An Experimental Investigation In The Characteristics Of Oxidation Of Coal Slime And Its Char And Fuel-NO_x Formation In A Fluidised Bed

Coal slime（CS）is a main waste in the coal washing process and has the characteristics of high water content,high viscosity and high impurity,thus an inferior fuel.The circulating fluidised bed combustion technology（CFB）is an effective way to utilize CS.In the operating temperature range of fluidised bed,the emission of fuel-nitrogen oxides and its control are a key challenge.Scholars at home and abroad have carried out much research on nitrogen oxide emission during coal combustion,but the conversion process of fuel-N is still not comprehensively understood.In this thesis work,the concentrations of O₂,CO,CO₂,N₂O,NO,NO₂ and N₂ in the oxidized products were accurately determined during CS combustion using an O₂/Ar mixture as oxidant in a small fluidised bed in the temperature range of 900-1300 K,mimicking the circulating fluidised bed operation.At the same time,the oxidation characteristics of CS and CS char as well as the emission characteristics of fuel-nitrogen oxides and N₂ are investigated.The main conclusions from this research are as follows:（1）In the study of the effect of temperature on oxidation characteristics of CS and its char,increasing temperature decreases the concentrations of O₂ and CO in flue gas,while increases the CO₂ concentration.At high temperatures,the oxidation rate of CS and CS char is accelerated,and the oxidation rate of both is controlled by diffusion.For CS char combustion,the absence of volatiles reduces its reactivity relative to that of the original CS and thus reduces the rate of O₂ consumption,resulting in lower CO₂ concentration and higher CO concentration than CS at all temperatures.In addition,the effect of initial oxygen concentration（3%,4%,5%,6%,8%）on the oxidation characteristics of CS was investigated in the temperature range of 900–1300 K.The results showed that with increasing initial oxygen concentration,the concentrations of both CO₂ and CO increase continuously at low temperatures.However,at 1300 K,the concentration of CO increases first and then decreases due to the more intense oxidation reaction.（2）The trend of N conversions to nitrogen oxides and N₂ under different conditions were revealed.At low temperatures,say 900 K,the N element in CS and CS char is mainly converted to N₂O and NO.With increasing temperature,the conversion ratio of N₂O decreases,but a small amount（less than 5%）is still generated.More N elements begin to be converted into N₂ and NO,and the conversion ratio of N₂ increases,albeit still lower than NO.At each temperature,an increase in the initial oxygen concentration promotes the conversion from N to NO,while decreases the conversion ratio to N₂.In the lower temperature range of 900-1100 K,the conversion ratio of N₂O firstly increases and then decreases.（3）Finally,the effects of different preparation conditions on the oxidation characteristics of CS char and the emission characteristics of nitrogen oxides and N₂ were studied.The results show that different coking conditions have a great influence on the concentration of CO in the oxidation process.At 900 K,the pyrolysis conditions in fluidized bed at low temperature for a long time are more conducive to the generation of CO,while the pyrolysis conditions in the Muffle furnace at high temperature for a long time are more conducive to the generation of CO.At 1300 K,different conditions have little influence on the concentrations of CO and CO₂.For the char prepared in the fluidised bed,with increasing pyrolysis temperature,the concentration of NO is increased,while the concentrations of NO₂,N₂O and N₂ are decreases.With increasing pyrolysis time,a further decrease in the total-N results in decreasing NO and N₂O concentrations.For the char prepared in the Muffle furnace,at lower operating temperatures,the concentrations of N₂O,NO₂ and N₂ increase with increasing pyrolysis temperature.At 1300K,the concentrations of all components decreased.With increasing pyrolysis time,the concentrations of all components showed a decreasing trend.