Combustion And Functional Groups Migration Characteristics Of Hydrothermally Upgraded Coal

Low-rank coal has high water content and low calorific value,which limits its use.In addition,nitrogen and sulfur in low-rank coal usually produce sulfur oxides and nitrogen oxides during combustion and use to pollute the environment.Based on this,this paper uses hydrothermal technology to upgrade low-rank coal,study the influence of upgrading on low-rank coal fuel characteristics,combustion pollutant emission characteristics and functional group migration characteristics,revealing the inherent relationship between functional group structure migration and pollutant discharge.The main results are as follows:(1)Hydrothermal upgrading can increase the calorific value of coal and reduce the volatile matter content.After hydrothermal upgrading at 380 °C,the calorific value of Guizhou coal was increased from 24.87 MJ/kg to 24.98 MJ/kg,and the volatile content was decreased from 8.33%.to 6.44%.Under this condition,the calorific value of Zhundong coal was increased from 25.61 MJ/kg to 27.42 MJ/kg,increased by 7.07%.The content of volatiles was decreased with the increase of hydrothermal temperature,which was decreased from 32.35% of raw coal to 23.60%,increased by 27.05%.(2)Based on the thermogravimetric analysis method,the effect of hydrothermal upgrading on coal combustion characteristics was studied.The ignition temperature of Zhundong raw coal by hydrothermally treated was higher than that of raw coal.The ignition temperature was also increased with the increase of hydrothermal temperature.The ignition temperature of the raw coal was 392.96 °C.After hydrothermally treated at 300 °C,the ignition temperature of the coal sample was increased to 393.93 °C,and the ignition temperature of coal sample by hydrothermally treated at 300 °C was further increased to 449.75 °C.After Guizhou coal was hydrothermally treated,the ignition temperature was also higher than the ignition temperature of the raw coal.The ignition temperature of Guizhou raw coal is 514.57 °C.After hydrothermally treated at 260 °C,the ignition temperature of treated coal was increased to 521.61 °C.After hydrothermally treated at 340 °C,the ignition temperature of treated coal was 530.15 °C.The ignition temperature of the two coal samples were both increased with the increase of the hydrothermal temperature,because the volatile matter content in the coal was decreased with the increase of the hydrothermal temperature.The combustion characteristic index is a comprehensive parameter measuring the behavior of coal combustion.A comprehensive analysis of the relationship between the combustion characteristics index of two coal samples and the hydrothermal temperature was found: when the hydrothermal temperature was below 300 °C,the combustion characteristic index of coal was increased with the increase of hydrothermal temperature.When the hydrothermal temperature was beyond 300 °C,the combustion characteristic index was decreased as the hydrothermal temperature increased.The combustion characteristic index of Zhundong raw coal was 4.99×10-8.After hydrothermally treated at 300 °C,the combustion characteristic index of Zhundong coal sample was increased to 6.71×10-8.The combustion characteristic index of coal sample after 380 °C hydrothermal heat decreases to 4.90×10-8.The burning characteristic index of Guizhou raw coal is 3.16×10-8.After 300 °C water heating,the combustion characteristic index of Guizhou coal sample increases to 4.47×10-8.The combustion characteristic index of Guizhou coal sample after 380 °C hydrothermal heat reduces to 2.46×10-8.It shows that when the hydrothermal temperature is less than 300 °C,the increase of hydrothermal temperature is conducive to the combustion of coal,and the excessively high hydrothermal temperature is not conducive to the combustion of hydrothermal coal.(3)In the fixed bed reactor,the pollutant emission characteristics of the upgraded coal combustion process were studied.The results showed that the pollutant emission of coal after the hydrothermal body decreased,and as the hydrothermal temperature increased,the amount of SO2 emitted by the hydrothermal coal combustion decreased gradually.reduce.The SO2 emission of Guizhou raw coal is 205.39 mg/g.When the coal sample passes 380 °C hydrothermal,the SO2 emission is only 93.96 mg/g,and the hydrothermal emission is only 45.75% of the raw coal.This is because part of the hydrothermal process.Sulfur contains the opportunity to form H2 S precipitated coal.Sulphides convert into sulfates in the hydrothermal process.In addition to the analysis of coal with water,the hydrothermal reaction converts partially unstable organic sulfur into thiophenes at 900 °C.At the combustion temperature of,some of the thiophenes have not been burned in the solids,reducing the amount of SO2 discharged into the air.Among them,saturated calcium hydroxide as an additive can greatly reduce SO2 emissions at 260 ° C hydrothermal temperature,but with the increase of hydrothermal temperature,the reduction is small,because calcium hydroxide is still after hydrothermal It will adhere to the surface of coal,and after SO2 is formed by combustion,SO2 is adsorbed to form calcium sulfite,thereby achieving the result of reducing SO2.(4)The migration characteristics of nitrogen,oxygen and sulfur-containing functional groups before and after thermal upgrading of two low-order coals were studied by XPS.The results show that hydrothermal upgrading can remove oxygencontaining functional groups,and with the increase of hydrothermal upgrading temperature,The content of each oxygen-containing functional group including a carboxyl group,a hydroxyl group,and a carbonyl group is lowered.The form of organic sulfur changed.As the hydrothermal temperature increased,the content of mercaptan and thioether decreased,and the conversion to more stable sulfone and thiophene was carried out.The content of sulfoxide and sulfone decreased,and it also converted to thiophene.N-6 is converted to N-5 in coal,and N-6 and N-5 are converted to N-Q.When the hydrothermal temperature reaches 380 °C,a large amount of N-X is formed,and all nitrogen-containing compounds are converted to N-X.(5)The effect of additives on the migration characteristics of functional groups was studied.The results showed that the addition of hydrogen peroxide can oxidize thiophene to sulfoxide and sulfone.At higher hydrothermal temperatures,organic sulfur can be oxidized to sulfate and discharged with water.In addition,the hydrothermal upgrading also converts some of the hard-to-remove organic sulfur into easily removed inorganic sulfur,and some of the sulfur-containing organic matter is broken and generates hydrogen sulfide gas to escape from the coal.With the increase of hydrothermal temperature,hydrogen peroxide as an additive has a great influence on the functional group.With the participation of hydrogen peroxide,the thiophene which is extremely difficult to decompose in coal will be oxidized to form sulfoxide and sulfone,and some thiophene will be directly converted into sulfate.