Identification Of Combustion State Of Coal Fires Based On Gaseous Product Generation Characteristics And Study Of Residual Structures

Coal fires in China are currently facing the problems of many new fire areas,rapid development and easy re-ignition.The main reason for this is that the state of hidden fires and their development trends are unknown,resulting in the inability to carry out timely and targeted prevention and control of coal fires during the critical period of its occurrence,development or re-ignition.Therefore,there is an urgent need to investigate the combustion status and residual structure of coal fires at different stages.In this thesis,the characteristics of multi-component gas generation and gas-tar component evolution during coal combustion and pyrolysis were elucidated by means of a fixed-bed on-line mass spectrometry system and gas-mass spectrometry experiments,and a method for identifying the combustion state in the fire area based on the multi-component gas and gas-tar generation characteristics was established.The structural characteristics of the extracted residual tar and residual coal were revealed by gas mass spectrometry and in situ diffuse reflectance Fourier transform infrared spectroscopy.The main conclusions are as follows:（1）The multi-component gas generation characteristics of the coal fire front at different oxygen concentrations are elucidated.During combustion and pyrolysis of the coal samples,the concentration of carbon and oxygen bodies were proportional to oxygen;alkane,olefin and alkyne gases were produced in a parabolic pattern,with methane peaking at 400-450°C and the rest of the gases peaking in the 340-380°C range and decreasing in concentration as the number of carbon atoms increased;benzene was produced in a lower concentration,reaching a maximum at the highest temperature;H₂increased exponentially with temperature and oxygen mainly influenced the concentration of hydrogen produced at high temperatures;sulphur containing gases were produced in a concentration proportional to the elemental S content of the coal samples.（2）A method for identifying combustion stages in the fire zone based on gas product generation characteristics is developed.The order of carbonaceous gas generation for lignite and long-flame coals is CH₄≈CO≈CO₂<CO（increasing temperature）<C₂H₆<C₃H₈<C₂H₄<C₃H₆<C₂H₂<CH₄（increasing temperature）<C₄H₈<C₆H₆<C₄H₁₀<C₅H₁₀,The initial generation temperature of C₆H₆ in gas coals is slightly higher than that of C₄H₁₀.A method for identifying the combustion stages in the fire zone is developed,and the temperature range for C₂H₂ production（125-150°C）is corrected for the first time,introducing C₄H₈,C₄H₁₀,C₅H₁₀ and C₆H₆ as gas indicators for high temperature coal fires.（3）The evolutionary characteristics of gaseous tar fractions at different temperature stages are elucidated.At 300-500°C,the content of aliphatic hydrocarbons in pyrolysis tar and combustion tar is inversely proportional to the reaction temperature,the content of aromatic hydrocarbons in pyrolysis tar increases and then decreases,reaching a peak at 350°C,while the content of aromatic hydrocarbons in combustion tar keeps increasing with temperature,the content of oxygen-containing organic matter in pyrolysis tar is proportional to the reaction temperature,while the oxygen-containing organic matter in combustion tar reaches a maximum at 450°C.Oxygen promotes the reaction and conversion of phenolic organic matter,which decreases the phenolic organic matter content in the tar and inhibits the condensation reaction to a certain extent,which facilitates the increase of aromatic hydrocarbon content in the tar,mainly naphthalene organic matter.（4）A method for identifying combustion stages in the fire zone based on the enrichment of gaseous tar products has been proposed.By analysing the total organic content and species in pyrolysis and burning tar at different temperatures,a model of the organic content in coal pyrolysis tar as a function of temperature is developed,and specific organic matter is found to be generated in burning and pyrolysis tar at different temperature stages,and a method for identifying the development stages of the fire zone using the organic content and species in gaseous tar is proposed.（5）The characteristics of the residual tar fractions in coal at different combustion stages in the fire zone are revealed.Coal pyrolysis and combustion residual tar fractions are mainly easily oxidised alkane organic compounds,which will enhance the combustion performance of coal in the fire zone,at 300-500°C.The lower the fire zone temperature,the more residual tar in the coal and the greater the risk of re-ignition in the extinguished fire zone.The alkane and phenol content of pyrolysis residual tar from lignite and low rank bituminous coals are positively and negatively correlated with reaction temperature,respectively,while the alkane content of medium rank bituminous coals is inversely proportional to reaction temperature and the phenol content is less affected by temperature.（6）The evolution characteristics of functional groups in residual coals in the fire zone and their relationship to the generation of gaseous products are revealed.The rate of phenolic hydroxyl depletion is proportional to the rate of phenolic organics production in the tar,and oxygen accelerates the breakage of phenolic hydroxyl groups in the coal.In pyrolysis reactions,aliphatic hydrocarbons are mainly produced as alkane,olefin and alkyne gas products,while they are also converted to CO and CO₂ in combustion reactions.The carbonyl group is the main source of CO and its rapid decline interval coincides with the CO peak temperature range.The decrease in the aromatic C-H bond leads to an increase in the aromatic hydrocarbon content of the tar;the change in the C=C bond indicates that condensation reactions occur in the coal during high temperature pyrolysis.The presence of oxygen increases the onset lowering temperature,reaction rate and burn-up temperature of the groups in the coal.There are 46 figures,31 tables and 91 references in this thesis.