Sequential Transformation Behavior Of Minerals And Environmental Stability Of Gasification Residue During Underground Coal Gasification

As an in-situ coal mining and conversion technology,underground coal gasification（UCG）is expected to become a strategic reserve technology of“coal-to-gas”in China,which is of great strategic significance for the construction of China’s“clean,safe and efficient”modern energy system.UCG is carried out in coal seams hundreds of meters or even more than one thousand meters underground,and the actual reaction conditions of UCG are difficult and costly.However,the mineral characteristics of UCG residues are closely related to the process conditions of the actual underground gasification reaction,and the characteristic high-temperature minerals are expected to indirectly indicate the actual reaction conditions in the underground coal seam.At the same time,the environmental stability of UCG residues has always been a key issue of concern,and it is also one of the bottlenecks in the application of underground coal gasification technology.Therefore,this paper systematically studies the sequential transformation behavior of minerals in coal and the environmental stability of gasification residues in the process of UCG,which has important theoretical and practical significance for the industrialization demonstration and application of UCG technology.In this thesis,Ulanqab lignite,Ulankarma hyper-iron coal（Fe₂O₃>15%）and Xinjiang Zhundong hyper-alkali coal（Ca O>15%）are selected as the research objects,and the decomposition process of UCG into raw coal pyrolysis process,semi-coke reduction process and residual oxidation process are divided according to the formation process of ash and slag.Using the method of combining experimental determination and thermodynamic simulation calculation,the sequential transformation behavior of typical minerals in the process of UCG is comprehensively and systematically studied.Especially,the co-transformation behavior of coal and rock minerals under the condition of coal partings was studied,and the correlation between characteristic high temperature minerals and the actual reaction conditions of UCG was established;At the same time,the migration and transformation mechanisms of trace elements in the UCG process are analyzed,and the exist speciation of trace elements in UCG products are further studied by step extraction method.Combined with the actual reaction conditions of UCG,the risk assessment code（RAC）is proposed to evaluate the risk level of UCG residues to the underground environmental to reflect the environmental stability of residues under different reaction atmospheres and temperatures.At the same time,the leaching characteristics of trace elements in the UCG products are investigated,and the types,concentrations,leaching rates and influencing factors of trace elements are identified.The transformation behavior of minerals and trace elements is supplemented by thermodynamic simulation software Fact Sage 7.3,the following conclusions are obtained:1)In the third chapter,the sequential transformation behavior of typical iron-bearing minerals in low-iron coal and hyper-iron coal during UCG was studied.The results show that the iron-bearing minerals in the coal are transformed from pyrite（Fe S₂）to pyrrhotite(Fe_1-xS)during the pyrolysis process.After pyrolysis of low-iron coal,orthoclase in raw coal disappears,while sanidine appears in semi-coke.In the process of pyrolysis of hyper-iron coal,kaolinite in raw coal decomposes gradually.The typical iron-bearing minerals found in the ash under reduced atmosphere in the temperature range from 900-1300℃involve magnetite（Fe₃O₄）,ferrous oxide（Fe O）,hercynite（Fe Al₂O₄）and augite（Fe Si₂O₆）.Magnetite changes to Fe O,and then,Fe O reacts with Al₂O₃ in the low-iron coal to produce hercynite at 1000℃,while Fe O reacts with Si O₂ in the hyper-iron coal to produce augite（Fe Si₂O₆）.This is consistent with the results obtained from the thermodynamic simulation results.The typical iron-bearing minerals during the oxidation process of residual-coke include hematite（Fe₂O₃）,hercynite,augite and sekaninaite(Fe₂Al₄Si₅O₁₈).When the oxidation temperature increases to 1400℃,hercynite and augite are converted to the thermodynamically-stable sekaninaite.2)Hyper-alkali coal has a typical transformation characteristic in the UCG process.In the fourth chapter,the transformation behavior of calcium-bearing minerals in the UCG process was investigated.The results indicate that the transformation behavior of calcium-bearing minerals is closely related to the reaction temperature and atmosphere,calcium‐bearing mineral is gradually converted from gypsum（Ca SO₄·2H₂O）in the raw coal into anhydrite（Ca SO₄）during the pyrolysis process.In the reduction stage,anhydrite reacts with the reducing gas（CO）to produce oldhamite（Ca S）,and the oldhamite is stably present in the reduction