Aggomeration And Combustion Characteristics Of Coal Slime

With the massive exploitation and utilization of coal in recent years,the quality of the existing coal is worse,and the environmental problems are very grim.Therefore,most of the coal is mined to be washed and separated,used by quality,coal slime is the byproduct of the coal preparation.The high moisture content and moisture holding capacity,high ash content and low calorific value limit the utilization of coal slime.The characteristics of high ash and low calorific value will affect the rapid burning out of coal slime and bring about certain boiler wear problems;the characteristics of high moisture will bring about adverse effects such as lower bed temperature of fluidized bed,lower combustion efficiency,Thermal burst and agglomeration phenomena occur at high temperatures.The agglomeration characteristics of coal slime will cause the particles to form agglomerates instead of being reduced to a fine powder and blown away,thereby improved combustion efficiency.However,an excessively large agglomerate is not conducive to the burnout of the coal slime,and is likely to cause the bed accumulation.studying the effects of different feed characteristics and operating conditions on the combustion characteristics of coal slime based on its unique characteristics,and proposing a low-energy technology suitable for stable combustion of coal slime are of great significance to the efficient and clean use of coal slime.In this paper,flotation tailings coal slime in three different areas of Shandong Jining,Shanxi Linxian,Shaanxi Zichang were used.The static combustion characteristics of coal slime and the evolution of the macroscopic and microscopic structures of the combustion process were studied using a horizontal tube furnace experimental system.With the use of a muffle furnace,the agglomeration of the coal slime under large-diameter feed conditions were studied,and bench-scale fluid bed reactor was used to study the effect of moisture on combustion and emissions characteristics of coal slime.Combined with multiple analytical testing methods and data analysis,a deeper understanding of the characteristics of coal slime has been achieved,and more valuable conclusions obtained through the data analysis.The static combustion experiment of slime was carried out in a horizontal tube furnace,the combustion characteristics of single coal slime particles in air at different furnace temperatures were studied.The temperature changes in the center of the coal slime particles and combustion images during the combustion process were obtained.The rapid pyrolysis of the coal slime resulted in the production of small molecules of volatile gas,tar and other pyrolysis products.Next,the pyrolysis caused an unusual change in the surface structure of the particles.The temperature changes in the different types of coal slime varied;however,those in the same coal slimes with different particle sizes at the same furnace temperature were similar.With increasing particle size,the diffusion of internal oxygen slowed down,which resulted in slower combustion processes and increases in the ignition delay and burnout times.The exothermic performance index varied for the different coal slimes and decreased with an increase in particle diameter,which was related to the contents of volatiles and ash in the coal slime.A high-definition camera was used to capture changes in the macrostructure of the particles in the process of combustion,and a large amount of educt at the stage of homogeneous combustion was observed.The main component of the educt was amorphous carbon,as determined by X-ray diffraction(XRD),which demonstrated the process whereby the organic carbon molecule structures are transformed from a complicated and disordered condensed aromatic nucleus macromolecular structure to ordered amorphous carbon molecules with simple structures inside coal slime particles.Using a conducting N2 adsorption test and analysis,this study found that there were a number of mesopores and a handful of micropores inside the coal slime particles.The pore structures experienced a process that varied from complicated at the initial moment to simple,to complicated and finally to simple.Scanning electron microscope(SEM)photos proved the change process of the above pore structures and found the phenomenon whereby pyrolytic products filled the pores at the stage of homogeneous combustion.The increase of feed diameter will promote the formation of larger agglomerate,and increase the rate of agglomeration.The agglomeration characteristics of coal slime is also affected by the shape of the feed material.The effect of the shape is mainly due to the difference in the specific surface area,and the lower the specific surface area of the shape,the higher the agglomeration rate;however,the excessively large surface area will also accelerate the process of evaporation of water in the inside of coal slime and promote agglomeration.The agglomeration characteristics of coal slime are significantly affected by the furnace temperature.As the furnace temperature increases,smaller agglomerates are formed,and the agglomeration rate decreases.Different types of coal slime agglomeration characteristics are affected by the properties of the coal slime's original grain size,volatile matter content,ash content,and composition.The coal slime with larger original granularity is beneficial to the formation of the agglomerates.In addition,the coal slime with lower content of volatile matter and higher content of ash is easy to form a agglomerates.But excessively high agglomeration rate is not conducive to the rapid burnout of slime.Combined with the shape of coal slime agglomeration,it can be judged that the process of coal slime agglomerates formation is mainly layer-by-layer thermal explosion.In order to further study the fluidized combustion characteristics of coal slime,the influence of moisture content on coal slime combustion and emission characteristics was studied in a fluidized bed combustion experiment.The basic combustion parameters of coal slimes with different moisture contents were obtained by using thermogravimetric analysis and indicate that moisture delayed the ignition of coal slime and that lowmoisture slime is more conducive to combustion.The effect of water content on the bed temperature change and NOx and SO2 emission are obtained in a bench-scale fluidized bed reactor,revealing that the lowest bed temperature decreases with the increase in moisture content.Moreover,with the increase in moisture content,the observed degree of blackening of the flue gas increases,and more CO is produced,which in turn leads to a lower combustion efficiency of the coal slime.However,the presence of moisture plays a positive role in the reduction of NOx and SO2.With the increase in moisture content,the amount of NOx and SO2 produced tends to decrease.