| The coal resources are abundant in our country,the soot particles derived from coal combustion will affect the temperature field distribution in furnace,which emissed into the atmosphere will produce huge harm to the environment and human body.The soot particles mainly form in the boundary layer of the coal particles,and the gasification of the alkali metal Na in coal crosses the boundary layer,these two processes overlap in the spatial area,and researches show that the alkali metal Na can inhibit the formation of soot,however,the research that combines these two processes is rarely reported.Therefore,to study the effect of alkali metal Na on the soot is of great significance for the efficient use of coal and reduction of atmospheric pollutants.In this paper,the experiment of rapid pyrolysis in pure N2 atmosphere was carried out on self-designed and constructed drop tube furnace.Under different content of Na,temperature and residence time,the physicochemical properties(such as formation,microscopic structure and chemical construction)of pyrolysis products of demineralized coal and synthetic coal would be comparatively analyzed to explore the influence of alkali metal Na on the conversion process from tar to soot.Full sampling of soot samples was carried out by a large flow sampling instrument,and the number and mass concentrations of the particles were measured on-line with a electric low pressure impactor(ELPI).The soot and tar were separated by methylene dichloride combined with ultrasonic shock.The content of metal ions in coal and soot were detected by a microwave digestion instrument combined with inductively coupled plasma aomic emission spectrometer(ICP-AES).The content of functional groups of soot samples were tested by fourier transform infrared spectroscopy(FTIR).The microstructure of soot was observed by transmission electron microscopy(TEM).The research results show as below: With the increase of temperature,the yields of soot derived from coal supported by alkali metal Na increase first and then decrease,and the tar yields decrease at first and then increase;at the temperature of 1373 K,1523K,1673 K,the soot yields are 0.97%?2.08%?1.62%,the tar yields are 0.58%?0.47%?0.63%,respectively.With the increase of residence time,the yields of soot derived from coal supported by alkali metal Na increase,and the tar yields decrease,and the total yields of soot and tar remains at a relatively constant value;at sampling muzze distance from the top of the furmace 0.8m,1.3m,1.8m,the soot yields are 1.59%?1.76%?2.08%,the tar yields are 1.18%?0.95%?0.47%,respectively.With the increase of contents of alkali metal Na,the soot yields decrease,the tar yields increase,and the total amount of soot and tar remain at a relatively constant value;at the contents of Na 0.57%,0.81%,1.35%,the soot yields are 2.12%?1.98%?1.39%,the tar yields are 0.49%?0.55%?0.94%,respectively;the change of both the average diameter of the basic soot particles and the polymerization process are not obvious;the contents of O-H and N-H decrease slightly,the contents of alkyl radical increase,the contents of C?C decrease,and the contents of C=C decrease.In the presence of alkali metal Na,the yields of soot derived from model compound pyrolysis increase from 4.32% to 5.50%,the tar yields decrease slightly;the change of both the average diameter of the parimary soot particles and the polymerization process are not obvious;the contents of O-H and N-H increase slightly,the contents of alkyl radical increase,the contents of C?C decrease,and the contents of C=C increase.The number and mass concentration distributions of soot derived from both coal and model compound present a single peak distribution,the peak value of number and mass concentration distributions at near 0.12?m and 0.2?m,respectively.This research indicates that alkali metal Na can both catalyze tar cracking and inhibit tar polymerization reaction,thus inhibiting the formation of soot derived from coal pyrolysis. |
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