ash.During the oxidation process,oldhamite is first transformed into Ca SO₄,and then Ca SO₄ is converted into Ca O.Finally,Ca O reacts with Al₂O₃ and Si O₂ to produce gehlenite（Ca₂Al₂Si O₇）at 1100°C.As the oxidation temperature rises to1400°C,gehlenite is transformed into the thermodynamically stable anorthite（Ca Al₂Si₂O₈）.With the further progress of the reaction,anorthite will co‐melt with iron‐bearing minerals.3)UCG is carried out in the underground coal bed,and its gasification object is non washed heterogeneous coal seam,which is mixed with different depth of gangue layer with inorganic minerals as the main component.In the fifth chapter,the influence of gangue intrusion on the sequential transformation behavior of minerals in UCG is studied.The results show that the intrusion of gangue will significantly increase the silicon aluminum content and silicon aluminum ratio in ash,thereby affecting the transformation behavior of minerals in the UCG process.The iron-bearing mineral present in raw coal is nontronite(Na_0.3Fe₂Si₄O₁₀（OH）₂.4H₂O),and the intrusion of gangue introduces new iron-bearing mineral glauconite(K（Fe,Al）₂（Si,Al）₄O₁₀（OH）₂).Pyrrhotite is the main iron-bearing mineral in the pyrolysis products.In the reduction stage,the content of Al₂O₃ in the bulk ash is increased due to the presence of the partings.The reaction of Fe O and Al₂O₃ in the coal with the partings produces hercynite above 1100℃.When the temperature increases to 1300℃,hercynite is converted to the thermodynamically-stable cordierite.In the oxidation stage,mullite is produced when the oxidation temperature reaches 1300℃,which could be used to reflect the reaction conditions in the coal seam.The intrusion of gangue will make it easy to form mullite,anorthite and other silicates in the coal ash,in addition,the intrusion of gangue will also lead to an increase in the melting temperature of the coal ash.4)The sixth chapter analyzes the migration and transformation of TEs（Hg,As,Se,Cd,Pb,Cr,F,Zn,Be,Ni,Ba,U）in the UCG process,the chemical speciation of TEs in the UCG residual and the effects of TEs on the underground environment.The results show that in the process of underground coal gasification pyrolysis,trace elements Pb and Ni will react with the S released by pyrite（Fe S₂）to form Pb S（s）and Ni S（s）,the main chemical speciation of Hg,As,Se,Pb,Cr,Cd in the semi-coke is the oxidizable form;The abundant clay minerals provide the possibility of the existence of various hazardous trace elements in the reduction ash.The oxidizable form of As is the main speciation in900-1300℃reduction ash.Cr,Cd,Se,and Pb have high reducible fraction in the reduction ash.The concentration of four TEs（F,Zn,Be,and Ni）decreased with the reduction temperature increase,while the content of Ba and U increased;For the oxidation ash and slag samples,Trace element Se mainly exists in the form of Pb Se（selenite）,and As is combined with the Fe Al₂O₄ in the oxidation ash and slag.The status of trace element F in coal is more complex,generally tends to combine with alkaline elements in coal,and may be adsorbed in layered minerals.The trace elements of As,Se,Pb,Cr,and Cd mainly exist in residual form.According to the risk assessment code（RAC）,the harmful degree of toxic elements to the underground environment decreases with rises of reaction temperature.When the reaction temperature reaches 1200℃,the harmful degree of toxic elements to the underground environment is range in the low risk.The results can provide scientific guidance for environmental risk assessment of underground coal gasification residuals.5)The trace elements（TEs）in the UCG residue may permeate into the underground water through the long-term leaching,causing the potential pollution of underground water.In chapter seven,the enrichment characteristics and leaching behavior of TEs in the simulated experimental residue of UCG.The results show that only about 1%of Hg remains in the UCG residue after undergoing the high temperature reaction.The trace elements Pb,Be,As and Se in coal undergo underground gasification and significant enrichment in the residue,and the mass concentration of As in the UCG residue is about8.5 times the mass concentration in gasified raw coal.The leaching rate of trace elements in the residues with different oxygen enriched volume fraction is very low,and the leaching amount is far lower than its original content in raw coal.In addition,the leaching level of trace elements in the residue is also affected by the oxygen enriched volume fraction of UCG.The leaching content of most trace elements decreases with the increase of oxygen enriched volume fraction.When the oxygen enriched reaction temperature is higher than 1200℃,the leaching level of trace elements in the residue in the leaching liquid at different p H（3.2,5.2,7.0）is lower than the relevant limits（GB5085.3-2007）,the gasification residue does not have leaching toxicity.