profile had a wide temperature range（300-700°C）.The C₂-C₄ start to be generated at around 300°C,and the generation rate is maximum at around 400°C,and the temperature range of the release profile is narrow（300～500°C）.CH₄ starts to release at around 400°C,and the generation rate is the largest at around 500°C.The peak of gas release profile of volatile shifted to higher temperature with the increase of pressure.The activation energy of coal samples in the fast condensation stage and fast pyrolysis stage gradually increased with the increase of pyrolysis pressure,and pre-exponential factor gradually decreased.In the slow pyrolysis stage,the activation energy decreases and then increases with the increase of pyrolysis pressure,and the pre-exponential factor increases and then decreases,mainly due to the plasticization of char in the slow pyrolysis stage.（2）Studies on the mechanism of CH₄ generation during pressurized pyrolysis show that when the temperature is below 500°C,the generation of CH₄ is dominated by the cleavage of methoxy and long fatty side chains on coal macromolecules,etc.When the pyrolysis temperature is higher than 600°C,CH₄ mainly comes from secondary reactions such as hydrogenation of coal char.The carbon crystallite structure study showed that the higher pyrolysis pressure reduced the density of marginal carbon atoms on the surface of the char and the aliphatic carbon hydrogenation reaction was the main reason for the increased CH₄ yield during pressurized pyrolysis.Curve-fitted of ¹³C NMR showed that the relative content of oxy-quaternary carbons was significantly reduced from 9.2%to 1.8%,and the pressure promoted the generation of CH₄ from the oxy-quaternary carbon chain as the active site in the aliphatic group.（3）The reaction of coal with carbon dioxide（α/%）of the raw coal sample was only34.8%at the gasification temperature of 800°C.When it was greater than 1000°C,αexceeded 90%and the raw coal showed good reaction of coal with carbon dioxide.The experiments of char pressurized gasification showed that the H₂ and CH₄ contents in syngas increased with the increase of pressure.Compared with atmospheric gasification,the syngas yield increased by 8.8%when the gasification pressure reached 3.0 MPa,in which the relative CH₄ content increased from 0.24%to 2.04%.The CH₄ was generated from the hydrogenation reaction of char with H₂ under pressurized conditions.The temperature was increased from 900°C to 1050°C,and the reaction rate of char was accelerated and the gasification time was reduced by half,but the CH₄ yield was decreased.At the gasification temperature of 1050°C,the reaction rate increased with the increase of vapor-to-oxygen ratio,which promoted the water gas shift reaction,and the content of H₂ and CO₂ in the syngas fraction increased significantly.When the vapor-to-oxygen ratio increased from 1:1 to 2.5:1,the converted hydrogen yield in syngas increased from 961.9 L/kg to 1118.6 L/kg.（4）The high-pressure impregnation catalysts Ca O,Ca（NO₃）₂,Fe（NO₃）₃ and KNO₃could effectively improve the syngas yield,and the CH₄ yields were increased by 5.5%,2.4%,8.7%and 0.7%,respectively.Considering the economic factors and catalytic effect,Ca O was chosen as the active component and sodium lignosulfonate was introduced as the dispersant to investigate the catalytic pyrolysis effect,and it was found that the CH₄ yield in pyrolysis gas could be increased by 9.8%.Carbon microcrystalline structure studies showed that the catalysts were all effective in inhibiting the thermal condensation of the basic structural units of the char in both the lateral and longitudinal directions.The mechanism of dispersant action based on FT-IR and XPS analysis showed that the dispersant promoted the combination of Ca with oxygen-containing functional groups of coal char to form calcium-containing complexes,which led to more uniform dispersion on the surface of char and better catalytic effect,while hindering the graphitization process of char.Therefore,sodium lignosulfonate as dispersant and Ca O as an active component can be used as catalysts for in situ catalytic pyrolysis process has a good catalytic effect.The study of char catalytic gasification showed that when K₂CO₃ and Ca O were used as catalysts,the gasification reaction time was shortened by 45.7%and 8.4%,respectively,and the reactivity index（α）was increased by 85.7%and 43.4%.In contrast,the gasification reaction time was shortened by 58.4%and the reactivity index（α）was increased by 167.9%when K/Ca was used as a binary composite catalyst,in which the CH₄ yield in syngas was increased by 26.5%.XRD and SEM analysis showed that Ca O could hinder the interaction of acidic minerals such as kaolinite with K salts,resulting in a significant coupling of K/Ca catalysts in the gasification process.