Mechanism Of Low Temperature Pyrolysis And Oxidation Of Low Rank Coal And Its Resistance To Oxygen Inhibition

With the transfer of the main battlefield of China’s coal mining to the west,coal spontaneous combustion has become a key factor restricting the safe mining of coal.From the perspective of coal macro characteristics and microstructure,this thesis deeply analyzed the low-temperature pyrolysis behavior of coal,revealed the influence mechanism of low-temperature pyrolysis behavior on reoxidation reaction,and studied the inhibition mechanism of inhibitor on coal low-temperature pyrolysis and oxidation reaction.The main conclusions are as follows:（1）The results of thermogravimetric and pyrolysis experiments show that there is an obvious interactive relationship between the thermogravimetric characteristics and characteristic temperature and the coal metamorphosis degree.The coal metamorphosis degree is negatively correlated with the mass loss and maximum thermogravimetric rate of pyrolysis reaction,but positively correlated with the characteristic temperature;the coal with high metamorphosis degree is difficult to enter the active decomposition stage because of its large gas adsorption capacity,few oxygen-containing functional groups and stable structure,leading to the dehydration and degassing reactions occur easily.In the process of coal heating,water evaporation is the key factor affecting the early heating of the sample.The coal with lower metamorphosis degree has more oxygen-containing functional groups and rich side chains,high heating rate and large amount of pyrolysis gas.（2）The microstructure test of raw coal and low-temperature pyrolysis coal shows that with the increase of pyrolysis temperature,the formation of alkyl radicals in coal molecular carbon skeleton and the escape of hydrocarbon gases reduce the content of fat structures;the value of X_BP increasesbecause of the condensation structure increased.The content of C-O structures with low dissociation energy decrease significantly with the increase of temperature;the content of C=O structuresdecreases first and then increases,and the turning point temperature is related to the coal metamorphosis degree;the generation and escape of CO₂ is the main consumption way of-COO-.Based on the carbon structure and functional group content,the macromolecular structure models of raw and pyrolysis coal are constructed.（3）Molecular dynamics calculation determines that the turning point temperatures of the primary and secondary pyrolysis reaction stages of NM,HLH and DH macromolecular models are 2000 K,2100 K and 2100 K respectively.The main reaction process in the primary pyrolysis stage includes:the alkyl ether bridge bonds with low dissociation energy break first,and then the C-C bonds and oxygen-containing functional groups decompose to generate gases and tar radical fragments to produce active sites.Among them,CO₂ is the earliest gas product,mainly from the desorption reaction of carboxyl and ester groups.The main reaction process in the secondary pyrolysis stage includes:decomposition and polymerization of char molecules.The reaction mechanism is char+tar→char+tar+intermediate products+free radicals.H₂O and H₂ are produced in large quantities at this stage,mainly from phenolic hydroxyl,H·and carbonyl groups.In this stage,CO₂ can be reduced to CO,H₂O and C₂H₂O under the action of high temperature.It is worth noting that the formation of CO and C₂H₄ runs through the whole pyrolysis reaction process,and the main sources are the decomposition of carbon based and long-chain aliphatic hydrocarbons respectively.（4）The in-situ FTIR test results show that the active sites of pyrolysis samples undergo oxidation reaction after contacting with oxygen,resulting in the quantity of oxygen-containing functional groups higher than that of raw coal,which increases the tendency of coal spontaneous combustion;through the TG-FTIR combination experiment,it is found that the low-temperature pyrolysis coal at about 200℃has the lowest combustion characteristics and the highest spontaneous combustion tendency.The pyrolysis oxidation chain reaction model determines that the peroxy radical generated by oxygen absorption reaction and the·OH radicals which are both reactants and products are the key active groups in the process of coal oxidation;the radical products in each stage of the chain reaction can participate in the reaction again,resulting in the shortening of alkyl carbon chain in coal and the release of CO,CO₂ and H₂O.（5）Through the evaluation of antioxidant inhibition effect,the antioxygen inhibition characteristics were clarified.The inhibition difficulties of low-temperature pyrolysis coal were revealed,and the key reaction stages and active groups to be inhibited were determined.The results show that the addition of BHT and TPPI can inhibit the reaction temperature,reaction rate and heat release power in the combustion stage.Among them,BHT,as a proton donor,can combine with a variety of active free radicals in coal to form stable compounds,so as to inhibit coal spontaneous combustion and inhibit various sites in pyrolysis oxidation reaction;Tpp I inhibits the chain reaction by capturing peroxy radicals and alkoxy radicals.The action site iss less than BHT,but the inhibition effect is better.