Study On Secondary Oxidation Characteristics Of Coal With Different Preoxidation Degree

Coal is an important energy source in China.In order to save resources and improve the coal's recovery ratio,it is necessary to unseal the enclosed fire rone or to repeat minning the residual coal in goaf.However,these coal seams have been oxidized by contacting with the air and the spontaneous combustion characteristics may have changed.As so called this type of coal's re-oxidation as the secondy oxidation.The secondy oxidation mean the raw coal has been pre-oxidationed and the temperature has dropped back to room temperature before it combustion secondly.In this paper,the coal were selected by three different mines and preoxidized to 80?,120?,160? and 200? respectively,then make all of the samples back to 30? in nitrogen atmosphere.The secondary oxidation and primary oxidation of coal with different preoxidation degree were studied.The result can provide the theoretical basis for the forecasting technology of spontaneous combustion of unsealing the fire or repeat minning the residual coal.The main contents are as follows:(1)The industrial parameters and the original molecular structure of the coal samples were tested by industrial analysis and KBr compaction method.It was found that the volatile content of coal samples which were pre-oxidized to 120 C are increased.In the molecular structure of different coal samples,the content of oxygen-containing structures are the most,then are aromatic hydrocarbons,aliphatic hydrocarbons are the least.The change of molecular structure of different preoxidized coal samples is complicated.Preoxidation to 120 C of coal samples of hydroxyl,carboxylic acid groups and aliphatic hydrocarbons' content are the highest,but ether and substituted benzene have no regular.(2)The TG/DSC-FTIR experiment was used to analyze the characteristic temperature,thermal effect and gas production of the primary oxidation and secondary oxidation process.The results show that the secondary temperature of the coal samples are mainly different before them breaked down.The characteristic temperature of coal with different preoxidation degree decreases with the increase of preoxidation degree,and decreases first and then increases.And usually the characteristic temperature of coal samples are the lowest when preoxidation is to 120?.In terms of thermal effect,the coal samples absorption heat first and then exothermiced in the process of oxidation.The exotherm of oxidative combustion of coal samples increases first and then decreases with the increase of preoxidation.But which preoxidation coal samples release the maximum heat are not sure as the coal' metamorphism are different.The coal of pre-oxidation to 160? in Da-Nan lake release the maximum heat.Gu-Bei coal and Yangquan coal are the sample of pre-oxidation to 120? which release the maximum heat.In the gas products,the gases of CO,CO2 and H2O(g)are increasing first and then reduced with the temperature increased.The amount of gas produced by three kinds of metamorphic coal increases first and then decreases with the increase of preoxidation.The coal of Pre-oxidation to 120? produce the largest amount of CO.The coal from Gu-Bei and Yangquan produce the most amount of H2O(g)for the samples pre-oxidized to 120?,Da-Nan lake produce the most amount of H2O(g)for the samples of pre-oxidized to 80? and 120?.(3)The Fourier transform method was used to study the molecular functional groups in the secondary oxidation process of coal samples.The results showed that the molecular structure of coal with different preoxidation degree was the same in the second oxidation process,but the amount of change was different.The molecular structure of different coal samples are common except a small amout of structures in the process of oxidation.The content of free-OH bond,ether bond,aliphatic hydrocarbon,aromatic ring C=C structure and aromatics-CH decrease with the increase of temperature,as the substitution of benzene and carbonyl bonds increases with the temperature increased.The total change of the molecular structure before and after oxidation in the secondary oxidation of coal samples with different preoxidation levels are common except a small amout of structures.The maximum change in the total content of coal's molecular structure are the samples of pre-oxidation to 80?-160?,and in these coal samples,the ratio of more than half the maximum are pre-oxidation to 120 ?coal samples.(4)The kinetics of coal samples was analyzed by improved KAS method.The result is that in the stage of water evaporation and gas desorption,the activation energy of coal samples was decreased with the temperature rise.The activation energies of the three kinds of metamorphic coal are the same as those of the different pre-oxidation degree,and the activation energy of the coal which pre-oxidized to 120? are the smllest,and the activation energy of the fresh coal and the coal of pre-oxidized to 200 ? are the largerst.In the stage of oxygen uptake,the activation energy increases with the temperature rise,and also the activation energy of the coal which pre-oxidized to 120? are the smllest.